A STUDY OF HIGH TEMPERATURE REACTIONS IN OXIDE-DISPERSION-STRENGTHENED MOLYBDENUM AT REDUCED OXYGEN PARTIAL PRESSURES

Transcription

1 A STUDY OF HIGH TEMPERATURE REACTIONS IN OXIDE-DISPERSION-STRENGTHENED MOLYBDENUM AT REDUCED OXYGEN PARTIAL PRESSURES A Thesis Presented to The Acdemic Fculty By Jelil Srh Mohmmed In Prtil Fulfillment Of the Requirements for the Degree Mster of Science in Mterils Science nd Engineering Georgi Institute of Technology July, 2004

2 A Study of High Temperture Rections in Oxide-Dispersion-Strengthened Molybdenum t Reduced Oxygen Prtil Pressures Approved By: Dr. Dvid N. Hill Dr. Jnet M. Hmpikin Dr. Robert F. Speyer Dr. Wyne L. Ohlinger Dte Approved July 8, 2004

3 ACKNOWLEDGEMENTS There re severl people who deserve my deepest grtitude, becuse without them this work would not hve been possible. First of ll, I would like to thnk Dr. Norm Hill for his guidnce, knowledge, nd for giving me the opportunity to work on this project. Also, the members of my thesis committee, Dr. Wyne Ohlinger, Dr. Jnet Hmpikin, nd Dr. Robert Speyer were gret help by giving their time to red my thesis nd provide vluble suggestions nd insight. The success of this reserch depended hevily on Richrd Shfer s skill nd expertise s he spent countless hours keeping the furnce nd other equipment running smoothly, in spite of ll of the chllenges we fced. I gretly pprecite the friendship of the other grdute students in the MSE deprtment, including Ksi, Chuck, Kip, Dwn, Mtt, Morgn, Chris, Will, Brent, Dn M., Rob, Shubhr, Dn C., Mike, Shwn, Lrry, nd Meliss. They hve ll mde these pst couple of yers enjoyble nd memorble. In prticulr, I would like to thnk Lur for lwys being so understnding nd supportive. Thnks re lso due to Erin for her motivting, cheerful ttitude (nd for her jelly bens). I m lso especilly grteful to Jeremy for hving so much fith in me nd for lwys being there for me. His unlimited ptience, kindness, nd encourgement kept me going during some of the most stressful nd difficult times. Finlly, I thnk my fmily for their love nd support throughout my eduction. My prents hve tught me some of life s most importnt lessons, nd they hve worked hrd to provide me with so mny gret opportunities in nd outside of school. Also, my sister, Nil, hs been very thoughtful, cring, nd encourging over the yers. My iii

4 fmily hs lwys believed in my bilities, sometimes even more thn I did, nd my ccomplishments would not hve been possible without their motivtion nd encourgement. iv

5 TABLE OF CONTENTS ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES SUMMARY iii vii viii x CHAPTER 1 INTRODUCTION 1 CHAPTER 2 LITERATURE REVIEW 3 Ferroelectrics 3 Dispersion Strengthening 4 Rre Erth Molybdtes 5 Phse Equilibri Studies 6 Preprtion Techniques 8 The L 2 O 3 -MoO 3 System 10 The L 2 O 3 -MoO 2 System 12 The L 2 O 3 -MoO 2 -MoO 3 System 12 The Y 2 O 3 -MoO x System 13 The ZrO 2 -MoO x System 17 The CeO 2 -MoO x System 18 CHAPTER 3 EXPERIMENTAL PROCEDURE 20 Preprtion nd Processing of Smples 20 Experimentl Setup 21 Temperture Control 24 Atmosphere Control 25 Chrcteriztion 27 CHAPTER 4 RESULTS AND DISCUSSION 28 LO 1.5 -MoO x system t 1200 C 29 Oxygen Prtil Pressures from 1x10-4 to 2x10-6 P 32 Oxygen Prtil Pressures from 2x10-6 to 3x10-7 P 32 Oxygen Prtil Pressure of 3x10-7 to 1x10-7 P 37 Oxygen Prtil Pressure of 1x10-7 to 8x10-9 P 42 Oxygen Prtil Pressure below 8x10-9 P 52 The LO 1.5 -MoO x system t 1000ºC 55 The YO 1.5 -MoO x system t 1200ºC 58 The YO 1.5 -MoO x system t 1000ºC 61 The ZrO 2 -MoO x system 62 v

6 CHAPTER 5 CONCLUSIONS 64 CHAPTER 6 RECOMMENDATIONS 66 APPENDICES 68 REFERENCES 71 vi

7 LIST OF TABLES Tble 3.1 Heting nd cooling rtes for furnce 25 Tble 4.1 Phses resulting from experiments with L 2 O 3 nd Mo t 1200 C 30 Tble 4.2 Phses resulting from experiments with L 2 O 3 nd Mo t 1000 C 57 Tble 4.3 Phses resulting from experiments with Y 2 O 3 nd Mo t 1200 C 58 Tble 4.4 Phses resulting from experiments with Y 2 O 3 nd Mo t 1000 C 61 Tble 4.5 Phses resulting from experiments with ZrO 2 nd Mo 63 vii

8 LIST OF FIGURES Figure 2.1 MoO 3 -L 2 O 3 Phse Digrm 11 Figure 2.2 MoO 3 -Y 2 O 3 Phse Digrm 15 Figure 3.1 Process Flow Digrm 21 Figure 3.2 Tube Furnce Used for Heting Smples 22 Figure 3.3 End Plte t Inlet of Furnce Tube with Pressure Guge 23 Figure 3.4 End Plte t the Outlet of the Furnce Tube 23 Figure 3.6 Prtil pressure of O 2 vs. frction of CO 2 in the gs flow 26 Figure 4.1 Phse equilibri in the LO 1.5 -MoO x system t 1200ºC 31 Figure 4.2 XRD Pttern for LAMO_53 34 Figure 4.3 XRD pttern for LAMO_10 35 Figure 4.4 XRD Pttern for LAMO_54 36 Figure 4.5 XRD Pttern for LAMO_60 39 Figure 4.6 XRD Pttern for LAMO_61 40 Figure 4.7 XRD Pttern for LAMO_62 41 Figure 4.8 XRD Pttern for LAMO_35 43 Figure 4.9 XRD Pttern for LAMO_34 46 Figure 4.10 XRD Pttern for LAMO_34 with Powder Diffrction File for Y 5 Mo 2 O Figure 4.11 XRD Pttern for LAMO_66 50 Figure 4.12 XRD Pttern for LAMO_31B 51 Figure 4.13 XRD Pttern for LAMO_20 53 Figure 4.14 XRD Pttern for LAMO_30 54 viii

9 Figure 4.15 Phse equilibri in the LO 1.5 -MoO x system t 1000ºC 56 Figure 4.16 XRD Pttern for Smple YMO_03 60 ix

10 SUMMARY Metl-oxide lloys contining Mo with rre-erth oxide re of interest for hightemperture structurl pplictions. It is unknown wht specific compounds, phses, nd crystl structures provide the properties of these high strength, deformble metl-oxides. This project intends to define phse equilibri of systems contining molybdenum nd rre-erth oxide, specificlly L 2 O 3, Y 2 O 3, nd ZrO 2 t high tempertures nd low oxygen prtil pressures. Preceding reserch defined rre-erth molybdte phses of 2:1 L:Mo compositions t 1200 C t oxygen prtil pressures rnging from P to 10-7 P. This study imed to identify phse boundries in the L 2 O 3 -Mo, Y 2 O 3 -Mo, nd ZrO 2 -Mo systems t 1000 C nd 1200 C t similr prtil pressures of oxygen. Experiments in the L system were extended to focus on phses t severl different L:Mo molr rtios. Preliminry experiments in the Y 2 O 3 -Mo nd ZrO 2 -Mo systems were designed to investigte the formtion of rre-erth molybdtes with vrious oxidtion levels of Mo. Phses nd compounds were produced using powder strting mterils rected in tube furnce under controlled oxygen prtil pressures. Chrcteriztion ws performed using x-ry diffrction, using published powder diffrction files for phse identifiction. Regions of stbility nd specific processing conditions were identified for mixedmolybdenum vlence phses in the L 2 O 3 -MoO x system t 1200ºC (L 12 Mo 6 O 35, L 4 Mo 2 O 11, nd L 6 Mo 2 O 14 ). A compound ws formed t 1:1.5 molr rtio of L:Mo between O 2 prtil pressures of 3x10-7 P nd 8x10-9 P for which the formul L 2 Mo 3 O 10 (with 2 Mo IV for 1 Mo VI ) is suggested. x

11 For smples run t 2:1 L:Mo molr rtio, XRD ptterns revel the existence of L-rich phse in equilibrium with Mo or Mo-rich phse. Bsed on previously reported compounds of the form Ln 5 Mo 2 O 12 with mixed molybdenum vlence of 3 Mo IV nd 1 Mo VI, this compound is believed to be L 10 Mo 4 O 24. Experiments in the LO 3 -Mo system t 1000ºC confirmed previous work, but generted no new informtion due to the stbility of Mo 2 C versus rre-erth molybdtes t low O 2 prtil pressures obtined in CO 2 -H 2 tmospheres. XRD ptterns from experiments with Y 2 O 3 reveled the existence of mixedmolybdenum vlence phse, Y 5 Mo 2 O 12, t both 1200ºC nd 1000ºC. Preliminry experiments in the ZrO 2 -Mo system did not yield ny rre-erth molybdtes. This study hs defined much of the phse equilibri in the L 2 O 3 -Mo systems, s well s specific processing conditions for some of the molybdtes formed t high tempertures t very low oxygen prtil pressures. Further work should concentrte on producing smples of these pure phses suitble for mechnicl testing, to help define the vribles tht contribute to the unique mechnicl properties of rre-erth molybdtes. Additionl reserch is lso necessry in the Y 2 O 3 -Mo nd ZrO 2 -Mo systems, prticulrly in the re of mixed-molybdenum vlence compounds, which my help elucidte the effect of crystllogrphy on plstic deformbility in these compounds. xi

12 CHAPTER 1 INTRODUCTION For yers, rre-erth molybdtes hve been known for their ferro-electric properties, but more recently they hve been hypothesized to form in oxide dispersion strengthened molybdenum (ODS-Mo) lloys. These lloys re produced by dispersing prticles of certin rre-erth oxides (REO) in molybdenum mtrix nd forming the mixture into composite ingot. During the high-temperture consolidtion process, the oxides my be converted into rre-erth molybdtes. During subsequent processing, the oxide molybdte phse(s) undergo(es) deformtion to form high-surfce-re ribbons tht serve to inhibit disloction movement, thus improving the mechnicl properties reltive to pure molybdenum or solution-strengthered lloys, such s Mo-Re. Rre-erth oxides used in oxide dispersion strengthening re observed to provide excellent strength nd disply deformbility. It hs been suggested tht this my be due to the presence of molybdte compounds rther thn pure dispersed rre-erth oxide prticles. The mechnicl properties of oxide dispersion strengthened molybdenum re somewht vrible, nd the formtion of vrious rre-erth molybdtes bsed on vritions in processing detils my explin, t lest in prt, why it is difficult to chieve highly reproducible behvior in processed lloys. It is lso considered likely tht molybdenum my disply rnge of oxidtion sttes (IV to VI) in these rre-erth molybdtes. It is unknown wht specific compounds nd ssocited crystl structures my provide these oxides with their high strength nd deformbility. Likewise, little is known of the phse equilibri in rre-erth oxide- (metllic) molybdenum systems due to the high tempertures nd low oxygen prtil pressures required to evlute them. The 1

13 primry gol of this reserch ws to study phse equilibri in systems comprised of metllic molybdenum in equilibrium with rre-erth oxides or molybdte compounds. The project hd the objective of identifying compounds nd specific oxidtion sttes of molybdenum over rnge of equilibrium conditions. At tempertures of 1000ºC nd 1200ºC, nd O 2 prtil pressures rnging from 10-4 P to P, the systems LO 1.5 -Mo, YO 1.5 -Mo, nd ZrO 2 -Mo were investigted. A survey of the literture is given in Chpter 2, covering bckground informtion on the properties of rre-erth molybdtes nd relted work on phse equilibri in these systems. The experimentl procedure is explined in detil in Chpter 3. Using powder strting mterils, smples of vrying REO-Mo molr rtios were mixed nd heted in tube furnce with controlled tmosphere. Vrious oxygen prtil pressures were used to equilibrte compounds displying Mo IV, Mo VI, nd mixed-vlence sttes. The oxygen prtil pressure in the system ws controlled using mixed flow of H 2 nd CO 2 gs. X- ry diffrction ws used to identify phses in the smples. Chpter 4 presents the results of the experiments, with discussion of their significnce. After identifying equilibrium phse compositions nd molybdenum oxidtion sttes s function of oxygen prtil pressure nd temperture, phse digrms were produced for the LO 1.5 -Mo system t 1200ºC nd 1000ºC. These re binry phse digrms tht represent the dt by mens of oxygen prtil pressure versus molr composition of MoO x. Also, tenttive formuls hve been given for new phses tht were identified using x-ry diffrction. The conclusions re summrized in Chpter 5. Finlly, suggestions for future work re provided in Chpter 6. 2

14 CHAPTER 2 LITERATURE REVIEW This chpter will serve s brod survey of the literture on rre-erth molybdtes. A brief discussion of the pplictions of rre-erth molybdtes will be presented first, followed by detiled ccount of reserch done on the phse equilibri of these systems. Mny phses nd compounds tht possess vrying nd mixed Mo vlences will be discussed in reltion to the phse equilibri nd published phse digrms. The dvncements in preprtion nd experimentl techniques for these mterils will lso be ddressed. The greter prt of this review will focus on the lnthnum molybdtes formed in both binry nd ternry systems. Phse equilibri of the Y 2 O 3 -MoO x system will lso be presented for comprison to the L 2 O 3 -MoO x system. Finlly, literture on the relted system, ZrO 2 -MoO x, will be exmined. Ferroelectrics Gret interest in the physicl nd structurl properties of rre-erth molybdtes ws triggered in the 1960 s in the field of ferroelectrics. In 1966, Borchrdt nd Bierstedt prepred new mteril, Gd 2 (MoO 4 ) 3, nd found tht it could be used s ferroelectric lser host. 1 After this discovery, much reserch ws imed towrd the synthesis nd chrcteriztion of new compounds incorporting other rre-erth oxides nd molybdenum. Most of the erly studies focused on compounds contining Mo VI O 3 insted of the reduced formuls tht incorporte Mo IV O 2. As reserch progressed into the lte 1970 s, discoveries were mde of rre-erth molybdtes contining mixed molybdenum vlences. Structurl nd crystllogrphic dt were chrcterized for these 3

15 mterils, nd the methods of preprtion were refined to include solid-stte rection, fused slt electrolysis, nd precipittion. Dispersion Strengthening Oxide dispersion strengthening hs been used for yers s strengthening mechnism for metls. An oxide phse is mixed into bse metl by vriety of methods including mixing, sol/gel processing, nd mechnicl lloying. The deformbility of the dispersoid prticles is dependent on the following fctors: oxide properties, prticle size, deformtion resistnce of the bse metl, nd the interction between the metl nd oxide. 2 Most oxide prticles behved s expected with molybdenum, either with no chnge or brekge during deformtion. Rre erth oxides, however, hve been found to plsticlly deform. In 1990, Endo, et l., experimentlly doped molybdenum wire with five different rre erth oxides (Y 2 O 3, L 2 O 3, Nd 2 O 3, Sm 2 O 3, nd Gd 2 O 3 ). They found tht doping incresed the recrystlliztion temperture nd reduced deformtion t high tempertures when compred to undoped wire. 3 A 1992 study focused on the deformtion of L 2 O 3 prticles in Mo mtrix due to the high prticle deformbility of L 2 O 3 s opposed to other rre-erth oxides. The smples were rolled into sheets nd drwn into wire to deform the oxides. Prticle deformbility ws found to be closely relted to the extent of rre-erth oxide dispersion strengthening in molybdenum. 4 Zhng, et l., mde frcture toughness mesurements on sintered Mo-L 2 O 3 lloys in The K IC of the lloy ws found to be 2.5 times tht of pure molybdenum. Chrcteriztion of the microstructure reveled similr grin structures nd impurity distribution in both the lloy nd the molybdenum, so there ws no physicl evidence tht could help explin the lloy s toughness. 5 Lnthnum oxide hs been found to hve beneficil effects in 4

16 dispersion strengthening of molybdenum; however, the fundmentl mechnism responsible for the strengthening of L 2 O 3 hs still not been identified. Rre Erth Molybdtes Prior to the study of phse equilibri of systems involving rre-erth oxides nd molybdenum, mny rre-erth molybdtes were synthesized nd chrcterized for the first time in the 1970 s. In 1970, Fournier, et l., published study of the Ln 6 MoO 12 compound contining Y or L s the rre-erth element combined with MoO 3, nd lso proposed severl new compounds, including L 2 Mo 4 O 15, L 2 Mo 3 O 12, L 2 Mo 2 O 9, L 4 MoO 9, Y 2 Mo 4 O 15, nd Y 6 MoO 12, bsed on vrious stoichiometric vlues. 6 Brixner, et l., lso prepred compounds with rre erth oxides nd MoO 3, determining the cell dimensions nd spce groups for compounds of the form Ln 2 (MoO 4 ) 3 where Ln=L, Ce, Pr, Gd, nd Sm. 7 They lso prepred nd determined lttice prmeters for Ln 2 MoO 6 type rre erth molybdtes for Ln= Ce through Lu, Y, nd L. 8 In 1977, LnMoO 5 compounds, the first identified with Mo IV vlence, were prepred by solid stte rection nd by the reduction of Ln 2 MoO 6 compounds. 9 Detiled elbortion on these solid-stte experimentl methods will be given lter in this literture review. Kerner-Czeskleb nd Tourne published the first nlysis of compounds displying multiple oxidtion sttes of molybdenum in rre-erth molybdtes in In study using ternry oxides, they found tht Mo VI nd Mo IV compounds were stble in specific oxygen pressure rnges t high tempertures, nd reported tht there is lso rnge of pressures in between where new type of compound exists in which Mo hs vlence of IV or V.. 10 They reported tht the multiple oxidtion sttes of Mo (IV, V, nd VI) cn be formed with rre erth oxides, nd tht these oxidtion sttes cn be stbilized so tht they will not reduce. 11 5

17 Phse Equilibri Studies Mny studies were done on the phse equilibri of systems contining rre erth oxides nd molybdenum oxides. In 1970, with their study of the Ln 6 MoO 12 compounds, Fournier, et l., published preliminry phse digrms for the L 2 O 3 -MoO 3 nd Y 2 O 3 - MoO 3 systems. They included in their digrms mny new compounds with vrying stoichiometric rtios nd provided x-ry diffrction dt for severl of these compounds. 6 Two yers lter, Nssu nd Shiever studied trivlent tungsttes nd molybdtes of the form Ln 2 (MO 4 ) 3. They chrcterized the phses nd determined crystllogrphic prmeters for vrious lnthnides. Of greter interest were the lnthnide molybdtes, which showed more structures thn the lnthnide tungsttes. Pseudo-binry tungsttemolybdte phse digrms nd structurl prmeters were given for compounds of L, Nd, Sm, Gd, Ho, nd In. 12 In mny spects of its chemicl behvior, tungsten is very similr to molybdenum, nd the behvior of its compounds cn be considered nlogous to the behvior of molybdenum compounds. However, for Mo IV compounds this comprison is not s effective becuse W IV is reltively unstble nd redily reduces to (metllic) W. A phse digrm of the Gd 2 O 3 -MoO 3 system including Gd 2 (MoO 4 ) 3 ws published in 1974 using results bsed on differentil therml nlysis nd x-ry diffrction. The compound Gd 2 (MoO 4 ) 3 ws found to melt congruently t its 1:3 stoichiometric rtio of Gd 2 O 3 :MoO 3. Additionl compounds were identified with 1:6, 1:4, nd 1:1 Gd 2 O 3 :MoO 3 rtio compositions. 13 In 1978, Kerner-Czeskleb, et l., conducted study of the Ln 2 O 3 -MoO 2 -MoO 3 system t 1473 K for 2:1 Ln:Mo rtio in n ttempt to find conditions of stbility for 6

18 vrious rre-erth molybdtes hving different oxidtion sttes of molybdenum. They found tht Ln 2 MoO 5 (Mo IV ) nd Ln 2 MoO 6 (Mo VI ) compounds exist for every Ln t specific oxygen prtil pressures. Also, the compounds tht were reported s hving Mo vlence between IV nd VI re only stble in restricted rnge of oxygen prtil pressures, nd do not exist t ll for the hevier lnthnides such s Yb or lnthnide-like metls, such s Y. It ws concluded tht the specific Ln element does ffect the oxygen prtil pressure rnge of stbility of the molybdtes. 10 A lter study looked specificlly t the lnthnides Pr nd Sm between 1000 nd 1400 K nd oxygen prtil pressures between 10-6 nd tm. New stble phses were formed; however, the voltility of MoO3 nd the unpredictbility of the oxidtion of Mo mde mny of the compositions difficult to prepre, so the dt is incomplete. Mny exmples were given of molybdtes formed with molr rtios nd structures tht re similr to those of relted rre-erth oxide tungsttes. 14 In 1981, phse equilibri in the Nd 2 O 3 -MoO 2 -MoO 3 system were reported between 1273 nd 1673 K for O 2 prtil pressures between 10-5 nd tm. Mny stble Mo VI compounds were found on the Nd 2 O3-MoO 3 binry, s well s other compounds reported s Mo V. Agin, the oxidtion of MoO 2 to MoO 3 presented problems in determining the compositions of some phses. 15 Phse equilibri ws estblished in the Ln 2 O 3 -MoO 2 -MoO 3 system for Ln=Gd, Eu, Tb in New stble compounds were identified in ll three systems. It ws concluded tht the higher the Ln to Mo rtio, the less likely the molybdte phse would be reduced, with the Mo VI compound Ln 6 MoO 12 being the most stble. 16 7

19 Preprtion Techniques In previously reported work, mny of the difficulties in experimenting with rreerth molybdtes rose from lck of dequte control in the processing of the mterils. As progress ws mde in the studies of phse equilibri, the preprtion techniques were refined nd the chemicl behvior of the elements ws more esily mnged. Erly processing ws done mostly in ir. Brixner, et l., described process of solid-stte rection of the desired rre-erth oxide nd MoO 3. A pure MoO 3 powder ws produced by decomposition of (NH 4 ) 6 Mo 7 O 24 4H 2 O. Ech of the rre-erth oxides ws lso specilly prepred before firing with MoO 3. L 2 O 3 ws believed to redily bsorb wter nd CO 2 so it ws pre-fired in ir. Ce 2 O 3 ws prepred in rgon gs or under vcuum to prevent reoxidiztion to CeO 2. Pr 6 O 11 ws used s strting powder becuse it esily loses excess oxygen t high tempertures. Stoichiometric mixtures of the powder strting mterils were then fired t tempertures up to 1600ºC, in seled tubes or cpsules to control the tmosphere. 7,8 A publiction by Kerner-Czeskleb, et l., described procedures for forming the Mo VI compound Ln 2 MoO 6. This procedure noted tht MoO 3 should be reduced to Mo in hydrogen prior to recting, to prevent excessive loss of molybdenum due to evportion of the voltile MoO 3. Equimolr Ln 2 O 3 nd Mo were processed in n lumin bot t 1200 º C. Pr 2 O 3 ws obtined by reduction from Pr 6 O 11 s done by Brixner, nd Tb 2 O 3 ws reduced from Tb 4 O 7. L 2 O 3 powder ws used s strting mteril nd there ws no mention of concerns tht L 2 O 3 bsorbs wter or CO 2. Although Brixner s concern ws vlid, no subsequent experiments reported pre-firing the L 2 O 3 in ir. Prtil pressures of O 2 rnged from 10-5 to tm s controlled by pproprite mixtures of CO 2 nd H 2 gs 8

20 flowing in horizontl tube furnce. 10 Similr methods were used in lter studies of the Ln 2 O 3 -MoO 2 -MoO 3 system for most of the rre-erth oxides. In the erly 1980 s, single crystl nd oriented polycrystlline smples of rre erth molybdtes (L, Nd, Y) with reduced molybdenum complexes were prepred using fused slt electrolysis. These specimens were more pproprite for vriety of chrcteriztion techniques. MoO 3 ws first clcined in ir t 475ºC, nd the rre erth oxides were clcined t 1000 º C. Stoichiometric mixtures were then weighed out nd melted in crucible. Pltinum foil electrodes were used to pss currents through the melt for minutes. The electrodes were then removed from the melt nd rpidly cooled in ir. The cthode would ttrct the product, seprting it chemiclly nd mechniclly from the surrounding smple. The desired product grew onto the cthode in the form of prismtic crystls. 17 This technique ws lso lter used to prepre L 2 Mo IV 2O 7 for n nlysis of electricl, mgnetic, nd structurl properties nd to compre it with other rre-erth molybdtes hving the Ln 2 Mo 2 O 7 formul. 18 A precipittion method ws developed by Hung, et l., to prepre Ln 2 (MoO 4 ) 3. First, the rre-erth oxide ws dissolved in HNO 3, nd then mixed with n mmonium prmolybdte solution. The prmolybdte cn be dded in excess, s its mount does not ffect the composition of the rre-erth molybdte formed. They then dded NH 4 OH to the solution to control the ph between 5.5 nd 7 while the precipitte formed. The precipitte ws then filtered, wshed, nd dried t 120ºC. The precipitte ws clcined in ir t 520ºC to yield the molybdte. 19 9

21 The L 2 O 3 -MoO 3 System The first projected phse digrm for the L 2 O 3 -MoO 3 system, shown in Figure 2.1, ws published in By experimenting with vrious molr rtios of L 2 O 3 to MoO 3, new compounds were produced nd they were chrcterized using x-ry diffrction. At the 1:4 rtio, tetrgonl L 2 Mo 4 O 15 ws reported, long with 1:3 monoclinic L 2 Mo 3 O 12, 1:2 cubic L 2 Mo 2 O 9, nd 1:1 tetrgonl L 2 MoO 6. 6 Over the following yers, reserchers discovered new compounds in this system tht were missing from the preliminry phse digrm. Alekseev, et l., reported the 2:1 compound L 4 MoO 9 nd found tht it hd hexgonl structure. 21 In 1972, the 3:1 compound L 6 MoO 12 ws produced nd chrcterized s cubic. 22 Kerner-Czeskleb nd Cros synthesized nd chrcterized two new compounds: the FCC pyrochlore L 10 Mo 2 O 21 with 5:2 molr rtio, nd orthorhombic L 6 Mo 2 O 15 with 3:2 molr rtio. They lso reevluted the crystl structure of L 6 MoO 12 nd reported it s rhombohedrl 23 10

22 Figure 2.1 MoO 3 -L 2 O 3 Phse Digrm 11

23 The L 2 O 3 -MoO 2 System Due to the instbility of the Mo IV oxide MoO 2 t tmospheric oxygen pressure conditions, there hve not been ny published phse digrms for the L 2 O 3 -MoO 2 system. The first compound reported in this system ws L 2 MoO 5, produced by Hubert from equimolr L 2 O 3 nd MoO 2. 9 This sme compound ws synthesized lter by Kerner-Czeskleb nd Tourne who determined it ws monoclinic. 10 Yers lter, the crystl structure ws revised to cubic fter n x-ry diffrction study. 24 Hubert lso reported 2:1 molr compound, L 4 MoO 8, with x-ry diffrction dt, but no evlution of crystl structure. 25 In 1987, n orthorhombic compound, L 2 Mo 2 O 7 ws prepred with 1:2 molr rtio of L 2 O 3 nd MoO 2. Synthesis ws performed using both fused slt electrolysis nd solid-stte rection. 18 The L 2 O 3 -MoO 2 -MoO 3 System Severl compounds hve been recognized s forming from mixture of molybdenum vlences, positioning them in the L 2 O 3 -MoO 2 -MoO 3 system. Hubert reported mixed-vlence compounds tht he produced by solid-stte rection of stoichiometric mixtures of L 2 O 3, MoO 2, nd MoO 3 powder. He synthesized the 3:4 molr L 2 O 3 :MoO x compound L 3 Mo 2 O 9. For this compound, 4:5 molr mixture of L 2 O 3 :MoO 2 ws heted to 1250ºC in rgon. 26 He lso produced nother 3:4 L 2 O 3 :MoO x compound, L 3 Mo 2 O 10, by heting mixture of L 2 O 3, MoO 2, nd MoO 3 to 1250ºC in rgon. 27 The cubic 5:6 L 5 Mo 3 O 16 compound ws lter prepred by heting the powder constituents (L 2 O 3, MoO 2, nd MoO 3 ) bove 1200ºC under vcuum. 28 Kerner-Czeskleb, et l., lter did extensive work in number of Ln 2 O 3 -MoO 2 -MoO 3 systems with L nd severl other rre-erth elements. All smples were produced using 12

24 solid-stte rection of powders in n tmosphere of controlled oxygen prtil pressure. In their experiments, they were unble to reproduce ny of Hubert s compounds but they discovered nd chrcterized new compounds. In their investigtion of stble phses with 1:1 L 2 O 3 :MoO x molr rtio, they found L 12 Mo 6 O 35 forming t prtil pressures between those defining the limits of stbility of L 2 MoO 5 nd L 2 MoO 6. They ccounted for the resulting molybdenum oxidtion number s two Mo V for four Mo VI. 10 A subsequent publiction ssigned the orthorhombic crystl structure to L 12 Mo 6 O 35 using x-ry diffrction dt. 11 There ws no evidence of Hubert s L 5 Mo 3 O 16 or L 7 Mo 4 O The 3:2 compound L 6 Mo 2 O 14 ws synthesized tht sme yer nd reported to be hexgonl. 23 In 1992, Gll nd Gougeon produced L 4 Mo 2 O 11 using 1:1 molr rtio of L 2 O 3 :MoO x. It ws formed by heting L 2 O 3, MoO 3, nd Mo in seled molybdenum crucibles to 1980 K. They proposed tetrgonl crystl structure, bsed on the presence of Mo 2 O 10 dimeric unit becuse they felt tht L 4 Mo 2 O 11 ws nlogous to the estblished structure of Nd 4 Re 2 O 11. It is known tht rhenium cn form mixed vlence oxides such s this with Re-Re dimers nd Re 2 O 10 groups tht cn link to ech other in Re-O network tht llows for electron delocliztion. The behvior of molybdenum is not understood this well. XRD dt ws on file for L 4 Mo 2 O 11 with the ICSD, but not published in the journl rticle. 29 The Y 2 O 3 -MoO x System The Y 3+ ion is similr to the L 3+ ion in rdius, so yttrium molybdtes hve often been studied long with the lnthnum molybdtes in mny of the sme experiments. Severl yttrium molybdtes hve been identified with molybdenum vlence of 6; 13

25 however, very few hve been found with vlence of 4 in comprison to the vriety of lnthnum molybdtes tht hve been discovered for both oxidtion sttes of molybdenum. Along with their digrm for the L 2 O 3 -MoO 3 system, Fournier, et l., lso published preliminry phse digrm for Y 2 O3-MoO 3, shown in Figure 2.2. Smples were prepred by mixing specific molr rtios of Y 2 O 3 with MoO 3 nd pressing them into pellets. The pellets were then rected t tempertures up to 1500ºC, in n unspecified tmosphere. Y 2 Mo 4 O 15 ws formed from 1:4 molr rtio nd found to be stble up to 830ºC. Also, 1:3 molr Y 2 Mo 3 O 12 nd equimolr Y 2 MoO 6 were reported stble up to 1310ºC. Crystl structures could not be estblished for these phses in the molybdenumrich portion of the phse digrm. A 2:1 Y 4 MoO 9 compound ws formed t 1400ºC nd determined to be hexgonl. Also, 3:1 Y 6 MoO 12 compound ws found to be hexgonl t low tempertures, while forming new high temperture FCC phse t 1500ºC. X-ry diffrction dt nd crystllogrphy informtion ws published for the Y 2 O 3 -rich phses. 6 Experiments conducted by Kerner-Czeskleb, et l., ttempted to form n equimolr Y 2 O 3 -MoO x mixed vlence compound between Y 2 MoO 5 nd Y 2 MoO 6 over rnge of oxygen prtil pressures. The study concluded tht there is direct trnsition from the Mo IV compound to the Mo VI compound with no mixed-vlence compound forming

26 Figure 2.2 MoO 3 -Y 2 O 3 Phse Digrm 15

27 Riemers, et l., synthesized Mo IV compound, Y 2 Mo 2 O 7, using 1:2 molr mixture of Y 2 O 3 nd MoO 2 in Powder neutron diffrction dt ws used to determine the crystl structure s cubic pyrochlore. Bond distnces were found to be in excellent greement when compred to similr compounds such s Y 2 Ti 2 O 7 nd Y 2 Sn 2 O In 1985, Y 5 Mo 2 O 12 ws synthesized vi electrochemicl reduction of lklimolybdtes, MoO 3, nd rre-erth oxides by Torrdi, et l. Their study initlly imed to use fused slt electrolysis to synthesize the Y 2 MoO 5 compound previously reported by Hubert 9 nd Kerner-Czeskleb. 10 The method enbled them to produce single crystls, which llowed for detiled diffrction studies. Crystl diffrction nlysis of Y 2 MoO 5 reveled tht the unit cell ctully contined 10 Y toms nd 4 Mo toms, yielding the chemicl formul Y 5 Mo 2 O 12. Supporting electricl nd mgnetic dt were published for this compound s well s for Gd 5 Mo 2 O 12. No other rre-erth molybdtes of this Ln 5 Mo 2 O 12 form were synthesized in the study. Y 5 Mo 2 O 12 ws found to hve n verge Mo oxidtion of 4.5, mking it the first yttrium molybdte reported to hve mixed molybdenum vlence. 31 A study in 1994 of the Y 2 O 3 -MoO x system investigted the synthesis of new yttrium molybdte with molybdenum vlence less thn six. Reserchers begn by prepring YMoO 4 (1:2 Y 2 O 3 :MoO 3, verge Mo vlence of 5) by solid-stte rection of Y 2 O 3 nd MoO 3 t 1200ºC. After creful nlysis using x-ry diffrction nd electron microscopy, they determined tht there were two phses present with the yttriummolybdenum rtios of 1:3 nd 5:8. The YMoO 4 composition now ppered to consist of binry mixture of Y 5 Mo 4 O 18 (5 Mo VI 3 Mo IV ) nd Y 2 Mo 3 O 10 (2 Mo VI 4 Mo IV ). The 16

28 reserchers were ble to isolte the compound believed to be Y 5 Mo 4 O 18 nd determined its tomic structure nd prmeters. No confirmtion could be mde of the exct formul of the other compound, ssumed to be Y 2 Mo 3 O The ZrO 2 -MoO x System A study of the zirconi-molybdenum oxide system provides good exmple for future work supplementing tht on the lnthnide molybdtes. The zirconi system involves tetrvlent metl for comprison with the yttrium oxide nd lnthnide oxide systems, which re trivlent. The existence of mixed molybdenum vlence zirconium molybdte could provide insight into the impct of ction crystllogrphy on behvior of the molybdte. The first reported zirconium molybdte ws prepred in 1967 when Trunov nd Kovb produced single crystl of ZrMo 2 O 8 nd published its x-ry diffrction dt. 33 The sme compound hs lso been lbeled Zr(MoO 4 ) 2 nd is Mo VI compound tht crystllizes in two phses: low temperture one nd high temperture one tht exists bove 913 K. Its composition nd structure hve been reported, verified by x-ry diffrction nd vibrtionl spectroscopy. 34 The crystllogrphy of the high temperture form ws lter re-exmined nd new crystl structure ws given with tomic prmeters. Zr(MoO 4 ) 2 ws synthesized by heting 1:2 stoichiometric mixture of ZrO 2 nd MoO 3 powder to 1123 K nd then quenching to prevent ny trnsition to the low-temperture form. 35 There is very limited informtion on zirconium molybdtes, nd so fr no mixed molybdenum vlence compounds hve been reported. One relted study of phse equilibri ws done in 1962 on the quternry system, Mo-Ti-Zr-O. Powders were rected in vcuum furnce t 1500ºC nd chrcterized by x-ry diffrction. The 17

29 quternry system ws presented s tetrhedron, on which the tringulr fces represented isotherml ternry systems. Experiments were run t mny different conditions, nd regions were recognized s single or multi-phse; however, the ctul composition nd formul of these phses were not identified. 36 The CeO 2 -MoO x System CeO 2 is nother rre-erth oxide tht rects with molybdenum in similr wys to L 2 O 3 nd Y 2 O 3. Mny of the studies of rre-erth molybdtes include cerium molybdtes. Although difficult to work with due to vrition of cerium oxidtion sttes, they hve been of gret interest due to the mny pplictions of cerium, such s refrctory mterils, luminescence, or ctlysis. In 1972, two studies published by Brixner, et l., involved synthesis nd crystllogrphic nlysis of cerium molybdtes. They prepred Ce 2 MoO 6 by combining CeO 2, Mo, nd MoO 3 powder in molr rtio of 6:1:2. This powder mixture ws pressed into pellet nd sintered in seled Pt cpsule between 1400 nd 1600ºC. Their discussion of the vlence sttes in Ce 2 MoO 6 concluded tht both Ce III nd Ce IV were present nd tht Mo hd reduced from Mo VI to Mo V during the rection. 8 Solid-stte rection ws lso used to prepre Ce 2 (MoO 4 ) 3, from CeO 2 nd MoO 3 powder strting mterils. 7 Also in the CeO 2 -MoO 3 system, CeMo 2 O 8 nd Ce 2 Mo 3 O 13 hve been produced nd chrcterized. 37 Brt nd Giordno published study of phse reltionships in the cerium-molybdenum oxide system in Ce 2 Mo 4 O 15 ws prepred by dding (NH 4 ) 6 Mo 7 O 24 4H 2 O to Ce(NO 3 ) 3 6H 2 O in vrious Ce-Mo rtios, drying the smples, nd clcining them in ir t 500 to 800ºC. The sme compound ws lso synthesized by clcining CeO 2, MoO 3, nd MoO 2 t 550ºC. A high temperture compound α-ce 2 Mo 3 O 13 nd lower temperture compounds β-ce 2 Mo 3 O 13 18

30 nd Ce 2 Mo 3 O were prepred vi solid-stte rection of CeO 2 nd MoO 3 in He or ir. Phse trnsformtion tempertures between ll of these phses were estblished nd prt of the equilibrium phse digrm of the Ce 2 O 3 -MoO 3 system ws presented. 38 This phse digrm greed well with the phse digrm for the L 2 O 3 -MoO 3 system published in 1970 by Fourier, et l. 6 19

31 CHAPTER 3 EXPERIMENTAL PROCEDURE The smples used in this study were ll produced using solid-stte processing of powder strting mterils. Experiments were trgeted towrds forming molybdtes t reduced oxygen prtil pressures nd high tempertures. Powders were heted to rection temperture in horizontl tube furnce under controlled tmosphere. A combined flow of H 2 nd CO 2 gs provided control of the oxygen prtil pressure inside the system. After preprtion, the smples were chrcterized using x-ry diffrction to determine the phses tht hd formed. Preprtion nd Processing of Smples All smples were prepred using powder strting mterils of Mo (H. C. Strck, <10µm, Stock # 46838) combined in stoichiometric rtios with L 2 O 3 (Alf Aesr, 99.99% pure, <10µm, Stock # 11264), Y 2 O 3 (Alf Aesr, 99.99% pure, <10µm, Stock # 11180), or ZrO 2 (Zirconi Sles (Americ), Inc., 99.7% pure, <5µm, Grde DK3-CL). Smples of pproximtely 0.5 cm 3 were produced to gurntee enough product to use in n x-ry diffrctometer. All weight mesurements were mde to four significnt figures using Mettler AE50 Anlyticl Blnce. Powders were mixed by grinding with mortr nd pestle until the smple ws uniform in color. The smple ws pcked into n lumin crucible nd compressed with stinless steel rod. A shllow rectngulr lumin bot ws used to hold the crucibles while they were heted in the furnce. As mny s four smples in crucibles could be processed during one tril. During the experiments, the smples were heted to 1000 or

32 degrees C. All smples were kept t the rection temperture for t lest 48 hours to ensure completion of the rection. The oxygen prtil pressure ws lso controlled to pressure between P nd 10-5 P during the rection by estblishment nd mintennce of the pproprite CO 2 :H 2 rtio. Experimentl Setup A process flow digrm of the experimentl pprtus is shown in Figure 3.1. The components of this digrm will be explined in detil lter in this chpter. Mss flow controller Furnce Flow rte control Electricl heter Thermocouple RGA Bubblers Computer Gs tnks Figure 3.1 Process Flow Digrm 21

33 The body of the furnce consisted of 2.75-inch dimeter mullite tube obtined from Hrrop Lbortories with 14 Globr silicon crbide heting elements running prllel to the tube evenly distributed round the outside. Figure 3.2 shows the exterior of the tube furnce. Figure 3.2 Tube Furnce Used for Heting Smples Circulr steel end pltes were fstened onto the ends of the mullite tube using 3 bolt clmps to retin ech plte. A pressure guge ttched to the inlet end plte indicted the equilibrium pressure inside the tube. Vcuum grese ws pplied to rubber o-ring which ws embedded into the end of the furnce tube to crete n irtight sel. These end pltes, seen in Figure 3.3 nd Figure 3.4, contined irtight fittings for the gs delivery nd exhust tubing. 22

34 Figure 3.3 End Plte t Inlet of Furnce Tube with Pressure Guge Figure 3.4 End Plte t the Outlet of the Furnce Tube 23

35 Within the tube, the smples were held in rectngulr lumin bot. This bot ws enclosed on either end by cylindricl het shields mnufctured from refrctory brick. The inner fces of these het shields were coted with lumin to provide reflection of het inwrd. Het tpe, refrctory brick, nd insultion were dded on the outside of the furnce tube to minimize het loss. A schemtic of the furnce nd its inner prts is shown in Figure 3.5. refrctory brick het shields lumin coting on het shield heting elements mullite tube refrctory brick spcers lumin bot smple holder Figure 3.4 Cross-sectionl Schemtic of Furnce Temperture Control The heting elements in the furnce hd the cpcity to het smples up to 1450 degrees C. A Type-S pltinum/pltinum-rhodium thermocouple ws plced ner the (xil) center of the furnce on the outside of the tube to monitor the temperture. Inside the tube, Type-K chromel-lumel thermocouple ws plced nd used long with the Type-S thermocouple to clibrte the furnce temperture. It ws determined tht ner 24

36 constnt temperture ws mintined over length of bout 4 inches. Smples in the bot were pushed into the center of this 4-inch re during ech tril. Heting schedules were set using computer softwre tht llows one to estblish heting rtes, tempertures, nd periods to be held t constnt temperture. The heting nd cooling rtes for experiments run up to 1200 C re provided in Tble 3.1. Tble 3.1 Heting nd cooling rtes for furnce Rte (ºC/min) Finl Temperture (ºC) 5 (heting) (cooling) (cooling) (cooling) 200 Atmosphere Control The tmosphere in the furnce ws controlled using combined flow of hydrogen nd crbon dioxide gs. Gs ws obtined from Airgs of ultr high purity. Thermodynmic clcultions, included in Appendix A, were mde to determine the combintion of gs flow rtes necessry to produce the desired oxygen prtil pressures inside the furnce t ech temperture. The plot in Figure 3.6 shows the vrition of prtil pressure of O 2 with the rtio of CO 2 to H 2 gs. 25

37 Prtil Pressure O2 (P) 1.00E E E E E E E E E E E ºC 1000ºC Mole frction of CO 2 in gs flow Figure 3.6 Prtil pressure of O 2 vs. frction of CO 2 in the gs flow A Visul Bsic progrm ws written to clculte the molr rtio of gses necessry to chieve the desired O 2 prtil pressure t given temperture. The progrm code is included in Appendix B. The desired molr rtios of CO 2 nd H 2 were used to determine corresponding volumetric flow rtes. A combined gs flow rte of pproximtely 120 cc/min ws mintined. Flow rte control ws provided by MKS mss flow nd n MKS model 247C controller to control the ctul flow rtes. Gses from the supply tnks were fed to the furnce through polyethylene tubes on the inlet end, nd flowed out of the system through stinless steel tubing t the exhust end. The outlet tubing ws connected through one flsk to bubbler into second flsk to void drwing ir or moisture into the system in the event tht vcuum ws creted. All of the experiments were strted in pure H 2 until the furnce pproched 800 degrees C to llow wter or orgnics to decompose, nd to prevent the formtion of 26

38 molybdenum oxide or molybdenum crbide from the CO 2. At 800 degrees, the CO 2 ws dded t flow rte tht corresponded to the desired gs proportions. A residul gs nlyzer (RGA) ws lso connected to the furnce to monitor the gs species in the system. The resulting RGA spectr were monitored in rel time using TWre32 Process Gs Chrcteriztion Softwre. This dt ws useful for detecting leks nd monitoring gs equilibrium during furnce runs. Chrcteriztion X-ry diffrction (XRD) ws used to chrcterize the phses present in the processed smples. After firing in the furnce, the smples were removed nd extrcted from the crucibles. Oftentimes the smples would emerge slightly sintered, so they were lwys ground into fine powder using mortr nd pestle. The resulting powder ws pcked into metl diffrction smple holder using glss slide s smooth pcking surfce. A Philips utomted powder diffrctometer (PW 1800) ws used to collect x-ry diffrction dt over rnge of 10 to 90 degrees 2θ. Dt cquisition ws performed using the Philips PW-1877 Automted Powder Diffrction progrm (version 3.6). After the scn ws complete, the rw dt files were exmined for clrity nd intensity nd pek serch ws performed. Any phses present were identified using the PW1876 PC- Identify progrm (version 1.0) in conjunction with the JCPDS Powder Diffrction File dtbse. In the PC-Identify progrm, restriction files were creted to limit the identified ptterns to ones contining only the elements known to be present in the smple. 27

39 CHAPTER 4 RESULTS AND DISCUSSION This chpter presents the phse equilibri results obtined from the experiments nd chrcterized by x-ry diffrction. All of the phses identified re presented in tbles with the corresponding experimentl conditions. X-ry diffrction dt re lso included for ll of the phses discussed. The lnthnum-molybdenum oxide experiments t 1000 C nd 1200 C yielded enough conclusive dt to construct isotherml phse digrms. Compounds observed in this system re plotted on chrt of oxygen prtil pressure versus composition for ech rection temperture. The relevnt x-ry diffrction ptterns re included s evidence for the phses shown in the digrm. Results from the experiments with yttrium nd zirconium re lso presented nd discussed with supporting x-ry diffrction ptterns. The specific compounds formed by rection of rre-erth oxides with molybdenum re dependent on the extent to which molybdenum oxidizes in the controlled oxygen prtil pressure tmosphere. As oxygen is dded, Mo will form MoO 2, with molybdenum vlence of IV, nd in the presence of dditionl oxygen, some frction of the Mo will form MoO 3, with molybdenum vlence of VI. For tht reson the dt for ech system re discussed in segments ccording to the prtil pressure of oxygen during the rection. In some of the preceding literture, rre-erth molybdtes hve been described s hving molybdenum vlences of IV, V, nd VI. The Mo V compounds published to dte hve not been well supported with structurl or 28

40 experimentl dt. As result, ny compound hving molybdenum vlence between IV nd VI is depicted in this study s mixed vlence compound of Mo IV nd Mo VI. LO 1.5 -MoO x system t 1200 C Phse identifiction ws performed using the PC Identify softwre for ll of the smples prepred in the LO 1.5 -MoO x system. Resulting phses, experimentl conditions, nd the percent of molybdenum in the smple re given in Tble 4.1 tbulting the results t 1200 C in order of decresing po 2 vlues nd decresing L:Mo rtio for ech po 2 vlue. Primry phses for ech processed smple re indicted by bold type. Phses identified here were used to construct the binry phse digrm seen in Figure 4.1. The x-xis represents the molr percent of molybdenum or molybdenum oxide in the smple, nd the y-xis is log scle of the oxygen prtil pressure t which the phses form. Lines re drwn in to represent stble compounds tht were determined by experiments or identified from the literture. As seen in Tble 4.1, severl smples were found to hve more thn two phses present fter processing. In some cses, the dditionl phses might be intermedite rection products indicting tht the smple hd not ttined equilibrium. The existence of three or more phses t equilibrium lso suggests tht the LO 1.5 -MoO x system would be better presented s the L-Mo-O ternry system, nd tht the binry presented is section of ternry phse digrm. For this study; however, it will be ssumed tht the binry presentttion of Figure 4.1 is sufficient representtion of the dt. 29

41 Tble 4.1 Phses identified from experiments with L 2 O 3 nd Mo t 1200 C Smple Nme L:Mo Rtio Molr % Mo O 2 Prtil Pressure (P) Process Time (hours) Phses Present LAMO_02 2: E L 2 MoO 6 LAMO_ : E L 2 O 3, unble to identify, L 6 MoO 12 LAMO_ : E L 6 Mo 2 O 14, L 2 MoO 5 LAMO_10 2: E L 12 Mo 6 O 35, L 6 Mo 2 O 14, L 5 Mo 3 O 16 LAMO_ : E L 12 Mo 6 O 35, L 5 Mo 3 O 16, L 5 Mo 4 O 14, L 2 Mo 2 O 7 LAMO_60 3: E L 6 Mo 2 O 14, L 10 Mo 4 O 24, L 4 Mo 2 O 11 LAMO_61 2: E L 4 Mo 2 O 11 LAMO_62 1: E L 2 Mo 3 O 10 LAMO_57 6: E L 6 Mo 2 O 14, L 2 O 3, L 2 MoO 5 LAMO_34 2: E L 10 Mo 4 O 24, L 2 Mo 3 O 10 LAMO_ : E L 10 Mo 4 O 24, L 2 Mo 3 O 10 LAMO_59 1: E L 2 Mo 3 O 10 LAMO_33 1: E Mo, unble to identify LAMO_33B 1: E Mo, unble to identify LAMO_66 1: E Mo, L 2 Mo 3 O 10, L 10 Mo 4 O 24 LAMO_67 1: E Mo, L 2 Mo 3 O 10 LAMO_35 6: E L 6 Mo 2 O 14, L 2 O 3, L 2 MoO 5 LAMO_32 2: E L 10 Mo 4 O 24 LAMO_32B 2: E L 10 Mo 4 O 24 LAMO_ : E L 10 Mo 4 O 24, L 2 Mo 3 O 10 LAMO_ : E L 10 Mo 4 O 24, L 2 Mo 3 O 10 LAMO_64 1: E Mo, L 2 Mo 3 O 10, L 10 Mo 4 O 24 LAMO_65 1: E Mo, L 2 Mo 3 O 10 LAMO_31 1: E Mo, L 2 Mo 3 O 10 LAMO_31B 1: E Mo, L 2 Mo 3 O 10 LAMO_20 2: E L 10 Mo 4 O 24 LAMO_30 1: E Mo, L 10 Mo 4 O 24 LAMO_30B 1: E Mo, L 10 Mo 4 O 24 LAMO_02B 2: E L 2 O 3, Mo 30

42 31 O 2 Prtil Pressure (P) 1.00E E E E E E E-10 LO1.5 + Mo mol% MoOx LO1.5 MoOx L 6 Mo IV Mo VI O L 12 Mo IV Mo VI 5O 35 L 6 Mo VI O 12 L 10 Mo VI 2O 21 L 4 Mo VI O 9 L 6 Mo VI 2O B Figure 4.1 Phse equilibri in the LO1.5-MoOx system t 1200ºC L 10 Mo IV 3Mo VI O 24 L 4 Mo IV Mo VI O 11 L 2 Mo IV 2Mo VI O 10 L 2 Mo VI O 6 L 2 Mo VI 2O 9 L 2 Mo VI 4O 15

43 Oxygen Prtil Pressures from 1x10-4 to 2x10-6 P Compounds formed in this region hve molybdenum vlence of VI. The following compounds hve been reported in the literture s stble Mo VI compounds depending on the L-Mo rtio: L 6 MoO 12, L 10 Mo 2 O 21, L 6 Mo 2 O 15, L 4 MoO 9, L 2 MoO 6, L 2 Mo 2 O 9, nd L 2 Mo 4 O ,21,8,6 Previous work done in this system verified the existence of ll of these phses in this pressure rnge; however, the lower O 2 prtil pressure limit could not be confirmed. The only smple formed s pure phse in this work ws L 2 MoO 6 using 2:1 molr rtio of L to Mo. L 2 MoO 6 ws found to be stble t oxygen prtil pressures s low s 2.645x10-6 P, therefore this is set s the stbility limit of the Mo VI compounds for compounds with MoO x greter thn pproximtely 10 mole percent. Oxygen Prtil Pressures from 2x10-6 to 3x10-7 P At 1200ºC in this pressure rnge, molybdenum begins to form mixed-vlence compounds. The x-ry diffrction pttern for smple LAMO_53, which ws formed from pproximtely 8.11% Mo t n O 2 prtil pressure of 4.6x10-7 P, is seen in Figure 4.2. The pek sizes show smll mount of the Mo VI phse L 6 MoO 12, but none of the other Mo VI compounds could be synthesized in this pressure rnge. As reported in the review of literture, Prevost-Czeskleb nd Tourne 16 found tht compounds with molybdenum vlence of VI re less likely to be reduced s the concentrtion of Mo decreses, with compounds of the form Ln 6 MoO 12 being the most stble. For this reson, the stbility limit for Mo VI compounds is inferred to be n O 2 prtil pressure of 3x10-7 P for Mo concentrtions less thn pproximtely eight to ten mole percent. 32

44 The smple run using 3.71:1 L:Mo rtio t n oxygen prtil pressure of 4.6x10-7 P (LAMO_55) ws found to contin primrily L 6 Mo 2 O 14, which is mixedmolybdenum vlence compound of 25 mole percent Mo composition (6LO 1.5-2MoO x ). There ws lso evidence of smll mount of L 2 MoO 5, in which Mo hs vlence of IV. This diffrction pttern, did not contin ny evidence of the Mo VI compounds L 10 Mo 2 O 21, L 4 MoO 9 or L 6 Mo 2 O 15 tht were observed t higher oxygen prtil pressures for similr rtio of L to Mo. This is interpreted s evidence tht the Mo VI compounds contining greter thn 15% Mo re not stble t oxygen prtil pressures below 2x10-6 P. Smple LAMO_10, with composition of 2:1 L:Mo, ws found to contin mostly L 12 Mo 6 O 35. This formul hs mixed molybdenum vlence with one Mo IV to five Mo VI. This high verge vlence of molybdenum is consistent with the fct tht the compound becomes stble close to the oxygen prtil pressure region where the Mo VI compounds re stble. The x-ry diffrction pttern for this smple is shown in Figure 4.3. Evidence of trces of the mixed vlence compounds L 6 Mo 2 O 14, which hs one Mo IV to one Mo VI, nd L 5 Mo 3 O 16, which hs one Mo IV to five Mo VI, were lso seen in the pttern. The primry phse ws gin the 2:1 compound L 12 Mo 6 O 35 (one Mo IV to five Mo VI ), which ws ccompnied by minor phses L 5 Mo 3 O 16 (one Mo IV to five Mo VI ), nd L 5 Mo 4 O 16 (seven Mo IV to one Mo VI ). 33