Technical Note

Transcription

1 Technical Note Mo an ructural Study nyb S J. M. Lotii P. M. Race 9 August 196 Lincoln Laboratory

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3 MASSACHUSETTS INSTITUTE OF TECHNOLOGY LINCOLN LABORATORY MAGNETIC AND STRUCTURAL STUDY OF THE SPINEL MnYt> 2 S 4 /. M. LONGO P. M. RACCAH Group 83 TECHNICAL NOTE AUGUST 1967 LEXINGTON MASSACHUSETTS

4 ABSTRACT The compound MnYb^S. has the normal spinel structure with no measurable mixing of atoms in octahedral and tetrahedral sites. The sulfur parameter is ± 0.002, which is lower than usually found in spinels. Magnetic susceptibility shows no magnetic order to liquid He temperature and a temperature dependence that is the simple sum of the theoretical contributions for Yb and Mn ions. Accepted for the Air Force Franklin C. Hudson Chief, Lincoln Laboratory Office in

5 MAGNETIC AND STRUCTURAL STUDY OF THE SPINEL MnYb^ I. INTRODUCTION Many compounds with the spinel structure are magnetic and therefore provide information about the signs and strengths of the interactions between magnetic ions. Previous studies of these interactions have been confined to transition-metal compounds with the spinel structure. Recently, rare-earth sulfur and selenium compounds with the spinel structure have been found. The rare-earth ions are on octahedral sites, and either a magnetic transition- 1 metal ion or a nonmagnetic ion is on the tetrahedral sites. Suchow and Stemple have prepared various rare-earth spinels having cadmium ions on the tetra- 2 hedral sites. Patrie, Flahaut, and Domange have studied rare-earth thio- spinels having iron, manganese, or magnesium on the tetrahedral sites. systems provide an opportunity to study the signs and magnitudes of the inter- These actions between the rare-earth ions and between the rare-earth and transition- metal ions. Since interactions between octahedral-site ions in spinels are, in general, relatively weak, this study represents an investigation of the 3+ magnetic interactions between an octahedral-site Yb ion and a tetrahedral- 2 + site Mn ion in the thiospinel MnYb? S.. Since these interactions are mediated via covalent mixing with the anions it was anticipated that the greater covalency of sulfur, relative to oxygen, would enhance the magnitude of their interactions. II. EXPERIMENTAL 2 MnYbpS. was first prepared as described by Patrie, et al. This involves heating a stoichiometric mixture of the oxides in a stream of FLS at about 1300 C. However, a more reproducible product was prepared by heating at 1300 C for 24 hrs. a stoichiometric mixture of ytterbium sulfide, manganese metal, and sulfur in an aluminum crucible that had been sealed under vacuum in a silicon tube. The samples were microcrystalline and olive-green in color. o They had a face-centered-cubic unit cell with a = A, which is in agree- ment with Patrie, et al.

6 The magnetic susceptibility x of a gram sample was measured in a field of 1 0 koe from liquid He to room temperature with a vibrating- sample magnetometer developed at this laboratory. shown in Fig. 1. The l/x vs T plot is At higher temperatures the curve obeys a Curie-Weiss law X A = C /(T - e ) (1 m nr p 40 MnYb 2 S d 24 < O 5 * TEMPERATURE ( K) Fig. 1. Inverse molar susceptibility vs temperature for MnYb^S,. with molar Curie constant C = C(Yb) + C(Mn) = Theoretical and m 4 experimental values for the contribution of the two ytterbium ions to C are C(Yb) = 2 x 2.58 = This leaves a C(Mn) = 4.36, which corresponds to a n^ = -s/ 8C(Mn) = N/ eff The theoretical value for Mn 2+ ions is n eff ^, n = g N/ S(S + 1) = *J 35. Magnetization vs magnetic field at both liquid N, and liquid He temperatures showed a linear dependence, characteristic of the paramagnetic state.

7 The relative x-ray diffraction intensities of a finely ground powder which passed a 325 sieve were measured with a General Electric X-RD5 diffractometer using Ni filtered Cuk radiation. In order to eliminate the long-term drift of the instrument, the intensity of every peak is expressed relative to the strongest peak (311), which was remeasured each time. Where two peaks could not easily be separated, their intensities are grouped into a single intensity. The struc- tural parameters were refined with the aid of a computer program developed 5 at this laboratory that is able to adjust all positional parameters, isotropic temperature factors, and site occupancies to obtain a minimum value for the factor R defined as R = z I K (hki) x 10 hki obs In the refinement, the theoretical form factors (Mn, Yb, S ) used were those calculated by Cromer and Waber. Both the real and the imaginary parts 7 of the anomalous-dispersion correction, as calculated by Cromer, were applied. Comparison of observed and calculated intensities is presented in Table I. III. DISCUSSION The magnetic-susceptibility data can be accounted for nicely by simple paramagnetism with so little interaction between the ions that there is no long-range magnetic order at liquid He temperature. The bend in the l/x vs (4) T plot at very low temperatures is similar to that found in YbF^ and is therefore attributed to the Yb 3+ ions. Subtraction of the ytterbium contribution leaves a straight line with a slope corresponding to the theoretical Mn contribution. The final refinement of parameters had an R = 8.Z. It gave a sulfur u parameter of ± 0.00Z and the isotropic temperature factors B-, = 0.03, B y, = 0.54, B g = Patrie, et al. 2 have reported a u = ± However, their result was based only on the ratio of intensities of two sets of

8 " ^ pn 0 ~ N " ** m 0 ^ ro rv> r^ -1 PO N ir N ~ r~ r^ 0 in in - O^ O a Ov CT- rv) f-^ - " ; ~ a- O 1. ro -T in»* -] ro -c ^ in O in* -r* r**> rv) V r* in O TH IT "* -1 -O t m V. * in tf\ -r in r- 0 r- vo x p- X in m r- so X r- r* "* <T~ tn r- 0 rp r^- ro s r- a* X r~ r- X n- c O r- X f>»h a- 0 X r^ 0- r\j O 5-0 N *- fo < u, a r\] O in in X O- ^o in «^> X ^D r^ 0 I s - vn c X rn r*i r0 ^-«H 2C r- m ~ X vo rn 0 0 ir T INJ O vo * «0 N r- x X O 0 s - in t-. 0 s - -r r- X ^H ro r*i r~ p) in r- <\J rvj in r- N in r- r*l t r-- 0* r- 00 X X O c- 0 0 c c 0 T4 ^-> ^_i fm rvj ro ^ r<r) ro rf c c< EH 00 CO T4 (M * Ln J IP ^_ -r vo a- T in O- ^* r- e < ~ a-,^' rn u *t r- N T-t M rp 0 00 TH X IP rn -r S rh i"> < 0 1 N ~ " *" &H X 'Si r^ -A M < O 0 1 r-o r*j * ^-l ^H 0 ^H ^-( in N ^-< * «-- 0 X ~) * X j < -o 5 -\ <» H Pfi w D C a, X r- s0 I s - <-* in 0 -J M 0 -> -] - O «O r^\ rsj ^f ^* (NJ ip 0 m rg in r-i 0 tn d y. ^ - r\i ~ N a -p N ** "f rn IM m I^J T in Tf ip 0 m N in so -r.-31 w * 5^ - r\j ro fm -r ~o * in -r in NO it> s ) in ^O LP X I s - X in vd X < - 0 ix ~ ^ ~ ^1 N X 0 c~- ** -r Tf {^1 T nj to ^J O rg O I - -Xi a^ '-I -c r* ro X a- X CT-NO m rn IP T in rv rv> O vd 0 DM * ~J vo X ^1 in' O N ^ 'j- ~j rf IT) X 0 pn in X O r^ in in r- O " r " N ru IN ro ^ <* -r -r -r in in in Lfl 0 0 so 0 I s - 1- I s - r- I s - S H J f\j r- 0 in N X in 0 0 a~ 0 ro O 0] r\j -r cr- -r so PJ r>j < t 0 ifl d vo 0 r\j pq NO LT W r-j,h -T 0- sc in ~ in fsj -r ** X ^ O ^j 0^ ^ LP ** X3 " * 1 en 0 (M 0-0 tn -J f\) cr- N -r X _ X "j 0 0 0] X pn 0 X O *' X 0 r^ O -0 * -1- CP LP ^ -r s6 cr- N 0 X N *H ^r I s N in -f so TH (SJ *4 *H ^ 14

9 reflections: 311/ and 531/ The relatively low value of the u parameter in this manganese spinel (MnIn2S4 has u = and MnCr^^ has u = ) is due to the larger size of Yb^+ relative to Cr-^+ or In^ +. In the spinel MnYb? S., the Mn ions occupy tetrahedral sites and the \b 3+ ions occupy octahedral sites. For ideal close packing of the sulfur ions, u = In general a u > is found in cubic spinels. Several attempts were made to reduce the factor R below 8.2. First, a fraction of the Mn 3+ and Yb ions were allowed to interchange between octahedral and tetrahedral sites. This gave no improvement, as might have been expected from the site preferences of these ions. Second, some tetrahedral Mn ions were allowed to move to empty, octahedral-site interstitial positions. improvement. This also gave no Finally, the concentration of sulfur was allowed to vary as a check for gross nonstoichiometry. The results showed each sulfur site having a concentration of 0.97 ± 0.05, with no significant lowering of the R factor. It is believed that a lower R factor was not attained because of surface attack of the sample by the atmosphere. ACKNOWLEDGMENT We would like to express our gratitude to Dr. J.B. Goodenough for the many helpful discussions and suggestions during the course of this work. Also, we wish to acknowledge the assistance of D. Batson in the preparation of materials and C.H. Anderson in the collection of intensity data.

10 REFERENCES 1. L. Suchow and N. R. Stemple, J. Electrochem. Soc. Ill (2), 191 (1964). 2. M. Patrie, J. Flahaut and L. Domange, Comptes Rendus Z58, 2585 (1964). 3. S. Foner, Rev. Sci. Instr. 30, 548 (1959). 4. S. Kern and P.M. Raccah, J. Phys. Chem. Solids 26, 1625 (1965). 5. P.M. Raccah and J.B. Goodenough, Phys. Rev. 153 (to be published 1967). 6. D.T. Cromer and J.T. Waber, Acta. Cryst. 18, 104 (1965). 7. D.T. Cromer, Acta. Cryst. 18, 17 (1965). 8. P.M. Raccah, R.J. Bouchard and A. Wold, J. Appl. Phys. 37 (3), 1436 (1966).

11 I'NCLAKSIMKl) Security Classification DOCUMENT CONTROL DATA - R&D (Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified) I. ORIGINATING ACTIVITY (Corporate author) Lincoln Laboratory, M.l.T. 3. REPORT TITLE Magnetic and Structural Study of the Spinel MnYb? S. 2a. REPORT SECURITY CLASSIFICATION Unclassified 2b. GROUP None 4. DESCRIPTIVE NOTES (Type of report and inclusive dates) Technical Note 5. AUTHOR(S) (Last name, first name, initial) I,ongo, John!\1. Raccah, Paul M. 6. REPORT DATE 9 August 1967 la. TOTAL NO. OF PAGES NO. OF REFS 8 8a. CONTRACT OR GRANT NO. AF 19 (628) PROJECT NO. 64Q L 9a. ORIGINATOR'S REPORT NUMBER(S) Technical Note b. OTHER REPORT NO(S) (Any other numbers that may bo assigned this report) ESD-TR AVAILABILITY/LIMITATION NOTICES This document has been approved for public release and sale; its distribution is unlimited. 11. SUPPLEMENTARY NOTES None 12. SPONSORING MILITARY ACTIVITY Air Force Systems Command, USAF 13. ABSTRACT The compound MnYb2S4 has the normal spinel structure with no measurable mixing of atoms in octahedral and tetrahedral sites. The sulfur parameter is ± 0.002, which is lower than usually found in spinels. Magnetic susceptibility shows no magnetic order to liquid He temperature and a temperature dependence that is the simple sum of the theoretical contributions for Yb 3 + and Mn 2+ ions. 14. KEY WORDS spinels rare-earth compounds transition metals magnetic interactions UNCLASSIFIED Security Classification