LUMINESCENCE' OF SOLID SOLUTIONS _OF THE SYSTEM CaMo04-~hMo04 ' AND OF SOME OTHER SYSTEMS

Transcription

1 R 4.S Philips Res. Rep~'2, 83-89, 947 LUMINESCENCE' OF SOLID SOLUTIONS _OF THE SYSTEM CaMo04-~hMo04 ' AND OF SOME OTHER SYSTEMS by F. A. KRÖGER Summary. Introduction A,s shown in a previous paper, the fluorescence of solid solutions (Zn, Mg) W0 4 can be interpreted simply as the superposition of the fluorescence of its two components; but in the systems (Ca, Pb) W0 4, (Sr, Pb) W0 4 and (Ba, Pb) W0 4 new emission bands were observed which were attributedtto tungstate groups with mixed surroundings of lead and calcium, strontium or barium ions. -In this paper the systems (Ca, Sr) W0 4, (Ca, Sr) Mo0 4, and (Ca, Mg)aWOo are shown to behave as (Zn, Mg) W0 4, whereas (Ca, Pb) Mo0 4 behaves as (Ca, Pb) W0 4 Below a certain critical temperature practically all tungstates and molybdates show fluorescence which has been shown to depend on the crystal structure and on the cations that are present in the lattice ). The influence of the cations can best he studied in solid solutions containing different cations, but possessing throughout the same crystal structure. As discussed in a previous communication the fluorescence of solid solutions (Zn, Mg) WO4 is nearly identical with that of the components 3), but in the systems (Ca, Pb) W0 4, (Sr, Pb) W0 4 and (Ba, Pb) W0 4, the fluorescence of the solid solutions cannot be explained as a simple superposition of the bands ofthe components: in the latter systems new bands appear which must be attributed to tungstate groups having a mixed surroundings consisting of calcium, strontium or barium, together with lead ). Continuing these investigations we.shall in the present:paper repo~.t- about -the systems (Ca, Pb) Mo0 4, (Ca, Sr) W0 4, (Ca, Sr) Mo0 4 and (Ca, Mg)s W (Ca, Pb) MoO. Calcium molybdate 'and lead molybdate crystalize in the scheelite.~stru«ture with nearly the same lattice dimensions, and they form an uninterrupted series of solid solutions of this structure. Pure CaMoO4 and PbMoO 4 were precipitated from solutions of calcium and lead nitrate by addition of a. solution of ammonium molybdate, and the mixed crystals were made by heating the precipitates together in the required proportion at a temperature of ~bout 850 C for about one hour. *) *) Thé preparations have been carried out by Mr A. Bosman in this laboratory.

2 84 2a. Fluorescence Pure CaMo0 4 shows fluorescence in a broad yellow band with maximum at 5350 A, pure PbMo0 4 in a 'green band with maximum at 520 'A; in solid solutions of these two substances atleast one ne", band appearp with a maximum at longer wavelengths; Fi{J. shows the spectral distribution ÀinA Fig.. Spectral distribution of the fluoresccnce of mixed crystals (Ca, Pb) Mo0 4./976 t!jast"" bands maximum of the _,-.,.. ~ I molybdate < t.ungstate ~ 4000 Ca mol.%, 75 Pb Fig. 2. Variation of the maximum of the fluore~cence band with the composition for (Ca, Pb) W0 4 and (Ca, Pb) Mo0 4 _ 497/7

3 J.UMINESCENCE OF SOLID SOLUTIONS OF THÈ. SYSTEM C.MoO~-Pbll00, of the emission at - 80' o C for various products, while. the variation in the position of the maximum with. the composition is shown in fig~ 2. In these observations CaMo0 4 was excited by À. = 2537 Á, but all systems containing lead were excited by À. = 3650 Á. For comparison the variation of the blue bands in the system CaW0 4 - Pb WO4 has also been plotted.. 2b. Reflection and excitation The reflection spectrum of the pure compounds and of various solid solutions is shown in fig. 3. While for CaMo0 4 nearly total absorption is Excitation ÀinA 4000 range / 3500 ~-+--+-i--ii-- -- orange-yett0wl ye/tow. gre II--4-i- --- ye ow,~ _BOï25 C. Reflection Cai''004-xmo/lPbfo{J, 2500H---I--..=...j -Reftection in% II o Fig. 3. Spectral distribution of the reflection and of th~ excitation of fluorescence in the system (Cn, Pb) Mo0 4 observed at wavelengths below 3000 Á, this limit is situated at longer wavelengths for products containing lead, shifting slightly towards the longwave side with increasing lead concentration. At a sufficiently low temperature (cf. section 2c) irradiation into the absorption bands indicated by the reflection gives rise to fluorescence. In CaMo0 4, absorption of À. < 3200 Á produces the ~ell-known yellow fluorescence of this substance. For products containing a few per cent of lead molybdate the excitation spectrum consists of two parts. Excitation by À. < 3200 Á gives rise to the yellow CaMo04 band together with an orange-yellow band, but excitation by 3200 < À. < 3800 Á produces the latter band only. With increasing le~d co~tent the yellow band disappears, while the other band gradually varies its position on the wavelength scale in the direction of shorter wavelengths. While products containing up to 60% lead molybdate are lumines-

4 86 F. A. KROGER cent at room temperature, are products of higher lead content only luminescent at low temperatui-es (cf. se?tion 2?). ',2c. Tempemsure dependence As shown in fig. 4 the temperature dependence of the fluorescence is different for excitation by,î,; = 2537 A (fig.4b) and À '--, 3650 Á (fig.4a) ,"", with À 2537 A -200 Fig. 4. Température dependence of the fluorescence of (Ca, Ph) Mo0 4 for excitation hy (a) J. = 3650 Á, (h) J. = 2537 Á; (c) definition ofthe quenching range LT = T 2 -T p In both cas~s the incorporation of lead in a.concentration below 25 mol. % causes an increase in the temperature at which the quenching is completed, but in the latter case (3650 Á) thia.increase is-much more pronounced. At higher lead concentrations the quenching points are lowered and gradually approach the ones of pure lead molybdate. The upper and lower limits of the quenching range Tl and T 2 - determined as shown in fi~. 4c - have been plotted in.fi~. 5 as a function ofthe composition, The spectral distribution of the fluorescence observed at the highest " temperatures is slightly different from that appearing at lower temperatures. For instance the fluorescence of CaMo0 4-0% PhMo0 4, excitated by À ' 3650 Á, is orange-red below 00 oe, but is yellow at higher temperatures. We infer from this fact that the system is more complex than has been assumed in section 2b, the incorporation of lead in CaMo0 4 causing two bands at-least. This mayalso explain the observed difference~ in temperature dependence for excitation by À = 2537 Á and À = 3650 Á. Each will probably be associated with its own absorption band. Excitation by

5 - LUMINESCENCE OF SOLID SOLUTIONS OF THE SYSTEM CaMoO,-PbMoO, 87 different wavelengths produces the sub-bands in different proportions. If each has its own.characteristic quenching,range, the total quenching curves will also be different. A s~ilar behaviour has previouslybeen observed for the systemca W0 4 - Pb W0 4 (fig. Sb). In 'this ca~e the complexity of the emission spectrum is. not indicated by a variation of the quenching curves with the exciting wavel_engths, but it is made probable by. a comparison of the corresponding barium-lead and strontium-lead systems.a] t Temperature inoe maxitwm -2'0U't----I fluorescence maximum nuorescence CaMoo OJ\ '5 PIJIoO. x(mol~ ~ Fig. 5. Quenching ranges of the fluorescence in (Ca, Ph) Mo0 4 and (Ca, Ph) W Ot~er systems Calcium and strontium tungstates form uninterrupted series of solid solutions..the same applies to molybdates. We have investigated the fluorescence in (Ca, Sr) W0 4 and (Ca, Sr) Mo0 4 In both cases the emission of the components is nearly identical, and the emission of the solid solutions is intermediate between that of the components, while the quenching ranges vary regularly with the coniposition (fig; 6).'.. We pave also studied the fluorescence of mixed crystals of the composition M3W06 in which M stands for Ca,Mg, Ca,Sr, Ca,Ba 2): Pure Mg3W06 does not exist, but solid solutio~s (C~,MghW0 6 can be prepared containing.up to about 50% Mg3W0 6 Over this range the emission remains unchanged,. white-blue. As may be seen in fig. 7, the quenching temp~ratures pass through a minimum at 30%, Mg3WOr" verv similar to what has been observed in the system ZnW0 4 -MgW0 4 3).'.

6 88.' F. A. KRÖGER t~r~_o_c~ ~ )(- tunqstetes. motybdetes. 00 ttuorescence quenched o maximum ttcorescence 50 mol.% 75 Sr W04. Mof4., 497,z, Fig. 6. Quenching ranges of the fluorescence of (Ca, SI') W0 4 and (Ca, Sr).Mo0 4, excited by Ä = 2537 Á. '. 00 fluorèscence quenched maximum' ttuorescence 50 moll ". Fig. 7. Quenching ranges of the fluorescence of (Ca, Mg)aWOo'

7 LUMINESCENCE OF SOLID' SOLUTIONS OF THE SYSTEM CaMoO,-PhMoO, 89 It is difficult to determine the proporties of the sys~ems (Ca,Sr)s WOGand {Ca,Ba)aWOs because the quenching ranges for the barium and strontium compounds lie so low that at -80 C the maximum efficiency is not reached. It c~uld he ascertained, however, that the quenching range ofthe fluorescence of the calcium c<!mpound is decreased by the incorporation of strontium and barium. 4. Discussion' 'Forniation of solid solutions of the type considered in this paper ~ay have two effects: ) a gradual change of the' properties of a certain emission band, 2) the appearance of a new emission band.. Effects of the first kind occur when the luminescent centre is not disturbed to such an extent that the electronic transition underlying the emission is essentially changed. Variation of the quenching point will be caused by a change Ut the interaction of the excit~d centre and its surroundings. In the frame-work of a theory: of the energy dissipation given by Mott and Seitz 4), properties ofthe configuration formed by the nearest neighbours, - I such as the mass of the atoms, the electric charge, the symmetry of the surroundings, interatomie distances, must be of gre!lt importance. If, on the other hand, the dissipation is supposed to be a property of the lattice as a whole (Möglich and Rompe 5)), the formation of a solid solution must vary this property for the lattice as a whole: The first assumption seems to be the most acceptable one. Under particular circumstances, the changed surroundings,~not only cause a variation in the interaction, but they willalso change the electronic transition in the' centre, either by causing a transition involving entirely new energy levels, or by changing the relative probabilities for the transition between levels already present. In the molybdates an'd tungstates with mixed cations studied so far, only the systems containing lead show such a new emission at intermediate compositions_. Systems containing calcium together with strontium, or zinc together with magnesium, do not show this effect. Eindhoven, January 94.7 REFERENCES ). F. A. Kröger, Some Aspects of theluminescence of Solidsv.Amsterdam- New York 947, in print, 'especially' chapter IV... ' 2) H. P. Rooksby & E. G. Steward, Nature 57, , ) F. A. Kröger, Philips Res. Rep. 2, 77-82, ) N. F. Mott, Proc. Roy. Soc. London (A) 67, , )938; R. W. Gurney & N. F. Mott, Trans. Faraday Soc. 35, 69-73, 939; F. Se it s, Trans. Faraday Soc. 35, 74-85, ) K. Birus, F. Möglich & R. Rompe, Phys. Z. 44, 22-29,943; M. Schön, Fo~schungen und Fortschritte, 9, 4-9, 943. '