the preparation of some ternary sulphides mr2s4 (m = ca, cd; r = la,sm,er) and the melt growth of...

Mat. Res. Bull . , Vol. 19, pp. 717-725, 1984. Printed in the USA. 0025-5408/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.

THE PREPARATION OF SOME TERNARY SULPHIDES MR2S 4 (M = Ca,Cd; R = La,Sm,Er) Ah~ THE

MELT GROWTH OF CaLa2S4

P.J. Walker and R.C.C. Ward Clarendon Laboratory

University of Oxford, U.K.

(Received February 28, 1984; Communicated by J. B. Goodenough)

ABSTRACT The Preparation of the binary sulphides CaS, La2S3, Er2S3, and Sm2S 3 in high purity powder form from the elements is described, with the subsequent synthesis of ternary sulphides with Th3P 4 and spinel structures. Single crystals of CaLa2S 4 and CaLa2S 4- La2S 3 solid solutions, up to ]0 mm3~volume, have been ~rown from the melt in sealed crucibles using the St~ber technique.

Introduction

The rare earth ternary sulphides of general formula MR2S4 (M = divalent metal ion, R = trivalent rare earth ion) are under investigation for anplication as infra-red window materials. The aim of the work here is to prepare high- purity materials, preferably in single crystal form, for intrinsic property assessment of these compounds.

Compounds of the MR2S 4 type crystallise in a variety of structures depending upon the sizes of the M and R ions (],2). Two cubic structures are found - these are of particular interest because I.R. windows could be manufactured by hot pressing. The Th3P4 structure is formed when both cations are large, e.~. M = Ca or Sr, R = La-Gd along the lanthanide series. When smaller cations are nresent the spinel structure can be formed, e.g. CdR2S4(where R = Sc and Ho-Lu at the end of the lanthanide series) and M~R2S4 (R = Sc and Tm-Lu). The par- ticluar compounds under study here are CaLa2S4, CaSm2S4 (Th3P 4 structure) and CdEr2S4 (spinel).

In the synthesis of these materials, particular attention has been paid to keeping the oxygen impurity levels as low as possible. For this reason the chosen preparative route is by direct synthesis from either the elements or pure metal sulphides in sealed ampoules, rather than the more usual method of

*Present address: GEC Ltd., Hirst Research Centre, Wembley

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718 P . J . WALKER, et al . Vol. 19, No. 6

reaction of the metal oxides in flowing H2S. Separate preparations are undertaken of the two binary constituents, MS and R2S3, which are then reacted to form the ternary compound.

Crystal growth from the melt in sealed tantalum crucibles has been attempted using the St~ber technique of directional solidification in a stationary system. This technique has previously proved applicable to several other high meltin~ materials e.g. Eu0, EuS. Experiments have been confined to CaLa2S4(Th3P ~ structure) because of the very high vapour pressure of the CdS component of the spinel compound.

Synthesis

Binary Sulphides

Of the required binary sulphides, only CdS could be obtained in the necessary pure form. The remainder have been prepared by reaction of the elements in silica ampoules sealed under high vacuum. The sesquisulphides of samarium and erbium, as well as calcium sulphide, CaS, were prepared in a straightforward manner by reaction between stoichiometric quantities of the elements. However, for several reasons, lanthanum sesquisulphide was found much more difficult to prepare in pure form; this synthesis is described in full detail below.

Calcium, samarium and erbium are all relatively volatile metals and can be obtained in pure, sublimed form (< 50 ppmw oxygen); the sources are given in Table ]. No further preparation of these metals was undertaken other than vacuum degassing to decrease the hydrogen impurity level in the rare earth metals. Calcium is also extremely reactive and tarnishes rapidly in air, and was therefore handled in an argon glovebag. The rare earth metals generally decrease in reactivity along the lanthanide series. Lanthanum tarnishes in air nearly as quickly as calcium, while erbium is stable in air (except in finely divided form) and samarium very nearly so.

Experimental details of the binary preparation are given in Table I. A vitreous carbon boat was used to prevent direct reaction between the silica and rare earth metal or calcium. When loading the reactants, the sulphur was kept outside the boat, so that the subsequent reaction was a controlled one between sulphur vapour and solid metal. This was particularly desirable in the case of the Ca-S reaction, which is extremely exothermic. It is interesting that all the CaS preparations at 250oc resulted in a uniform,black powder, which on heating to 750°C converted to the normal yellow-white colour of CaS with the evolution of a very small quantity of sulphur. The resultant CaS is in the form of a fine powder: an ampoule is shown in Figure I after reaction. The Sm2S 3 and Er2S 3 preparations were carried out at a higher temperature, 600oc, with a short period at 800oc to ensure complete reaction.

La2S 3 preparation

Lanthanum metal exhibits the properties of high reactivity and very low vapour pressure. Purification by distillation is not practical, therefore, and cormnercial lanthanum (Rare Earth Products Ltd.) contains an oxygen level typically ~ 2000 ppmw which is unacceptable for this work. It has proved possible to reduce the oxygen concentration to = I00 ppmw during processing in a cold-boat zone refiner (as described for praseodymium metal in reference (6)). Use is made of the fact that the solubility of oxygen in molten lanthanum is extremely low, so that nearly all the oxygen impurity exists as solid

Vol. 19, No. 6 TERNARY SULPHIDES 719

FIG. ]. Preparation of polycrystalline CaS (Boat dimensions: 55 mm lon~ x 16 mm wide)

TABLE I.

Details of Binary Syntheses

Starting Components

Ca:S Ca from Ames Laboratory, Iowa, U.S.A.

2Er : 3S

2Sm:3S

La:2S

Note :

Sublimed- grade Er (Rare Earth Products Ltd. Widnes, Cheshire)

Sublimed- grade Sm (REP Ltd.)

Zone refined La (see text)

LaS 2 reduction

Typical Preparation Conditions

3 days at 250°C

24 hrs at 750°C

6 days at 600°C

15 hrs at 800°C

6 days at 600oc

16 hrs at 800°C

6 days at 600oc

16 hrs at 800°C

Product

black powder

CaS (yellow-white)

Er2S 3 (tan)

monoclinic, D-structure (3)

~-Sm2S 3 (red)

orthorhombic (3,4)

~-LaS 2 (brown) monoclinic (5)

~-La2S 3 (rust)

Sulphur was Johnson Matthey Chemicals Ltd. "Specpure" or BDH Ltd. "Optran" grade, used in broken lump form. If the cor~mercial product was in powder form, this was fused under continuous high vacuum prior to use.


particles of La203, which are less dense than the melt and form a crust which can be physically removed after cooling.

An alternative source of lanthanum was also used. Material obtained from Ames Laboratory, Iowa University, U.S.A., was sufficiently pure to be used without any further purification (oxygen content ~ 50 ppmw).

The method of direct synthesis which was used for Sm2S 3 and Er2S 3 was found, in the case of lanthanum, to result in the formation of a LaS2/La2S3/La mixture. This is attributed to the following factors:- the relative stability of the disulphide in this system, the low vapour pressure of La, and the fact that the La is in lump form.

In fact the reaction to LaS 2 will go to completion at 600°C if sufficient sulphur is present. It was therefore decided to prepare La2S 3 by reduction of LaS2, using a sealed silica ampoule with one end held at room temperature as a condensation point for the evolved sulphur. The LaS 2 synthesis and reduction (details in Table 1) were carried out in the same ampoule.

Unless the La metal is initially contained in a carbon boat, the end product is a mixture of ~-La2S3 (the required orthorhombic low temperature form) and ~-La2S3 structures, as identified by powder X-ray diffraction. The tetra- gonal ~ structure, normally stable between 900oc and 1300°C (2), can incor- porate a proportion of oxygen to form Lal0Sl5-x0x (0 < x < I) which stabilises the structure at low temperature (7). Elemental oxygen analysis, by an inert gas fusion technique (8) of samples prepared here suggests that the B structure is formed when the oxygen level is ~ 500 ppmw, (i.e. x ~ 0.06). The two structures are easily distinguished by colour - ~-La2S3 is a deep rust colour while Lal0SlS_x0 x is light yellow. Only ~-La2S3 was used for the CaLa2S~ experiments. Two samples are shown in Figure 2: the correct ~ phase shows a much darker and uniform colour.

FIG. 2. ~-La2S3 and a mixture of ~-La2S 3 and La10Sl5_x0x


It should be noted that a recent report by Schevciw and White (9) gives the colour of ~-La2S3 as light yellow. The sulphide in this case was prepared by the reaction of the oxide in flowing H2S. Our results would suggest that an oxysulphide phase has been produced by this method, as this colour corresponds to Lal0SlS_x0x, even though the reaction time was extended. It is interesting to note that previous workers (10) have suggested that it is difficult to achieve 100% conversion to the sulnhide by this route.

Ternary Sulphides

Rare earth ternary sulphides were prepared by reaction of stoichiometric quantities of the binary constituents in tantalum crucibles sealed by argon arc welding. Starting powders were mixed and packed into the crucible inside an argon glovebag. The required reaction temperatures are in the range ]250°C - ]500°C (see Table 2). Note that CaS and La2S 3 material was used directly in CaLa2Sq melt growth runs (see section 3 below) and not to prepare powder.

CaSm2S 4 was studied here as an alternative to CaLa2S4, because Sm2S 3 is much easier to prepare in pure form than La2S 3. However, the CaSm2S4 product was black in colour, thought to be due to charge transfer effects associated with Sm ions. This effect may lead also to absorption in the infra-red and thus limit the usefulness of CaSm2S 4 as a window material.

The spinel compound, CdEr2S~, proved straightforward to prepare in the sealed crucible. This contrasts with the acute problem of loss of CdS encountered in the alternative method of preparation from the mixed oxides in flowing H2S. However, this same problem was apparent during hot pressing trials of samples prepared here. It appears that this could rule out the application of many of the spinel sulphides.

TABLE 2.

Details of Ternary Syntheses

Starting Components

Er2S 3 + CdS

Sm2S 3 + CaS

Preparation Conditions

48 hrs at ]250°C

60 hrs at 1450°C

Products (X.R.D.)

CdEr2S4, spinel-structure

(+ trace CdEr4S 7 (I]))

CaSm2S~, Th3P4 -structure

(+ trace CaS)

La2S 3 + CaS used directly for melt experiments (below)

Crystal Growth

Melt Growth of CaLa2S ~

The crystal growth of CaLa2S 4 has been attempted using the St~ber technique, which has been used successfully in the past in this laboratory for the growth of europium chalcogenides and other high melting materials (]2).


A resistance-heated furnace, employing a cylindrical tungsten mesh element, is operated under high vacuum. The crucible, made of tantalum and sealed by arc welding, stands on a tungsten plate which is supported on tungsten pins providing a thermal link to the water-cooled base of the chamber. A vertical temperature gradient is thereby set up in the crucible, so that when the furnace is programmed down from a temperature at which the whole charge is molten, directional solidification is achieved. The method relies on the high diffusion rates which are obtained at temperatures ~ 2000oc to prevent spurious nucleation. Full details of the furnace are given in reference (12).

Little data on the melting points of the ternary sulphides in sealed systems is available. During preliminary experiments in the present work, the melting point of SrLa2S 4 material supplied by BDH Ltd. was determined to be 1930°C. It was assumed that the melting point of CaLa2S 4 would also be close to this value. All growth runs involved cooling from 2000°C : the products revealing that the CaLa2S ~ had indeed been molten at this temperature. A typical growth run involved heatinglto 2000°C, then cooling at |0°C hr -I to ]500oc hr -I followed by 50oc hr- to room temperature. The vertical temperature gradient is estimated = 2oc mm-l; crucibles were |3 mm inside diameter with a charge length typically 15 rmn after melting.

The starting binary sulphide powders were mixed and packed into the crucible in an argon glovebag, the crucible having been acid-cleaned and then vacuum baked at ~ |250oc.

Results of melt growth

As well as the stolchlometric case, i.e. 50 mole % La2S 3 - 50 mole % CaS, melts containing a slight excess of La2S 3 were used. The high temperature form of La2S3 (y-type) has the Th3P~ structure and is expected to form a range of solid solution with CaLa2S4.

The products were examined by metallography and SEM (EDAX) to look for gross precipitation, by EPMA to assess point to point variation in stolchiometry, and by bulk chemical analysis to obtain the percentage composition. In the latter, Ca and La were determined by atomic absorption and emission respec- tively and a gravlmetric method was used for sulphur. The main points are su~marlsed in Table 3 but several features are common to all melts:

i) the products were dark grey in colour, indicative of sulphur deficiency. Analytical results indicated that the sulphur was ]-2 wt% below the expected figure.

ii) sulphur was lost from the crucible at high temperatures, presuma- bly due to diffusion along grain boundaries.

iii) some reaction with the tantalum crucible occurred, leading to tantalum precipitates in the product along the grain boundaries.

iv) no evidence has been found in the samples of any further secondary precipitation and in particular no evidence was found for the presence of excess CaS or oxysulphide. Preliminary melt growth experiments using batches of CaLa2S 4 and SrLa2S 4 prepared from the oxides via H2S treatment had previously shown both of these features (13).

v) Analysis showed that only sulphur was lost from the crucibles and not one of the binary sulphides.


TABLE 3.

La2S3-CaS Melt Data

INITIAL MELT SIZE OF SECONDARY COMPOSITION CRYSTALS PRECIPITATES LATTICE PARAMETER (~)

50 mole % La2S 3- Less than ] mm 3 tantalum 8.680

50 mole % CaS

52 mole % La2S 3- up to ] mm 3 tantalum 8.686

48 mole % CaS

(y-La2S3 : 8.73])

(CaLa2S4 : 8.683)

The largest single crystals were obtained from starting stoichiometries containing a slight excess of La2S 3. A Laue photograph of one such crystal with orientation close to <]|0>, is shown in Figure 3 and indicates a good quality sample. This sample is a solid solution of y-La2S3-CaLa2S4- chemical analysis and X-ray data indicate a composition close to 52 mole % La2S 3. Althou~h the EPMA results remain qualitative, because of the lack of suitable standards, they did not reveal any significant variation in stoichiometry across the length and breadth of the sample boules.

FIG. 3. Laue photograph of CaLa2S 4 (see text)


Melt Growth using inert liner

In an attempt to overcome the interaction between the sulphide melt and the tantalum crucible, some runs were carried out using tantalum crucibles with a closely fitting vitreous carbon liner. The preparation of the samples and the growth conditions were kept as close to the previous attempts as possible to permit a direct comparison of the results. On completion of the runs there was no sign of interaction between the carbon and either the tantalum or the melt - the sample separated easily from the liner. This was never the case without the liner. The liners should be re-usable although the extensive recrystallisation of the tantalum does not allow further runs with the same crucible. Samples obtained were, again, grey in colour, but showed several improvements; details are in Table 4. Lattice parameters were determined by powder X-ray diffraction.

TABLE 4.

Melt data using Vitreous Carbon Liner

INITIAL MELT SIZE OF SECONDARY COMPOSITION CRYSTALS PRECIPITATES LATTICE PARAMETER (~)

50 mole % La2S 3- about I rmn 3 None 8.680

50 mole % CaS

55 mole % La2S 3- up to I0 mm 3 None 8.689

45 mole % CaS

The most noticeable improvements were the lack of tantalum precipitates and the large increase in the size of the crystals (particularly in the solid solution case). These samples were subjected to rigorous chemical analysis and were sectioned into five pieces to check for stoichiometry variations. The results did not show up any detectable variations in the boules.

Annealing Experiments

Several small crystals (up to I mm 3) from a boule of composition 52 mole % La2S 3 - 48 mole % CaS ( no liner used) and a polycrystalline slice 5 rm~ diameter and 1.5 mm thick (containing crystals up to 3 mm long) from a 55 mole % La2S 3 - 45 mole % CaS composition (liner used) were annealed in flowing H2S at ll00oc to try and replace the sulphur lost during melting. For the small crystals, some lightening was apparent after 6 hours and a further I0 hours produced pale yellow, almost transparent crystals. The slice showed some lightening but was still grey: it was subsequently placed in a vitreous carbon boat and sealed in a silica ampoule in the presence of excess sulphur and an atmosphere of hydrogen. After annealing at l;00oc for 4 weeks the sample was still opaque but much lighter in colour. A sample taken from this and ground to a fine powder for analysis was brown/yellow - previous samples had always been grey/black in this form. Chemical analysis showed the sulphur had been replaced virtually to the expected figure. (Before annealing, S = 27.04 wt%; after annealing, S = 28.13 wt%. Expected S = 28.29 wt%).


Conclusion

The combination of the low-oxygen route of preparation of the ternary sulphide CaLa2S~, coupled with the use of a vitreous carbon liner for melt growth, has resulted in the growth of small crystals of CaLa2S ~ and larger crystals of solid solutions of y-La2S3-CaLa2S~, close to CaLa2S4. The size of the crystals was limited by the continual loss of sulphur from the tantalum crucible during the growth as this non-equilibrium condition would lead to multi- ple nucleation and the formation of small crystallites. Although the sulphur can be replaced by annealing, the crucible compatability remains the major problem. It is possible that vapour deposited tungsten crucibles may be less susceptible to recrystallisation and grain boundary diffusion. The cost of these is much higher than tantalum and preliminary experiments without a liner indicated a much greater degree of attack by the melt than with tantalum (]3). However, the combination of the inert liner with tungsten crucibles may pro- vide the answer.

Acknowledgements

This work was supported by Procurement Executive, Ministry of Defence, spon- sored by DCVD. The authors wish to thank Mrs. R. Harverson for undertaking the chemical analyses, the analytical staff of the Geology Department, Oxford, and Dr. G. Garton, Dr. J. Savage (RSRE, Malvern) for helpful discussions.

1.

References

O. Muller and R. Roy, The Major Ternary Structural Families (Springer- Verlag, 1974).

2. J. Flahaut, ch. 31 of Handbook on the Physics and Chemistry of Rare Earths, Vol. 4 (North-Holland, ]979).

3. A.W. Sleight and C.T. Prewitt, Inorg. Chem. Z, 2282-8 (1968).

4. C.T. Prewitt and A.W. Sleight, Inorg. Chem. 7, 1090-3 (1968).

5. S. Benezeth, M. Guittard and J. Flauhaut, J. Solid-State Chem. 37, 44-48 (]981).

6. R.C.C. Ward, D.Phil Thesis, University of Oxford, (1981).

7. P. Besancon, J. Solid-State Chem. i, 232-240 (]973).

8. Analysis carried out by A.E.R.E., Harwell.

9. O. Schevcin and W.B. White, Mat. Res. Bull. 18, 1059 (1983).

10.

II.

J.R. Henderson, M. Moramoto, E. Loh, J.B. Gruber, J. Chem. Phys. 47, 3347 - 3356 (1967), and references therein.

C. Adolphe and P. Laruelle, Bull. Soc. Fr. Mineral. Cristal. 91, 2]9-232 (1968).

12. K.G. Barraclough, J. Crystal Growth, 36, 347-350 (1976).

13. P.J. ~#alker and R.C.C. Ward, Unpublished results.

the preparation of some ternary sulphides mr2s4 (m = ca, cd; r = la,sm,er) and the melt growth of...

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