Phase behaviour of Li2MoO4 at high pressures and temperatures

Solid State Communications, Vol.5, pp. 405-409, 1967.

Pergamon Press Ltd. Printed in Great Britain

PHASE BEHAVIOUR OF Li2 MoO4 AT HIGH PRESSURES AND TEMPERATURES J. Liebertz Philips Zentrallaboratorium GmbH, Laboratorium Aachen and C. J. M. Rooymans Philips Research Laboratories, Eindhoven, Netherlands (Received 17 March 1967 by G. W. Rathenau) Under normal conditions Li2MoO4 has the phenactte structure. A transition to the spinel structure at high temperature, as mentioned by Goldschmic~,could not be confirmed. It appeared, however, to be possible to realize this transition by the application simultaneously of high temperatures and high pressures. The P. T. diagram of Li2MoO4 Is given. The quasi-unary transition curve shows a negative value of dP/dT, but it does not Intersect the T-axis before the melting point. The phase transition causes a density increase of about 26%, in accordance with the values already observed for some other phenacite-spinel transitions. Introduction

other phenacites transform under pressure into spinel, it looked possible that also for the cornpounds Li2 BeF4, L12W04 and Li2 MoO4 a highpressure phase of the spinel type could occur.

GOLDSCHMIDT ~ believed, without going into experimental details, that he had proved that Li2 MoO4, L12W04, Li2SO4 and Li2BeF4,whlch have a phenacite or similar structure at room temperature, change to a spinel structure at a higher temperature. His conclusions, however, have not been confirmed by later workers In this field,

The relevant data of the phenacite compounds under consideration are collected in Table 1. This contribution will deal with the P. T. diagram of Li2MOO4.

According to Belyaev 2 LI2MoO4 undergoes no transformation up to the melting point. Recent research by Hahn et al. ~ led to the same 2 conclusionthe mentions forexistence Li2 BeF4.of aP~i~ high-temperature LI2 WO4 Belyaev modification but says nothing of its structure. Li 2SO4 changes over to a cubic phase at ambient pressure and approx 575 °C;its structure, however, bears no relationship to spinel. ~

Method of experiment lithium Themolybdate substance supplied used for by theDr. experiment Th. Schucharct was GmbH & Co, Munich. The high-pressure expertments were carried out by means of an apparatus previously descrIbed 10 with two opposed anvils of tungsten carbide, at temperatures up to 650°C and pressures up to 20 kbar, over periods varying from 2 to 60 hr. At the end of the experiment, the sample was cooled under pressure, and the pressure subsequently released. The structure of the phases present in the sample after this treatment was determined by means of a Philips X-ray difiractometer, (Cu Ka~.rad1atlon). The resuks are plotted in Fig. 1.

So far only Li2SO4 has been Investigated under high-pressure conditions. Up to 25 kbar Pistorlus ~ could only observe the two modifications already appearing at normal pressure, I. e. no spinel.

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PHASE BEHAVIOUR OF Li2 MoO4

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In accordance with Ref. 2 the examination by the polarisation microscope showed that phenacite continues to exist at normal pressure up to the melting point which was observed at 705°C. It was further observed that at this temperature the density of the solid phase Is greater than that of the melt. This means that the melting temperatures of phenacite will increase with pressure.

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The P.T. diagram of LI2MoO4. Spinel phase, starting from phenacite.

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Phenaclte phase, unchanged. () Mixture of spinel and phenacite, starting from phenacite.

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Spinel phase, starting from spinel prepared at 20 kbar and 400°C.

~ Mixture of phenacite and spinel phase, starting from spinel prepared at 20 kbar and 400°C. To enable the study of LI 2MoO4 at normal pressure up to the melting point, a microscope with a specially prepared heating table was provided. A homogeneous distribution of temperature was ensured by means of a silver insert in a smaU platinum-wound furnace, Results The high-pressure experiments showed that spinel forms under suitable conditions even at relatively low pressures. The relevant results are reproduced in the P. T. diagram of Fig. 1. As the diagram shows, the phenacite-spinel transformation can roughly be represented by a straight line with negative slope. In the X-ray diagrams isolated traces of spinel were found in the assumed phenacite region, a fact that can be explained by the inevitable lack of homogeneity in pressure due to the process employed. (The pressure indication refers to total force divided by the die cross-section).

It follows from the P.T. diagram (Fig. 1) that the phenacite-spinel transformation curve cannot cut the T-axls but must end In a triple point. Its position above the melting temperature resuks from the positive slope of the phenacite melting point curve. The melting curve of the spinel phase must lie within the region bounded by the extrapolation of the other two curves, within this limit It is arbitrarily drawn in Fig. 1. The lattice constant of the spinel phase is The corresponding values a = 8.434 ±0.3/mol. 003 A and hence the molar volume V 45. 17 cm forphenacite”area=14.3381;c=9.588A and V = 57. 12 cm3/mol. Ignoring compression and thermal expansion there is a difference in molar volume of 1~V= - 12. 0 cm3/mol or approx 21%. If the system is regarded as quasi-unary in accordance with the experiments the entropydifference can be calculated with the help of the Clausius-Clapeyron equation: dP —

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For the given ~V, and a dP/dT value of 7 bar/degree in accordance with Fig. 1, the entropy-difference ~ S is found to be 2. 0 cal. degree ~ mol —

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Because of the great difference in scattering power of Li and Mo, the cation distribution in the spinel phase could be ascertained simply by making a rough comparison of the measured X-ray intensities with the values calculated, leaving angle-dependent factors out of account. The reflexes 220, 222 and 400 are especially characteristic. It followed that the high-pressure modification of Li2 MoO4 belongs to the normal spinels just as the molybdates Na2M0O4 12 and Ag2 MoO4. 12 The Mo-ions therefore are tetrahedrally surrounded whilst the Li-ions are in octahedral coordination.

408

PHASE BEHAVIOUR OF Li2MoO4

Discussion As already mentioned, the phenacitespinet transformation has been realized with Meyer ~give an account the spinel phaseand of substance other than Li2of MoO4. Neubaus LiGaGeO 4 and LIAIGeO4, without, however, mentioning the P. T. conditions. According to Gaines, Perrotta and Stephenson the transformation curve of L1A1GeO4 can be described by the equation P = 0. 018T-4. 5 (P in kbar, T in °C)and hence has a positive slope. The transition curve intersects the temperature axis at 250°C. It would imply that in this case spinel would be the low-temperature stable modification, in contrast with the observations made for LI2MoO4. To test this rather unlikely behaviour a sample of LIA1GeO4 in the spinel structure, obtained at 400°C and 40 kbar, was given a hydrothermal treatment at 200°Cand 1.4 kbar. ‘~

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The sample did partially convert to phenacite, which cannot be explained with the P. T. diagram of Ref. 7. Another compound which may be mentioned is LIZnVO4 Which9 ISQuite transformed recently into the authors preparing the spinelpolyspinel atsucceeded 400°Candin3OIthar. morph of Zn2GeO4.

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Our investigations Into the high pressure behaviour of LI2WO4 have only just begun. It can be said, however, that three modifications probably exist, none of them having a spinel structure. The accordance found by Pistorius ‘~ between the high-pressure polymorphs of Na2WO4 and Na2MoO Is therefore not present for the lithium compounds. We should like to express our thanks to Mrs. Vermeulen-Heck and Miss Rigter for their valuable help in carrying out the high-pressure experiments.

References 1.

GOLDSCHMIDT V. M., Geochemlsche Verteilungsgesetze der Elemente. Norske VidenkapsAkademi, Oslo, VII, 81, 104 and 107 (1926).

2.

BELYAEV I.N., Russ. J. Inorg. Chem. ~

3.

HAHN Th., BIELEN H., EYSEL W. and WEBER F., Chemie d. Erde 22. 175 (1962).

4.

FORLAND T. and KROGH-MOE J., Acta Chem. Scand. ~J.,L565 (1957).

5.

PISTORIUSC.W.F.T., Z. Phys. Chem. N. Folge28,262(1961).

6.

NEUHAUS A. and MEYER H. J., Naturw. 52, 639 (1965).

7.

GAINES A.M., PERROTTA A.J. und STEPHENSON D.A., J. Am. Ceram.Soc. 49, 516 (1966).

8.

ROOYMANS C. J. M., unpublished.

9.

BLASSE G., Inorg. Nucl. Chem. 25, 136 (1962).

602 (1961).

10. LIEBERTZ J. and ROOYMANS C. J. M., Z. Phys. Chem. N. Folge 44, 242 (1965). 11. WYCKOFF R.W.G., Crystal Structures, vol.Ifl, p.134, Interscience, New York (1965). 12. LINDQUIST I., Acta Chem.Scand. ~ 1066 (1950). 13. DONOHUE J. und SHAND W., J. Amer. Chem. Soc.

~,

222 (1947).

14. PISTORIUS C.W. F.T., J. Chem. Phys. ~j2 4532 (1966). 15. LIEBERTZ J. and ROOYMANS C.J.M. to be published. Li2MoO4 hat unter normalen Bedingungen bekanntlich Phenaklt-Struktur. Bei hOheren Temperaturen soll nach

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PHASE BEHAVIOUR OF Li2MoO4

Goldschmidt eine Umwandlung In die Spinell-Struktur erfolgen, was jedoch durch neuere Untersuchungen nicht bestUtigt werdèn konnte. Dagegen war es möglich, die Spinell-Phase von Li2MoO4 bei gleichzettiger Anwendung hOherer Drucke und Temperaturen zu erhalten. Das P. T. Dlagramm von Li2MoO4 wurde bestlmmt. Die Phasengrenze Phenakit/Spinell zeigt einen negativen dP/dT-Wert und schneidet die T-Achse oberhalb des Schmelzpunktes. Mit der Phasentransformation ist etne DichteerWihung von Ca. 26% verbunden, in Ubereinstimmung mit den fur andere Phenaklt/Spinell-Umwandlungen gefundenen Werten. -

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