Beam lnterections with Materials 8 Atoms

Nuclear Instruments and Methods in Physics Research B 133 (1997) 24-27 ELSEVIER

Olivine-spine1 phase transformations in Mn2SiSe4 at high pressure

Andrzej Grzechnik a5*, Paul F. McMillan ‘, Guy Ouvrard b

’ Materials Reseurch Group in High Pressure Synthesis. Department of’ Chrrnistry arid Biochrrnistry, Arkonu Stute University. Tempe, AZ 85287-1604. USA

h Institut des MatCriuux de Nantrs. Luhoratoirr de Chimie dcs Solides. UMR 110. 2 yue de la Houssini&e, 44072 Nantes, C6de.u 03, France

Abstract

High pressure behavior of MnzSiSe,+ with the olivine structure is investigated using X-ray diffraction. The pressure- induced olivine-spine1 phase transition in the 2040 kbar range is accompanied by formation of intermediate spinelloid compounds. 0 1997 Published by Elsevier Science B.V.

The olivine structure (Puma space group) is a common one for materials with the M1M2TX4 stoichiometry, where T = Si. Ge, Sn (a tetrahedral site); X = 0, S, Se, and Mf+ and Mi+ are cations at octahedral sites [l]. Many oxides with the olivine structure are known to transform to the spine1 structure (F&KY space group) at high pressure, with no change in the cation coordination [2]. In olivines, oxygen atoms form a distorted hexagonal close packing whereas spinels have a face-centered oxygen packing [3]. Both olivine and spine1 struc- tures are taken at ambient pressure by many chal- cogenides X = S, Se, Te [4-61. However, the olivine-spine1 transition at high pressure has not

yet been demonstrated for sulfide or selenide com- pounds.

*Corresponding author. Present address: Ecole Normale

SupCrieure de Lyon. 46 allee d’ltalie, 69364 Lyon CCdex 07.

France.

0168-583X/97/$37.00 0 1997 Published by Elsevier Science B.V. All

PIISO168-583X(97)00545-4

MnZSiSe4 is known to have the olivine structure at ambient pressure [4], and exhibits antiferromag- netism at low temperature [6].

In this study, we have investigated Mn2SiSe4 at high pressures using X-ray diffraction measure- ments in a diamond anvil cell. We present evidence for the occurrence of a first-order olivine-spine1 phase transition in MnzSiSed at high pressure. There also appear to be transformations to inter- mediate (spinelloids) phases, in the intermediate pressure range. The results of our in situ Raman measurements are presented elsewhere [7].

The high-pressure X-ray diffraction patterns

were recorded in an energy dispersive configura- tion on the wiggler x-17-c line of the National Synchrotron Light Source at Brookhaven. The polychromatic X-ray beam was collimated by tungsten carbide slits to a size of 40 x 40 pm’. The diffracted beam was analyzed with a Canberra planar germanium detector at the 28 angle of 12” for energies between 5 and 70 keV. The resolution was about 200 eV at 20 keV. The sample was finely

rights reserved.

A. Grzechnik et al. f NW/. Instr. and Meth. in Phys. Res. B 133 (19971 24-27 25

ground and loaded into a Mao-Bell-type cell with type I diamonds, brilliant cut with 350 urn culets, and a sample chamber diameter of 150 urn. Pres- sures were determined from the Au internal stan- dard [8]. To determine the actual pressure gradients, we calibrated the same cell assembly from the shift of the RI ruby fluorescence line [9]. The pressure gradients, within the 40 x 40 urn2 area, did not exceed 5 kbar.

In situ X-ray diffraction patterns were recorded up to 106 kbar (Fig. 1). The patterns at low pres- sures are dominated by reflections from the olivine structure, which shows a series of sharp peaks. The relative intensity of peaks from the minor impurity phase (the cl-MnSe phase as determined by powder diffraction at ambient pressure, outside the dia- mond cell) are enhanced due to preferred orienta- tion effects. Upon compressing, the patterns show not only a decrease in the number of reflections but also an increase in the linewidths of the peaks. The disappearance of the peaks due to the olivine form of MnlSiSed and appearance of new peaks at various pressures indicates the occurrence of trans- formations with a series of intermediate phases. The major changes in the structure of Mn2SiSe4

.z? z s .c

Fig. 1. EDXRD patterns of MnzSiSe4 as function of pressure

(compression). Stars indicate Au peaks. Ka and KP, are the

X-ray emission lines of Se.

at high pressures occur at about 40 kbar. This is consistent with the second phase transformation observed in our in situ Raman experiments [7]. Hyde et al. [2] demonstrated that structural chan- ges from the olivine structure into spine1 structure undergo through a series of structures correspond- ing to the hypothetical o phase and spinelloids [lo-141. We suggest that the phases observed in the intermediate pressure range could represent MnzSiSe4 spinelloid-type compounds.

The pattern recorded at 106 kbar is indexed with Fd3rn symmetry of a spine1 form of MnzSiSed (Table 1). The peaks that are not indexed corres- pond to the MnSe impurity and a new phase MnSe? occurring from the decomposition of MnzSiSe4 by heating in the synchrotron beam. Similar thermal decomposition has been noted for Fe$iS, [15]. Cubic cL-MnSe undergoes a high-pressure transition from ZnS sphalerite-type into the NiAs type at 70 kbar [16]. The peaks at about 3.06 and 2.76 A are related to the low- and high-pressure phases of MnSe, respectively, that coexist to 106 kbar. We find no evidence for vibrational modes expected for the MnSez with the pyrite structure in the Raman spectrum [17], so this impurity cannot represent a significant pro- portion of the sample [7], although its X-ray reflec- tions appear quite strong in the diamond cell synchrotron experiment.

The pressure dependence of the unit-cell vol- ume of MnzSiSed is shown in Fig. 2. A structural olivine-spine1 phase transition takes place at 40- 50 kbar. The associated relative volume change be- tween the olivine and spine1 phases is 17%.

The observed relative intensities of the peaks for the spine1 phase are not compatible with those predicted for the normal Mn,SiSe, spine1 [ 181. This could be due to the energy-dispersive technique used in this study but could also indicate disorder- ing of the Mn’+ and Si4+ cations over the tetrahe- dral and octahedral sites. Spinelloids [lo-141, the intermediate phases between olivine and spine1 [2], have disordering at both octahedral and tetra- hedral sites. The partial Mn occupancy of the 8a and 16d sites in the spine1 structure [18], i.e., the mixed normal-inverse character of the high-pres- sure phase, can be preserved from partial Mn oc- cupancies in the intermediate low-symmetry

26 A. Gtzechnik et ul. I Nucl. Instr. and Meth. in Phys. Res. B 133 (1997) 24-27

Table 1 Observed and calculated d spacing (A) for MnlSiSeJ at 106 kbar

Ml Energy (keV) doss III, &I,

1 I 1 12.792 4.636 3.19 4.6407

19.387 3.059 5.32

220 20.889 2.839 100.00 2.8419

21.518 2.756 7.44

22.42 I 2.645 2.12

311 2.4235 177 ___ 25.773 2.301 42.55 2.3204

400 29.504 2.010 17.02 2.0095

33 I 3 I .970 1.855 21.28 1.8440 422 36.095 1.643 1.07 I .6407

511 38.285 1.549 17.02 1 .S469

333

440 41.792 1.419 9.58 1.4209

53 I 1.3587

442 1.3397

620 46.733 I.269 5.32 1.2709

533 I .2258

622 49.01 I 1.210 5.32 1.2118

444 51.036 1.162 1.06 1.1602

551 52.500 1.1296 2.13 1.1255

711

642 55.476 1.069 2.13 1.0741

553 56.913 1.042 2.13 1.0465 731

800 59.185 1.002 1.06 I .0048

Ft/3m. a = 8.038(9) A.

phases. The broadness of the peaks could be relat- ed to this disorder.

The hysteresis observed upon decompressing both in Raman and X-ray data is related to the first-order pressure induced phase transitions in MnzSiSe4. We could not recover high or interme- diate phases from our diamond anvil cell experi- ments. The decompressed sample always showed

f 600 c

I I I, I I1 1 I I ” 1 ’ j 1 j 1” 0 20 40 60 60 100 120

Pressure (kbar)

Fig. 2. Evolution with pressure of the unit-cell volume of

MnzSiSej as determined from EDXRD measurements.

the X-ray pattern characteristic of the olivine structure. We then attempted to synthesize recov- erable samples of the high-pressure phases using multiple anvil and piston cylinder devices at high pressures and high temperatures. However, the re- covered samples held at high pressure (20-100 kbar) and high temperature (200-900°C) were mixtures of products of MnZSiSe4 decomposition into a-MnSe, MnSez, Se, and SiSez. Finally, we carried out high-pressure synthesis of Mn2SiSe4 at 90 kbar and room temperature for nine days. The X-ray pattern of the decompressed sample ex-

hibits the presence of the ambient pressure olivine phase accompanied by intermediate high pressure phases of Mn2SiSe4. These experiments indicate that the spinel-structured form of Mn2SiSe4 is un- likely to be recoverable to ambient pressure under these conditions. The energy barriers for the se- quence of displacive phase transitions from the oli- vine to spine1 structures, through intermediate

A. Grzechnik et al. I Nucl. hstr. and Meth. in Plqx Rex B 133 (1997) 24-27 21

spinelloids phases [2], must be on the order of kT N 2.5 kJ/mol (k is the Boltzmann constant; T is room temperature). However, these first-order olivine-spine1 transformations in Mn2SiSe4 are thermally activated and would become more slug- gish at lower temperatures. Future attempts with the decompression of the high-pressure phases of MnzSiSe4 and possibly its other chalcogenide ana- logs in the temperature of liquid nitrogen [19] could be more successful.

References

PI

PI

131 [41

[51

F. Hanic, M. Handlovic, K. Burdova, J. Majling, J. Cryst.

Spectr. Res. 12 (1982) 99.

B.G. Hyde, T.J. White, M. O’Keeffe, A.W.S. Johnson, 2.

Krist. 160 (1982) 53.

M. O’Keeffe, B.G. Hyde, Struct. Bond. 61 (1985) 77.

V.G. Rocktaschel. W. Ritter, A. Weiss, 2. Naturforsch. 19

(1964) 958.

J. Fuhrmann, J. Pickardt. Acta Crystallogr. C 45 (1989)

1808.

161

[71

181 [91

PO1

1111 [121 u31

iI41

u51 [‘61

1171 1181

I191

S. Jobic. F. Bodenan, P. Le Boterf, G. Ouvrard. J. Alloys

Comp. 230 (1995) 16.

A. Grzechnik, P.F. McMillan, G. Ouvrard, J. Phys. Chem.

Solids, in press.

D. Heinz, R. Jeanloz, J. Appl. Phys. 55 (1984) 885.

J.D. Barnett, S. Block, G.J. Piermarini. Rev. Sci. Instr. 44

(1973) 1.

C.B. Ma, K. Sahl. E. Tillmans. Acta Crystallogr. B 31

(1975) 2137.

C.B. Ma, E. Tillmans, Acta Crystallogr. B 31 (1975) 2139.

C.B. Ma, K. Sahl, Acta Crystallogr. B 31 (1975) 2142.

K. Horioka, K.I. Takahashi. N. Morimoto. H. Horiuchi, Acta Crystallogr. B 37 (1981) 635.

K. Horioka. M. Nishiguchi. N. Morimoto, H. Horiuchi,

Acta Crystallogr. B 37 (1981) 638.

B. Kamb, Amer. Mineral. 53 (1968) 1439.

L. Cemic, A. Neuhaus, High Temp. High Press. 4 (1972)

97.

B. Muller. H.D. Lutz. Sol. State Commun. 78 (1991) 469.

R.W.G. Wyckoff, Crystal Structures, vol. 3. 2nd ed..

Robert E. Krieger Publishing Company, Malabdr, 1981.

K. Leinenweber, U. Schuelke, S. Ekbundit, P. McMillan,

submitted to High Pressure-Temperature Research: Prop-

erties of Earth and Planetary Materials.