HIGH PRESSURE INVESTIGATIONS OF a-SnWO4, b-SnWO4 AND Sn10W16O46
Wulf Depmeier1, Detlef Bischke1, Karsten Knorr2, Burkhard Annighöfer2
1Mineralogisches Institut der
Universität Kiel, Olshausenstr. 40, D-24098 Kiel, Germany
2Hahn-Meitner-Institut,
Glienickerstr. 100, D-14109 Berlin, Germany
Keywords: Sn10W16O46, SnWO4, high pressure, diamond anvil cell
Sn10W16O46 is hexagonal (SG P63/m) with a=7.667(4), c=18.640(4) A, a-SnWO4 is orthorhombic (SG Pnna) with a=5.6270(3), b=11.6486(7), c=4.9973(3) A and its high-temperature form a-SnWO4 is cubic (SG P213) with a=7.2989(3) A [1,2,3]. a-SnWO4 and Sn10W16O46 were prepared by heating a stoichiometric mixture of Sn, WO3 and W in evacuated silica tubes at 1000 and 1100 K. The high-temperature form b-SnWO4 could be obtained by rapidly quenching the sealed silica tubes from above 840 K. All compounds have stereoactive Sn2+ lone pairs and the aim of our experiments was to study structural changes induced by lone pairs which possibly become inactive under high pressure.
X-ray powder investigations were performed applying a diamond anvil cell (DAC) mounted on an imaging plate system. For neutron powder experiments the Kiel-Berlin-pressure-cell II [4] was used. The samples were mixed with NaCl or CaF2 as internal pressure standards. The pressure determination was carried out with the well-known equation of state (EOS) of Decker [5] and Hazen and Finger [6]. The experimentally determined pressure dependence of the unit cell volume were fitted with the 3rd order Birch-Murnaghan EOS [7]:
p(V)=1.5 b0[(V0/V)7/3- (V0/V)5/3] (1+ 3/4 (b'-4)[(V0/V)2/3-1])
Fig. 1 Pressure dependence of the unit cell volume. The solid lines are fits with the 3rd order Birch-Murnaghan-EOS.
In the pressure range from 0 to 6 GPa the unit cell volume of b-SnWO4 shows a pressure behaviour typical of many solid state compounds (Fig. 1). The bulk-modulus and its pressure dependence were calculated to b0=27.0(3) GPa and b'=4.7(2). According to the measurements of Sn10W16O46 and a-SnWO4 we obtained a nearly linear pressure dependence of the compressibility up to 4 and 5 GPa (Fig. 1). The bulk-moduli of these compounds are in the same order of magnitude and were calculated to 86.4(10) and 74.8(7) GPa. A pressure dependence of the bulk-moduli was not taken into account.
Fig. 2 Pressure dependence of the lattice parameters of a-SnWO4 (a) and Sn10W16O46 (b). Linear approximations are plotted as dotted lines. Linear bulk-moduli are given.
In contrast to b-SnWO4, where cubic symmetry only allows isotropic compressibilities, we observed an anisotropic pressure behaviour of the lattice parameters for a-SnWO4 and Sn10W16O46 (Fig. 2). With regard to the lattice parameters of b-SnWO4, the pressure dependence decreases in order a, c, b. A linear approximation a=a0(1-bp) with bi=1/b resulted in the linear bulk-moduli bb=294(20), bc=250(40) and ba=212(30) GPa. In Sn10W16O46 the lattice parameter c shows the smallest pressure dependence. The calculated linear bulk-moduli are ba=212(25) and bc=270(30) GPa.
The lattice parameter a of Sn10W16O46 shows a weak change of the slope of its pressure dependence at ~2.8 GPa (Fig. 2b). A suspected phase transition could not be verified. In the DAC-measurements of a-SnWO4 we obtained an additional peak at ~ 4 GPa and 12.3 o 2q (MoKa) which has not been identified yet. Single crystal investigations actually being carried out, are hoped to give more information.