Twinned crystal structure and compressibility of TlTeVO 5

Abstract

The high-pressure behavior of TlTeVO 5 has been investigated in situ using single-crystal X-ray diffraction in a diamond anvil cell. This material is structurally stable at least to 7.11 GPa, the highest pressure reached in this study. TlTeVO 5 is twinned both at ambient and high pressures ( Pna2 1, Z=4). The twinning law relating the two individuals is equivalent to a rotation of approximately 3° around the [ 1 2 ¯ 0 ] direction. Within errors, no changes in the orientation of the two individuals are observed as a function of pressure. The refined twin volume fractions do not change within estimated standard deviations, either. The material is the most and the least compressible along the c and a axes, respectively. The P– V data could be fitted by a Murnaghan equation of state with B 0=32(1) GPa, V 0=504.4(4) Å 3, and B′=7.97(43). The most important effect of pressure is the increase of the coordination numbers for the Tl and Te atoms. The Tl–O distance nearly parallel to the [ 001 ] direction is the most sensitive structural feature to pressure, resulting in the anisotropic compressibility. The long Te–O distances decrease, while the short ones are constant or even become slightly longer. Such a pressure-induced change of the coordination is interpreted as due to increasing uniformity of the oxygen atoms surrounding the cations and to decreasing activity of the electron lone pairs. The change is accompanied by an increase of the pseudosymmetry of the structure with respect to the centrosymmetric space group Pnna.