High-pressure single-crystal X-ray diffraction of Tl2SeO4

The effect of pressure on the crystal structure of thallium selenate (Tl2SeO4) (Pmcn, Z=4), containing the Tl+ cations with electron lone pairs, has been studied with single-crystal X-ray diffraction in a diamond anvil cell up to 3.64 GPa at room temperature. No phase transition has been observed. The compressibility data are fitted by a Murnaghan equation of state with the zero-pressure bulk modulus B0=29(1) GPa and the unit-cell volume at ambient pressure V0=529.6(8) A3 (B'=4.00). Tl2SeO4 is the least compressible in the c direction, while the pressure-induced changes of the a and b lattice parameters are quite similar. These observations can be explained by different pressure effects on the nine- and 11-fold coordination polyhedra around the two non-equivalent Tl atoms. The SeO42- tetrahedra are not rigid units and become more distorted. Their contribution to the compressibility is small. The effect of pressure on the isotypical oxide materials A2TO4 with the β-K2SO4 structure is discussed. It appears that the presence of electron lone pairs on the Tl+ cation does not seem to influence the compressibility of Tl2SeO4. - Graphical abstract: Pressure dependence of normalized lattice parameters and unit-cell volumes in Tl2SeO4 (Pmcn, Z=4). The solid line is the Murnaghan equation of state