Pressure-induced phase transitions in β-BaTeMo2O9

High-pressure Raman scattering study of monoclinic β-BaTeMo2O9 single crystal was performed. This study showed the onset of two reversible second-order phase transitions near 3.1 GPa and 7.0 GPa associated with weak changes of the structure. The pressure dependence of Raman bands suggests that the phase transitions might involve decrease of the long Mo-O and Te-O bonds involved in formation of the Mo-O-Mo and Te-O-Mo bridges, whereas the short Mo-O and Te-O bonds are less affected. These modifications probably result in tilting on the MoO6 and TeO4 polyhedra in both phase transitions, although the presence of a strong A(LO) mode at 13 GPa suggests that the new phases are noncentrosymmetric. Different behaviour was reported previously in literature for α-BaTeMo2O9, which exhibited a pressure-induced phase transition associated with a very large change in the coordination sphere of Te4+. Origin of such different stability of the both BaTeMo2O9 polymorphs under compression is discussed.