High-pressure structural phase transition on Bi14MoO24

In situ high-pressure powder synchrotron X-ray diffraction studies on tetragonal Bi14MoO24 have been carried out up to 11.5 GPa at room temperature. A tetragonal (I4/m) to monoclinic (C2/m) structural phase transition has been identified around 5 GPa and possible evidence for a second phase transition is observed at the highest pressure of this study. Both phase transitions are reversible upon decompression. The evolution of the lattice parameters, axial ratio, and volume under pressure have been determined for both the low pressure tetragonal and high-pressure monoclinic phases. A volume collapse of 2.7% observed at the tetragonal to monoclinic phase transition indicates it to be of first-order character. The phase transition involves a shear deformation perpendicular to the b-axis and a displacement of the Mo atoms along the b-axis. The details of the mechanism of the tetragonal-monoclinic transition are discussed. The bulk modulus of the low-pressure tetragonal and highpressure monoclinic phases have been estimated to be 67(7) GPa and 64(3) GPa, respectively. The linear compressibilities of both phases show an anisotropic response to pressure. The obtained results are compared with the high-pressure behavior of its isomorph counterpart Bi14CrO24 and with high-temperature behavior on Bi14MoO24.