High-pressure properties of TiP₂O₇, ZrP₂O₇and ZrV₂O₇

High-pressure synchrotron X-ray powder diffraction studies of TiP 2 O 7 , ZrP 2 O 7 and ZrV 2 O 7 have been performed. The ZrV 2 O 7 structure undergoes a reversible transition at 1.38–1.58 GPa from cubic α- to pseudo-tetragonal β-ZrV 2 O 7 that displays an orthorhombic 2 × 3 × 3 supercell. At pressures above 4 GPa, ZrV 2 O 7 becomes irreversibly X-ray amorphous. No such transformations are observed for TiP 2 O 7 and ZrP 2 O 7 , which compress smoothly up to the highest investigated pressures (40.3 and 20.5 GPa, respectively). These differences in high-pressure properties are discussed in terms of the negative thermal expansion of ZrV 2 O 7 . The bulk moduli at ambient pressure ( B 0 ) for TiP 2 O 7 , ZrP 2 O 7 , α-ZrV 2 O 7 and β-ZrV 2 O 7 were estimated to be 42 (3), 39 (1), 17.0 (7) and 20.8 (10) GPa, respectively.