High-pressure properties of TiP₂O₇, ZrP₂O₇ and ZrV₂O₇

High-pressure synchrotron X-ray powder diffraction studies of TiP₂O₇, ZrP₂O₇ and ZrV₂O₇ have been performed. The ZrV₂O₇ structure undergoes a reversible transition at 1.38–1.58 GPa from cubic α- to pseudo-tetragonal β-ZrV₂O₇ that displays an orthorhombic 2 × 3 × 3 supercell. At pressures above 4 GPa, ZrV₂O₇ becomes irreversibly X-ray amorphous. No such transformations are observed for TiP₂O₇ and ZrP₂O₇, 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 ZrV₂O₇. The bulk moduli at ambient pressure (B₀) for TiP₂O₇, ZrP₂O₇, α-ZrV₂O₇ and β-ZrV₂O₇ were estimated to be 42 (3), 39 (1), 17.0 (7) and 20.8 (10) GPa, respectively.