Unusual 180° P−O−P Bond Angles in ZrP2O7

Abstract

The structure of cubic ZrP₂O₇ at room temperature has been solved and refined using a combination of modeling and high-resolution neutron powder diffraction data. The cell edge is 24.74 Å, the space group is Pa3̄, and Z is 108. For those P₂O₇ units not on a 3-fold axis, the P−O−P angles range from 134° to 162°. Two crystallographically distinct P₂O₇ groups are on three fold axes with P−O−P angles thus constrained to be 180° on average. The structure of cubic ZrP₂O₇ was also refined from data taken at 227, 290, 371, 435, and 610 °C. The 3 × 3 × 3 superstructure present at room temperature disappears at about 290 °C, and all P−O−P angles of P₂O₇ are then constrained by symmetry to be 180° on average. The exceptionally low thermal expansion shown by ZrP₂O₇ above 290 °C is likely related to the unusual P−O−P angle.