Unusual 180 degrees P-O-P bond angles in ZrP2O7

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 Angstrom, the space group is Pa(3) over bar 3, and Z is 108. For those P2O7 units not on a 3-fold axis, the P-O-P angles range from 134 degrees to 162 degrees. Two crystallographically distinct P2O7 groups are on three fold axes with P-O-P angles thus constrained to be 180 degrees on average. The structure of cubic ZrP2O7 was also refined from data taken at 227, 290, 371, 435, and 610 degrees C. The 3 x 3 x 3 superstructure present at room temperature disappears at about 290 degrees C, and all P-O-P angles of P2O7 are then constrained by symmetry to be 180 degrees on average. The exceptionally low thermal expansion shown by ZrP2O7 above 290 degrees C is likely related to the unusual P-O-P angle.