Unusual 180 degrees P-O-P Bond Angles in ZrP(2)O(7)

The structure of cubic ZrP(2)O(7) 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 Pa&thremacr;, and Z is 108. For those P(2)O(7) units not on a 3-fold axis, the P-O-P angles range from 134 degrees to 162 degrees. Two crystallographically distinct P(2)O(7) groups are on three fold axes with P-O-P angles thus constrained to be 180 degrees on average. The structure of cubic ZrP(2)O(7) 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 P(2)O(7) are then constrained by symmetry to be 180 degrees on average. The exceptionally low thermal expansion shown by ZrP(2)O(7) above 290 degrees C is likely related to the unusual P-O-P angle.