High-temperature in situ synchrotron X-ray powder diffraction experiments were carried out to investigate the phase transition mechanism of Zr2P2O9 (2ZrO2·P2O5 or “Z2P”). Linear thermal expansion coefficients were calculated for the low-temperature phase (α-Z2P) and the high-temperature phase (β-Z2P) from temperature-dependent changes in lattice parameters. The crystal structures of α- and β-Z2P were determined as a function of temperature by performing Rietveld crystal structure refinements. The structural changes at the phase transition are accompanied by an increase in the average atomic distance between neighboring Zr atoms. The occurrence of shorter metal–metal distances in α-Z2P is interpreted to result from stronger interactions between partly occupied valence orbitals of the d0 metal atoms. The bond valence method was used to calculate the valence sums of the atoms of α- and β-Z2P, respectively, considering also contributions resulting from covalently bonded atoms. As the bond strength between the metal atoms in Z2P decreases with the transition into the high-temperature phase, notably, the metal–metal interactions are regarded to constitute a prerequisite for the stabilization of the α-phase over the energetically favored β-phase.
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