Abstract
1. An investigation of the thermal variations of the IR spectrum of ZrP2O7 permits us to relate the phase transition at 300° to inversion of the P2O7 anions and indicates the existence of several minima of the potential function of the vibration δPOP, symmetrical with respect to the configuration with ∠POP = 180°. 2. The IR spectra of Mn2P2O7, Sc2Si2O7, and Er2Si2O7 give evidence of a centrally symmetrical form of the anion X2O7 and do not exhibit transition to the bent configuration all the way down to −150°. 3. The possibility of a change in form of the potential function of the vibration δXOX in X2O7 ions is assumed within broad limits: from centrally symmetrical but, possibly, highly anharmonic (or even with a small central maximum) to a configuration with a barrier around 0.1–0.01 eV at < XOX = 180°, and to a curve with one minimum at XOX ≠ 180° (the “usual” bent ions X2O7). 4. Redistribution of the dπ — pπ bonds in the anion X2O7 with increasing fraction of covalent character in its interaction with the cations leads to an increase in the equilibrium value of < XOX and promotes inversion of the anion.
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