Polymorphism of molecules and complex ions in oxygen compounds of silicon and phosphorus

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.