Polymorphism of molecules and complex ions in oxygen compounds of silicon and phosphorus. Report 2. On the mechanism of the phase transition in Mg₂P₂O₇

Conclusions

• 1. The IR spectrum of Mg₂P₂O₇ crystals was investigated and interpreted in the temperature range from −150 to 150°. In the low-temperature form of Mg₂P₂O₇, the intrinsic symmetry of the P₂O₇ ions does not exceed C_s or C₂ at a value of the angle POP no greater than 150–160°. The spectrum of the high-temperature modification of Mg₂P₂O₇ formally satisfies the selection rules for a centrally symmetrical configuration of the anion (< POP=180°).

• 2. A consideration of the characteristics of the spectrum of the high-temperature modification and the changes in the bands of the internal vibrations of the P₂O₇ anion with changing temperature leads to the conclusion of a statistical character of the central symmetry of the anion. It is assumed that the potential function of the vibration δPOP possesses two minima, while the mechanism of the phase transition in Mg₂P₂O₇ is related to thermal excitation of inversion of the anion.