A vibrational study of phase transitions in Fe₂P₂O₇ and Cr₂P₂O₇ under high-pressures

The vibrational properties of synthetic iron diphosphate (Fe₂P₂O₇) and chromium diphosphate (Cr₂P₂O₇) are studied under high-pressure conditions between ~22 and ~30 GPa, respectively. Each compound's structural response to pressure and pressure-induced phase transitions are characterized. The chromium-bearing sample shows coalescence of infrared bands occurring near 6 and 17 GPa: these may be associated with increases in the local symmetry of the P₂O₇ group. The iron sample undergoes a first-order phase transition near ~9 GPa, and a possible phase transition near 5.5 GPa. At 9 GPa, the initially single, strong symmetric PO₄ stretching mode splits into four modes, and the sole asymmetric PO₄ stretching mode splits into two bands. These changes indicate the presence of multiple tetrahedral environments within a larger volume unit cell, and the relative frequencies of the split vibrations indicate a P₂O₇ environment with a markedly narrowed P–O–P angle. The difference between the behavior of the iron and chromium compounds is probably generated by the smaller iron ion producing a discontinuous decrease in the P–O–P angle at lower pressures than in the analogous chromium compound. Our results demonstrate that the dimerized P₂O₇ group remains stable under compression to over 20-30 GPa at 300 K.