A vibrational study of phase transitions in Fe 2 P 2 O 7 and Cr 2 P 2 O 7 under high‐pressures

The vibrational properties of synthetic iron diphosphate (Fe 2 P 2 O 7 ) and chromium diphosphate (Cr 2 P 2 O 7 ) are studied under high‐pressure conditions between ~22 and ~30 GP a, 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 GP a: these may be associated with increases in the local symmetry of the P 2 O 7 group. The iron sample undergoes a first‐order phase transition near ~9 GP a, and a possible phase transition near 5.5 GP a. At 9 GP a, the initially single, strong symmetric PO 4 stretching mode splits into four modes, and the sole asymmetric PO 4 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 P 2 O 7 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 P 2 O 7 group remains stable under compression to over 20‐30 GP a at 300 K.