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

Abstract

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.

Compressibility of structural modulation waves in the chain compounds BaCoX 2O7 (X = As, P): a powder study

“…Indeed, the symmetric modes at 580-620 cm À1 in BCAO and 750-800 cm À1 in BCPO transform to a single peak at the critical phase transition P c for both compounds. The unusual square shape of these modes observed in the starting modulated phase is not observed in other non-modulated monoclinic diphosphate phases (Blanc et al, 2018); thus we believe that this unusual shape is a result of the modulation that also impacts on the X 2 O 7 molecular chain in the structure. The sudden narrowing of the width to the single vibrational mode at the transition shows that the molecular unit adopts a more symmetrical packing as a result of the disappearance of the modulation.…”

“…Based on the literature, the most intense modes of the P 2 O 7 molecular double tetrahedra unit consist of two (symmetric and antisymmetric) stretching modes between 700 and 1000 cm À1 that involve the P-O bond of the P-O-P bridging the two tetrahedra, and two, symmetric and antisymmetric, stretching modes between 1000 and 1300 cm À1 that involve the P-O bond of the PO 3 tetrahedra (Blanc et al, 2018;Zhang & Brow, 2011;Baran et al, 1986). These modes are indicated by arrows in Fig.…”

Nanoarchitectonics of Mesoporous M2P2O7 (M = Mn(II), Co(II), and Ni(II)) and M2–xCoxP2O7 and Transformation to Their Metal Hydroxides with Decent Charge Capacity in Alkali Media

“…All intense diffraction lines are well resolved; some characteristic intense lines originated from the (110), (001), (021), (20–1), (130), (220), (131), (31–1), (112), (22–2), (202), (400), and (35–1) planes and were observed at 17.22°, 20.09°, 29.00°, 30.38°, 34.17°, 34.75°, 41.63°, 43.65°, 47.09°, 49.18°, 55.31°, 56.86°, and 70.31°, 2θ, respectively. Note also that, in the β-phase, the bridging P–O–P angle is almost 180° and linear, , as clarified from the IR data (see later). All other weaker lines are also visible in the patterns of the sample, calcined at 700 °C.…”

“…ATR-FTIR spectra of all the samples display similar features in their amorphous phase. The spectra show broad peaks with shoulders in the P–O stretching (1100 cm –1 ) and bending regions (500 cm –1 ) and two distinct peaks at 900 and 750 cm –1 due to asymmetric and symmetric stretching of the bridging P–O–P units, respectively, , see Figure . The peak at 750 cm –1 is sensitive to the structure type because in the β-M 2 P 2 O 7 phase, the P–O–P angle is 180° and linear.…”