Crystal chemistry of alkali metal diphosphates and diarsenates

Abstract

The following work aims to provide a deeper insight into the structural chemistry of M4X2O7 (M = K, Rb, Cs; X = P, As). Extensive structural characterizations were performed with single-crystal X-ray diffraction (SCXRD). The phase transitions were investigated with differential scanning calorimetry (DSC) and by temperature-dependent powder X-ray diffraction (TPXRD). Dehydration and condensation reactions starting from M2HXO4·nH2O precursors were elucidated with thermogravimetry (TG). DSC and TG experiments demonstrate that M2HXO4·nH2O dehydrate and condensate in multi-step reactions yielding the diphosphate and diarsenate. Within the investigated temperature range, all M4X2O7 phase transitions are solid-to-solid and of reversible nature. All transitions could be detected via DSC and TXRPD. K4P2O7, K4As2O7, Cs4P2O7 and Cs4As2O7 display one transition at 237, 407, 291 and 232 °C, respectively, according to DSC. In contrast, Rb4P2O7 and Rb4As2O7 feature two transformations, at 201 and 212 °C, and 289 and 315 °C, respectively. M4X2O7 adopt a hexagonal aristotype structure with space group P63/mmc as the high-temperature (HT) phase. The connected XO4-units of the X2O74--anions adopt an ecliptic conformation and the bridging X-O-X bond is linear. The room temperature (RT) phases are in most cases monoclinic and can be derived from the aristotype structure. The loss of the ecliptic conformation and bending of the bridging bond away from 180 ° during cooling displaces the surrounding atoms affording structures of lower symmetry. In this case, Rb4P2O7 and Cs4As2O7 are isotypic and acquire the space group P21; Rb4As2O7 C2/c; and Cs4P2O7 Cc. K4P2O7 and K4As2O7 are isotypic and remain hexagonal with P61/P65. The interim phases of Rb4P2O7 and Rb4As2O7 are yet to be structurally characterized. However, SCXRD experiments indicate that in both cases the symmetry is not intermediate to that of the hexagonal HT phase and their RT structures. During the investigation, a second RT K4P2O7 polytype with space group Cc was discovered. Upon heating it transforms into the hexagonal HT phase. All structures can be described as an alternating stack of the same two layers with exception of the Cs4P2O7 RT phase. Most structures are strongly disordered with some displaying order/disorder (OD) features.