The vibrational spectra of α-alkaline earth pyrophosphates

Raman spectra and middle infrared absorption spectra (4000-200 cm-1) were measured for α-Ca2P2O7, α-Sr2P2O7, and α-Ba2P2O7. These spectra were interpreted using factor group analyses, and band assignments were made. Noncoincidences in the infrared and Raman spectra of α-Sr2P2O7 indicated that this material possesses the centrosymmetric space group D162h-Pnma rather than the noncentrosymmetric space group, C92ν-Pna21. The similarities between the vibrational spectra of α-Ba2P2O7 and α-Sr2P2O7 suggested that α-Ba2P2O7 is isostructural with α-Sr2P2O7. Differences were noted in the vibrational spectra of monoclinic α-Ca2P2O7 and the two orthorhombic α-phases, α-Sr2P2O7 and α-Ba2P2O7, which were related to differences in their crystal structures. The P2O74--anions of all three compounds contained nonlinear POP bond angles. © 1978