Reassessment of vibration spectra in alkaline earth metaphosphate crystals

The Raman and infrared (IR) spectra for the alkaline earth metaphosphate crystals with chains were assessed by density functional theory (DFT) calculations. The assignments of peaks of the DFT calculated Raman and IR spectra were consistent with reported assignments of phosphate crystals and glasses except for the assignment of the peaks around 1100 cm−1 of the calculated IR spectra. Peaks around 1100 cm−1 of IR spectra for phosphate crystals and glasses have been assigned to asymmetric stretching vibrations of bonds between phosphorus atoms and non-bridging oxygens (NBO) of Q1 units. However, the calculated IR spectra showed that symmetric stretching vibrations of P–NBO bonds and asymmetric stretching vibrations of –P–O–P– bonds of Q2 units also result in peaks around 1100 cm−1. In addition, the calculated Raman spectra showed a strong correlation between cation field strength and the Raman shift of intense peaks assigned to symmetric stretching vibrations of P–NBO bonds of Q2 units.