Reassessment of vibration spectra in alkali phosphate crystals

Abstract

The Raman and infrared (IR) spectra for the alkali metaphosphate crystals composed of Q² units and the alkali pyrophosphate crystal composed of Q¹ units were assessed by density functional theory calculations. The assignments of peaks of the 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 for the alkali metaphosphate crystals. 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 the peaks around 1100 cm⁻¹ result from not only asymmetric stretching vibrations of P–NBO bonds of Q¹ units but also symmetric stretching vibrations of P–NBO bonds and asymmetric stretching vibrations of –P–O–P– bonds of Q² units.