Exploring the vibrational Raman modes of P–O–P bonds in oligomeric sodium metaphosphates: a comprehensive DFT study

This work presents an in-depth density functional theory (DFT) study aimed at elucidating the Raman vibrational patterns of P–O–P bonds in oligomeric structures of sodium metaphosphate (NaPO₃)n structures. To investigate their vibrational behavior, DFT methods were employed on three oligomeric crystal structures: monoclinic, triclinic, and orthorhombic, as well as a non-periodic cyclic structure. Vibrational frequencies, computed using several exchange-correlation functionals and basis sets, were compared with the available Raman spectra in the literature, showing good agreement. The normal modes were analyzed, and the role of external conditions (laser, temperature, and pressure) was computed and discussed. The PBE0 functional combined with the DZVP provided the best geometry, while the TZVP basis set provided the best overall agreement with the experimental vibrational results, accurately capturing the majority of key vibrational peaks, including the critical P–O–P bond stretching modes. These findings clarify the origin and characteristics of vibrational modes in (NaPO₃)n structures and the role of external factors common in the search for complex phosphate materials.