Structures and vibrational frequencies of the cyclic polyphosphate ion (P4O12)4− and the chain polyphosphate ion (PO3)44- studied by combined DFT calculation and Raman spectroscopy

Raman spectra of LiCe(PO3)4 and KCe(PO3)4 powder samples were recorded at room temperature in the frequency range 200-1400 cm−1. In order to characterize the structure of the phosphate anion, we have performed a comparative study with the isomorphic compounds and we have undertaken a density functional theory DFT calculation of the geometries and the vibrational frequencies in the two possible configurations the (PO3)44- chain and the (P4O12)4− ring. To minimize the computational time needed for a run, we first carried out semi-empirical calculation PM3 and then we used DFT-B3LYP methods with 6-311G∗ and basis sets and the hybrid B3PW91 method. The comparative study of calculated structures and frequencies with the experimental data of LiCe(PO3)4 and KCe(PO3)4 on one hand and of KDyP4O12 on the other hand shows that the chain structure is unambiguously characterized by the appearance of well resolved and intense bands at 681 cm−1 and 1180 cm−1 associated to stretching P-O-P bridged oxygen and to asymmetric PO2 stretching, respectively, whereas in the cycle form these modes appear around 800 cm−1 and 1220 cm−1, respectively.