Raman spectra of a-Ta2O5: a first-principles analysis

We present a first-principles study of the vibrational spectra, i.e. vibrational density of states(VDOS) and Raman spectra, of amorphous tantala. Model structures of upto three-hundredsatoms size have been generated by means of classical and ab-initio molecular dynamics and subsequentlyused for a series of ab-initio calculations of the vibrational spectra, including tests withdifferent kinds of exchange-correlation functionals and pseudopotentials. Our study confirms thatthe vibrational modes above ∼550 cm−1 consist mainly of Ta-O stretching motion of O atoms,meanwhile Ta motion gives a major contribution to the VDOS only below ∼200 cm−1. In particular,the vibrational modes underlying the main Raman peak at ∼670 cm−1 are related to Ta–Obond stretching motions and a major contribution to the peak appears to come from twofold Oatoms. Decomposition of the twofold O contribution to the VDOS in terms of rocking, bending,and stretching weights shows that above ∼550 cm−1 rocking and bending fade away, while thestretching component becomes largely dominant. Finally, by means of local projectional analysiswe infer that vibrational modes with frequencies above 550 cm−1, feature mostly asymmetric andsymmetric (A1) stretching modes of TaOn (n = 5, 6, 7) polyhedra. The latter decomposition, inparticular, may explain the origin of the experimental Raman doublet at ∼830 and ∼940 cm−1observed with 785 nm laser in Favaro et al. Class. Quantum Gravity 41, 105009 (2024).