Vibrational spectra of a − Ta 2 O 5 : A first-principles investigation

We present a first-principles study of the vibrational spectra, i.e., vibrational density of states (VDOS) and infrared (IR) and Raman spectra, of amorphous tantala ( a − Ta 2 O 5 ) . Three model structures of up to 336 atoms in size have been generated by means of classical and molecular dynamics and subsequently used for a series of calculations of the vibrational modes and the relevant Raman and IR coupling tensors. For the assessment of size-dependent effects on the calculated vibrational spectra, we employed a machine-learned potential that enabled the calculation of the VDOS with accuracy for models containing up to a few thousand atoms. Raman and IR coupling tensors have been analyzed in a local reference frame, which provides direct insight into correlations with local structural parameters like the Ta–O–Ta angles and O–Ta bond lengths. Fair agreement is recorded with available experimental dielectric functions and Raman spectra for both the largest model structure and the fully generated model, in contrast with a third model, the Raman spectrum of which may appear to represent a structure approaching the onset of crystallization. In this study, we confirm that the 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 . The vibrational modes underlying the main Raman peak at ∼ 670 cm − 1 are related to Ta–O bond stretching motions, and a major contribution to the peak appears to come from twofold-coordinated O atoms, whereas the Raman weight of threefold-coordinated O atoms is maximized in the range ∼ 500 – 550 cm − 1 . Moreover, an analysis of the twofold-coordinated O contribution to the VDOS—resolved into rocking, bending, and stretching components—shows that, beyond ∼ 550 cm − 1 , the rocking and bending contributions vanish, leaving stretching as the dominant mode. Above 400 cm − 1 , the contribution of threefold-coordinated O atoms to the VDOS is essentially given by an in-plane Ta–O stretching motion which exhibits maxima around 450 – 500 cm − 1 , while oxygen out-of-plane motion is relevant mostly around 300 cm − 1 . Finally, by means of further local projectional analysis, we infer that vibrational modes with frequencies above 550 cm − 1 feature mostly asymmetric and symmetric stretching modes of TaO n ( n = 5 , 6 , 7 ) polyhedra. The latter decomposition allows us to explain the origin of the experimental Raman band above ∼ 780