Phonons and related spectra in bulk and monolayerV2O5

The phonons at the zone center are calculated for the layered material ${V}_{2}{\mathrm{O}}_{5}$ using density functional perturbation theory and are compared with experimental infrared and Raman spectra from literature. We find that significantly better agreement is obtained when the pseudopotential treats vanadium semicore states $3s$ and $3p$ as bands. Remaining discrepancies are shown to be related to the local density approximation overestimate of the bond-stretch mode frequencies. The vibrational mode patterns are studied and presented in detail. The same method is then applied to well-separated monolayers. We find significant changes in some phonon frequencies. In particular, the highest-frequency vanadium vanadyl-oxygen stretch modes show a blue-shift of order 6%--8%. The origin of this blue-shift is analyzed in terms of the force-constant changes between bulk and monolayer and is found to arise from a delicate balance of short- and long-range dipolar force components. The latter are affected by the changes in dielectric constants and in the Born-effective charges. The red-shifts of some of the low-frequency modes are found also to be related to a reduction in force constants within the layer, rather than the removal of interlayer bonds. In this case, it is primarily motions along the $x$ direction that are affected. The static and high-frequency dielectric constants are both found to be reduced in the monolayer.