Non-empirical calculations of properties of KNbO$_3$ and RbNbO$_3$ crystals

The present study investigates the electronic and mechanical properties of potassium and rubidium niobates forming temperature-dependent lattices in cubic, tetragonal, orthorhombic or rhombohedral syngonies. The calculations are based on the framework of density functional theory (DFT) using the HSE06 functional. The dispersion curves for these perovskites were obtained, and it was found that in all phases except rhombohedral KNbO$_3$ has imaginary modes, indicating the instability of the structures.However, for RbNbO$_3$ such a picture was not observed, and except for the phase with cubic unit cell imaginary frequencies in the dispersion dependences were absent. The electronic bands, high-frequency dielectric permittivity, and lattice formation energy, along with other relevant properties, have been calculated for all phases of both RbNbO$_3$ and KNbO$_3$ crystals.