Electronic structure, anisotropic elastic and thermal properties of monoclinic Ca2Nb2O7

The electronic structure, anisotropic elastic and thermal properties of monoclinic Ca2Nb2O7 have been investigated by density functional theory (DFT) calculations and further are verified by experimental results. It has been shown that the monoclinic Ca2Nb2O7 is a direct band gap insulator with the calculated band gap of 3.07 eV which is comparable with the experimental value of 3.33 eV. The bottom of the conduction band (CB) is dominated by the 4d orbitals of the Nb atoms and the 2p orbitals of O atoms, while the top of valence band (VB) mainly consists of the 2p orbitals of O atoms. Calculated sound velocities of different directions show that the faster mixed mode (v+) is much larger than that of slower mixed mode (v- ) and pure transverse mode (vt ) in both [100] and [001] directions. The pure longitudinal mode vl has the greatest sound velocity among the three acoustic modes in the [010] direction. According to Clarke's model, monoclinic Ca2Nb2O7 has low limit thermal conductivity with 1.43 Wm−1K−1 at high temperature, and minimum thermal conductivity in (100), (110), (010) and (001) planes sensitively depends on the directions.