First-principles calculation of the effects of Li-doping on the structure and piezoelectricity of (K_0.5Na_0.5)NbO₃ lead-free ceramics

The crystal structures of the lead-free piezoelectric ceramics (K_0.5Na_0.5)NbO₃ and (K_0.5Na_0.5)_0.94Li_0.06NbO₃ prepared by a solid-state method were investigated using first-principles calculations. The calculated values of piezoelectricity were in good agreement with the experimental data. We found that the primary contribution to piezoelectricity in this material comes from the hybridization of the O 2p and Nb 4d orbitals, which causes a change in the Nb-O bond length and the distortion of the Nb-O octahedral structure. Analysis of the band structure and the total density of states revealed that Li-doped (K_0.5Na_0.5)NbO₃ enhances hybridization of the O 2p and Nb 4d orbitals. This hybridization enhancement further reduces the Nb-O1 bond length and enhances the distortion of the Nb-O octahedron along the [001] direction, which may be the main reason for the improvement of the piezoelectric properties. In addition, the piezoelectric coefficients are calculated here, which show the same trend as the experimental results.