First principles study on lattice vibration and electrical properties of layered perovskite Sr₂M₂O₇ (M=Nb, Ta)

In this paper, we performed calculations to investigate the dielectric, piezoelectric properties, Born effective charge (BEC), and spontaneous polarization of Sr₂M₂O₇, the method used in our study was a well-known density functional theory based on first-principles. The optimized results were in good agreement with previous experiments and calculations, which indicates that our calculated method is reasonable. The research we have done suggested that greater piezoelectric components of Sr₂Nb₂O₇ were e₃₁ and e₃₃, and the contributions were derived from the A₁. By studying the Born effective charge, it could be seen that the valence of ions changed, and the O of Sr₂Nb₂O₇ were most obviously that caused by the covalent character of ions and the hybridization of O-2p and Nb-4d. The spontaneous polarization of Sr₂Nb₂O₇ in the[001] direction is 25 μC/cm², while for Sr₂Ta₂O₇, there was no spontaneous polarization in the paraelectric state. Finally, the effect of pressure on the piezoelectric properties were also investigated, the polarization of Sr₂Nb₂O₇ decreased linearly with the increase after pressure. All our preliminary results throw light on the nature of dielectric, piezoelectric properties, Born effective charge, and spontaneous polarization of Sr₂M₂O₇, it was helpful for experimental research, the development of new materials, and future applications.