First-principles study on phase transition and ferroelectricity in lithium niobate and tantalate

The phase transitions and ferroelectricity of LiNbO₃ and LiTaO₃ have been investigated theoretically from first principles. The phonon analyses and the molecular dynamics simulations revealed that the ferroelectric phase transition is not conventional displacive type but order-disorder type with strong correlation between cation displacements. According to the evaluated potential energy surfaces around the paraelectric structures, the large difference in ferroelectricity between the two oxides results from the little difference in short-range interionic interaction between Nb-O and Ta-O. As the results of the crystal orbital overlap population analyses, the different short-range interaction originates from the difference in covalency between Nb4d-O2p and Ta5d-O2p orbitals, particularly d_xz-p_x/d_yz-p_y orbitals (π orbitals), from the electronic point of view.