LiNbO_3和LiTaO_3晶体电子结构、平面声波和光折射特性

Electronic Structure, Plane Acoustic Velocity and Refractive Property of LiNbO_3 and LiTaO_3

本研究基于密度泛函理论的第一性原理超软赝势平面方法计算了LiNbO_3和Li TaO_3的晶格参数、电子结构和弹性常数,并利用Christoffel方程研究了二者平面声波特征。结果表明:两者的理论计算晶格参数和弹性常数与实验值接近,禁带宽度分别为3.78和3.98 e V,导带底和价带顶主要由O-2p和Nb-4d(Ta-5d)态电子贡献。化学键理论揭示Li和Nb(Ta)与O原子之间有两种成键类型。电荷布局分析结果显示有两种相应的重叠布居数,Nb(Ta)–O键呈现强共价键作用,并且Nb–O(Ta–O)键长小于Li–O键长。LiNbO_3和Li TaO_3晶体平面声波有两支横波和一支纵波,纵波速度大于横波速度,在xy平面呈现六重对称性,在xz和yz平面各向异性程度强于xy平面,沿[001]、[001]晶向上两支横波振动速度相等。最后利用模守恒赝势(Norm-conserving)计算了介电常数和静态折射率,计算表明LiNbO_3晶体的折射性能和非寻常光(e光)离散程度均强于Li TaO_3晶体。

Lattice parameters, electronic structures and elastic constants of lithium niobate and lithium tantalate were calculated with the plane wave pseudopotential method based on the first-principles density functional theory. The results show that calculated lattice parameters and elastic constants are in consistent with the corresponding experimental values. It was found that the bottom of the valence band and the top of the conductive band are mainly determined by electron orbits of O-2p and Nb-4d （Ta-5d）. The chemical bonds theory indicate that Li, Nb （Ta） and O atoms have two types of bonds, and the Mulliken population analysis exhibits that there are two corresponding bond populations. The Nb-O （Ta-O） covalence is stronger than that of Li-O, and band length shorter than that of Li-O. Moreover, the planar acoustic velocities, studied by Christoffel equation, shows that the three-dimensional images of the planar acoustic wave consisting of a longitudinal wave and two transverse waves, indicating the anisotropic feature. The velocity of the longitudinal wave is larger than those of the two transverse waves. In xz andyz planes, not only the plane projections of the planar acoustic waves show the stronger anisotropy than those in xy plane which have a six-fold symmetry, but also the velocities of the two transverse waves are equal in [001] and [00T] directions. Thecalculated static dielectric constants and optical permittivity indicate the refractive index of LiNbO3 is stronger than that of LiTaO3.