Hybrid density functional theory insight into the stability and microscopic properties of Bi-doped LiNbO3 : Lone electron pair effect

Abstract

It has recently been reported that Bi-doped ${\mathrm{LiNbO}}_{3}$ exhibits more excellent photorefractive properties than the traditional Fe doping. Bi-induced structural and physical properties remain unverified by either experiment or theory, however. Thus, here the basic characteristics of Bi-doped ${\mathrm{LiNbO}}_{3}$, such as the preferable Bi doping site, local lattice distortion, and the effect of Bi doping on the electronic structure and optical properties, are investigated by density functional theory with a hybrid functional. In particular, we focus on the effect of a Bi lone electron pair on the structural distortion and polaronic behavior of ${\mathrm{LiNbO}}_{3}$. The calculated results show that Bi substitutional Li in its +4 charge state (${{\mathrm{Bi}}_{\mathrm{Li}}}^{4+}$) and Bi substitutional Nb in its neutral state (${{\mathrm{Bi}}_{\mathrm{Nb}}}^{0}$) are energetically preferable in the majority of ${\mathrm{LiNbO}}_{3}$ samples. The incorporation of Bi could form a small bound electron polaron in ${\mathrm{LiNbO}}_{3}$. The strongly polarized localization of the Bi $6{s}^{2}$ lone electron pair around the Bi center dominantly contributes to the large local lattice relaxation and the huge energy gain of ${{\mathrm{Bi}}_{\mathrm{Li}}}^{2+}$ that result in the negative $U$ effect. A new ${{\mathrm{Bi}}_{\mathrm{Li}}}^{4+/2+}$ photorefractive center that is 2.2 eV deeper than the intrinsic ${{\mathrm{Nb}}_{\mathrm{Li}}}^{4+/2+}$ photorefractive center is introduced by Bi doping.