AI 中文总结
本研究通过第一性原理DFT计算，明确了电场边界条件下LiTaO$_3$的铁电-反铁电相转变机制，其转变临界电位移低于LiNbO$_3$，更便于实验观测。
AI 中文摘要
LiTaO$_3$的铁电相和反铁电相在不同电场边界条件下存在竞争。第一性原理密度泛函计算表明，高对称参考结构存在两种相关的不稳定$\boldsymbol{\rm \u0393}$模式：频率为$181i$ cm$^{-1}$的极性$\boldsymbol{\rm A_{2u}}$模式，会凝聚为铁电（FE）相；频率为$103i$ cm$^{-1}$的反极性$\boldsymbol{\rm A_{2g}}$模式，会凝聚为反铁电（AFE）相。在短路条件下，FE相为全局极小值，势阱深度约为144 meV；在开路条件下，未被屏蔽的退极化场会使FE相失稳，AFE相成为基态，势阱深度约为19 meV。频率为$48.3i$ cm$^{-1}$的弱不稳定纵光学模式对应的势阱仅约0.24 meV，无热力学意义。绘制自由能随电位移$D$的变化曲线，确定二阶FE$\boldsymbol{\rm \u2194}$AFE转变发生在$D_c=0.051$ C/m$^2$处，该值低于LiNbO$_3$对应的0.07 C/m$^2$，因此实验上更易实现。
英文摘要
Ferroelectric and antiferroelectric phases of LiTaO$_3$ compete under different electric boundary conditions. First-principles density-functional calculations show that the high-symmetry reference structure supports two relevant unstable $Γ$ modes: a polar $A_{2u}$ mode at $181i$ cm$^{-1}$ that condenses into the ferroelectric (FE) phase, and an antipolar $A_{2g}$ mode at $103i$ cm$^{-1}$ that condenses into the antiferroelectric (AFE) phase. Under short-circuit conditions the FE phase is the global minimum, with a well depth of $\approx 144$ meV. Under open-circuit conditions the FE phase is destabilized by the unscreened depolarization field and the AFE phase becomes the ground state, with a well depth of $\approx 19$ meV. A weakly unstable longitudinal-optic mode at $48.3i$ cm$^{-1}$ yields a well of only $\approx 0.24$ meV and is thermodynamically irrelevant. Mapping the free energy versus electric displacement $D$ locates a second-order FE$\leftrightarrow$AFE transition at $D_c=0.051$ C/m$^2$, lower than the corresponding $0.07$ C/m$^2$ in LiNbO$_3$ and therefore more accessible experimentally.
