AI 中文总结
研究金红石TiO₂高压转变中CaCl₂型相作用，用全电子密度泛函等理论表明其在13.5 GPa有铁弹性不稳定性，虽有相关途径，但该相仅微弱稳定，是瞬态或弱亚稳中间体，受热力学竞争抑制，为实验观察提供解释。
AI 中文摘要
我们研究了CaCl₂型（Pnnm）相在金红石TiO₂高压转变中的作用，其实验特征一直难以捉摸。与其他金红石型氧化物的类比表明存在这样一个中间相，但TiO₂通常直接转变为更高配位相，如斜锆石。我们使用全电子密度泛函框架结合密度泛函微扰理论表明，金红石TiO₂在13.5 GPa时经历铁弹性不稳定性，其特征是正交应变的发展和双阱能量景观。这种不稳定性与C11 - C12弹性组合的软化和B1g声子模式的凝聚有关，涉及TiO6八面体的协同旋转，使对称性降低到Pnnm结构。尽管有明确的弹性和动力学途径，但焓计算表明，CaCl₂型相相对于金红石仅微弱稳定，与竞争的铌钽铁矿和斜锆石相比，能量上仍不利。因此，Pnnm相不是稳定的高压多晶型，而是作为瞬态或弱亚稳中间体存在。这些结果表明，CaCl₂型相代表了金红石TiO₂的固有铁弹性响应，但受到热力学竞争的抑制，为其难以捉摸的实验观察提供了一致和统一的解释。
英文摘要
We investigate the role of the CaCl2-type (Pnnm) phase in the high-pressure transformation of rutile TiO2, whose experimental signature has remained elusive. While analogies with other rutile-type oxides suggest such an intermediate, TiO2 typically exhibits a direct transformation to higher-coordination phases such as baddeleyite. Using an all-electron density functional framework combined with density functional perturbation theory, we show that rutile TiO2 undergoes a ferroelastic instability characterized by the development of an orthorhombic strain and a double-well energy landscape at 13.5 GPa. This instability is associated with the softening of the C11 - C12 elastic combination and the condensation of a B1g phonon mode, involving coordinated rotations of TiO6 octahedra that lower the symmetry to the Pnnm structure. Despite this clear elastic and dynamical pathway, enthalpy calculations show that the CaCl2-type phase is only weakly stabilized relative to rutile and remains energetically unfavorable compared to competing columbite and baddeleyite phases. Consequently, the Pnnm phase does not emerge as a stable high-pressure polymorph but instead exists as a transient or weakly metastable intermediate. These results demonstrate that the CaCl2-type phase represents the intrinsic ferroelastic response of rutile TiO2, yet is suppressed by thermodynamic competition, providing a consistent and unified explanation for its elusive experimental observation.
