Ferroelastic instability in rutile TiO2 and thermodynamic suppression of the CaCl2-type phase

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.