Orthorhombic Symmetry and Anisotropic Properties of Rutile TiO₂

The symmetry of the material is an important factor determining its properties. In this work, we demonstrate both experimentally and by numerical simulations that the actual symmetry of the rutile phase of TiO2 is orthorhombic, described with space group Pnnm, in contrast to what it is commonly believed that rutile TiO2 has a tetragonal symmetry, described with space group P4(2)/mnm. We present very precise first-principles calculations for the determination of the structural properties of rutile TiO2 and highlight the relevance of using the revised regularized SCAN meta-GGA density functional for the interpretation and analysis of neutron and synchrotron radiation diffraction measurements. The lowering of the symmetry has a small but not negligible influence on the elastic, vibrational, and optical properties of rutile TiO2.