Comment on “Orthorhombic Symmetry and Anisotropic Properties of Rutile TiO 2 ”

R ecently, Gonzales Szwacki et al. 1 investigated the crystal structure of the rutile form of TiO 2 (rutile-TiO 2 2 ) at ambient conditions ("room temperature" and normal pressure) and temperatures below.This was done based on evaluation of powder X-ray and neutron diffraction (PXRD and PND) experiments and based on first-principles calculations on structures (pertaining to the nonvibrating material at 0 K).The authors 1 concluded that at these ambient conditions, rutile-TiO 2 has the orthorhombic CaCl 2 -type structure (Strukturbericht notation C35) with Pnnm space group symmetry instead of the classical rutile-type structure (Strukturbericht notation C4) with P4 2 /mnm symmetry.2 The main arguments were characteristically hkl-dependent peaks widths in the PXRD/PND data, which were attributed to reflection-splitting in accordance with lattice-parameters values a ≠ b, discarding an alternative interpretation in terms of anisotropic microstrain broadening.Characteristic displacements of the O atoms appear to accompany the lattice distortion when evaluating the PXRD/ PND data in terms of the CaCl 2 -type structure.Moreover, the first-principles calculations seemed to imply that orthorhombic atomic geometries can have lower energies than more highly symmetric tetragonal ones.The tetragonal rutile-type structure of rutile-TiO 2 is an iconic inorganic structure type.3 Its invalidation under ambient conditions would affect textbook knowledge.Indeed, already Tomei and Goletti 4 questioned the conclusion of an orthorhombic CaCl 2 -type structure based on undetectable effects predicted for some optical properties.In their reply, the original authors 5 acknowledged "lower optical effects", and once again emphasized their theoretical and experimental findings.Moreover, the authors addressed the relevance of domains in orthorhombic rutile-TiO 2 , which obstructed macroscopic measurements.In the present comment, the author more generally questions the interpretation of their PXRD/PND data by Gonzales Szwacki et al. 1 in terms of presence of a CaCl 2 -type TiO 2 .The present author acknowledges to be able to reproduce the results of the Rietveld refinements from ref 1 based on the published diffraction data as it concerns both the CaCl 2 -type structure model and the alternatively presented rutile-type structure model with anisotropic microstrain broadening.The main point of this comment is that in the view of the present author, the latter tetragonal structure model with anisotropic microstrain broad-