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Combining a precise ab initio electron band structure calculation of the TiO2rutile structure with the temperature evolution of the Ti mean-squaredisplacements, we reproduce a puzzling temperature increase of the electricfield gradient at Ti sites in TiO2, observed experimentally. Our method employsa procedure of averaging two quadrupole electron density components (L = 2)inside a sphere vibrating with the Ti nucleus at its center, where the keyfactor introducing the temperature dependence is the square root of theDebye-Waller factor. Although the Debye-Waller factor always reduces thecorresponding Fourier component, in TiO2 due to the interplay between terms ofopposite signs, it results in a net increase of the whole sum with temperature,leading to the growth of the electric field gradient. Quantitatively, we findthat the increase of electric field gradient is only half of the experimentalvalue, which we ascribe to anharmonic effects or a strong oxygen positioninfluence. In addition, our method reproduces the unusual temperaturedependence of the asymmetry parameter eta, which first decreases withtemperature, goes to zero and then increases.

Ab initio based description of the unusual temperature increase of the electric field gradient at Ti sites in rutile TiO2