The unusual temperature dependence of the electric field gradient at titanium sites in rutile (TiO₂)

47 Ti and 49 Ti NMR measurements were performed in single-crystal rutile between 150 and 1500 K. The NMR spectrum is due to second-order quadrupole-perturbed central transitions (m= 1 / 2 to - 1 / 2 ) and consists of a doublet for each of the isotopes resulting from the different orientations of the EFT tensor at the two Ti sites in the unit cell. From the angular dependence of the NMR spectrum the magnitudes of the largest eigenvalue V zz and the asymmetry parameter eta of the EFG tensor were determined. At room temperature one finds: mod V zz mod =(2.2+or-0.1)*10 20 V m -2 , eta =0.19+or-0.01. While V zz is only weakly dependent on temperature the asymmetry parameter eta reveals an uncommonly strong dependence on temperature caused by the non-isotropic thermal expansion of rutile.