Microwave Acoustic Relaxation in Reduced Rutile

The microwave acoustic attenuation in vacuum-reduced rutile is determined at low temperatures for longitudinal and transverse modes propagating along various directions in a single crystal. Vacuum reduction at a temperature of 850°C changes the resistivity radically but induces no appreciable changes in the attenuation of any type of mode when compared to the stoichiometric state. Reduction at 1050°C leads to a large anisotropic change in the attenuation of some of the transverse waves but has no effect on the longitudinal modes. The change in attenuation occurs only for transverse modes associated with the $C_{44}$ elastic constant and appears to be due to particle motion in the vicinity of the defect rather than due to band electrons. The particle moves between sites which are inequivalent under the strain associated with the $C_{44}$ modulus and may be a titanium ion or a polaron.