Internal Friction in Rutile Containing Ni or Cr

Rutile containing Ni gives an internal-friction peak centered near 50°C at 2 kHz characterized by an activation energy $E=15.3\mathrm{}\pm 2.4$ kcal/mole and a characteristic relaxation time ${\tau }_0=3\mathrm{}\times {10}^{-15\mathrm{}\pm 1.7}$ sec; this peak occurs for tensile stress along [100] or [110]. A peak centered near 200°C with $E=21.4\mathrm{}\pm 2.1$ kcal/mole and ${\tau }_0=8\mathrm{}\times {10}^{-15\mathrm{}\pm 1.0}$ sec occurs for stress along [100] but not for stress along [110]. Rutile containing Cr gives similar peaks characterized by $E=13.5\mathrm{}\pm 2.4$ kcal/mole and ${\tau }_0=2\mathrm{}\times {10}^{-14\mathrm{}\pm 1.6}$ sec and by $E=22.2\mathrm{}\pm 1.5$ kcal/mole and ${\tau }_0=4\mathrm{}\times {10}^{-15\mathrm{}\pm 0.9}$ sec. Light reduction in vacuum decreases the amplitude of the low-temperature peak and enhances the high-temperature peak in both cases. Heavy reduction of rutile containing Cr removes both peaks but they reappear upon subsequent reoxidation. The evidence suggests that the motion of interstitial cations is involved and that reduction causes the formation of a compound defect. A tentative interpretation is given in terms of unpaired ${\mathrm{Ni}}^{3+}$ interstitials for the low-temperature peak and of interstitial pairs composed of ${\mathrm{Ni}}^{3+}$-${\mathrm{Ti}}^{3+}$ for the high-temperature peak in Ni-doped specimens; the corresponding peaks in Cr-doped rutile are tentatively interpreted in terms of ${\mathrm{Ti}}^{4+}$ interstitials and of interstitial pairs consisting of ${\mathrm{Ti}}^{4+}$-${\mathrm{Ti}}^{3+}$. It is possible to have two charge-compensating ${\mathrm{Cr}}^{3+}$ on adjacent substitutional sites and preserve the last two models.