In reduced ${\mathrm{LiTaO}}_3$, an axially symmetric EPR signal with ${\mathit{g}}_{\mathrm{\parallel }}$=1.948±0.003 and ${\mathit{g}}_{\mathrm{\perp }}$=1.827±0.003 has been identified. It is attributed to ${\mathrm{Ti}}^{3+}$ (3${\mathit{d}}^1$) by comparison with the already known EPR data of ${\mathrm{Ti}}^{3+}$ in ${\mathrm{LiNbO}}_3$. We are able to explain the g values of this center in ${\mathrm{LiNbO}}_3$, as well as in ${\mathrm{LiTaO}}_3$, by a model calculation involving a dynamic pseudo Jahn-Teller effect. Spin-orbit coupling, lattice vibration, pseudo Jahn-Teller interaction, and the Zeeman term are treated on equal footing. Intervalence transfer from ${\mathrm{Ti}}^{3+}$ to ${\mathrm{Nb}}^{5+}$, forming a ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ polaron, has been identified. This transfer can be stimulated either by visible light or by thermal activation. By investigating the temperature dependence of the thermally activated intervalence transfer, we have determined the ionization energy of ${\mathrm{Ti}}^{3+}$ with respect to ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ to be 0.11 eV ±0.02 eV.

• Received 10 November 1993

DOI:https://doi.org/10.1103/PhysRevB.49.5845