Investigation of the optical-absorption bands of ${\mathrm{Nb}}^{4+}$ and ${\mathrm{Ti}}^{3+}$ in lithium niobate using magnetic circular dichroism and optically detected magnetic-resonance techniques

The magnetic circular dichroism (MCD) of the absorption of ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ and ${\mathrm{Ti}}_{\mathrm{Li}}^{3+}$ centers in ${\mathrm{LiNbO}}_3$ has been selectively measured by applying optically detected magnetic resonance. The attribution of a well-known broad and unstructured absorption band peaking at 1.6 eV to the ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ bound small polaron is now unambiguously confirmed. In the MCD spectrum of the isoelectronic ${\mathrm{Ti}}_{\mathrm{Li}}^{3+}$ center, bands show up, which closely resemble the MCD bands at 1.6 eV of this bound small polaron. This striking similarity is explained by a cluster model, representing both defects. Either ${\mathrm{Ti}}_{\mathrm{Li}}$ or ${\mathrm{Nb}}_{\mathrm{Li}}$ is at the center of this cluster. In both cases, the small polaron is bound to the cluster, and its MCD bands correspond to intervalence transfer transitions within the constituents of the cluster. A study of the spin-orbit coupling of the molecular orbitals of the cluster allows one to analyze the structure of the MCD bands at 2.9 eV of ${\mathrm{Ti}}_{\mathrm{Li}}^{3+}$ have no counterpart in the ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ spectrum. These bands are assigned to transitions to excited states, which are specific to the impurity and are related to the 10Dq transitions known for the crystal field states of a ${\mathit{d}}^1$ ion.