INVESTIGATION OF THE OPTICAL-ABSORPTION BANDS OF NB(4+) AND TI(3+) IN LITHIUM-NIOBATE USING MAGNETIC CIRCULAR-DICHROISM AND OPTICALLY DETECTED MAGNETIC-RESONANCE TECHNIQUES

The magnetic circular dichroism (MCD) of the absorption of Nb(Li)4+ and Ti(Li)3+ centers in LiNbO3 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 Nb(Li)4+ bound small polaron is now unambiguously confirmed. In the MCD spectrum of the isoelectronic Ti(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 Ti(Li) or Nb(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 connected with these transitions. Well-structured MCD bands at 2.9 eV of Ti(Li)3+ have no counterpart in the Nb(Li)4+ spectrum. These bands are assigned to transitions to excited states, which are specific to the impurity and are related to the 10 Dq transitions known for the crystal field states of a d1 ion.