Charge states and optical transitions of vanadium in Bi4Ge3O12 identified by MCD and ODMR

The spectroscopic properties of V-doped Bi₄Ge₃O₁₂ have been investigated in detail via several complementary techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly modified under UV-illumination and the initial state can be restored optically with visible light. Optical absorption and magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UV-illumination while a paramagnetic one is created. The latter shows a very characteristic S-shaped MCD pattern in the near-IR, which is readily assigned to the ²E→²T₂ internal transition of tetragonal V⁴⁺ centers at the Ge sites. This assignment was further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at 35 GHz. The g tensor of V⁴⁺ was found to be anisotropic with principal values g_||=1.81±0.03 and g=1.94±0.02. Two additional MCD bands in the visible spectral region could be attributed to V⁴⁺ via ODMR measurements. Ionization thresholds for holes (V⁵⁺) and electrons (V⁴⁺) were determined by optical absorption experiments and the V^4+/5+ donor level was positioned 1.9 eV below the conduction band. The presence of the V^4+/3+ acceptor state in the forbidden band is also suggested.