Electron paramagnetic resonance and optical absorption study of V4+ centres in YVO4 crystals

Electron paramagnetic resonance (EPR) has been used to characterize three distinct V4+ centres in undoped Czochralski-grown yttrium orthovanadate (YVO4) crystals. These EPR signals are observed at low temperatures, and their average c-axis splittings between adjacent V-51 hyperfine lines are 40 G, 123 G, and 140 G. We refer to these centres as [V4+](A), [V4+](B), and [V4+](C), respectively. The [V4+](A) and [V4+](B) centres are present in as-grown crystals. Exposure at 77 K to ionizing radiation (x-rays or an ultraviolet laser beam) destroys these centres and creates the [V4+](C) centres. The as-grown state of the crystal is restored upon returning to room temperature. Angular dependence data are used to determine the principal values and principal directions of the g tensor and the V-51 hyperfine tensor for each of the centres. We suggest that the [V4+]A centre is a V4+ ion adjacent to an oxygen vacancy and that the [V4+](B) centre is a V4+ ion substituting for a Y3+ ion (i.e. a vanadium antisite defect). The [V4+](C) centre is assigned to a V4+ ion at a regular vanadium site with a nearby stabilizing defect, possibly a Zr4+ on a Y3+ site. In as-grown crystals, there is a correlation between the number of [V4+](A) centres and the intensity (at 380 nm) of a broad near-edge optical absorption band. This band, now associated with oxygen vacancies, gives YVO4 a 'yellow' appearance.