  Vanadium ions charge states and their incorporation in the yttrium aluminum
garnet Y3Al5O12 (YAG) lattice were studied by the correlated optical absorption
and electron paramagnetic resonance (EPR) measurements. In as-grown crystals,
the occupation of the V3+ at both the octahedral and tetrahedral aluminum sites
was proven. The V3+ to V4+ charge transformation was observed after annealing
in air, whereas annealing in the hydrogen atmosphere resulted exclusively in a
slight weakening of the V3+ absorption bands due to partial recharge of these
ions. Spin Hamiltonian parameters of the V3+ and V4+ ions at the tetrahedral
sites including the zero field splitting and the 51V hyperfine constants have
been determined using the high-frequency, up to 300 GHz, EPR measurements. From
the analysis of the spin Hamiltonian parameters in the framework of the crystal
field theory, the ground state energy levels splitting of the V3+ and V4+ ions
were calculated. The charge distribution over the tetrahedral V3+ and its
nearest oxygen surroundings was found to be strongly inhomogeneous whereas the
tetrahedral V4+ ion concentrated the charge with very weak participation of
surrounding ligands. Furthermore, the correlation of the optical and EPR data
allowed the proper assignment of the optical absorption peaks in YAG:V
crystals.
