EPR study of the luminescent triplet state of VO/sup 3-/4 in YVO4 and YP/sub 0.96/V/sub 0.04/O4 single crystals at 1.2 K

An EPR study with optical detection is reported of the luminescent triplet state of the VO/sup 3-/4 ion in single crystals of YVO4 and YPO4 doped with VO/sup 3-/4 . The experiments in an external magnetic field and in zero field can be fitted by a spin Hamiltonian for an orbitally nondegenerate triplet state. Signals originate from four symmetry-related species; the directions of the principal axes of the four ZFS tensors indicate a lowering of symmetry of the VO/sup 3-/4 ion in accordance with a static Jahn--Teller effect by which the tetrahedron tends to a trigonal pyramid on excitation. For YVO4 the ZFS parameters are: chemical bondDchemical bond = 0.58 +- 0.04, chemical bondEchemical bond = 13.72 +- 0.04 GHz, and for YPO4: chemical bondDchemical bond = 1.12 +- 0.04, chemical bondEchemical bond = 12.98 +- 0.04 GHz (z axisparallelV--O bond and yperpendicularac or bc crystallographic plane). For the mixed crystal the lifetimes of the individual spin levels were determined: tau/sub x/ = 0.35, tau/sub y/ = 3.3, tau/sub z/ = 1.4 ms