Electron spin resonance of Cr5+ in YPO4 and YVO4

The ESR of CrO3−4 has been studied at 4.2 K in single crystal YPO4 and YVO4. The g values and 53Cr hyperfine constants are g∥ = 1.9772, g⊥ = 1.9525, A∥ = 8.5×10−4 cm−1, A⊥ = 19.6×10−4 cm−1 for YPO4 and g∥ = 1.9773, g⊥ = 1.9511, A∥ = 8.6×10−4 cm−1, A⊥ = 18.7×10−4 cm−1 for YVO4. The resonances show an isotropic superhyperfine splitting due to the two nearest neighbor yttrium ions. The PO4 and VO4 tetrahedra are distorted to D2d symmetry by a c axis stretch and a crystal field model predicts a dx2–y2 ground state for the d1 electron on CrO4 in these hosts. But a dz2 state is observed. We attribute the inversion to strong covalency. The g and A values cannot be explained with D2d symmetry and the width of the superhyperfine lines is orientation and magnetic field dependent. We believe these anomalies are due to motional effects.