EPR and optical absorption studies of vanadyl impurity in zinc potassium phosphate hexahydrate single crystal

Electron paramagnetic resonance (EPR) study of VO²⁺ doped zinc potassium phosphate hexahydrate single crystal is carried out. The angular variation of the spectra is studied in the three crystallographic planes. The principal value of spin Hamiltonian parameters g and A and the direction cosines which principal axes make with the crystallographic axes are determined. The observed values are site I: g_∥=1.9664±0.0002, g_⊥=1.9973±0.0002, A_∥=150±2×10⁻⁴, A_⊥=60±2×10⁻⁴ cm⁻¹; site II: g_∥=1.9276±0.0002, g_⊥=1.9921±0.0002, A_∥=155±2×10⁻⁴ and A_⊥=62±2×10⁻⁴ cm⁻¹. By comparison of direction cosines of g from EPR with the direction cosines of different bonds obtained from crystal structure data it is ascertained that the VO²⁺ ion occupies Zn²⁺ substitutional sites. The optical absorption study of the crystal at room temperature is also carried out. The bands observed in the optical absorption spectrum are attributed to d-d transitions. The EPR results together with the optical data are employed to estimate the molecular orbital (MO) coefficients. These MO coefficients (also called bonding coefficients) are further used to discuss the nature of bonding of VO²⁺ ion with different ligands in the crystal.