Optical Absorption Spectra of Vanadyl Ion (Impurities) in Three Host Crystal Compounds

Vanadyl ions (V0 2 + ) occur coordinated to other groups both in solids and liquids, bringing the total coordination to five or six. Usually the metal ion is surrounded by a distorted octahedron of oxygen ligands. However, the behaviour of the unpaired electron in the vanadyl complexes is dominated by the strong V = 0 bonding; as a result, most of the complexes possess very nearly a square pyramid geometry (C4 r) . The EPR and optical data can be correlated to establish the symmetry and the bonding of the vanadyl complexes. The EPR studies of V 0 2 + ion in Tutton's salts [ 1 5 ] have shown that in all these hosts, substitutional pentahydrate vanadyl complexes [ V = 0 ( H 2 0 ) 5 ] 2 + are formed. These complexes have been found to show very nearly C4[. symmetry on the basis of optical absorption studies [6-8] . The results of single crystal EPR studies of V 0 2 + ions doped in Magnesium Ammonium Sulphate Hexahydrate [MASH] (Mg(NH 4 ) 2 (S0 4 ) 2 -6H 2 0) [1], Rubidium Sulphate [RS] (Rb 2 S0 4 ) [9] and Potassium dihydrogen phosphate [KDP] (KH 2P0 4 ) [10] have shown the presence of multiple sites of the substitutional complexes in these hosts. In RS and KDP the substitutional vanadyl complexes are associated with the appropriate charge compensating vacancies. Since usually the single crystal EPR spectrum of the vanadyl ion consists of lines from multiple sites, making the spectrum much more complex, the spin Hamiltonian parameters (SHP) are more easily determined by crushing the crystal into a fine powder and by recording the polycrystalline spectrum [ 6 9 , 11]. The powder EPR spectra recorded by us on a Varian E109 X-band EPR spectrometer both at RT and at LNT have shown that in MASH,