B. V. R. Chowdari

EPR studies of Mo3+ ion in single crystals of NH4Al(SO4)2⋅12H2O

Electron paramagnetic resonance studies of Mo3+ ion in NH4Al(SO4)2⋅12H2O single crystals are made at X‐band frequencies and at room temperature. It is found that there are four magnetically inequivalent complexes of Mo3+ ion in the lattice which exhibit tirgonal symmetry. The angular variation of the fine structure lines is studied in the crystallographic (111) and (100) planes. The z axes of the complexes are found to be along the (111) directions. The spectra are analyzed using the fine structure spin‐Hamiltonian and the parameters are found to be g∥=1.974, g⊥=2.009, and D=506.8×10−4 cm−1. Paramagnetic resonance of the polycrystalline samples is also studied and the usual spin‐Hamiltonian analysis is made. A model is proposed for the position of the Mo3+ ion in the lattice of ammonium alum.

Electron paramagnetic resonance studies of VO2+ in single crystals of alkali halides

Abstract Results of electron paramagnetic resonance studies of VO2+ in KBr and KI are reported over a wide temperature range. At room temperature there is an isotropic eight-line pattern attributed to a freely rotating VO2+ molecular ion in the crystal. At low temperature an anisotropic spectrum attributed to the hindered rotation of VO2+ ion is obtained. The spin-Hamiltonian parameters at room and liquid-nitrogen temperatures are determined, and study of their dependence on the lattice constant shows that as the lattice constant increases, the hyperfine parameter (A0) increases while g0 and linewidth decrease.

Study of vanadium complexes in potassium oxalate monohydrate crystals

The optical absorption of VO2+ doped K2C2O4:H2O has been studied at 300 and 77 °K. The existence of vanadyl ion in the crystal was verified by IR absorption study. Four optical peaks corresponding to d‐d transitions and an intense peak at 28 900 cm−1 due to charge transfer have been recorded at 300 °K. The identity of charge transfer band was established by polarization studies. At 77 °K the band at 15 384 cm−1 showed vibronic structure with a separation ∼755 cm−1. The spectra have been interpreted in terms of both molecular orbital (MO) theory and crystal field theory. Radiation damage studies of VO2+ doped K2C2O4⋅H2O revealed the formation of VO2. The two bands due to d‐d transitions in VO2 and other optical peaks due to interband transitions have been recorded for parallel and perpendicular polarizations of light. This evidence for the formation of VO2 has been discussed.

Optical properties of dichromate centers in some lattices

Abstract The optical absorption spectra of dichromate centers in potassium alum, ammonium alum, KDP and ADP have been studied. Different molecular orbital schemes have been considered to explain the observed spectrum and the scheme proposed by Viste and Gray has been preferred. Coupling between the electronic states and the totally symmetric stretching vibrational mode is found to be present.

CrO43- centers in NH4Al(SO4)2.12H2 O single crystals

Abstract Electron paramagnetic resonance and optical absorption spectra of radiation damage products of undoped and CrO 4 2- doped ammonium alum are studied. On X-irradiation at room temperature several paramagnetic species like CrO 4 3- , SO 3 − and O 3 − are formed in the doped crystals. In the undoped crystals SO 3 − and O 3 − are the major radiation damage products.

Thermal depolarization in dichromate doped KDP and ADP single crystals

Abstract Thermal depolarization current in KDP and ADP crystals doped with Cr2O2−7 have been studied in the temperature range of 80–300K. The single peak observed at 200K in KDP has been explained by considering the proton conduction in (H2PO4)− sub-lattice. The peaks observed at 235 and 210 K in ADP are considered to be due to the proton conduction in(H2PO4)− and (NH4)+ sublattices, respectively. The relevant relaxation parameters for the dipoles in both the crystals have been obtained. The results are compared with conductivity and dielectric lose measurements in these crystals.

EPR studies of the hydroperoxy radical in x-irradiated single crystals of BaCl2·2H2O

Electron paramagnetic resonance studies of X-irradiated single crystals of BaCl2-2H2O have been made at room temperature. Four resonance lines are observed which can be grouped into two hyperfine doublets. Angular variation studies were made in the crystallographic ab, bc and ac planes. There are two magnetically equivalent but physically inequivalent magnetic complexes in the crystal lattice which are mutually perpendicular to each other. The paramagnetic species is identified as the hydroperoxy radical and a detailed discussion is made at the end.

EPR studies of a molybdenyl complex in single crystals of NH4Cl

Abstract Electron paramagnetic resonance of [MoOCl 5 ] 2− has been studied in single crystals of NH 4 Cl. At room temperature the interaction of the unpaired electron with both the even and odd isotopes of molybdenum has been observed. The existence of metal-halogen π bonding is established by the observation of the ligand superhyperfine interaction at liquid nitrogen temperatures. Various possible models corresponding to the different spatial configurations of the molybdenyl complex in the lattice are considered to explain the experimentally observed features. The spectra are analysed using the usual spin-hamiltonian corresponding to tetragonal symmetry. The spin-hamiltonian parameters obtained are: g ∥ = 1.964, g − = 1.945, A ∥ = 75.53 × 10 −4 cm −1 , A ⊥ = 38.42 × 10 −4 cm −1 .

Molybdenyl centers in NH4Cl single crystals

Molybdenyl centers are studied in single crystals of NH4Cl using a set of correlated experimental techniques like EPR, optical absorption, infrared absorption, conductivity, and dielectric loss. EPR studies on NH4Cl single crystals doped with (MoOCl5)2− are made over a wide temperature range. At room temperature the resonance due to even isotopes of Mo and the hyperfine interaction due to the odd isotopes are observed. At 77 K the ligand hyperfine interaction due to 35,37Cl is also observed. The spectra are analyzed using a suitable spin Hamiltonian. An LCAO–MO method is used to interpret all the results. In the optical absorption spectra the bands corresponding to the d–d transitions and charge transfer transitions are observed. Spin–orbit coupling constant is estimated using the EPR and optical results. Molecular orbital parameters are calculated taking into account the spin–orbit effects of the ligands also. The calculated bonding coefficients are correlated with the EPR results and the nature of bindi...

Electrical conductivity and infrared absorption of VO2+ doped alkali halides

Abstract The electrical conductivity and IR absorption of vanadyl doped alkali halides are reported. In alkali halides vanadyl is found to rotate in the lattice. The electrical conductivity measurements give evidence for the presence of a bound cation vacancy. The energy of association between the impurity and the vacancy is ≈0.2 eV. There is a shift in the “knee” point for different impurity concentrations and in the IR absorption the band intensities of V—O stretching are different for different concentrations of vanadyl. The vibrational frequencies observed in the host lattices are very different from the free vibrations of V—O in VOSO 4 . The deviations are explained by considering the molecular orbital picture of vanadyl complexes. The metal-hallide d π  π * overlap is found to be dominant compared to the metal-oxygen d π - p π interaction.