G. S. Sastry

Electron spin resonance studies on Cu2+ doped Na2Zn(SO4)2.4H2O

Electron spin resonance studies at room temperature (302 K) have been carried out on Cu2+ doped Na2Zn(SO4)2.4H2O. The electric field symmetry around Cu2+ is found to be rhombic. From the esr data it is found that the z axis of the rhombic field falls along Zn-O(H2O(1)) direction. The principal g values and the hyperfine constants are given.

Structural phase transitions in langbeinites: an EPR study

The mechanism of structural phase transitions in langbeinites has been examined by EPR, taking Mn2+:Rb2Cd2(SO4)3 and VO2+:(NH4)2Cd2(SO4)3 as case studies. These studies suggest that the softening of SO42- rotational modes is primarily responsible for structural phase transitions in this family. The vanadyl dopant, which forms rigid oxygen octahedral coordination, has been found to stabilise the low-temperature phase, locally, at temperatures much higher than the actual transition point. Furthermore, indications were obtained to suggest that the transition points, -144 and -170 degrees C, are probably associated with the onset and completion, respectively, of the slowing down of SO42- rotations in the timescale of EPR.

Proton conduction and phase transitions in orthorhombic N2H6SO4 single crystals

Electrical conductivity, coulometric, DTA and DSC studies are made of orthorhombic N2H6SO4 single crystals. The conductivity plot and the DTA and DSC thermograms show a phase transition at 483K, which exhibits thermal hysteresis. During cooling a multiplet structure is observed in the DSC thermogram. Activation energies obtained from the conductivity reveal that protons may be the charge carriers. This is confirmed by hydrogen gas being evolved at the cathode of the coulometer; this is analysed by gas chromatography.

ESR investigations of VO2+ doped ferroelectric tris sarcosine calcium chloride

ESR studies of vanadyl ions doped in ferroelectric tris sarcosine calcium chloride single crystals are carried out. The room temperature spectra indicate four magnetically inequivalent complexes which become equivalent along the crystallographic axes. The z axes of two of these correspond to two Ca–O bond directions, suggesting substitutional sites for the paramagnetic ion. The molecular orbital coefficients are estimated from the ESR and optical absorption data which indicate the complex to be ionic except for the extremely low value for e*π2. A new band is observed in the optical absorption spectrum which might be due to the b2↔a transition. The equivalency at room temperature is lost at liquid nitrogen temperature.

Optical absorption and electron paramagnetic resonance of Cu2+ ions in a low dimensional CaCu(HCOO)4 single crystal

Abstract Optical absorption and electron paramagnetic response (EPR) of Cu 2+ ions in a CaCu(HCOO) 4 single crystal have been studied. Analysis of EPR spectra using crystal field theory indicates that Cu 2+ ions are in tetragonally distorted (elongated) octahedral sites. The optical absorption in the UV-visible region are characterized by a charge transfer band and cubic field splitting of the 3d energy level of the Cu 2+ ion, but in the near-IR the absorptions at 4225, 4310, 5000, 5714 and 6622 cm −1 are attributed to the transitions between the admixed quantized energy levels of copper and oxygen orbitals.

EPR studies of phase transitions in (NH4)3H(SO4)2 single crystals

The successive phase transitions reported in triammonium hydrogen disulphate (TAHS) are investigated by EPR using Cu2+ and Mn2+ ions as probes. At room temperature, in the spontaneously strained monoclinic phase, Cu2+ and Mn2+ ions were found to be in two magnetically inequivalent sites. At the phase II (monoclinic) to (+140 degrees C) phase I (trigonal) transition the magnetic inequivalence of Cu2+ ions vanished, resulting in a broad single line at g=2.240+or-0.002 due to a possible dynamic Jahn-Teller effect that may be present for the 2E state of the Cu2+ ion in trigonal symmetry. This transition exhibited a hysteresis of about 45 degrees with the reverse transition appearing at +95 degrees C on cooling. The II to III transition resulted in the splitting of the Cu2+ resonance into two components with a temperature-dependent intensity ratio and separations. These results are consistent with the model that the Cu2+ ion occupies the NH4+ (I) position at room temperature, and takes an off-centred position in phase III with an associated electronic dipole moment, responding to the onset of antiferroelectric ordering.

Electron spin resonance of Cu2+ in hexaaquo magnesium dihydrogen ethylenediaminetetraacetic acid

ESR studies on Cu2+ doped in Mg(H2O)6H2EDTA crystals are reported, in which the hyperfine structure for the complex Cu2+(H2O)6 at room temperature is observed for the first time. This has been attributed to the large scale hydrogen bonding involving the water protons in this crystal. The spin Hamiltonian parameters are found to be considerably different from the corresponding values of Cu2+(H2O)6 in other matrices, due to the large covalent bonding present in this crystal. gz=2.286+or-0.002 gy=2.095+or-0.002 gx=2.193+or-0.002 Az=87+or-2G Ay=77+or-2G Ax=28+or-2G.

Transport properties and thermal studies on pure and Cu2+-doped potassium hydrogen maleate single crystals

Conductivity, thermally stimulated depolarisation (tsdc) and thermal studies are carried out on pure and Cu2+-doped potassium hydrogen maleate single crystals. Bothdta anddsc reveal first-order phase transition at 503 K.dsc further shows that the crystal decomposes even before the transition is complete. A ‘knee’ is observed around 505 K in conductivity plots at this transition. Thetsdc of Cu2+-doped system gave a single peak at 241 K and the reasons are analysed. Activation energies from conductivity andtsdc suggest proton conduction. However no direct evidence in coulometric experiment could be observed. But thermoelectric power measurements reveal the charge carriers to be positive.