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Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time TM, were carried out, in the 20-200 K temperature range, on an AsO44- paramagnetic probe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP), and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the 14N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP, and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number mI of 75As. The temperature dependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb0.5(NH4)0.5H2PO 4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample. © IOP Publishing Ltd.

Thermally stimulated luminescence of natural mineral petalite (LiAlSi4O10) crystals was investigated and the possible electron/hole traps responsible for thermoluminescence (TL) emission were identified using optical absorption and electron paramagnetic resonance (EPR) measurements. The glow curves for natural samples, obtained with a heating rate of 4C s-1, show two glow peaks at 160°C and 330°C. Pre-annealed and subsequently irradiated samples give rise to three glow peaks at 175°C, 340°C and 435°C. An isochronal thermal study established correlation between the first glow peak and Ti3+-like trap centre and E'1-centre and Al-O--Al-like recombination centres. The second TL peak at 340°C may be related to the E'1 centres formed on irradiation. A mechanism for the observed TL is suggested.

Polycrystalline MgB2, synthesized using a conventional solid-state reaction route, has been investigated for its magnetic field dependent microwave absorption using a standard X-band EPR spectrometer. The changes in microwave absorption as a function of temperature and/or magnetic field were recorded by following the microwave reflectivity of a sample-loaded cavity. The modulated low-field microwave absorption signal was found to be similar to that observed in high-temperature superconducting materials. The field dependent direct microwave absorption in the 0-10 kG range was used to determine Hc1 values at different temperatures, and the value of Hc1(0) was found to be 250 G. The absorbed microwave power has been found to obey a (H)1/2 dependence with a change of slope indicating a transition from a strongly pinned flux lattice to the flux flow regime.

Low and high field dependent microwave absorption is studied in various sintered and melt textured HTSs. The low field loss is found to originate from weak links and the high field loss is due to thermally assisted flux flow. Melt textured superconductors are found to be best suited for low field device applications due to the drastic reduction in low field induced loss when compared to sintered samples. Melt textured superconductors with 211 inclusions are found to show reduced high field losses due to pinning of field at the secondary defects created by 211 particulates. Various parameters of the critical state such as Jc and the pinning force constant, Kx, are determined from the microwave absorption study.

Some of the Chevrel phase compounds are reported to exhibit unusual cationic mobilities, particularly those with transition metals. EPR evidence is presented for mobile ions in -doped . Evidence is also obtained for the existence of ions, giving possible support for the model correlating the site change with the reaction during ionic conductivity.

By using EPR of probe ions in (x = 0 and 0.05), clear evidence has been obtained for the existence of dynamic fluctuation between cubic and tetragonal phases in the vicinity of the phase transition temperature. This was manifested as a total absence of the EPR of in the temperature range for x = 0.05, clearly showing that there is a time-dependent interaction serving as an additional and more effective pathway of spin relaxation. The time scale of the fluctuation is estimated to be .

The mercurous ion Hg+ is produced by gamma irradiation of Hg2+-doped ammonium chloride single crystals, and electron paramagnetic resonance studies of it have been conducted in the 10-300 K temperature range. It is shown that Hg+ occurs substitutionally at the NH4+ site. For odd isotopes of mercury, 199Hg and 201Hg, hitherto unreported transitions have been observed and are found to be highly sensitive to variations in the hyperfine coupling constant A, and thereby to the temperature dependence of A. The temperature dependence of A (for odd isotopic species) has shown an anomalous increase below the lambda transition of ammonium chloride. The observation of new transitions has enabled the hyperfine coupling constants to be determined accurately and also the hyperfine anomaly for 199Hg+ and 201Hg+ to be re-estimated.

Electron paramagnetic resonance studies of Rb1-x(NH4)xH2PO4 (x=0, 0.5, 0.8, 0.9, 0.95 and 1.0) containing an AsO44- probe gave a clear indication of the fluctuations between ferroelectric (FE) and antiferroelectric (AFE) regions in the mixed crystals right from room temperature. This has become possible because the AsO44- probe has different 75As hyperfine coupling constants in pure RDP (FE) and APP (AFE) crystals (x=0 and 1). This fluctuation is manifested as the averaging of the 75As hyperfine coupling constant, with compositional dependence of the form (A)mixed crystal= (1-x)AFE+xAAFE. The characteristic fluctuation time T

We report electron paramagnetic resonance (EPR) results on natural feldspar grains extracted from sediment samples of three widely different source regions. The XRD analysis indicated that the samples had albite, microcline and Ca/Ba–orthoclase as the major constituents. The EPR spectrum of all the samples exhibited an intense line around geff = 4.3. This is characteristic of Fe3+ with a zero field splitting that is significantly larger than the microwave quantum. More interesting was the presence of a strong EPR signal in the region of geff = 2.54–2.7. On irradiation with gamma rays the position of the line shifted to higher magnetic field. The possible origin of this line and its unusual behavior with dose is explained as being due to the interaction of a hopping 'hole' on oxygen with iron centers. The possibilities considered include (1) the valence fluctuation of an Fe3+ center at a tetrahedral site, (2) oscillation of FeO4− between two resonant forms, Fe2+O− and Fe3+O2−, and (3) exchange coupling between Fe3+, Fe2+ and O−. The concentration of the latter is dose dependent and, consequently, it results in the observed dose dependent line shift.

By taking 75As hyperfine coupling constants for AsO44- in hydrogen bonded crystals of RbH2PO4 and (NH4)H2PO4 as fingerprints of ferroelectric (FE) and antiferroelectric (AFE) phases respectively, it is shown that the variation of the coupling constant as a weighted average of Rb+ and NH4+ concentrations in Rb1-x(NH4)xH2PO4 clearly proves the existence of FE and AFE fluctuations in the mixed crystal.

Thermoluminescence (TL) glow curves of gamma-irradiated SrCl2 doped with Eu, Gd, Tb and Am in trivalent form have been obtained in the temperature ranges 77-300K and 300-600K. The glow peaks in the lower temperature range are accounted for by the successive bleaching of Vk-type trapped hole centres. Electron paramagnetic resonance (EPR) studies of Eu-doped samples in the temperature range 300-500K have been carried out in detail and the results strongly suggest that the TL glow peak around 500K is followed by the trapping of an electron by Eu3+ and the TL glow around 420K is due to the thermal destruction of OH0 centres. The TL glow curves have been analysed to obtain the trap depth and frequency factors.

Microwave absorption studies of the new high-Tc superconducting systems Tl-Ba-Ca-Cu-O (Tc( rho =0)=118 K) and Bi-Ca-Sr-Cu-O (Tc( rho =0)=84 K) were conducted using an X-band EPR spectrometer. The main features observed were (i) a sharp change in the leakage current at Tc; (ii) intense low-field (

Tetramethyl ammonium tetrachloroferrate (TMATC-Fe) has been examined by differential scanning calorimetry (DSC) to test the prediction of a phase transition in this compound by Gibb and his co-workers (1965) around 239K from Mossbauer studies. DSC has given clear evidence for the occurrence of a first-order transition at 238+or-1K during cooling, which has been found to occur at 246K during heating, exhibiting a hysteresis of 8K. Additional transitions, at 261, 271, 281 and a glassy one at 289K were seen during heating. During cooling, in addition to the 238K peak, a transition around 242K and two weak transitions at 285 and 267K were also observed. By suitable thermal cycling, pairs of transitions responsible for the same process during heating and cooling were identified wherever possible. Data for samples with different thermal and degassing history are also presented. Correlation with Mossbauer data on this compound is also presented.

Trap level spectroscopy studies of alkaline-earth sulphate (CaSO4 and SrSO4) phosphors doped with 239Pu were carried out using electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) techniques. Internal (in situ) alpha -irradiation (from the alpha -decay of 239Pu) and external gamma -irradiation studies carried out using EPR revealed the formation of radicals SO2-, O-, O3-, SO4-, etc, in these matrices under different conditions. SO2- dimer formation is detected for the first time in alkaline-earth sulphates on gamma -irradiation at 77 K. The thermal stabilities of the radical ions (monitored using EPR) and their correlation with TSL glow peaks helped to identify the thermally stimulated free-radical reactions which result in luminescence and were characterised by the activation energy and frequency factor obtained from the analysis of TSL glows.

For pt.I see ibid., vol.17, p.5851 (1984). Trap-level spectroscopic studies of SrSO4 phosphor doped with 241Am were conducted using ESR and thermally stimulated luminescence (TSL) techniques. The formation of electron and hole traps due to internal alpha -irradiation (from the decay of 241Am) and external gamma -irradiation were distinguished by carrying out the studies on pure SrSO4 and also on SrSO4 doped with europium (lanthanide analogue of Am). The ESR studies on these samples have shown that the paramagnetic radicals SO4-, SO3-, O3-, S5+, SH2- were stabilised in SrSO4 under different dopant and irradiation conditions. The radical SH2- is detected for the first time. From the ESR studies of thermal stabilities of radical ions produced and their correlation with TSL peaks, the role of paramagnetic molecular species in the electron and hole trapping and thermally stimulated free radical reactions were identified and the corresponding activation energies and frequency factors were determined.

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.

A versatile thermoluminescence (TL) unit, adapted for glovebox operation has been designed and built to study the radioactive recombination and self activation process in inorganic materials with radioactive elements as activators. The TL unit has provision for recording the TL output in two temperature ranges, 100-320K and 300-750K. The unit incorporates an electronic temperature programmer which can control the rise in temperature of a Kanthal strip at different constant rates in the ranges mentioned above. The salient features of the instrument include five linear heating rates viz: 0.1, 0.25, 0.5, 1.0 and 2.5K/s and a facility to maintain the temperature of the sample for a long time within +or-1K at any desired value in the two ranges to enable isothermal decay studies. Furthermore, the unit has the advantages of attaining the lower temperatures (100-320K range) without the cryogenic liquid (liquid N2) or cooled gas coming directly into contact with the sample, thereby avoiding radioactive contamination of the evaporating liquid. Glow curves for the standard phosphors, CaF2, TLD-100 and Dy:CaSO4, obtained in both temperature ranges are found to be in good agreement with those reported in the literature.

For pt.II see ibid, vol.21, p.5913 (1988). Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) studies were conducted on gamma -irradiated SrSO4:UO22+ to elucidate the role of electron and hole traps in thermally stimulated reactions and to obtain the trap parameters (the trap depth and frequency factor). For samples annealed at 970 K (samples A), intense hydrogen atom signals were seen in EPR, and it was found that the thermally stimulated reaction between H0 atoms and SO4- is responsible for the luminescence glow at 380 K. In the close vicinity of this temperature, the EPR spectrum of hydrogen exhibited features of chemically induced dynamic nuclear polarisation, suggesting that this reaction proceeds through pair formation. The hydrogen atom intensities were considerably smaller for samples quenched at 1170 K (samples B) but appeared to have participated in the formation of SH2. In both samples A and samples B, a trapped hole centre with g=1.995 formed owing to gamma -irradiation at 300 K. The temperature stability of this crucially depended on the presence of H0 and also SO4u-. The intense glow peak at 420 K present in both samples was identified as due to the reaction between SO4SuS- and UO2+.

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.

The structural phase transitions in LiKSO4 have been examined by EPR, at X band, using a paramagnetic SO4- probe. The orientation of the SO4- probe ion is modified with respect to that of the parent SO42- ion. The authors show that this modified orientation and its temperature-dependent motion has misled earlier workers into suggesting an unusual sequence of commensurate-incommensurate transitions in LiKSO4. Their measurements also support the occurrence of another transition in this compound at 130 K, as predicted by neutron diffraction work. This transition appears to trigger an anti-ferro-distortion resulting in the rotation of SO4- by +or-10 degrees about the c axis; all of the sulphates remain chemically equivalent. LiKSO4 exists below 130 K as a mixed-phase compound down to 77 K.