G. C. Upreti

EPR observation of a high temperature phase transition in Zn(ClO4)2·6H2O:Mn2+

Abstract The electron paramagnetic resonance of Mn2+ doped in single crystals of Zn(ClO4)2·6H2O has been studied at X-band in the temperature range 300–380 K. A phase transition, observed at ∼ 346 K, is reported for the first time.

EPR study of a new low-temperature phase transition in Zn(ClO4)2·6H2O

Abstract From a temperature variation EPR study of Mn 2+ doped single crystals of Zn(ClO 4 ) 2 ·6H 2 O phase transition has been detected at T 2 ~290 K. The phase relationships in this crystal are as follows. Phase I transforms at T 2 ~346K to Phase II, which in turn transforms to Phase III at T 2 ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn 2 1 both above and below T 2 . The water-perchlorate sublattice symmetry below T 2 is found to be lower than the P 6 3 mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T 2 , is reflected through the observed temperature dependence of the rhombic distortion parameter E . It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.

Electron paramagnetic resonance study of Mn2+ doped ammonium iodide single crystals

ERP of Mn2+ doped in ammonium iodide single crystals has been studied at room temperature (∼300 K) and at X band (∼9.3 GHz). An axial complex with the following spin‐Hamiltonian parameters has been observed: g∥=2.0014±0.0005, g∥=2.0028±0.0005, b02=−1611±2 G, b04=6±1 G, ‖A‖=‖B‖=90±1 G. Some unusual features observed in the spectra at X band due to the large value of b02 have been discussed.

Electron paramagnetic resonance study of Cu2+-doped ammonium iodide single crystals

Abstract Room-temperature (≈ 300 K) EPR studies of Cu 2+ -doped in NH 4 I single crystals are reported. The Cu 2+ ion is found to substitute for NH 4 + and get associated with a second or fourth neighbour cation vacancy. The spectra for the two distinct types of centres have been analysed and the parameters presented.

Q‐band EPR study of Mn2+ doped single crystals of NH4I

EPR of Mn2+ doped ammonium iodide single crystals has been studied at room temperature (∼300 K) and at Q band (∼35.25 GHz). An axial complex has been observed. The spin‐Hamiltonian analysis has been carried out and the obtained parameters reported. From the analysis of observed forbidden hyperfine transitions, the quadrupole coupling constant Q has been estimated.

EPR detection and study of new phase transitions in Mg(ClO4)2·6H2O

Abstract A temperature dependent EPR study of Mn2+ impurity-doped crystals of Mg(ClO4)2·6H2O has led to the detection of two critical temperatures T1 ~ 335K and T2 ~ 324K where the EPR characteristics undergo distinct changes. The monoclinic distortion of the water octahedron around the metal ion shows little temperature dependence between T1 and T2 and T2 and T3 ~ 272K, while it is found to be comparatively strongly temperature dependent above T1 and below T3, decreasing as T approaches T1 from the high temperature side and as T approaches T3 from the low temperature side. At room temperature the water-perchlorate symmetry seems to deviate slightly from the perfect P63mc symmetry proposed from previous X-ray measurements by West. Although the space symmetry group of the unit cell appears to remain unchanged at T1 and T2, the temperatures T1 and T2 are suspected to be related to structural phase transitions in this crystal.

EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn2+ and Cu2+ ions doped calcium cadmium acetate hexahydrate (CaCd(CH3COO)4•6H2O) have been reported. The investigation has been carried out in the temperature range between room temperature ( 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH3 groups which become frozen at 128 K. The incorporation of Cu2+ and Mn2+ probes at Ca2+ and Cd2+ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca2+ and Cd2+ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.

Electron paramagnetic resonance study of vanadyl ion doped in α-calcium formate single crystal

Abstract Results of an EPR study of vanadyl ion (VO2+) doped in single crystals of α-calcium formate are reported. Impurity sites in the unit cell and the molecular structure of the vanadyl complexes in the crystal are discussed. The spin-Hamiltonian parameters are determined from single-crystal EPR data at room temperature and from powder EPR data at room temperature and at liquid nitrogen temperature. The g and A tensors are found to be axially symmetric. From the optical and EPR data, the MO bonding coefficients and crystal field parameters are calculated and discussed. Two distinct vanadyl complexes at substitutional sites (Ca2+) and one interstitial vanadyl complex have been identified.

Low‐temperature X‐band EPR study of Mn2+‐, Cu2+‐, and Co2+‐doped NH4I single crystals

X‐band EPR of Mn2+,Cu2+, and Co2+ ions doped in ammonium iodide single crystals have been studied at low temperatures. The Mn2+ and Cu2+ spectra show the presence of only one type of complex for each with features similar to those observed at room temperature with no unusual changes. The results on Co2+ ion, reported here for the first time, reveal the presence of three equivalent Co2+ complexes having tetragonal symmetry with their tetragonal axes orthogonal to each other. The charge‐compensation mechanism and possible vacancy complexes are discussed, and the spin‐Hamiltonian parameters reported.

EPR of Mn2+ in NH4Br Single Crystals

The EPR of Mn 2+ in NH 4 Br single crystals has been studied at X-band in the temperature range from 573 K to 77 K. At room temperature, an isotropic spectrum (spectrum-I) comprising a sextet superimposed on a broad line is observed. On heating the crystal up to 400 K the sextet disappears due to explulsion of Mn 2+ impurity from the crystal. On lowering the temperature, the isotropic spectrum-I disappears about at 230 K and a new spectrum (spectrum-II) appears below this temperature. The spectrum-II is highly anistropic and unusual and is believed to be due to very large zero field splitting of Mn 2+ . Spectrum-I is attributed to Mn 2+ ions substituting for NH 4 + with charge compensating NH 4 + vacancies far away from Mn 2+ . The transformation of spectrum-I into spectrum-II is attributed to the structural transformation of NH 4 Br.