Y. Ravi Sekhar

Radiation damage studies of CrO42−‐doped alums

Radiation damage studies on NH4Al(SO4)2⋅12H2O and KAl(SO4)2⋅12H2O single crystals doped with chromate centers are presented. Similar studies on undoped alums are also reported for comparison. EPR and optical absorption results are correlated to identify and characterize the species produced on x irradiation of these crystals. Formation of CrO43− ions seems to be the most interesting feature in the doped crystals. The radicals associated with host lattices are identified as SO3−, O3−, and SO2−. The effect of impurity on the stabilization of these radicals is discussed. Results regarding the growth and decay of all these centers during irradiation and the possible kinetics of radiolysis are presented.

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.

Radiation damage of CrO42− doped KH2PO4 single crystals

Radiation damage studies of CrO42− ions doped in KH2PO4 crystals are presented. The optical absorption spectra of these crystals before irradiation show characteristic bands around 3600 and 2700 A. The optical spectra after irradiation show new absorption bands in the region 5000–7000 A. EPR spectra of the irradiated crystals consist of a single line at RT which shows splitting at low temperatures. By a systematic correlation of the optical absorption and EPR results at different stages of irradiation, the radiation damage product of these crystals is identified as CrO43−. The analysis of EPR spectra shows axial symmetry for CrO43− in the paraelectric phase with g∥ = 1.955 and g⊥ = 1.972 and orthorhombic symmetry in the ferroelectric phase with gx = 1.983, gy = 1.976, and gz = 1.945.

EPR studies on an ‘‘unperturbed’’ CrO43− center in K2SO4 crystals

Electron paramagnetic resonance of CrO2−4 doped K2SO crystals has been studied after x‐irradiation at 77 K. The resulting species, a CrO43− center, has the principal g values gx=1.8743, gy=1.8507, and gz=1.9386. The EPR data indicate a small distortion of CrO43− tetrahedra. A temperature variation study of the spectra in the range 1.6–77 K shows that they broaden rapidly above 10 K and no motionally averaged spectrum is observed. The experimental results are discussed in terms of several effects including a possible Jahn–Teller effect.

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.

Electronic absorption spectra of MnO4− ions in single crystals of alums

Abstract The electronic absorption spectra of MnO 4 − ions in the single crystals of KA1(SO 4 ) 2 ·12H 2 O and NH 4 A1(SO 4 ) 2 ·12H 2 O at room temperature and liquid nitrogen temperature are reported. The assignments of the absorption bands observed at around 5200 A, 3600 A, 3000 A and 2300 A have been made in a consistent manner. The molecular orbital energy level scheme given by Johnson and Smith on the basis of a spin-unrestricted SCF Xα cluster model has been used in conjunction with Slaters transition state theory to interpret the optical absorption spectra.

A paramagnetic Cl–O defect produced by x irradiation of KBr:ClO−4

Experimental results on KBr: ClO−4 single crystals, before and after x irradiation at 77 K, are reported. Infrared and polarized Raman spectra show clearly that the perchlorate molecules are isolated in the KBr matrix and possess Td symmetry. EPR spectra, at 9.5 and 36 GHz, of irradiated crystals at 4.2 K indicate the formation of ClO2−4 in trigonal symmetry. The g and 35Cl hyperfine tensors are found to be strictly axial and collinear. Temperature variation of the EPR spectra presented no motional averaging while uniaxial stress applied to the crystals yielded negative results. The structure of the ClO2−4 molecule is discussed in terms of a trigonal Jahn–Teller effect and a probable dissociation into (ClO−3–O−) complex.

Vibrational spectra of MnO4- isolated in KBr crystals: an illustration of pure Td symmetry in the solid state

A solid state example that can clearly illustrate the rigorous application of IR and Raman selection rules in a pure Td symmetry.