K. G. Subhadra

Systematic hardness measurements on mixed and doped crystals of rubidium halides

Efforts are made to improve the hardness of rubidium halide crystals by (i) solid solution hardening and (ii) impurity hardening. Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr-RbI and KI-RbI) and rubidium halide crystals doped with Sr2+ ions. The composition dependence of the hardness of mixed crystals follows the law ΔHv=K x(1− x),where ΔHv is the enhancement in hardness,K a constant andx and (1 −x) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentrationC of the dopant according to the law, ΔHv+6 =k Cm,wherek andm are constants. The relative efficacy of the two methods of hardening is discussed.

X-ray determination of the mean amplitudes of vibration and debye temperatures of some rare earth monochalcogenides

The x-ray diffraction intensities of Bragg reflections have been measured at room temperature for thulium selenide, samarium sulphide, samarium selenide and samarium telluride. On the basis of a common amplitude approximation, the Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. The values of the Debye temperatures and mean amplitudes of vibration are 176±16°K, 0·185 ± 0·017 Å (TmSe), 155 ± 7°K, 0·244 ± 0·012 Å (SmS), 153 ± 14°K, 0·221 ± 0·020 Å (SmSe) and 151 ± 20°K, 0·204 ± 0·027 Å (SmTe).

X-ray diffraction studies of RbBr-RbI mixed crystals

Mixed crystals of RbBr and RbI have been prepared from melt. Using an X-ray diffractometer powder patterns have been recorded. The lattice constants show slightly positive deviations from Vegard’s law. From the integrated intensities, the mean Debye-Waller factor has been determined. The Debye-Waller factor shows a highly non-linear composition dependence with positive deviations from linearity, the values for intermediate compositions exceeding those for the end members. The Debye temperatures calculated from the Debye-Waller factors show a non-linear composition dependence with negative deviations from linearity.

X-ray determination of the mean Debye-Waller factors, amplitudes of vibrations and the Debye temperatures of CdO, PbS and MnS

Integrated intensities of Bragg reflections of CdO, PbS and MnS have been measured at room temperature using an x-ray powder diffractometer. From a least square treatment of the intensity data the mean Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. Calculations were made using scattering factors pertaining to neutral atoms as well as doubly ionised ions. The results from the two sets of calculations agreed within limits of errors.

Debye-Waller factors and Debye temperatures of alkali halide mixed crystals

The Debye-Waller factors of KxRb(1-x)I and NaClxBr(1-x) mixed crystals have been determined from X-ray diffraction intensities. The mean Debye-Waller factor is found to vary nonlinearly with the composition with positive deviations from linearity. The Debye temperatures (calculated from the Debye-Waller factors) are found to vary slightly non-linearly with composition with negative deviations from linearity. The Debye temperatures of seven alkali halide mixed crystal systems are critically compared with values predicted from six laws for composition variation. Using the estimated standard deviation as the criterion for the goodness of the fit, it is shown that the inverse cube (Kopp-Neumann) law provides the best description for the composition dependence of Debye temperatures of mixed crystals.

X-ray determination of Debye–Waller factors of NaBr and NaI

The Debye-Waller factors of NaBr and NaI have been determined from X-ray diffraction intensities measured at room temperature with a powder X-ray diffractometer. The resulting values are: B + = 1.55 (15) and B - = 1.14 (10) A 2 for NaBr and B + = 2.63 (20) and B - = 1.81 (15) A 2 for NaI.

Systematic hardness measurements on CsCl x Br(1−x) and NH4Cl x Br(1−x) mixed crystals

Vickers hardness measurements have been made on polycrystalline blanks of CsClxBr(1−x) and single crystals of NH4ClxBr(1−x). The composition dependence of hardness is highly nonlinear in both systems and follows an empirical model that includes a lattice contribution and a disorder contribution. The Gilman-Chin parameter (H/C44) has been calculated and its significance discussed.

X-ray determination of the Debye-Waller factors and Debye temperatures of europium monochalcogenides

X-ray powder diffractograms of EuS and EuTe have been recorded. The integrated intensites have been measured and corrected for TDS. From an analysis of the intensity data, Debye-Waller factors and Debye temperatures have been evaluated.

Systematic hardness measurements on single crystals and polycrystalline blanks of cesium halides

Vickers and knoop hardness measurements were carried out on CsBr and CsI single crystals. Polycrystalline blanks of CsCl,CsBr and CsI were prepared by melting and characterized by X-ray diffraction. Vickers hardness measurements were carried out on these blanks. The hardness values were correlated with the lattice constant and the Schottky defect formation energy.

Melting temperatures of some alkali halide mixed crystal systems

The melting points in the KCl-KBr, RbCl-RbBr and KBr-RbBr mixed crystal systems have been measured using the microscopic method. In all the three systems, melting points vary nonlinearly with composition, with negative deviations from linearity and maximum deviation at about the equimolar concentration. The observed composition dependence of the melting points is explained qualitatively in terms of the enhanced concentration of point defects and dislocations in mixed crystals.