Robust Rietveld refinement in the presence of impurity phases

A modified least-squares analysis is presented that allows reliable structural parameters to be extracted from a powder diffraction pattern even in the presence of a substantial unmodelled impurity contribution. The algorithm is developed within the context of Bayesian probability theory. Experimental points that fall above those calculated, and are thus more probably from impurity peaks, are systematically down-weighted. This approach is illustrated with a two-phase example.

ESTIMATION OF THE EFFECTIVE DIELECTRIC RESPONSE FROM HOPPING ACTIVATION ENERGY IN THE VICINITY OF INSULATOR-METAL TRANSITION IN SEMICONDUCTORS

The effect of the energy overlap integral, due to the partial overlap of the adjacent wave functions of impurities, on the dielectric response ε of n-type semiconductor is examined. The proposed model suggests that ε diverges as the overlap increases, i.e. as the impurity concentration n is increased and essentially goes to a non-vanishing value as n approaches a critical value nc, defining the insulator-metal transition. It also predicts that ε→k (the host material dielectric constant) when n becomes low enough such that the impurity contribution to ε is negligibly small.

Low Temperature Heat Capacity and Thermal Expansion of KCI Containing a Low Concentration of Li+

The linear thermal expansion coefficient a from 180 mK to 36 K and the heat capacity per gram cp from 0�5 to 10 Kare reported for KCl containing approximately 100 ppm Li+. Contributions to a and cp attributable to the impurity have been identified and analysed on the basis of a model in which the Li + ions reside about 0�05 nm from the equilibrium cationic site in any of the (111) directions. There is consistent quantitative agreement with a Schottky-like term that arises from the eightfold degeneracy of the level splitting; the splitting parameter is 1� 5 K. The large (- 160) Grtineisen parameter associated with the impurity contribution is evidence for a quantum-mechanical tunnelling process, and agrees with values derived from independent measurements of the spectroscopic and dielectric responses of this system under pressure.

The magnetic susceptibilities of palladium and palladium-rhodium alloys from 1.85 to 293 °K

Rapid increases in the susceptibilities of palladium-rich alloys have been observed at low temperatures. From the difference in susceptibilities of two specimens of nominally pure palladium, these increases appear to be due to impurities and are consistent with the assumption that the impurities give rise to a form of super paramagnetism with carriers of moments of the order of 9 Bohr magnetons. This forms the basis for the correction of the measured susceptibilities of the alloys for impurity contribution and the extrapolation of the susceptibility values to 0 °K. By combining these results with the results of measurements of the electronic contribution to the specific heat, the variation of Stoner’s exchange parameter θ' with composition is deduced. The form of this experimentally deduced variation suggests that the previously reported peak in the density of states curve for palladium lies closer to the Fermi level for palladium than hitherto supposed.