Model calculations for H^-impurity centres in alkali halides

Abstract

An existing consistent set of model potentials for alkali halides formulated for convenient use in defect applications is extended to incorporate hydrides. The additional parameters required are fitted to the limited available experimental data for perfect alkali hydride crystals. Calculations of local-mode frequencies for isolated H-substitutional (Ucentre) and interstitial impurities, H-H-, H-D-, H-cation-impurity and H-anionimpurity pairs in alkali halides are reported. These calculations, which are based on large periodic defect cells, involve no further adjustable parameters and are in good agreement with results of infrared absorption measurements found in the literature.

Crystal anharmonicity in Li(H,D) and Na(H,D) systems

“…Unlike all other pressure derivatives considered here, dC;,/dp does not involve C,,, in the expression and hence is not determined by the dominant nearest-neighbour repulsive interactions. In spite of this our calculation for dC', l/dp is much better than the estimates made by Bowman [13] and HS [14] potentials and is in reasonable agreement with the available observed value. In fact, both Bowman [13] and HS [14] potentials yield a very low value.…”

“…In spite of this our calculation for dC', l/dp is much better than the estimates made by Bowman [13] and HS [14] potentials and is in reasonable agreement with the available observed value. In fact, both Bowman [13] and HS [14] potentials yield a very low value. Although potentials due to Fischer et al [30] give reasonable values of these properties for LiH, the calculated value of dC;,/dp is not satisfactory (more than 200% higher).…”

“…The various lattice sums for different orders are collected in Table 1. The results of calculation of different quantities are shown in Tables 2, 3, and 4 along with those obtained by us using potentials reported by other workers [5,13,14,30,311. In order to facilitate complete comparison, SOECs reported previously by us [I] have also been included [31] and [43].…”

“…The Bowman potential [I31 again yields a negative value whereas those due to GS [31] and HS [14] give slightly better values. Like PS [5], HS [ 141, and ZP [33], our calculations for dB/dp are in good agreement with the available observed values, whereas the Bowman [I31 potential again gives smaller values.…”

Hydrides and Deuterides of Lithium and Sodium. I. Model Potentials and Their Use in Perfect Lattice

“…The significant differences between different potentials in predicting both cation migration and, even more important, the relative values of the Schottky and Frenkel formation energies in ionic compounds [12,221 serve to emphasize the importance of using potentials that simulate both the dielectric and elastic properties as closely as possible. The lattice constants found by HS [20] for NaH and CsH are higher. Also the use of a nonintegral value of Z is less transparent physicaliy and introduces various problems associated with the definition of the energies of charged defects [lo].…”