Interionic Forces and Elastic Properties of Na₂S

Abstract

A n interionic potential model is proposed for crystals with antifluorite structure which consists of long-range Coulomb and three-body interactions (TBI), and short-range dispersive and repulsive interactions up to second-neighbour ions. This potential is used to calculate the cohesive energy, second-order elastic constants, and their pressure derivatives of Na,S. The calculated values of elastic constants and cohesive energy agree well with their measured data which shows that the TBI contribute significantly to these properties.Es wird ein Interionenpotentialmodell fur Kristalle mit Antifluoritstruktur vorgeschlagen, das aus weitreichenden Coulomb-und Dreikorper-Wechselwirkungen (TBI) und kurzreichweitigen dispersiven und repulsiven Wechselwirkungen bis zu den zweiten Ionennachbarn besteht. Dieses Potential wird benutzt, urn die Kohasionsenergie, die elastischen Konstanten zweiter Ordnung und deren Druckableitungen fur Na,S zu berechnen. Die berechneten Werte der elastischen Konstanten und der Kohasionsenergie stimmen gut mit ihren gemessenen Werten uberein, was zeigt, daI3 die TBI betriichtlich zu diesen Eigenschaften beitragen.

Ab initio Hartree–Fock study of lithium and sodium sulfides: electronic and scattering properties

“…This last result is confirmed by examining the p valence band of sulfur (9-11 in Li2S, 17-19 in Na2S), where the participation of lithium is two times greater than sodium. It can be noticed that this highly ionic character constitutes a significant dis-crepancy with other results obtained for Na2S and deduced either from experiment (0.72 e) (Bfihrer & Bill, 1980) or from an interatomic potential model (0.96 e) (Agnihotri et al, 1984). (ii) As for highly ionic compounds [see for example, MgO (Causfi, Dovesi, Pisani & Roetti, 1986a)], the band structure is very simple, since most of the bands are in fact single levels attributable to pure or nearly pure orbitaSs.…”

“…In a few theoretical studies (Agnihotri, Singh & Sanyal, 1984;Jain & Shanker, 1982;Thakur & Pandey, 1975;Agarwal, Sharma, Sharma, Sharma & Shanker, 1977;Holbrook, Sabry-Grant, Smith & Tandel, 1990), interionic potentials are discussed and used to calculate binding energy, elastic constants, bulk modulus, Debye temperature and electron affinities of chalcogenide ions. It can be noted, therefore, that, among this data, there is nothing concerning the electronic structure of the two sulfide compounds.…”

“…This result seems rather surprising: indeed, it is customary to think that sulfur leads to less ionic bonds than oxygen (Shannon, 1981) because of its weaker electronegativity and larger polarizability. A comparative study with the homologous Na2S compound becomes interesting to generalize this property, in spite of some results given in the literature (B/ihrer & Bill, 1980;Agnihotri et al, 1984).…”