Application of XDM to ionic solids: The importance of dispersion for bulk moduli and crystal geometries

Relativistic ab initio calculations of inter-ionic potential energies are used to develop a reliable non-empirical method for predicting the properties of ionic solids containing the heaviest ions. A physically realistic method for describing the non-negligible differences between free and in-crystal ion wavefunctions is described. Functions are presented for describing the partial quenching, arising from ion wavefunction overlap, of the standard long-range form of the inter-ionic dispersive attractions. These attractions are shown to be distinct from the contributions to the inter-ionic potentials that arise from that portion of the electron correlation energy which is nonzero solely because of overlap of the ion wavefunctions. The results presented for NaCl, MgO and the fluorides of Li, Na, Ag and Pb show that these modifications overcome the deficiencies of previous calculations. Ab initio predictions of the closest cation-cation and anion-anion short-range interactions, which are not available from semi-empirical fits to experimental data, are presented. The non-point coulombic interactions between pairs of anions, derived by adding the dispersive attractions to the short-range interactions, are compared with previous semi-empirical and approximate ab initio results. The uncorrelated short-range inter-ionic potentials computed exactly are compared with those predicted from electron-gas theory. The use of the electron-gas approximation to describe any of these potentials degrades the quality of the predicted crystal properties.

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Molten fatty acid salts as model ionic liquids. I. Thermodynamic and transport parameters of some organic sodium salts

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1971.0068 ◽

1971 ◽

Vol 322 (1550) ◽

pp. 281-299 ◽

Cited By ~ 36

Keyword(s):

Electrical Conductivity ◽

Transport Parameters ◽

Sodium Propionate ◽

Isotropic Liquid ◽

Enthalpy Changes ◽

Physical Measurements ◽

Ionic Melts ◽

Ionic Solids ◽

Steep Decrease ◽

Transition Points

Procedures are described for maintaining good chemical stability in molten alkali-metal carboxylates, up to about 350 °C. Valid physical measurements can be made and the fluids can be used up to about this temperature, above which spontaneous decomposition of the anions is difficult to repress. Molten sodium propionate has a useful liquid range of about 60 °C and sodium isobutyrate of about 90 °C. Sodium n -butyrate transforms into a ‘liquid crystal’ at about 250 °0 and into the isotropic liquid at 324 °C. For sodium isovalerate corresponding transition points are respectively 188 and 280 °C. Thermodynamic measurements are reported of volume and enthalpy changes at transition and melting points. Transport parameters measured include the viscosity and the electrical conductivity. The viscosity of these ionic melts undergoes a steep decrease at the transition from mesomorphic to isotropic liquids. Jumps in ionic conductivity are found both at the melting and clearing points. Even for the isotropic liquids, the ratio of viscosity to electrical conductivity is exceptionally high, compared with the other ionic melts. Mechanisms of melting for these ionic solids are discussed.

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