Alkali Metal Bonding Energy and Activation Energy for dc Conductivity in Porous and Glassy Solid Oxides

Abstract

A model for the alkali metal bonding energy in some solid oxides based on a simplified approach of the electronegativity equalization method is proposed. From this model, no intensive numerical calculation is required. The bonding energy can be written as a linear combination of three independent energy contributions, namely, an electrostatic term, a covalent term, and a polarization term, that are expressed as a function of the alkali metal cation radius. The model is then used to fit the evolution of the activation energy for the dc conductivity with the various alkali metal cations in the following test cases: a clay mineral (montmorillonite), two zeolites (faujasites Fau-X and Fau-Y), three glassy oxides (a silicate with two alkali contents and a triborate). Two typical behaviors can be distinguished: (i) the considered solid oxide is either porous or supposed to have enough free volume for the displacement of the cation, in which case the model can be applied, and (ii) the compactness of the structure is such that the analytical expression for activation energy for the dc conductivity contains an additional term, in which case the model cannot be directly used. However, if this additional energy is known, then it is possible to estimate the alkali metal bonding energy and to properly reproduce it from the model. Finally, it is shown that, when the model is applicable, it yields relevant information about both the nature of the alkali metal bond and the microscopic mechanism involved in the dc conductivity. This information is in nice agreement with what we know about the structure of the considered solid oxides.