The size effect of alkali ions on structural variations in layered silicate materials

Abstract

Computer modelling techniques were used to investigate the structures and stabilities of a range of mixed alkali silicate materials with formula KLi_(1−x)M_xSi₂O₅, K_(1−x)M_xLiSi₂O₅, where M = Na, Rb, Cs, x = 0, 0.25, 0.5, 0.75, 1.0 and K_(1−x)M_xSi₂O₅ (M = Li, Na, Rb, Cs), Li_(1−x)M_xSi₂O₅ (M = Na, K, Rb, Cs), where x = 0, 0.25, 0.5, 0.75, 1.0 as well as the materials formed by substituting the lithium ions in the Li₂Si₂O₅ structure with a set of guest alkali ions, that is, Li_2(1−x)M_2xSi₂O₅, where M = Na, K, Rb, Cs and x = 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, 1.0. A newly developed computer program based on symmetry arguments is employed to identify equivalent configurations and hence eliminate unnecessary duplication of calculations, although many hundreds of calculations still needed to be carried out to simulate the complete range of different configurations for the mixed compounds. The results of our calculations show that in the wide range of concentrations investigated, the mixed alkali solid solutions based on KLiSi₂O₅ always retain the original structure of the disilicate material when the composition was varied, with six-membered rings of silica tetrahedra linked by the alkali ions to form pseudo-continuous channels throughout the structure. On the other hand, the replacement of lithium ions by bigger alkali ions in the Li₂Si₂O₅ material causes significant distortion of the original structure, and the symmetric chair formation of the material is lost, although the ring structure of the silicate sheet remains. Although solid solutions with different cations are often thermodynamically favourable, calculations of ion exchange reactions from aqueous solution show that only K–Na exchange would occur to any significant extent.