The extended Zintl–Klemm concept, ionic strength I and assessment of the relative stability of lattices using the stability enhancement ratio S

This article examines the comparison between the classical formulations used to describe silicates and that derived from the application of the extended Zintl–Klemm concept (EZKC). The ionic strength, I, for 25 silicate lattices is calculated taking into account both formulations, and the results show that, in every single one of the examples, the ionic strength of the Zintl polyanion is higher than that of the classical model which assigns a formal charge of 4+ for silicon. Our earlier study, firstly applied to the germanate (NH₄)₂Ge^[6][Ge^[4]₆O₁₅] [Vegas & Jenkins (2017). Acta Cryst. B73, 94–100] and to the polyanion [Ge^[4]₆O₁₅]⁶⁻ equivalent to the pseudo-As₂O₅ derived from it, explained satisfactorily the charge transfer that takes place in the Zintl compounds. The value of I = ½∑n_iz_i² for the Zintl polyanion was greater than for the compound as formulated in the classical way. In that article, a meaningful relationship was found between the electron transfers as defined by the EZKC and the ionic strength I of the anion [Ge^[4]₆O₁₅]⁶⁻ ≡ Ψ-As₂O₅. Because the ionic strength, I, of a lattice is directly proportional to the lattice potential energy, U_POT, the higher the I the greater the U_POT; thus it is harder to break up the lattice into its constituent ions and hence the lattice itself is more stable, giving support to the idea that the application of the EZKC and the resulting electron shifts yields structures which are inherently thermodynamically more stable than the starting configuration.