Generalized Stress-Redox Equivalence: A Chemical Link between Pressure and Electronegativity in Inorganic Crystals

The crystal structure of many inorganic compounds can be understood as a metallic matrix playing the role of a host lattice in which the nonmetallic atomic constituents are located, the Anions in Metallic Matrices (AMM) model stated. The power and utility of this model lie in its capacity to anticipate the actual positions of the guest atoms in inorganic crystals using only the information known from the metal lattice structure. As a pertinent test-bed for the AMM model, we choose a set of common metallic phases along with other nonconventional or more complex structures (face-centered cubic (fcc) and simple cubic Ca, CsCl-type BaSn, hP4-K, and fcc-Na) and perform density functional theory electronic structure calculations. Our topological analysis of the chemical pressure (CP) scalar field, easily derived from these standard first-principles electronic computations, reveals that CP minima appear just at the precise positions of the nonmetallic elements in typical inorganic crystals presenting the above metallic subarrays: CaF₂, rock-salt and CsCl-type phases of CaX (X = O, S, Se, Te), BaSnO₃, K₂S, and NaX (X = F, Cl, Br, I). A theoretical basis for this correlation is provided by exploring the equivalence between hydrostatic pressure and the oxidation (or reduction) effect induced by the nonmetallic element on the metal structure. Indeed, our CP analysis leads us to propose a generalized stress-redox equivalence that is able to account for the two main observed phenomena in solid inorganic compounds upon crystal formation: (i) the expansion or contraction experienced by the metal structure after hosting the nonmetallic element while its topology is maintained and (ii) the increasing or decreasing of the effective charge associated with the anions in inorganic compounds with respect to the charge already present in the interstices of the metal network. We demonstrate that a rational explanation of this rich behavior is provided by means of Pearson-Parr's electronegativity equalization principle.