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: CaF 2 , rock-salt, and CsCl-type phases of CaX (X = O, S, Se, Te), BaSnO 3 , K 2 S, 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. INTRODUCTIONThe literature on the theories and formalisms describing chemical bonding in inorganic crystal structures is very extensive, 1−6 and the models can typically be classified into either classical or quantum types. Among them, the approach of Pauling has been the paradigm for describing and rationalizing the crystal structures of ionic compounds over the last century. 7,8 The limitations of the ionic model, which have also led to a number of misconceptions about the crystal structure and the bonding network, were discussed by O'Keeffe and Hyde using alternative approaches. 9,10 These authors put the emphasis on the description of the structures of oxides as oxygen-stuffed alloys, since their cationic sublattices adopt the structures of either elements or simple alloys. Interestingly, this concept can also be applied to the naked metallic structure if the valence electrons localized in the empty spaces of the structure are conceived as coreless 49 pseudoanions. In fact, the term electride was introduced af...

An intensive research work was carried out in the frame of a RFCS (Research Found for Coal and Steel) project, to investigate the influence of different deep cryogenic treatments (DCT) on hardness, strength, toughness and wear resistance of AISI M2. Short and a long classical DCT, providing the soaking at temperature close to the boiling point of liquid nitrogen (−196 °C) for 6 h and 20 h, respectively, were carried out prior to and after tempering. Furthermore, a third short DC route, providing temperature cycling between RT and low temperature was also considered. Care was taken to avoid stabilization of retained austenite or self tempering due to wait at room temperature prior to DCT and/or tempering. All treatments were calibrated to get 840 HV 10, in order to compare the properties of steel with the same reference hardness. DCT does not allow the complete transformation of retained austenite in the investigated high speed steel, due to the stabilizing effect of alloying elements. Tempering is necessary to completely transform this phase and to allow proper secondary hardening. If carried out after quenching, DCT shifts the secondary hardness peak to lower temperature, evidencing the need to adjust the tempering parameters to avoid overtempering. The microstructure didn't show any significant influence of DCT in terms of carbides distribution, due to the conditioning of martensite at low temperature. The same can be also concluded for the other properties (toughness, tensile strength and wear resistance), which are practically the same for samples having the same hardness.

Tw od istinct points on the potential energy curve (PEC) of apairwise interaction, the zero-energy crossing point and the point where the stretching force constant vanishes, allowu st oa nticipate the range of possible distances between two atoms in diatomic,m olecular moieties and crystalline systems.W es howt hat these bond-stability boundaries are unambiguously defined and correlate with topological descriptors of electron-density-based scalar fields,a nd can be calculated using generic PECs.C hemical databases and quantum-mechanical calculations are used to analyzeafull set of diatomic bonds of atoms from the s-p main block. Emphasis is placed on the effect of substituents in C À C covalent bonds,c oncluding that distances shorter than 1.14 or longer than 2.0 are unlikely to be achieved, in agreement with ultra-high-pressure data and transition-state distances, respectively.Presumed exceptions are used to place our model in the correct framework and to formulate ac onjecture for chained interactions,w hicho ffers an explanation for the multimodal histogram of O À Hdistances reported for hundreds of chemical systems.

Bond and lone pairs are identified by the Chemical Pressure formalism providing correlations between ligand electronegativity and molecular activity within the VSEPR-LCP model.