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.

Polymer composites possess an integrated combination of structures and properties associated with the host matrix and the fiber material and thus hold the potential of being high-strength materials. In general, the load transfer from the matrix to the fiber depends upon the strength of bonding at the interface, which characterizes the mechanical strength. In this work, first-principles calculations based on the density functional theory are employed to provide the molecular-level description of the interface formed by resins (i.e., diglycidyl ether of bisphenol A (DGEBA) and 4′-bismaleimidodiphenylmethane (BMPM)) or hardeners (i.e., diethyl toluene diamine (DETDA) and o,o′-diallyl bisphenol A (DABPA)) with graphene (or boron nitride (BN) monolayer). The results show that the interaction strength between a resin (or hardener) and graphene is mainly governed by the nature of bonding at the interface, and subsequently, the mechanical response follows the hierarchical order of the interaction strength at the interface; the transverse stiffness of BMPM/graphene is higher than that of DGEBA/graphene. Moreover, the change in the polarity of the surface from graphene to the BN monolayer improves the superior interfacial strength and thereby a higher transverse stiffness of both resin and hardener composites at the molecular level. These results emphasize the need to use computational modeling to efficiently and accurately determine molecular-level polymer/surface combinations that yield optimal mechanical performance of composite materials. This is especially important in the design and development of high-performance composites with nanoscale reinforcement.

ELF superbasins (dark-yellow stick and ball circuits) reveal how borate clusters (green and red spheres) play the role of crystal defects interrupting the electric conductivity of metallic silver (grey spheres).

CeScO 3 is a promising perovskite-type material that presents the characteristic to remain highly stable upon compression, contrary to other perovskite compounds that often undergo phase transformations under pressure. In contrast with the structural behavior of CeScO 3 , the influence of pressure on other of its physical properties, such as electronic, vibrational, atomic, and polyhedral bulk and elastic properties, is still unknown. In this work, we propose to fill this gap by a combination of computational quantum-mechanics methodologies based on density-functional theory (DFT) and high-pressure Raman spectroscopy experiments. In particular, the influence of pressure in the crystal structure has been studied, up to 40 GPa, and compared with previous experiments showing that DFT properly describes the changes induced by pressure in CeScO 3 . Calculations have also been used to obtain phonon frequencies and their pressure dependence and to propose a mode-symmetry assignment. From Raman experiments, we have obtained the frequency and pressure dependence of lattice vibrations involving changes in polarizability, validating phonon calculations, which give not only Raman-active but also infrared-active and silent modes. In addition, phonon-dispersion and elastic-constant calculations are consistent with the structural stability of an orthorhombic perovskite-type up to 40 GPa. Finally, we provide a description of the electronic band structure, showing that CeScO 3 has a much smaller band gap than other scandates because of the role of Ce f-electrons. Such electrons also cause a closing of the electronic band gap under compression.

Understanding the stability limit of crystalline materials under variable tensile stress conditions is of capital interest for technological applications. In this study, we present results from first-principles density functional theory calculations that quantitatively account for the response of selected covalent and layered materials to general stress conditions. In particular, we have evaluated the ideal strength along the main crystallographic directions of 3C and 2H polytypes of SiC, hexagonal ABA stacking of graphite and 2H-MoS2. Transverse superimposed stress on the tensile stress was taken into account in order to evaluate how the critical strength is affected by these multi-load conditions. In general, increasing transverse stress from negative to positive values leads to the expected decreasing of the critical strength. Few exceptions found in the compressive stress region correlate with the trends in the density of bonds along the directions with the unexpected behavior. In addition, we propose a modified spinodal equation of state able to accurately describe the calculated stress–strain curves. This analytical function is of general use and can also be applied to experimental data anticipating critical strengths and strain values, and for providing information on the energy stored in tensile stress processes.