Chemical pressure-chemical knowledge: squeezing bonds and lone pairs within the valence shell electron pair repulsion model

Stability, Chemical Bonding, and Electron Lone Pair Localization in AsN at High Pressure by Density Functional Theory Calculations

The covalent bonding framework of crystalline single-bonded cubic AsN, recently synthesized under high pressure and high temperature conditions in a laser-heated diamond anvil cell, is here studied by means of density functional theory calculations and compared to single crystal X-ray diffraction data. The precise localization of the nonbonding electron lone pairs and the determination of their distances and orientations are related to the presence of characteristic structural motifs and space regions of the unit cell dominated by repulsive electronic interactions, with the relative orientation of the electron lone pairs playing a key role in minimizing the energy of the structure. We find that the vibrational modes associated with the expression of the lone pairs are strongly localized, an observation that may have implications for the thermal conductivity of the compound. The results indicate the thermodynamic stability of the experimentally observed structure of AsN above ∼17 GPa, provide a detailed insight into the nature of the chemical bonding network underlying the formation of this compound, and open new perspectives to the design and high pressure synthesis of new pnictogen-based advanced materials for potential applications of energetic and technological relevance.

A Chemical-Pressure-Induced Phase Transition Controlled by Lone Electron Pair Activity

The chemical pressure approach offers a new paradigm for property control in functional materials. In this work, we disclose a correlation between the β → α pressure-induced phase transition in SnMoO₄ and the substitution process of Mo⁶⁺ by W⁶⁺ in SnMo_1-xW_xO₄ solid solutions (x = 0-1). Special attention is paid to discriminating the role of the lone pair Sn²⁺ cation from the structural distortive effect along the Mo/W substitution process, which is crucial to disentangle the driven force of the transition phase. Furthermore, the reverse α → β transition observed at high temperature in SnWO₄ is rationalized on the same basis as a negative pressure effect associated with a decreasing of W⁶⁺ percentage in the solid solution. This work opens a versatile chemical approach in which the types of interactions along the formation of solid solutions are clearly differentiated and can also be used to tune their properties, providing opportunities for the development of new materials.

Chemical pressure in functional materials

Chemical pressure, a strange but familiar concept, is a lattice internal force caused by lattice strain with chemical modifications and arouses great interest due to its diversity and efficiency to synthesize new compounds and tune functional materials. Different from physical pressure loaded by an external force that is positive, chemical pressure can be either positive or negative (contract a lattice or expand it), often through flexible and mild chemical synthesis strategies, which are particularly important as a degree of freedom to manipulate material behaviors. In this tutorial review, we summarize the features of chemical pressure as a methodology and demonstrate its role in synthesizing and discovering some typical magnetically, electrically, and thermally responsive functional materials. The measure of chemical pressure using experimental lattice strain and elastic modulus was proposed, which can be used for quantitative descriptions of the correlation between lattice distortion and properties. From a lattice strain point of view, we classify chemical pressure into different categories: (i) chemical substitution, (ii) chemical intercalation/de-intercalation, (iii) size effect, and (iv) interface constraint, etc. Chemical pressure affects chemical bonding and rationalizes the crystal structure by modifying the electronic structure of solids, regulating the lattice symmetry, local structure, phonon structure effects etc., emerging as a general and effective method for synthesizing new compounds and tuning functional materials.

Metavalent bonding in chalcogenides: DFT-chemical pressure approach

Understanding the chemical bond nature has attracted considerable attention as it is crucial to analyze and comprehend the different physical and chemical properties of materials. This work is considered a complementary part of our previous work in studying the nature of different types of bonding interactions in a wide variety of molecules and materials using the DFT Chemical Pressure (CP) approach. Recently, a new type of chemical bond, the metavalent bond (MVB), has been defined. We show how the CP formalism can be used to analyze and study the establishment of MVB in two chalcogenides, GeSe and PbSe, in a similar fashion as the electron localization function (ELF) profiles. This is accomplished by analyzing the CP maps of these two chalcogenides at different pressures (up to 40 GPa for GeSe and 10 GPa for PbSe). The CP maps show distinctive features related to the MVB, providing insights into the existence of such chemical interaction in the crystal structure of the two compounds. Similar to ELF profiles, CP maps can visualize and track the strength of the MVB in GeSe and PbSe under pressure.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen

Chemical reactivity between As and N₂, leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single‐crystal synchrotron X‐ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P2₁3 space group, in which each of the two elements is single‐bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non‐bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Tutorial on Chemical Pressure Analysis: How Atomic Packing Drives Laves/Zintl Intergrowth in K3Au5Tl

The tight atomic packing generally exhibited by alloys and intermetallics can create the impression of their being composed of hard spheres arranged to maximize their density. As such, the atomic size factor has historically been central to explanations of the structural chemistry of these systems. However, the role atomic size plays structurally has traditionally been inferred from empirical considerations. The recently developed DFT-Chemical Pressure (CP) analysis has opened a path to investigating these effects with theory. In this article, we provide a step-by-step tutorial on the DFT-CP method for non-specialists, along with advances in the approach that broaden its applicability. A new version of the CP software package is introduced, which features an interactive system that guides the user in preparing the necessary electronic structure data and generating the CP scheme, with the results being readily visualized with a web browser (and easily incorporated into websites). For demonstration purposes, we investigate the origins of the crystal structure of K3Au5Tl, which represents an intergrowth of Laves and Zintl phase domains. Here, CP analysis reveals that the intergrowth is supported by complementary CP features of NaTl-type KTl and MgCu2-type KAu2 phases. In this way, K3Au5Tl exemplifies how CP effects can drive the merging for geometrical motifs derived from different families of intermetallics through a mechanism referred to as epitaxial stabilization.