Revealing Tendencies in the Electronic Structures of Polar Intermetallic Compounds

The quest for solid-state materials with tailored chemical and physical features stimulates the search for general prescriptions to recognize and forecast their electronic structures providing valuable information about the experimentally determined bulk properties at the atomic scale. Although the concepts first introduced by Zintl and Hume–Rothery help to understand and forecast the bonding motifs in several intermetallic compounds, there is an emerging group of compounds dubbed as polar intermetallic phases whose electronic structures cannot be categorized by the aforementioned conceptions. These polar intermetallic compounds can be divided into two categories based on the building units in their crystal structures and the expected charge distributions between their components. On the one hand, there are polar intermetallic compounds composed of polycationic clusters surrounded by anionic ligands, while, on the other hand, the crystal structures of other polar intermetallic compounds comprise polyanionic units combined with monoatomic cations. In this review, we present the quantum chemical techniques to gain access to the electronic structures of polar intermetallic compounds, evaluate certain trends from a survey of the electronic structures of diverse polar intermetallic compounds, and show options based on quantum chemical approaches to predict the properties of such materials

The Mineral Stützite: a Zintl-Phase or Polar Intermetallic? A Case Study Using Experimental and Quantum-Chemical Techniques

Differing reports regarding the structural features of the mineral stützite, Ag_5–<i>x</i>Te₃ (−0.25 ≤ <i>x</i> ≤ 1.44), and the quest for tellurides with low-dimensional fragments stimulated our impetus to review this system by employing experimental as well as quantum-chemical methods. Determination of the crystal structures for three samples with compositions Ag_4.72(3)Te₃ (<b>I</b>), Ag_4.66(1)Te₃ (<b>II</b>), and Ag_4.96(2)Te₃ (<b>III</b>) revealed considerable positional disorders for the Ag and Te sites and previously unknown structure models for <b>I</b> and <b>II</b>, which differ from that of <b>III</b> through the stacking sequences of honeycomb-fashioned Te layers. The crystal structures comprise [Te@Ag₉]@Te₁₄ units in the forms of bicapped hexagonal Te antiprisms that enclose Te-centered tricapped trigonal Ag prisms, while each Te atom is encapsulated by Ag atoms assembling diverse types of coordination polyhedra. The vibrational and electronic properties were determined for three models approximating the actual crystal structure of stützite by means of techniques based on first principles. From analyses of the electronic structures and projected crystal orbital Hamilton populations (pCOHP), it is clear that the amounts and distributions of the Ag atoms within the Te network should be influenced by the subtle interplay between the attempts to achieve an electronically favorable situation with a gap at <i>E</i>_F and minimize the occupations of antibonding states

Revealing the Nature of Chemical Bonding in an ALn2Ag3Te5-Type Alkaline-Metal (A) Lanthanide (Ln) Silver Telluride

Although the electronic structures of several tellurides have been recognized by applying the Zintl-Klemm concept, there are also tellurides whose electronic structures cannot be understood by applications of the aforementioned idea. To probe the appropriateness of the valence-electron transfers as implied by Zintl-Klemm treatments of ALn2Ag3Te5-type tellurides (A = alkaline-metal; Ln = lanthanide), the electronic structure and, furthermore, the bonding situation was prototypically explored for RbPr2Ag3Te5. The crystal structure of that type of telluride is discussed for the examples of RbLn2Ag3Te5 (Ln = Pr, Nd), and it is composed of tunnels which are assembled by the tellurium atoms and enclose the rubidium, lanthanide, and silver atoms, respectively. Even though a Zintl-Klemm treatment of RbPr2Ag3Te5 results in an (electron-precise) valence-electron distribution of (Rb+)(Pr3+)2(Ag+)3(Te2&minus;)5, the bonding analysis based on quantum-chemical means indicates that a full electron transfer as suggested by the Zintl-Klemm approach should be considered with concern