The chemical bonding of main-group MgAgAs-type compounds is analyzed with quantum chemical direct-space techniques. A new bonding concept is developed that unites the former ionic bonding and polyanionic network models. Polar and nonpolar contributions to the bonding are extracted by the combined analysis of electron density and electron localizability. A direct-space representation of the 8 - N rule is introduced. In this approach, the anions' heteropolar bonds are treated as a superposition of covalent (nonpolar) and lone-pair closed-shell (polar) contributions. The relation between covalent (nonpolar) and lone-pair (polar) character is obtained with the ELI-D/QTAIM basin intersection technique. This ratio depends on the constituting elements. On basis of this approach, MgAgAs-type compounds are compared with Zintl phases, where covalent bonds and lone pairs are spatially separated.
MgAgAs-type "half-Heusler" compounds are known to realize two out of three possible atomic arrangements of this structure type. The number of transition metal components typically determines which of the alternatives is favored. On the basis of DFT calculations for all three variants of 20 eight- and eighteen-valence-electron compounds, the experimentally observed structural variant was found to be determined by basically two different bonding patterns. They are quantified by employing two complementary position-space bonding measures. The Madelung energy E((M)(QTAIM)) calculated with the QTAIM effective charges reflects contributions of the ionic interactions to the total energy. The sum of nearest-neighbor delocalization indices ςnn characterizes the covalent interactions through electron sharing. With the aid of these quantities, the energetic sequence of the three atomic arrangements for each compound is rationalized. The resulting systematic is used to predict a scenario in which an untypical atomic arrangement becomes most favorable.
Chemical bonding models are one of the most powerful tools in chemistry and provide essential guidance in the understanding of composition and structure of chemical compounds, as well as in the development of new preparation routes. Facing the tremendous diversity of crystal structures and properties of intermetallic compounds, it is highly desirable to make the predictive power of chemical bonding models also available for this field of inorganic chemistry. Within the framework of quantum-chemical position-space analysis the concept of the 8 - N rule is recovered and extended for a consistent and quantitative treatment of heteropolar bonding situations as in compounds of the MgAgAs type and their relatives. A first evaluation of the predictive capabilities of the position-space view is obtained in the analysis of 51 zinc-blende, wurtzite and rock-salt-type compounds. An outlook on future investigations is given and modifications of main-group elements and (pseudo) main-group compound families are classified within the presented model framework.
The chemical bonding of transition metal compounds with a MgAgAs-type of crystal structure is analyzed with quantum chemical position-space techniques. The observed trends in QTAIM Madelung energy and nearest neighbor electron sharing explain the occurrence of recently synthesized MgAgAs-type compounds, TiPtGe and TaIrGe, at the boundary to the TiNiSi-type crystal structure. These bonding indicators are used to identify favorable element combinations for new MgAgAs-type compounds. The new phases-the high-temperature VIrGe and the low-temperature HfPtGe-showing this type of crystal structure are prepared and characterized by powder X-ray diffraction and differential thermal analysis.
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