Half-Heusler goes Zintl: about segregated and hidden lone pairs - Publication selected for ACS Editors’ Choice appears in Inorganic Chemistry Journal  

April 14, 2015

The MgAgAs-type or so-called half-Heusler intermetallic phases are often discussed as 8- or 18-valence electron compounds, keeping in mind the full population of the according atomic shells. Traditionally the Half-Heusler phases of main group elements A’AE are considered as a zink-blende arrangement [AE] with atoms A’ located in the voids. In the paper of D. Bende, F. R. Wagner and Yu. Grin it is shown on the basis of position-space bonding analysis, that this focus on the covalent interactions in the [AE] partial structure is not justified for all A’AE members of the family. In general, polar bonding between the most electronegative element E and its eight (4A + 4A’) nearest neighbours has to be considered. This raises the question if and how this large coordination number can be placed into the framework of the 8–N rule and the pseudo-atom concept. It is shown that with the decomposition of a polar bond population p into a mixture (mixing ratio x) of a covalent part x*c and a “hidden lone pair part” (1-x)*lp’ a consistent treatment of main group half-Heusler phases, zink-blende phases and classical Zintl phases with homoatomic polyanions is possible within the framework of the 8–N rule. The formulated position space variant of the 8–N rule provides a new framework for analysis of non-tetrahedral partial structures with higher coordination numbers and even compounds with non-octet atomic species.


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