Dresden 2006 – scientific programme
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MA: Magnetismus
MA 14: Magnetic Thin Films II
MA 14.2: Talk
Tuesday, March 28, 2006, 10:30–10:45, HSZ 03
Covalent bonding and the nature of band gaps in some half-Heusler compounds — •Claudia Felser — Johannes Gutenberg - Universität, 55099 Mainz, Germany
Half-Heusler compounds XYZ, also called semi-Heusler compounds, crystallize in the MgAgAs structure belonging to the space group F 4 3m. A systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18- electron half-Heusler compounds through first-principles density functional calculations is reported. The most appropriate description of these compounds - from the viewpoint of electronic structures - is found from a YZ zincblende lattice stuffed by an X ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN may be written Li+ + (MgN)−, and (MgN)*, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds may similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti4+ + (CoSb)4−; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and 18-electron compounds, when X is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of Y and Z. What is particularly exciting is that this simple idea of a covalently bonded YZ lattice can also be extended to the very important magnetic half-Heusler phases; these are described as valence compounds, but only in one spin direction. The local moment in these magnetic compounds resides on the X site.