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Abstract:
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 zeta(nn)
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.
