  We have carried out a detailed study of the chemical bonding for two
room-temperature stable platinum silicide phases, tetragonal alpha-Pt_2Si and
orthorhombic PtSi. An analysis of the valence electronic charge density reveals
surprising evidence of covalent three-center bonds in both silicide phases, as
well as two-dimensional metallic sheets in alpha-Pt_2Si. These elements of the
bonding are further analyzed by constructing valence force field models using
the results from recent first principles calculations of the six (nine)
independent, non-zero elastic constants of alpha-Pt_2Si (PtSi). The resulting
volume-, radial-, and angular-dependent force constants provide insight into
the relative strength of various bonding elements as well as the trends
observed in the elastic constants themselves. The valence force field analysis
yields quantitative information about the nature of the chemical bonding which
is not easily discernable from the more qualitative charge density plots. More
generally, this study demonstrates that the detailed variations in the elastic
constants of a material contain useful information about the chemical bonds
which can be extracted using valence force field models. Inversely, these
models also allow identification of specific elements of the chemical bonding
with particular trends in the elastic constants, both within a given material
and among a class of related materials.
