On the Bindings in the Mixture PtSnM

As an example from amore extensive investigation, the binding analysis of the mixture PtSnM (M = undetermined mole number) is described. A phase becomes stable when a lattice-like correlation of the valence electrons (with cell b) and a lattice-like correlation of the peripheral core electrons of Pt (cell e) are in an energetically favourable commensurability to the crystal cell a and to one another. Surprisingly PtsSn (of the CU3Au type) is not isodesmic to Cu, as with the Pt'" electrons the Sn!" electrons are not in correlation; the binding may be of the FF2 type which is also met in brass-Iike phases. For PtSn (NiAs type) a binding homeotypic to FF2 is found in which a strongly compressed em correlation exhibits good commensurability. In Pt2Sn3 (homeotype of NiAs) the eCH correlation is relaxed. In PtSn2(CaF2 type) the FU2 binding exists and in PtSN4 (homeotype of CaF2) the F'BZ binding, i. e. a FB2 binding with a noticeable amount of Hund insertion in the b correlation. It becomes apparent that the analysis of the binding is a new and effective instrument for developing structure systematics. The bindings classify Pt3Sn to the brass-Iike phases, PtSn and Pt2Sn3 to the NiAs-like phases, and PtSn2 and PtSn4 to the Si-like phases. The knowledge of the bindings provides arguments which explain energetically the found crystal structures and afford new viewpoints for isotypism and heterotypism of homologic or quasi-homologic phases. As an example the phases of the mixture PtPbM are briefly considered.