The theory of the structural properties of the A-15 compounds is developed starting with a picture of linear chains of atoms of the transition elements. The twofold-degenerate electronic term is shifted and split when interchain transfer is taken into account. A fine structure of the electronic spectrum and the density of states are examined. The latter has two singularities at the Fermi level corresponding to an estimated variation of the composition by several percent. Among the samples showing the martensitic transformation, one of these singularities is supposed to be responsible for the peculiar properties observed, and a theory of the transformation from the cubic to the tetragonal phase is suggested. The temperature dependence of the phonon spectrum has been studied in detail. The theory explains the softening of the phonon modes at large wave vectors in some directions, and predicts features of the phonon spectrum whose observation could be crucial to the theory.

On the theory of the structural properties of A-15 type materials

Dynamics of anharmonic lattices: Solitons and the central-peak problem in one dimension

Degradation universality and fröhlich lattice instability of A-15 superconductors

Influence of chemical composition within the range of homogeneity on phase transition and transition temperature of V3Si single crystals. II. On the defect structure of V3Si single crystals

In an attempt to understand the structure of high T c Nb 3 Ge films we have utilized tunneling, x-ray diffraction and transmission electron diffraction. Tunneling experiments indicate that such films have an appreciable highly disordered component. In x-ray diffraction, only two anomalous diffraction peaks are observed which are not sufficient to identify the unknown phase. The complex electron diffraction patterns are interpreted most easily as evidence for superlattice formation. Finally we show that large lattice expansions and depressions of T c are produced when Nb 3 Ge films are exposed to hydrofluoric acid, presumably due to inclusion of hydrogen.

S.T. Chui

Papers

Structural anomalies in cubic (A15) V$sub 3$Si and Nb$sub 3$Sn