Stress effects on the anisotropic thermal expansions of "martensitic" A15 compounds

Precision capacitance dilatometry provides a sensitive measure of the therm al strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V3Si and Nb3Sn, are well known to undergo distortion from their cubic structures at room temperatur e to tetragonal structures (c/a > 1 for V3Si and c/a < 1 for Nb3Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour rec orded for these materials has been attributed to a strongly anharmonic latt ice potential manifesting itself in unusually high, and strongly temperatur e-dependent, Gruneisen parameters. Further studies on polycrystalline mater ial revealed this anomalous expansion to be highly anisotropic at temperatu res for which, according to conventional diffraction data, the materials ar e cubic. This behaviour was linked to control of sample morphology by a res idual stress field resulting from sample preparation. ore recent experiments, in which the transformation morphology has been con trolled by the application of external stresses to single crystal V3Si and polycrystalline samples of Nb3Sn and Nb-3(Sn1-xSbx), have confirmed the occ urrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation. We link this departure from cubic symmetry with the well-known soft-mode ch aracter of these materials and the associated "central peak" scattering whi ch is also observed well above the transformation temperature. We are led t o propose that the "central peak" is the precursor to a Bragg reflection fo r the transformation structure. This coincidence between "central peak" sca ttering and the reciprocal lattice for the transformed phase in Ti-Ni has b een termed a "ghost lattice".