Study of the tetragonal-to-tetragonal phase transition of theA15pseudobinary alloyNb3Sn1−xSbxby neutron and x-ray scattering

Abstract

High-momentum-resolution neutron and x-ray scatterings have been carried out on recently grown ${\mathrm{Nb}}_3{\mathrm{Sn}}_{1-x}{\mathrm{Sb}}_x$ single crystals. It was shown that, in addition to the well-known cubic-to-tetragonal martensitic phase transition at $T_M$, another phase transition takes place at $T_t$ between two tetragonal phases with the identical space group $D_{4h}^9$ but with opposite tetragonal distortions $a>c$ and $a<c$. The transition at $T_t$ in the Sb concentration range $0.05\lesssim x\lesssim 0.15$ was found to be of the first order. In addition, the magnitude and direction of the sublattice displacements of the Nb atom were determined in both tetragonal phases. The Nb atom shifts so as to avoid the closer contact with its four neighboring Sn or Sb atoms. The tetragonal-to-tetragonal transition observed in the present system offers a rare example in which no orthorhombic phase exists between two tetragonal phases with opposite distortions. Conditions for the absence of an intermediate phase are discussed by Axe and Yamada.