The Relationship Between the Martensitic Phase Transition and the Superconducting Properties of A15 Compounds

Abstract

Since the discovery by Batterman and Barrett^l in 1964 of the 21 K structural transition in V₃Si, soon followed by the discovery of a similar transition in Nb₃Sn,² there has been intensive investigation, both theoretical and experimental, of its nature, particularly of its relationship to electronic structure and its coupling to lattice distortions and phonons. [This work is summarized in two excellent reviews by Weger and Goldberg³ and by Allen.⁴] The transition consists of a tetragonal distortion of the originally cubic unit cell (a shear strain distortion) accompanied by a dimerization of the atoms along the transition metal chains (an optic mode distortion); it is usually called the “martensitic” transition, probably because it is diffusionless, although it is more typical of soft-mode transitions⁴ than it is like “classical” martensitic phase transitions.⁵ Since both the structural transition and superconductivity in these compounds are thought to be a consequence of strong electron-lattice coupling, it was thought that an understanding of the relatively simpler structural transition might provide insight into the origin of the relatively high-temperature superconducting transition in intermetallic compounds with the A15 structure. Unfortunately a definitive understanding of either the structural or the superconducting transition in these materials still remains elusive.

Work performed under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. De-AC02-76CH00016.