Martensitic Transition in the A15 Superconductor Ti₃Ir

Transport, thermodynamic, XRD and high-pressure measurements on high-quality Ti₃Ir samples reveal a phase transition around T* = 90 K which we identify as a Martensitic transition leading from the cubic to a tetragonal structure with c/a <1. Next to V₃Si and Nb₃Sn_1-xSb_x, Ti₃Ir is only the third material among all the A15 superconductors displaying a clearly resolved martensitic transition at a unprecedently high temperature. With the application of high pressure, T_c increases while T* decreases. Thus, Ti₃Ir does not follow the correlation between pressure derivatives and tetragonal distortion seen for Nb₃Sn and V₃Si. Furthermore, with pressure the resistivity decreases at temperatures above T* while below it is essentially pressure independent. We attribute this unusual behavior to the strong, pressure dependent coupling between electrons and a G₃ optical phonon. Above T*, pressure induced stiffening of the G₃ phonon reduces the electron-phonon coupling and the resistivity. Below T*, this phonon freezes out causing the dimerization along the Ti-chains, a concomitant reduction of the over-all resistivity and the removal of the source of pressure dependence.