Premartensite to martensite transition and its implications for the origin of modulation inNi2MnGaferromagnetic shape-memory alloy

We present results of a temperature-dependent high-resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnGa}$. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval $(\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature, and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector $\mathbf{q}$ shows smooth analytic behavior with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phase. The existence of the incommensurate 7M-like modulated structure down to 5 K suggests that the incommensurate 7M-like modulation is the ground state of $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnGa}$ and not the Bain distorted tetragonal $\mathrm{L}{1}_{0}$ phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.