First-principles study of lattice instabilities in the ferromagnetic martensite Ni2_2MnGa

The phonon dispersion relations and elastic constants for ferromagnetic Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and symmetry-related directions displays a dynamical instability at a wavevector that depends on $c/a$. Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic phase the computed tetragonal shear elastic constant, C$^\prime$=(C$_{11}-$C$_{12}$)/2, is close to zero, indicating a marginal elastic instability towards a uniform tetragonal distortion. We conclude that the cubic Heusler structure is unstable against a family of energy-lowering distortions produced by the coupling between a uniform tetragonal distortion and the corresponding $[110]$ modulation. The computed relation between the $c/a$ ratio and the modulation wavevector is in excellent agreement with structural data on the premartensitic ($c/a$ = 1) and martensitic ($c/a$ = 0.94) phases of Ni$_2$MnGa.Comment: submitted to Phys. Rev.

GEOMETRICAL DISTINCTIONS OF FERMI SURFACE AND PRE-MARTENSITE PHENOMENA IN beta -ALLOYS WITH THERMOELASTIC MARTENSITE TRANSFORMATIONS

There the study objects are Cu-Zn, Ni-Al, TiNi alloys and others, which experience the thermoelastic martensite transformations. The study purpose is to ascertain the connection between the distinctions of electronic structure and the pre-martensite phenomena in the alloys with the thermoelastic martensite transformations. The substantial role of geometry of Fermi surface in the structurel instability of systems under study, which is exhibited by the moderation of phonon frequencies and of elastic modules, by the forming of intermediate and martensite structures, has been brought out. The obtained results may be used for the interpretation of pre-martensite phenomena, for the performance of designs of first principle for the phonon spectra, for the construction of microscopic theories of martensite transitionsAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio