Symmetry-restricted anharmonicities and the CsCl-to-7R martensitic structural phase transformation of the NixAl1−x system

Abstract

We suggest that the CsCl-to-7R structural transition undergone by ${\mathrm{Ni}}_{\mathrm{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathrm{x}}$, where 0.6≲x≲0.65, may be attributed to the anharmonic interaction between the ${\Sigma }_4${011} phonons and the homogeneous strains associated with the (1/2($c_{11}$-$c_{12}$) elastic constant. We show that the strains predicted by this theory are compatible with those of the experimentally observed 7R monoclinic unit cell. Also, we purport that the temperature dependence of the elastic constant (1/2($c_{11}$-$c_{12}$) is responsible for the very strong dependency of the martensitic start temperature on Ni concentration. Further, if above the transition the minimum of the dip of the ${\Sigma }_4$ phonon occurs at ${\xi }_0$(0,1,1), we show that, since, in principle, there must be a wave-vector dependence of the Landau coefficients appearing in the free-energy expansion, the modulation in the martensitic phase must correspond to ${\xi }^{\mathrm{*}}$(0,1,1), where (i) ${\xi }^{\mathrm{*}}$ cannot be equal to ${\xi }_0$ and (ii) ${\xi }^{\mathrm{*}}$ need not correspond to a commensurate lock-in value.