Stacking solitons inω-phase systems and quasielastic scattering

The unusual x-ray diffuse scattering in Zr-Nb alloys is explained in terms of lattice displacements described by "stacking solitons." These constitute a structural defect in columnar $\omega$-phase regions embedded in a bcc matrix. We model this situation as a one-dimensional system in which a discrepancy between the $\omega$ and bcc lattice constants drives the formation of stacking solitons. The density of these solitons depends on composition and temperature; in particular we suggest a commensurate-incommensurate transition at ∼17-wt.% Nb, so that the ground state of the higher-percent Nb alloys contains a static array of stacking solitons along the $\omega$-phase column. The dynamics of such configurations is consistent with the very narrow (quasielastic) inelastic scattering observed in these alloys and has the character of a "central peak."