Point-defect-induced nucleation of theωphase

The coherent, elastic-diffuse neutron scattering from a ${\mathrm{NbO}}_{0.020}$ and a ${\mathrm{NbO}}_{0.009}$ single crystal has been determined experimentally in absolute units. We demonstrate that the lattice distortions around one single oxygen impurity give rise to an intensity map which has the same qualitative features as observed in ${\mathrm{Zr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Nb}}_{\mathrm{x}}$ alloys. Model calculations for the diffuse scattering have been performed within the framework of Kanzaki forces. It is shown that a so-called ‘‘3f ’’ force field with one free parameter is able to reproduce the absolute values of the entire observed scattering pattern. The complementary interpretation of the experimental data as short-range-order scattering demands defect-induced ω-phase embryos which have a volume of approximately ten unit cells. A real-space interrelation between the Kanzaki force model and the picture of defect-induced, local domains of ω phase could be achieved by demonstrating that the local, defect-induced lattice distortions which are associated with the proposed Kanzaki force field contain static fluctuations of ω phase. It is shown that the attraction of the second-nearest-neighbor Nb shell around the oxygen atom is directly correlated to the structure of the locally condensed phase.