Investigation of the mesostructure of transition-metal monogermanides synthesized under pressure

The mesostructure of transition-metal monogermanides Mn$_{1-x}$Co$_{x}$Ge is studied by small-angle neutron scattering in a wide range of concentrations $x$ = 0–0.95. These compounds were synthesized under high pressure and are metastable under normal conditions. The experimental dependences $I(Q)$ obtained for the whole series of samples in the range of transferred momenta (6 $\times$ 10$^{-2}$ nm$^{-1}<Q<$ 2.5 nm$^{-1}$) are described by the power dependence $Q^{-n}$ with an exponent $n$ = 2.99 $\pm$ 0.02, uniquely related to the fractal properties of the system under study. The dependence obtained indicates that the superatomic structure of the samples is characterized by the presence of defects with a spatial organization described by a fractal model with a logarithmic dependence of the correlation function of the defect density. It is interesting to note that such defects are absent in the isostructural FeGe compound, i.e., the experimental dependences of the intensity are described well by the expression $Q^{-n}$ with an exponent $n$ = 4.1 $\pm$ 0.1, which demonstrates the presence of crystallites with a uniform density distribution inside and a sharp boundary characterizing the surface.