Short-range chemical environment versus long-range chemical homogeneity analyses in high-entropy transition metal AlB2-type diboride solid solutions

Calculations on high-entropy diborides bring to speculate about the effects of lattice distortion on properties such as hardness, but none have provided any support for their assertions about strain within the unit cells. At the same time, studies on the homogeneity of distribution of metal atoms on the lattice sites remain sparse at best, with computational investigations suggesting segregation of some species to grain boundaries. We carried out an extensive and systematic study based on high-resolution synchrotron diffraction (HR-SD) and extended X-ray absorption fine structure analysis to test these points. Our analysis unequivocally shows that the picture of a random distribution of atoms with local strain on the d-metals site well describe the data. Additionally, a linear trend is observed between the average structure and the first neighbor distances, suggesting that any description of the properties of such materials should go beyond the simple dichotomy between long-range order and local structure. The long-range chemical homogeneity was also extensively investigated in several single-phase materials based on HR-SD and SEM-EDS: increasing lattice ?-strains resulted strongly correlated to an increasing lack of long-range chemical homogeneity. The thermal expansion study based on HR-SD analysis of a variety of high-entropy AlB2-type diboride solution solutions containing up 7 group IV-V-VI transition metals was finally done.