On the structural relaxation around Cr3+ along binary solid solutions

The structural relaxation occurring along binary solid solutions (i.e. intended as the measure of the mismatch between average and local bond distances at a crystallographic site) was reappraised with the aim of revealing possible correlations of bulk properties with the departure of short-range from the long-range crystal structure. For this purpose, fourteen end-terms belonging to solid solutions, all characterized by Al3+-Cr3+ replacement at octahedral site, were selected from literature among those of mineralogical interest (i.e. clinopyroxene, garnet, spinel, corundum, and perovskite, using alumoniobite as a check). Along with X-ray diffraction and electronic absorption spectroscopy data used to define the structural relaxation coefficient, bulk properties were available (density and bulk modulus) or were calculated (deviation from the closest sphere packing) for each end-term. The structural relaxation coefficients exhibit a linear inverse correlation with the difference between crystal field strengths of end-members (D10Dq): the larger the deviation of local arrangement from average structure, the larger D10Dq. Evidence that the structural framework, encompassing polyhedral arrangement and anion packing, affects the structural relaxation is presented and discussed. Solid solutions based on dense structures tend to relax less than those with a less compact atomic arrangement. The greater the difference in compressibility between end-members, the more the structure will behave as predicted by Vegard’s law.