A first record of strong structural relaxation of TO4 tetrahedra in a spinel solid solution

Optical absorption spectroscopy and X-ray structural refinements of seven different spinel single crystals on the (Mg1-xMnx)Al2O4 solid solution (x = 0.02-1.00) evidences exceptionally strong relaxation (epsilon = 0.83) of Mn-IV(2+)-O bonds. Our single-crystal structure refinements demonstrate that the ideal Mn-IV(2+)-O bond distance in fully ordered galaxite (MnAl2O4) should be 2.050 angstrom, which is 0.014 angstrom longer than previously suggested, and that structural parameters are mainly affected by the variations occurring at the TO4 tetrahedron. The very strong structural relaxation observed around the T site may be explained by the fact that the TO4 polyhedra of the spinel structure share only corners with neighboring MO6 octahedra and are fully isolated from neighboring TO4 tetrahedra. This provides structural flexibility around the T site and allows for considerable local T-O bond distance variations.