A first record of strong structural relaxation of TO (sub 4) tetrahedra in a spinel solid solution

Optical absorption spectroscopy and X-ray structural refinements of seven different spinel single crystals on the (Mg (sub 1-x) Mn (sub x) )Al (sub 2) O (sub 4) solid solution (x=0.02-1.00) evidences exceptionally strong relaxation (epsilon =0.83) of (super IV) Mn (super 2+) -O bonds. Our single-crystal structure refinements demonstrate that the ideal (super IV) Mn (super 2+) -O bond distance in fully ordered galaxite (MnAl (sub 2) O (sub 4) ) should be 2.050 Aa, which is 0.014 Aa longer than previously suggested, and that structural parameters are mainly affected by the variations occurring at the TO (sub 4) tetrahedron. The very strong structural relaxation observed around the T site may be explained by the fact that the TO (sub 4) polyhedra of the spinel structure share only corners with neighboring MO (sub 6) octahedra and are fully isolated from neighboring TO (sub 4) tetrahedra. This provides structural flexibility around the T site and allows for considerable local T-O bond distance variations.