Local mean chromium–oxygen distances in Cr3+-centered octahedra of natural grossular-uvarovite garnet solid solutions from electronic absorption spectra

The crystal field parameter 10Dq [cm -1 ] of octahedral chromium obtained as the energy of the 4 A 2g → 4 T 2g band of Cr 3+ in the electronic absorption spectra, EAS, of a series of 13 natural grossular-uvarovite garnets with compositions close to the binary join Ca 3 (Al 1-x Cr x ) 2 Si 3 O 12 , depend on the chromium fraction x Cr of the solid solutions as 10Dq [cm -1 ] = (-515.51 .x Cr ) + 16579 (r = 0.984) The 10Dq [cm -1 ] -values were evaluated in terms of local mean octahedral distances, R local (Cr-O ), [A] with the result that R local (Cr-O) = 0.01262 . x Cr + 1.9812 (r = 0.982) (a) Equation (a) is in very good agreement with mean distances of individual octahedra R i calculated for the case X Cr(individual) = 1.00 from the distance relations obtained in X-ray structure refinements of low symmetry chromian garnets (Wildner and Andrut, 2001). This proves again, that the EAS-method to derive local 3d N -ion-oxygen distances is reliable. R local (Cr-O) = f(x Cr ) and the corresponding R i -function deviate strongly from the Vegard line of the crystal averaged function R average (Cr-O) = f(x Cr ), the virtual crystal model. This indicates significant structural relaxation around Cr 3+[6] in the garnet solid solutions studied. The relaxation coefficient calculated from the data, e lim xCr→0 = 0.82, is closer to e = 1.0 for full relaxation, i.e. the hard sphere model, than to e = 0.0 for absent relaxation in the virtual crystal model. This is proposedly related to the uptake of strain around octahedral chromium, in the large (CaO 8 ) polyhedra interconnected with the octahedra of the grossular-uvarovite garnets.