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 4A2g → 4T2g band of Cr3+ in the electronic absorption spectra, EAS, of a series of 13 natural grossular-uvarovite garnets with compositions close to the binary join Ca3(Al1– x Cr x )2Si3O12, depend on the chromium fraction xCr 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, R̅ (Cr–O), [Å] local with the result that

R̅ (Cr–O) local = 0.01262 · x Cr + 1.9812 (r = 0.982) (a)

Equation (a) is in very good agreement with mean 〈Al/Cr—O〉; distances of “individual” octahedra R̅i calculated for the case XCr(individual) = 1.00 from the distance relations obtained in X-ray structure refinements of low symmetry chromian garnets (Wildner and Andrut, 2001). The proves again, that the EAS-method to derive local 3d N-ion–oxygen distances is reliable.

R̅ (Cr–O) local = ƒ(x Cr) and the corresponding R̅ i-function deviate strongly from the Vegard line of the crystal averaged function R̅ (Cr–O) average = ƒ(x Cr), the “virtual crystal model”. This indicates significant structural relaxation around Cr3+[6] in the garnet solid solutions studied. The relaxation coefficient calculated from the data, ∊ lim x Cr→0 = 0.82, is closer to ∊ = 1.0 for full relaxation, i.e. the “hard sphere model”, than to ∊ = 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 (CaO8) polyhedra interconnected with the octahedra of the grossular-uvarovite garnets.