Anomalous birefringence in andradite-grossular solid solutions: a quantum-mechanical approach
Résumé
The static linear optical properties (refractive indices, birefringence and axial angle) of andradite-grossular (Ca₃Fe₂Si₃O₁₂-Ca₃Al₂Si₃O₁₂) solid solutions have been computed at the ab initio quantum-mechanical level through the Coupled Perturbed Kohn-Sham scheme, using an all-electron Gaussian-type basis set. Geometry relaxation after substitution of 1-8 Al for Fe atoms in the primitive cell of andradite yields 23 non-equivalent configurations ranging from cubic to triclinic symmetry. Refractive indices vary quite regularly between the andradite (1.860) and grossular (1.671) end-members; the birefringence d and the axial angle 2V at intermediate compositions can be as large as 0.02° and 89°, respectively. Comparison with experiments suffers from inhomogeneities and impurities of natural samples; however, semi- quantitative agreement is observed.