Andradite-uvarovite solid solutions. An ab initio all-electtron quantum mechanical simulation with the CRYSTAL code

Andradite−uvarovite (Ca3Fe2Si3O12−Ca3Cr2Si3O12) solid solutions have been investigated at an ab initio quantum-mechanical level by using an all-electron Gaussian-type basis set and the hybrid B3LYP functional in its unrestricted formulation. Only ferromagnetic phases have been considered. All possible nonequivalent geometrical configurations resulting from the substitution of Cr atoms for x = 1−8 Fe atoms in the 16a site in the garnet primitive cell have been fully optimized (cell parameters and fractional coordinates of 80 atoms). As the lattice parameters of the two end-members are very similar (11.99 and 12.05 Å for uvarovite and andradite, respectively), geometry rearrangements at the various x are extremely small, the largest excess volume being 0.15 Å3 and the largest excess energy 3.68 kJ/mol. Thermodynamic functions are calculated from the configurational contribution to entropy and disregarding the vibrational contribution, which is expected to be negligible. Almost ideal miscibility is predicted.