Effect of next-nearest neighbour interaction on oscillator strengths in garnets

Abstract
The oscillator strength of the octahedral Fe3+6A1(t3e2) to 4A1, 4E(t3e2) transition in the garnet andradite Ca3Fe23+Si3O12, is f=3.5*10-6. In schorlomite where 25% of the tetrahedral Si4+ ions are replaced by Al3+ or Ti4+ the oscillator strength is an order of magnitude greater. Substitution in the tetrahedral site destroys the inversion symmetry at the next-nearest neighbour octahedral site, and it is suggested that this primarily accounts for the observed increased absorption intensity for schorlomite.
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