The synthetic Cr (super 2+) silicates BaCrSi ₄ O ₁₀ ; the missing links in the gillespite-type ABSi ₄ O ₁₀ series

Universitaet Bayreuth, Bayerisches Geoinstitut, Bayreuth, Federal Republic of Germany

The new Cr (super 2+) -containing silicate compounds BaCrSi ₄ O ₁₀ and SrCrSi ₄ O ₁₀ were synthesized both from alkali-borate fluxes and by high-T subsolidus solid-state reactions. The gillespite-type crystal structures (space group P4/ncc, Z = 4) were determined from single-crystal X-ray diffraction data. The unit-cell parameters are a = 7.4562(4), c = 15.5414(4) Aa for SrCrSi ₄ O ₁₀ , and a = 7.5314(3), c = 16.0518(4) Aa for BaCrSi ₄ O ₁₀ . Comparison with previously published data shows that A (= Ba, Sr, Ca) cation substitution in ABSi ₄ O ₁₀ gillespite-type compounds mainly affects the c lattice parameter whereas the substitution of the B (= Cu, Cr, Fe) site leads to only small changes, mainly in a. The Cr (super 2+) cation occupies a square-planar coordinated site unique in oxide crystal chemistry, with a Cr-O bond length of 1.999+ or -0.002 Aa in all three Cr compounds. The rigidity of these bonds leaves the CrSi ₄ O ₁₀ layers within the structure with only one significant degree of freedom, that of rotation of the four-membered Si ₄ O ₁₀ rings in response to substitution on the A cation site. The magnitudes of these rotations are independent of the identity of the B cation. In addition the AO ₈ polyhedron becomes more elongated //c with increasing radius of the A cation. The increasing aplanarity of the O(3)X ₃ , configuration is almost exclusively determined by occupational changes on A, whereas the aplanarity of the square-planar BO(3) ₄ group can be related to the positional shifts induced by the individual substitutions on both A and B sites. Polarized optical absorption spectroscopy was conducted on (hk0) sections of SrCrSi ₄ O ₁₀ and BaCrSi ₄ O ₁₀ .

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