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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