THE SYNTHETIC CR2- THE MISSING LINKS IN THE GILLESPITE-TYPE ABSI(4)O(10) SERIES( SILICATES BACRSI4O10 AND SRCRSI4O10 )

The new Cr2+-containing silicate compounds BaCrSi4O10 and SrCrSi4O10 w ere synthesized both from alkali-borate fluxes and by high-T subsolidu s solid-state reactions. The gillespite-type crystal structures (space group P4/ncc, Z = 4) were determined from single-crystal X-ray diffra ction data. The unit-cell parameters are alpha = 7.4562(4), c = 15.541 4(4) Angstrom for SrCrSi4O10, and alpha = 7.5314(3), c = 16.0518(4) An gstrom for BaCrSi4O10. Comparison with previously published data shows that A (= Ba, Sr, Ca) cation substitution in ABS(4)O(10) gillespite-t ype compounds mainly affects the c lattice parameter whereas the subst itution of the B (= Cu, Cr, Fe) site leads to only small changes, main ly in alpha. The Cr2+ cation occupies a square-planar coordinated site unique in oxide crystal chemistry, with a Cr-O bond length of 1.999 /- 0.002 Angstrom in all three Cr compounds. The rigidity of these bon ds leaves the CrSi4O10 layers within the structure with only one signi ficant degree of freedom, that of rotation of the four-membered Si4O10 rings in response to substitution on the A cation site. The magnitude s of these rotations are independent of the identity of the B cation. In addition the AO, polyhedron becomes more elongated // c with increa sing radius of the A cation. The increasing aplanarity of the O(3)X, c onfiguration is almost exclusively determined by occupational changes on A, whereas the aplanarity of the square-planar BO(3)(4) group can b e related to the positional shifts induced by the individual substitut ions on both A and B sites. Polarized optical absorption spectroscopy was conducted on (hkO) sections of SrCrSi4O10 and BaCrSi4O10. Absorpti on bands at similar to 19500, similar to 14900, and similar to 22070 c m(-1) could be assigned to B-5(1g) --> B-5(2g), B-5(1g)--> (5)A(1g) (E perpendicular to C), and B-5(1g) --> E-5(g), (E // c) spin allowed d- d transitions for Cr2+ in a square-planar configuration. The crystal-f ield stabilization energies of 13110 +/- 150 and 13220 +/- 180 cm(-1) are indistinguishable for both compounds reflecting the very similar C rO4 geometries.