LINKED 4-MEMBERED SILICATE RINGS - VIBRATIONAL ANALYSIS OF GILLESPITE- BAFESI4O10 AND IMPLICATIONS FOR GLASS STRUCTURE

Raman spectra and lattice dynamics calculations are presented for the silicate mineral gillespite, BaFeSi4O10, which contains sheets of link ed four-membered silicate rings. The results are analyzed in relation to earlier work done on the isolated four-membered ring silicate BaCuS i2O6 and to published claims that vibrational modes of four-membered r ings are responsible for the sharp D-1 ''defect line'' observed in the Raman spectra of SiO2 glass. The crystal structure of gillespite (spa ce group P4/ncc or D-4h(8)) consists of puckered Si4O12 rings, where e ach SiO4 tetrahedron is linked to two neighboring tetrahedra within a ring and to a third tetrahedron within a different ring. The rings are linked to each other in a staggered configuration to form sheets, whi ch are also bonded together by Ba2+ and Fe2+. The calculation adjusts the bond bending and bond stretching force constants so that calculate d fundamental mode frequencies best fit observed fundamental frequenci es in the Raman spectra; eigenmodes associated with each calculated fu ndamental mode are then generated. Some eigenmodes calculated are uniq ue to the gillespite structure, but many of the more localized four-me mbered ring modes are similar to those calculated for the four-membere d rings in BaCuSi2O6 and for the three-and six-membered rings in the c yclosilicates studied earlier. The results for gillespite indicate tha t the Raman-active A(1g) mode at 450 cm(-1) is a four-membered ring br idging oxygen breathing mode that is mixed with other minor ring defor mation displacements; because of this mixing, the calculated frequency of this mode is 45 to 61 cm(-1) lower than that calculated for bridgi ng oxygen binding breathing modes of puckered four-membered rings in B aCuSi2O6 and in simulated glass structures.