Linked four-membered silicate rings: vibrational analysis of Gillespite |
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| Authors: | David A McKeown Michael I Bell |
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| Institution: | (1) Department of Chemistry, Howard University, 525 College Street, N.W., Washington, DC 20059, USA Fax: +1(202) 319-4469, US;(2) Naval Research Laboratory, Dynamics of Solids Branch, Code 6680 Washington, DC 20375-5320, USA, US |
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| Abstract: | Raman spectra and lattice dynamics calculations are presented for the silicate mineral gillespite, BaFeSi4O10, which contains sheets of linked four-membered silicate rings. The results are analyzed in relation to earlier work done
on the isolated four-membered ring silicate BaCuSi2O6 and to published claims that vibrational modes of four-membered rings are responsible for the sharp D
1“defect line” observed in the Raman spectra of SiO2 glass. The crystal structure of gillespite (space group P4/ncc or D
) consists of puckered Si4O12 rings, where each 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, which are also bonded together by Ba2+ and Fe2+. The calculation adjusts the bond bending and bond stretching force constants so that calculated fundamental mode frequencies
best fit observed fundamental frequencies in the Raman spectra; eigenmodes associated with each calculated fundamental mode
are then generated. Some eigenmodes calculated are unique to the gillespite structure, but many of the more localized four-membered
ring modes are similar to those calculated for the four-membered rings in BaCuSi2O6 and for the three- and six-membered rings in the cyclosilicates studied earlier. The results for gillespite indicate that
the Raman-active A
1g
mode at 450 cm−1 is a four-membered ring bridging oxygen breathing mode that is mixed with other minor ring deformation displacements; because
of this mixing, the calculated frequency of this mode is 45 to 61 cm−1 lower than that calculated for bridging oxygen binding breathing modes of puckered four-membered rings in BaCuSi2O6 and in simulated glass structures.
Received: 9 April 1997 / Revised, accepted: 19 August 1997 |
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