Crystal structure and optical spectroscopy (300 to 2200 nm) of CaCrSi_4O_(10)

Abstract

Crystals of CaCr^(2+)Si_4O_(10) were synthesized at 1 atm and 1400-1350°C in a closed system with fO_2 < 10^(-16) bar. CaCrSi_4O_(10) crystallizes in the space group P4/ncc with a= 7.378(5)Å and c = 15.119(2)Å. The structure was refined to R = 0.026 (R_w = 0.031) based on 546 reflections. CaCrSi_4O_(10) is a structural analog of gillespite and other ABSi_4O_(10) compounds, and although not itself a mineral, has been shown to exhibit structural and compression properties nearly identical to those of gillespite. The most interesting aspects of the structure are the square-planar coordination of Cr^(2+), and the Si-Ol-Si bond angle of 176.8°. Apparently, Cr^(2+) is stable in the gillespite structure because it is readily accepted into the square-planar site, in contrast to other C^(2+) compounds that are distorted from typestructures as a result of the Jahn-Teller effect in C^(2+). The optical absorption spectrum of CaCrS_4O_(10) has bands at 511 and 670 nm polarized ⊥ c and at 453 nm ‖ c, with all absorption occuring only through vibronic coupling. Diamond cell experiments indicate that CaCrSi_4O_(10) remains tetragonal to at least 50 kbar, with compression behavior identical to that of gillespite.

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