Crystal structure and optical spectroscopy (300 to 2200 nm) of CaCrSi4O10

Crystals of CaCr²⁺Si₄O₁₀ were synthesized at 1 atm and 1400–1350°C in a closed system with fo₂ < 10⁻¹⁶ bar. CaCrSi₄O₁₀ 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. CaCrSi₄O₁₀ is a structural analog of gillespite and other ABSi₄O₁₀ 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 Cr²⁺, and the Si–Ol–Si bond angle of 176.8°. Apparently, Cr²⁺ is stable in the gillespite structure because it is readily accepted into the square-planar site, in contrast to other Cr²⁺ compounds that are distorted from type-structures as a result of the Jahn–Teller effect in Cr²⁺. The optical absorption spectrum of CaCrSi₄O₁₀ has bands at 511 and 670 nm polarized ⊥ c and at 453 nm || c, with all absorption occuringonly through vibronic coupling. Diamond cell experiments indicate that CaCrSi₄O₁₀ remains tetragonal to at least 50 kbar, with compression behavior identical to that of gillespite.