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3.3 d. Crystal structures of BaCrSi₄O₁₀ and SrCrSi₄O₁₀: two new Cr²⁺ silicates (R. Miletich and D.R. Allan)

The electronic high-spin (t_2g)³(e_g)¹ configuration of divalent chromium atoms and the resulting Jahn-Teller distortion make the crystal chemistry of Cr²⁺-containing silicate phases of interest for understanding the chemistry of the Earth's interior. Cr²⁺ silicates are known only for a few compounds such as olivine and pyroxene solid-solutions, the thenardite-type Cr₂SiO₄ and the gillespite-type CaCrSi₄O₁₀ compounds. The two new BaCrSi₄O₁₀ and SrCrSi₄O₁₀ phases, analogous to the existing ACuSi₄O₁₀ (A = Ca, Sr, Ba) series, were synthesized as part of ongoing investigations into the influence of the Jahn-Teller cation on the high-pressure phase transition which occurs in gillespite (BaFeSi₄O₁₀).

Synthesis experiments in the respective MeO-SiO₂-Cr₂O₃-Cr⁰ systems were performed in evacuated quartz-glass tubes using excess chromium metal to keep fO₂ below the Cr₂O₃-Cr⁰ buffer. Subhedral platy single crystals of pink to salmon-red colour and up to 150µm in size were grown from Li₂B₄O₇ and Na₂B₄O₇ fluxes in the temperature range 800° - 1020 °C at P<10^-3 bar. Solid state reactions at 1350° - 1450 °C, without additional fluxes, yielded almost single-phase products of the Cr²⁺ compounds.

Crystal structure investigations by X-ray diffraction studies confirmed that both BaCrSi₄O₁₀ and SrCrSi₄O₁₀ possess the tetragonal ABSi₄O₁₀ gillespite-type crystal structure. Lattice parameters of the ACrSi₄O₁₀ series are shifted towards slightly smaller values compared to those of the equivalent Cu series (Fig. 3.3-4) owing to the smaller ionic radius of Cr²⁺. The
 

Fig. 3.3-4: Lattice parameters a, c for the synthesized ACr2+Si4O10 and ACuSi4O10 phases (A= Ba, Sr, Ca; gill= gillespite, BaFe2+Si4O10)

 
predominating changes in a, induced by the substitution of Cr²⁺ for Cu²⁺, arise from the alignment of the square-planar CrO₄ configurations parallel to (001). The <Cr^[4pl]-O3> bond distances are 1.992(5) and 1.995(5) Å, which are in close agreement with the expected bond length of 1.986 Å for ideally 4-coordinated Cr²⁺ as derived from bond-valence calculations.