High-pressure synthesis and crystal structure of iron sp3-carbonate (Fe2[C4O10]) featuring pyramidal [C4O10]4- anions

The behavior of iron carbonates at high pressures is relevant for geological processes occurring in Earth interiors. Here, cubic iron sp³-carbonate Fe₂[C₄O₁₀] was synthesized in diamond anvil cell by reacting Fe₂O₃ and CO₂ at 65(4) GPa and 3000(±500) K, simulating the environment of localized thermal anomalies in the mantle. The crystal structure, determined by in situ single-crystal X-ray diffraction, features pyramidal [C₄O₁₀]^4- anions. The experimental crystal structure corresponds to a structural model from density functional theory calculations. Experimentally determined values for zero-pressure volume V₀ and bulk modulus K₀ are: V₀ = 1059(17) ų, K₀ = 160(18) GPa, The DFT-calculated Raman spectrum, modeled with zinc substituting iron, matches the experimental one, supporting the structural model's accuracy. Fe₂[C₄O₁₀] remained stable upon decompression down to 25 GPa, below which it amorphized. DFT calculations also reveal a spin crossover of Fe²⁺ cations at 95 GPa, which is significantly higher than in other Fe²⁺-containing carbonates.