High-pressure synthesis and crystal structure of iron sp-carbonate (Fe[CO]) featuring pyramidal [CO] anions.

The behavior of iron carbonates at high pressures is relevant for geological processes occurring in Earth interiors. Here, cubic iron sp-carbonate Fe[CO] was synthesized in diamond anvil cell by reacting FeO and CO 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 [CO] anions. The experimental crystal structure corresponds to a structural model from density functional theory calculations. Experimentally determined values for zero-pressure volume V and bulk modulus K are: V = 1059(17) Å, K = 160(18) GPa, The DFT-calculated Raman spectrum, modeled with zinc substituting iron, matches the experimental one, supporting the structural model's accuracy. Fe[CO] remained stable upon decompression down to 25 GPa, below which it amorphized. DFT calculations also reveal a spin crossover of Fe cations at 95 GPa, which is significantly higher than in other Fe-containing carbonates.