Synthesis of Crystalline Calcium Bicarbonate

Bicarbonate reactions, especially those involving calcium, play prominent roles in the carbon cycle and biochemical processes, contributing to the largest carbon pool on Earth. However, although they were hypothesized over two centuries ago, all attempts to isolate calcium bicarbonate Ca(HCO₃)₂, both in nature and in the laboratory, as well as other bicarbonates of multivalent metals as pure compounds, have failed and invariably resulted in the formation of carbonates. In this study, we synthesized Ca(HCO₃)₂ crystals by modifying the bonding environment to increase the stability of the bicarbonate ions and determined their crystal structure. Ca(HCO₃)₂ crystals with direct calcium-bicarbonate bonds share an identical rhombohedral crystal structure with calcite. We further determined that the polarization of calcium-bonded bicarbonate ions in polarization-favorable environments (e.g., water solvation) inhibits the isolation of Ca(HCO₃)₂, as it polarizes the hydroxy group to destabilize the calcium-bicarbonate ion pairing, which is a prerequisite for Ca(HCO₃)₂ crystallization. These findings, particularly the first-ever resolved crystal structure of Ca(HCO₃)₂, provide fundamental insight into metal-bicarbonate interactions and the formation of ionic compounds, advancing the ability to direct CO₂-involved reactions.