ACBC to Balcite: Bioinspired Synthesis of a Highly Substituted High‐Temperature Phase from an Amorphous Precursor

Energy‐efficient synthesis of materials locked in compositional and structural states far from equilibrium remains a challenging goal, yet biomineralizing organisms routinely assemble such materials with sophisticated designs and advanced functional properties, often using amorphous precursors. However, incorporation of organics limits the useful temperature range of these materials. Herein, the bioinspired synthesis of a highly supersaturated calcite (Ca_0.5Ba_0.5CO₃) called balcite is reported, at mild conditions and using an amorphous calcium–barium carbonate (ACBC) (Ca_{1? x} Ba _x CO₃·1.2H₂O) precursor. Balcite not only contains 50 times more barium than the solubility limit in calcite but also displays the rotational disorder on carbonate sites that is typical for high‐temperature calcite. It is significantly harder (30%) and less stiff than calcite, and retains these properties after heating to elevated temperatures. Analysis of balcite local order suggests that it may require the formation of the ACBC precursor and could therefore be an example of nonclassical nucleation. These findings demonstrate that amorphous precursor pathways are powerfully enabling and provide unprecedented access to materials far from equilibrium, including high‐temperature modifications by room‐temperature synthesis.