Structural Basis for Metastability in Amorphous Calcium Barium Carbonate (ACBC)

ML Whittaker and WH Sun and KA DeRocher and S Jayaraman and G Ceder and D Joester, ADVANCED FUNCTIONAL MATERIALS, 28, 1704202 (2018).

DOI: 10.1002/adfm.201704202

Metastable amorphous precursors are emerging as valuable intermediates for the synthesis of materials with compositions and structures far from equilibrium. Recently, it was found that amorphous calcium barium carbonate (ACBC) can be converted into highly barium-substituted "balcite," a meta-stable high temperature modification of calcite with exceptional hardness. A systematic analysis ACBC (Ca1-xBaxCO3 center dot 1.2H(2)O) in the range from x = 0-0.5 is presented. Combining techniques that independently probe the local environment from the perspective of calcium, barium, and carbonate ions, with total X-ray scattering and a new molecular dynamics/density functional theory simulations approach, provides a holistic picture of ACBC structure as a function of composition. With increasing barium content, ACBC becomes more ordered at short and medium range, and increasingly similar to crystalline balcite, without developing long-range order. This is not accompanied by a change in the water content and does not carry a significant energy penalty, but is associated with differences in cation coordination resulting from changing carbonate anion orientation. Therefore, the local order imprinted in ACBC may increasingly lower the kinetic barrier to subsequent transformations as it becomes more pronounced. This pathway offers clues to the design of metastable materials by tuning coordination numbers in the amorphous solid state.

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