Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO₃. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO₃ crystallization pathways and CaCO₃/(bio)polymer nanocomposites. ACC can be synthesized by liquid–liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate “impurities” were added in the form of monetite (CaHPO₄) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state ³¹P, ¹³C, and ¹H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first—and fast—reaction step, the CO₃^2– anions are protonated by the HPO₄^2– groups. The formation of unprecedented hydrogen carbonate (HCO₃^–) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO₄^3– unit can fill space more efficiently. In a competing second—and slow—reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO₃^– anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration.

中文翻译：

---

监视机械化学反应揭示了一个新的ACC缺陷变异的含的HCO形成₃ ^-阴离子封装由非晶合金

非晶态碳酸钙（ACC）是结晶CaCO ₃的生物矿化中的重要前体。实际上，它用作存储材料或永久性结构元件，其寿命受有机基质的调节。ACC在材料科学中的意义主要与我们对CaCO ₃结晶途径和CaCO ₃ /（生物）聚合物纳米复合材料的理解有关。ACC可以通过液相分离得到，通常可以用大分子稳定。我们通过在行星式球磨机中研磨方解石来制备ACC。磷酸盐以杂质（CaHPO ₄）取代碳酸根阴离子，从而通过取代紊乱稳定ACC。磷酸根阴离子不能简单地替代碳酸根阴离子。它们经历剪切驱动的酸/碱和缩合反应，其中需要化学计量的（10％）磷酸盐含量才能完成非晶化。磷酸根阴离子产生应变网络，在动力学上阻碍ACC重结晶。通过定量傅里叶变换红外光谱和固态³¹ P，¹³ C和¹ H幻角旋转核磁共振光谱研究了BM-ACC的非晶化反应和结构，这对局部环境的对称性变化非常敏感。在第一步（快速反应）中，CO ₃^2–阴离子由HPO ₄ ^2–基团质子化。前所未有的碳酸氢盐的（HCO形成₃ ^- ）和正磷酸盐阴离子似乎是反应的驱动力，这是因为磷酸基团具有更高的库仑能量和四面体PO ₄ ^3-单元可以更有效地填充空间。在竞争性的第二步和慢速反应步骤中，焦磷酸阴离子在缩合反应中形成。在较高的碳酸盐含量下不会形成焦磷酸盐。高应变导致如此大的能量势垒，从而抑制了任何反应。我们的发现有助于理解碳酸钙的机械化学非晶化，并通常强调杂质对非晶相稳定的作用。我们的方法允许的新的无定形碱土缺陷合成变体包含唯一HCO ₃ ^-的阴离子。我们的方法概述了为各种碳酸盐/磷酸盐系统获得新的无定形固体的总体策略，这些固体有望作为骨再生的生物材料。