Abstract Sub-µm CaCO3 (calcite; CC) particles were converted to calcium monohydrogenphosphate dihydrate (DCPD) and hydroxyapatite (HAp) via soaking treatments in K2HPO4 solutions with varied pH (3–12) and concentrations (0.1–1.5 M) at 37°C for up to 10 days. DCPD was derived from the solutions with pH ≤ 6; while hollow HAp was yielded when pH ≥ 7 in assemblies of petal-like crystallites. Results of magic angle spinning (MAS) and cross-polarization magic angle spinning (CP-MAS) NMR studies have shown that the HAp lattice has only PO4 2− but no HPO4 2− at B (phosphate) sites. Trace amounts of CO3 2− have occupied both A (OH) and B (PO4) sites, and H2O is adsorbed on surface crystallites. The primary crystallite size of HAp derived from Scherrer equation increases quickly in a 12 h period and becomes gradually stable afterward. Samples of particles soaked within 3 h in a temperature range of 20–80°C were analyzed by X-ray diffraction. It is shown that the rate constant of 1 M solution is about an order of magnitude greater than that of 0.1 M solution and the apparent activation energy is 33 kJ/mol. In this work, the conversion of CC to HAp can be quantitatively controlled to solve the problem of slow degradation of HAp. Graphical abstract

中文翻译：

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亚微米碳酸钙（方解石）颗粒在 K2HPO4 溶液中转化为空心羟基磷灰石团聚体

摘要 亚微米 CaCO3（方解石；CC）颗粒通过在 37°不同 pH（3-12）和浓度（0.1-1.5 M）的 K2HPO4 溶液中浸泡处理，转化为二水合磷酸氢钙 (DCPD) 和羟基磷灰石 (HAp) C 最多 10 天。DCPD 来源于 pH ≤ 6 的溶液；而当 pH ≥ 7 时，花瓣状微晶集合体中会产生空心 HAp。魔角旋转 (MAS) 和交叉极化魔角旋转 (CP-MAS) NMR 研究的结果表明，HAp 晶格在 B（磷酸盐）位点处只有 PO4 2- 但没有 HPO4 2-。微量的 CO3 2- 占据了 A (OH) 和 B (PO4) 位点，并且 H2O 被吸附在表面微晶上。由谢勒方程导出的 HAp 的初级微晶尺寸在 12 小时内迅速增加，之后逐渐稳定。通过 X 射线衍射分析在 20-80°C 温度范围内浸泡 3 小时内的颗粒样品。结果表明，1M溶液的速率常数比0.1M溶液的速率常数大一个数量级，表观活化能为33kJ/mol。在这项工作中，可以定量控制CC向HAp的转化，以解决HAp降解缓慢的问题。图形概要