Five lead-arsenate apatites (mimetites)-Pb₅(AsO₄)₃X—where X denotes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and hydroxide (OH)—were synthesized via precipitation from aqueous solutions. The crystal structures were determined through Rietveld refinement of powder synchrotron X-ray data. All the compounds crystallized in the hexagonal class symmetry (space group P6₃/m). The Rietveld refinement indicated that mimetite-Cl, -Br, -I, and -OH had an anion deficiency at position X. Substitution of halogens in a mimetite structure brought about systematic changes in unit-cell parameters, interatomic distances, and metaprism twist angles φ, proportional to the substituted halogen’s ionic radius. Mimetite-OH did not follow the linear correlations determined within the series. Twist angle φ, a useful device for monitoring changes in apatite topology, ranged from 20.34° for mimetite-F to 11.42° for mimetite-I. The geometric method has been proposed for determining the diameter of hexagonal channels hosting halogens in apatites. A comparison of the results with halogenated pyromorphites showed similar systematic trends: the substitutions in mimetites have comparable effect on the interatomic distances as in their phosphorous analogues.

中文翻译：

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砷酸铅磷灰石（Mimetites）–Pb5（AsO4）3X中氟，氯，溴，碘和氢氧根取代的结构评估

五铅砷酸磷灰石（mimetites）-Pb ₅（ASO ₄）₃ X - ，其中X表示氟（F），氯（Cl），溴（Br），碘（I）和氢氧化（OH）-were通过合成从水溶液中沉淀出来。通过粉末同步加速器X射线数据的Rietveld精修确定晶体结构。所有化合物均以六角形对称性结晶（空间群P 6 ₃ / m）。Rietveld精炼表明，Mimetite-Cl，-Br，-I和-OH在位置X处具有阴离子缺陷。卤素取代为Mimicite结构后，系统地改变了晶胞参数，原子间距离以及与取代卤素的离子半径成比例的变棱镜扭转角φ。Mimetite-OH没有遵循该系列中确定的线性相关性。扭曲角φ是监测磷灰石拓扑变化的一种有用的设备，其范围从mimetite-F的20.34°到mimetite-I的11.42°。已经提出了用于确定磷灰石中容纳卤素的六边形通道的直径的几何方法。将结果与卤代焦亚铁酸盐进行比较，显示出相似的系统趋势：隐钙铁矿中的取代对原子间距离的影响与其在磷类似物中的影响相当。