Lead phosphate oxyapatite, Pb₁₀(PO₄)₆O, with <i>c</i>-double superstructure

Abstract Single crystal specimens of lead phosphate oxyapatite, Pb 10 (PO 4 ) 6 O, were prepared and the structure was refined in <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>P</m:mi> <m:mover accent="true"> <m:mn>6</m:mn> <m:mo>‾</m:mo> </m:mover> </m:mrow> </m:math> $P\overline{6}$ space group. The crystal structure was a superstructure of aristotype apatite structure doubled in the c -axis. This superstructure was characterized by change in width of the anion channel, namely change in sizes of regular A 2 triangles as ‘large – middle – small – middle – (large – )’ along &lt;001&gt; in 2 × c of the parent structure. A half of in-channel (guest) O 2− were located at either one of two positions ( X 1 site positions) split from the center of the smallest triangle ( A 2c triangle) along &lt;001&gt; with 50 % probability each. The rest scattered over four positions on the c -axis near two middle-size triangles in the cell with far less probabilities. Attraction between A 2c-site Pb 2+ and X 1-site O 2− made A 2c triangle the smallest, while 6s2 lone electron pairs of Pb 2+ hindered O 2− to locate at the center of A 2c triangle. This commensurately modulated structure was required to balance different sizes of A 2 triangles and least distortion of (PO 4 ) 3− complex anion simultaneously in the host structure.