Spatial distribution of 6 <i>s</i> ² lone electron pair in Pb ₄ Na(PO ₄ ) ₃ and stereochemical activity of the 6 <i>s</i> ² electron cloud in lead-bearing apatites

Single crystals of lacunary apatite structure-type compound Pb₄Na(PO₄)₃ were prepared with a flux/solid-state-reaction technique and consecutive melt-growth to examine spatial distribution and stereochemical activity of the 6s² lone electron pair of Pb²⁺ using X-ray diffraction. The target compound crystallized in space group P6₃/m with unit-cell edge lengths a = 9.7345 (12) Å and c = 7.2130 (18) Å without any indication of configurational ordering of Pb²⁺ and Na⁺ at the crystallographic A1 site. The anion channel running through the structure was confirmed as vacant. Residual density attributable to the deformed 6s² electron cloud was found 0.6 Å apart from the A2-site position on the A2 triangle normal to c. The lack of a channel-site anion induced shrinkage of the unit cell and the O3 trigonal antiprism in the xy plane, and an increase in the twist angle of the A1O₆ trigonal metaprism compensated for the shrinkage of the latter to keep the volume of the A1O₉ coordination polyhedron constant. Systematic comparison of the size of the A2 triangle in Pb₄Na(PO₄)₃ and other apatite-type compounds indicates no contribution from the 6s² electron orbital on the size of the A2 triangle and limited space-filling ability of the orbital. A2 cations are attached on the periphery of a large void formed by the in-plane distortion of the hexagonal close packed arrangement of (BO₄)^3- complex anions. In other words, a face-sharing array of A2 octahedra was inserted `as a tube' through the framework. The sizes of A1O₉ and BO₄ polyhedra and their framework define the maximum size of the A2 triangle, while the triangle can easily be shrunk by attraction from an anion, such as F^-, in the channel.