Spatial distribution of 6s2 lone electron pair in Pb4Na(PO4)3 and stereochemical activity of the 6s2 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.