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

Single crystals of lacunary apatite structure-type compound Pb 4 Na(PO 4 ) 3 were prepared with a flux/solid-state-reaction technique and consecutive melt-growth to examine spatial distribution and stereochemical activity of the 6 s 2 lone electron pair of Pb 2+ using X-ray diffraction. The target compound crystallized in space group P 6 3 / m with unit-cell edge lengths a = 9.7345 (12) Å and c = 7.2130 (18) Å without any indication of configurational ordering of Pb 2+ and Na + at the crystallographic A 1 site. The anion channel running through the structure was confirmed as vacant. Residual density attributable to the deformed 6 s 2 electron cloud was found 0.6 Å apart from the A 2-site position on the A 2 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 A 1O 6 trigonal metaprism compensated for the shrinkage of the latter to keep the volume of the A 1O 9 coordination polyhedron constant. Systematic comparison of the size of the A 2 triangle in Pb 4 Na(PO 4 ) 3 and other apatite-type compounds indicates no contribution from the 6 s 2 electron orbital on the size of the A 2 triangle and limited space-filling ability of the orbital. A 2 cations are attached on the periphery of a large void formed by the in-plane distortion of the hexagonal close packed arrangement of ( B O 4 ) 3− complex anions. In other words, a face-sharing array of A 2 octahedra was inserted `as a tube' through the framework. The sizes of A 1O 9 and B O 4 polyhedra and their framework define the maximum size of the A 2 triangle, while the triangle can easily be shrunk by attraction from an anion, such as F − , in the channel.