Polysomatic apatites

Abstract

Certain complex structures are logically regarded as intergrowths of chemically or topologically discrete modules. When the proportions of these components vary systematically a polysomatic series is created, whose construction provides a basis for understanding defects, symmetry alternation and trends in physical properties. Here, we describe the polysomatic familyA5NB3NO9N + 6XNδ(2 ≤N≤ ∞) that is built by condensingNapatite modules (A5B3O18Xδ) in configurations to createBnO3n + 1(1 ≤n≤ ∞) tetrahedral chains. Hydroxyapatite [Ca10(PO4)6(OH)2] typifies a widely studied polysome whereN= 2 and the tetrahedra are isolated inA10(BO4)6X2compounds, butN= 3A15(B2O7)3(BO4)3X3(ganomalite) andN= 4A20(B2O7)6X4(nasonite) are also known, with theXsite untenanted or partially occupied as required for charge balance. The apatite modules, while topologically identical, are often compositionally or symmetrically distinct, and an infinite number of polysomes is feasible, generally with the restriction being that anA:B= 5:3 cation ratio be maintained. The end-members are theN= 2 polysome with all tetrahedra separated, andN= ∞, in which the hypothetical compoundA5B3O9Xcontains infinite, corner-connected tetrahedral strings. The principal characteristics of a polysome are summarized using the nomenclatureapatite-(A B X)-NS, whereA/B/Xare the most abundant species in these sites,Nis the number of modules in the crystallographic repeat, andSis the symmetry symbol (usuallyH,T,MorA). This article examines the state-of-the-art in polysomatic apatite synthesis and crystallochemical design. It also presents X-ray and neutron powder diffraction investigations for several polysome chemical series and examines the prevalence of stacking disorder by electron microscopy. These insights into the structure-building principles of apatite polysomes will guide their development as functional materials.