Structural derivation and crystal chemistry of apatites

Abstract

The crystal structures of the [A(1)(2)][A(2)(3)](BO(4))(3)X apatites and the related compounds [A(1)(2)][A(2)(3)](BO(5))(3)X and [A(1)(2)][A(2)(3)](BO(3))(3)X are collated and reviewed. The structural aristotype for this family is Mn(5)Si(3) (D8(8) type, P6(3)/mcm symmetry), whose cation array approximates that of all derivatives and from which related structures arise through the systematic insertion of anions into tetrahedral, triangular or linear interstices. The construction of a hierarchy of space-groups leads to three apatite families whose high-symmetry members are P6(3)/m, Cmcm and P6(3)cm. Alternatively, systematic crystallographic changes in apatite solid-solution series may be practically described as deviations from regular anion nets, with particular focus on the O(1)-A(1)-O(2) twist angle phi projected on (001) of the A(1)O(6) metaprism. For apatites that contain the same A cation, it is shown that phi decreases linearly as a function of increasing average ionic radius of the formula unit. Large deviations from this simple relationship may indicate departures from P6(3)/m symmetry or cation ordering. The inclusion of A(1)O(6) metaprisms in structure drawings is useful for comparing apatites and condensed-apatites such as Sr(5)(BO(3))(3)Br. The most common symmetry for the 74 chemically distinct [A(1)(2)][A(2)(3)](BO(4))(3)X apatites that were surveyed was P6(3)/m (57%), with progressively more complex chemistries adopting P6(3) (21%), P3; (9%), P6 (4.3%), P2(1)/m (4.3%) and P2(1) (4.3%). In chemically complex apatites, charge balance is usually maintained through charge-coupled cation substitutions, or through appropriate mixing of monovalent and divalent X anions or X-site vacancies. More rarely, charge compensation is achieved through insertion/removal of oxygen to produce BO(5) square pyramidal units (as in ReO(5)) or BO(3) triangular coordination (as in AsO(3)). Polysomatism arises through the ordered filling of [001] BO(4) tetrahedral strings to generate the apatite-nasonite family of structures.

Dissolution and solubility of the arsenate - phosphate hydroxylapatite solid solution [Ca5(PxAs1 - xO4)3(OH)] at 25°C

“…[9] Expansion of the average size of (As,P)O 4 tetrahedra is also reflected in the widely considered metaprism twist angle, giving the rotation of the two triangular faces formed by O1 and O2 about Ca1. [30] The trend of increasing twist with increasing As content up to As fraction 0.66 is consistent with the distortion between the (As,P)-O distances, where the (As,P)-O1 distance increases more rapidly than the (As,P)-O2 distance over the intermediate concentration range, which would serve to accentuate the metaprism twist in this same range. [9] This distortion or twist could result in the increasing of 1200 1000 800…”

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

“…The triclinic distortion has been attributed to a size mismatch between the La3.33+2x/3(GeO4)6 framework and the La6O2 channels leading to underbonding at the channel La atoms, which is relieved by the triclinic distortion, through twisting of the GeO4 tetrahedra [20,21,39]. In prior work, we have shown that Y can be selectively doped into the La sites within the La3.33+2x/3(GeO4)6 framework altering the diameter, and hence relieving the triclinic distortion [18,37].…”

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…Namely, the crystal structure of HA is well described elsewhere (Elliott 1994;White and Dong 2003;Mathew and Takagi 2001), the detailed analysis of the electronic structure, bonding, charge transfer, optical and elastic properties is also available Rulis et al 2004;Snyders et al 2007;Ching et al 2009), while the readers interested in Posner's clusters are referred to still other papers (Treboux et al 2000;Kanzaki et al 2001). A shell model was developed to study the lattice dynamics of HA (Calderin et al 2005), while a cluster growth model was created to illustrate its growth (Onuma and Ito 1998).…”

“…An anhydrous phase diagram CaO-P 2 O 5 at temperatures within 200-2200°C is shown in Fig. 3 (Kreidler and Hummel 1967;Carayon and Lacout 2003). Table 4 comprises crystallographic data of the existing CaPO 4 (Elliott 1994;White and Dong 2003;Mathew and Takagi 2001). The most important parameters of CaPO 4 are the ionic Ca/P ratio, basicity/acidity and solubility.…”

“…In addition, it is the most thermally stable CaPO 4 with the melting point at *1650°C (Tõnsuaadu et al 2012). Perhaps, such ''extreme'' properties of FA are related to the specific position of F -ions in the center of Ca(2) triangles of the crystal structure (Elliott 1994;White and Dong 2003). Due to its properties, FA is the only CaPO 4 that naturally forms large deposits suitable for the commercial use (McConnell 1973;Becker 1989;Rakovan and Pasteris 2015;see also Fig.…”