Apatites hexagonal apatite structure

Figure 8.3 illustrates the hexagonal apatite structure. One set of calcium ions is in 6-fold coordination with oxygen atoms belonging to the PO4 tetrahedra. Thus, as... 

Apatites and related compounds, most notably hydroxyapatite [HAP, Caio(P04)6 (0H)2], are of considerable interest due to their potential as biomaterials, adsorbents and ion-exchange materials [10]. Their hexagonal apatite structure consists of Ca sites surrounded by P04 tetrahedra the OH ions occupy columns parallel to the hexagonal axis (Figure 5.1). The crystal structure features two nonequivalent Ca sites one set of Ca ions is aligned in columns (site I, Cai), while the other set forms equilateral triangles centered on the screw axis (site II, Can). 

Ternary rare earth arsenates having hexagonal apatite structure have been prepared by Escobar and Baran (1982a). As with phosphates and vanadates their composition can vary widely. 

Fig. 9. Projection of the hexagonal apatite structure of Sm dSiftN2024 along the c-axis (Gaude et al. 1975b).

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c-axis. In its pure form, F , OH , or Cl occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

Fluorapatite (FA) corresponds to the chemical formula Caio(P04)eF2 and crystallises in the hexagonal space group PGs/m, with Z = 1 and unit-cell parameters a = b = 9.367 A and c = 6.884 A [1] (Fig. 2). From a structural viewpoint, fluorapatite is often considered as a crystalline model for other apatites and is seen as a reference apatitic array [2]. It is one of the very first apatite structures to have been solved. It has been thoroughly studied since the 1930s [3] and is well documented in the literature. In particular, Sudarsanan et al. [1] reported the single crystal refinement of X-ray diffraction (XRD) data, and the detailed description of atomic positions and local symmetry is fully available [4,5],... 

The apatite family of minerals is a common feature to many of the minerals shown in Table 3. In nature, the apatite mineral structure conforms to the 6/m class of minerals with hexagonal crystal structure and the generic formula Me5(X04)3Z where Me is Ca, Sr, Ba, Cd, and Pb (typically), X = P, As, V, Mn, and Cr and Z = OH, F, Cl, and Br. In addition to caibon-ate apatite, chloroapatite, chloropyromorphite, fluoroaptite, fluoropyromorphite, hydroxyapatite, and hydroxypyromorphite, the family includes abukumalite ((Ca,Th,Ce)5(P04, Si04)3(0H,F)),... 

Figure 2. Depiction of possible anion positions in the hexagonal temaiy apatite structure. Stippled planes represent minor planes at z = each containing a triangle of Ca2 atoms...

Schriewer MS, Jeitschko W (1993) Preparation and crystal structure of the isotypic orthorhombic strontium perrhenate halides Sr5(Re05)3X (X = chloride, bromide, iodide) and structure refinement of the related hexagonal apatite-like compound barium perrhenate chloride (Ba5(Re05)3Cl). J Solid State Chem 107 1-17... 

Some substitutions which can occur, wholly or in part, in natural or synthetic apatites are listed in Table 5.19. Many minerals and many synthetic orthophosphates adopt an apatite-type crystal structure which usually has hexagonal symmetry or a closely related structure. Only very recently has the successful synthesis of the iodide Ca,o(P04)6l2 been reported, moreover it is believed that introduction of the radioactive isotope T may give a product with important medical uses [87]. 

Apatite compounds all have a similar type of hexagonal crystal structure which was originally worked out by Mehmel andNaray-Szabo in 1930 [24-26] (Figures 5.5 and 5.6). They include a whole... 

According to recent investigations into the structure of OAp, there exists a Unear chain of 0 ions paraUel to the c-axis, with each one followed by a vacancy (Alberius Helming et al., 1999) (Figure 10.11b). Calculations using density-functional theory with local-density approximation (DFT-LDA) and first-principles pseudopotentials (Calderin et al., 2003) postulated the existence of a hexagonal c-empty structure Caio(P04)6D2 vvith a stable total-energy minimum. Hence, the defect apatite structure appears to be insensitive to the choice of anion (OH , F , ) in the c-axis column that is, if these are removed completely, the structure... 

HA displays ionic character, and its crystalline structure can be describe like a compact hexagonal packing of oxygen atom with metals occupying the tetrahedral and octahedral holes of the periodic network. The basic apatite structure is hexagonal with space group Pbs/m and approximate lattice parameters a = 9.4 and c = 6.9 A, being the fluorapatite... 

Fig. 3. Splitted view of atoms along the c axis of the hexagonal structure showing the two possible fluoride ion locations. In stoichiometric fluorapatite, fluoride ions locate in the equilateral triangle formed by Ca(ll) ions. In type B carbonate apatite, the replacement of P04 ions by ions creates an oxygen atom vacancy which may be occupied by a second kind of fluoride ion (adapted from Ref. [4]). (See Colour Plate Section at the end of this book.)...

Enamel contains well-oriented apatite crystallites, of sizes up to 1 nm in maximum dimension (Elliott, 2002). Ultra-high-resolution photos of thin sections of developing enamel show the unmistakable hexagonal outlines of apatite (Figure 31(A)). Powder X-ray diffraction analysis which can be used to accurately identify the crystal structural characteristics of any solid shows that the material is clearly apatite and that there is preferred orientation of the crystallites. These... 

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