**ii.** Ca(II)O5Z(O) polyhedra in ninefold coordination

The length of Ca(2)-F bond is 2.311 Å (**Table 7**). As was described in **Section 1.2**, each fluoride anion is surrounded by three Ca atoms (**Fig. 16**(**a**)) at one level, and in addition, Ca-O columns are linked with PO4 3− groups forming hexagonal networks. This arrangement gives very stable structure to fluorapatite while the structure of hydroxylapatite is lightly expanded and less stable to compare with fluorapatite. F anions lie in special position at the intersections of hexads with mirror planes at *z* = ¼ and ¾ (**Fig. 16**(**b**)) [39],[147],[148].

**Fig. 16.** Schematic depiction of a portion of four unit cells of the fluorapatite structure projected onto the (0001) plane. (a) Solid, dashed and dotted lines indicate bonds in different polyhedra. (b) The arrangement of Ca- and O-triangle on the screw axis (*Z* anion column) and attached PO4 3− group [39].

The structure field of fluorapatite **Fig. 17** was investigated by KREIDLER and HUMMEL [149]. It is defined within the limits of radii of ions occupying the positions of phosphorus (*R*p) and calcium (*R*c) and is in the following range:

**i.** 0.29 ≤ *R*p ≤ 0.60 Å

**i.** Ca(I)O9 polyhedra in sevenfold coordination

are linked with PO4

**ii.** Ca(II)O5Z(O) polyhedra in ninefold coordination

28 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

with mirror planes at *z* = ¼ and ¾ (**Fig. 16**(**b**)) [39],[147],[148].

The length of Ca(2)-F bond is 2.311 Å (**Table 7**). As was described in **Section 1.2**, each fluoride anion is surrounded by three Ca atoms (**Fig. 16**(**a**)) at one level, and in addition, Ca-O columns

structure to fluorapatite while the structure of hydroxylapatite is lightly expanded and less stable to compare with fluorapatite. F anions lie in special position at the intersections of hexads

**Fig. 16.** Schematic depiction of a portion of four unit cells of the fluorapatite structure projected onto the (0001) plane. (a) Solid, dashed and dotted lines indicate bonds in different polyhedra. (b) The arrangement of Ca- and O-triangle on

3− group [39].

the screw axis (*Z* anion column) and attached PO4

3− groups forming hexagonal networks. This arrangement gives very stable

**ii.** 0.95 ≤ *R*c ≤ 1.35 Å

The size limitations of ions occupying the calcium positions were less well defined because upper limit radius of ion was not found from the following reasons:


From practical proposes, the upper limit of *R*c was set to 1.35 Å, which is slightly greater than ionic radius of barium. The lower limit of *R*c was set to 0.95 Å so that both normal and distorted phases were included in the fluorapatite structure field.

**Fig. 17.** The structure field of fluorapatite (Ahrens's radii): filled circles = apatite; open circles = no apatite [149].

The structure of monoclinic polymorph of fluorapatite is related to the space group P21/B with the crystallographic parameters *a* = 9.488, *b* = 18.963, *c* = 6.822Å, *β* = 119.97° and *Z* = 6 [145].
