**Abstract**

A number of prepared alkaline-earth-rare-earth silicates and germanates also have the structure of apatite type. The fifth chapter of this book then continues with descrip‐ tion of synthetic compounds of apatite structure. Attention will be directed to descrip‐ tion of rare-earth element bearing apatites and oxyapatites. The structure, properties and preparation of apatite-type silicates, germanates and borates were described. This chapter gives also description of oxygen-rich apatites, which are promising material for electrolytes in solid oxide fuel cells and sensors and explain the basic concepts between structure and conductivity of these compounds. The additional information about application of apatites is given in the last chapter of this book. Furthermore, N-apatite, REE vanadocalcic apatite and apatite type yttrium phosphates were described.

**Keywords:** Apatite, Oxyapatite, Conductivity, Silicates, Germanates, Borates, N-apa‐ tite, Apatite-type yttrium phosphates

In this chapter, the preparation and the properties of synthetic phases of apatite are given; the geological role is described in **Section 7.3**. The ideal general formula of an apatite-type oxide may be written as M10(XO4)6O2 (M = alkaline-earth and/or rare-earth element, X = Si, Ge, P, V, …). The structure (**Fig. 1**) can be described in terms of a "microporous"1 [1] framework (A(1)4(XO4)6) composed of face sharing M(1)O6 trigonal meta-prismatic columns, which are corner connected to MO4 tetrahedra. This framework allows some flexibility to accommodate remaining M(2)6O2 units [2].

<sup>1</sup> Microporous material is defined as containing pores with the diameters >2 nm. The materials with the pore diameter in the range from 2 to 50 nm and higher than 50 nm are termed as mesoporous and macroporous, respectively. In combina‐ tion with nanotechnology, the term nanoporous material is often used. Despite the fact that there is not clear definition, usually the pores with the size from 0.1 to 100 nm are considered. In other words, nanoporous covers the range from microporous to macroporous [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Fig. 1.** Illustration of "microporous" M(1)(XO4)6 framework of the apatite (M10(XO4)6O2) structure (a): tetrahedra MO4, M(1) cation at the center of trigonal meta-prism. Remaining M(2)6O2 units occupy the cavities within this framework (b): large spheres are M(2) cations and small spheres are O anions [2].
