**Author details**

Petr Ptáček

the silicon nitride shows excellent oxidation resistance. Formed SiO2 then reacts with the grain

but in the case of MgO, the formed layer did not act as protection [136]. Y4.67(SiO4)3O apatite

The synthesis and physicochemical study of rare-earth-containing vanadocalcic oxyapatites where the pair Ca2+ and □ was substituted by Ln3+ and 1/2O2− was described by BENMOUSSA et al [139]. This substitution leads to lanthanum and praseodymium dioxyapatites Ca8Ln2(VO4)6O2, where Ln = La and Pr. Regarding rare earths such as neodymium, samari‐ um, europium, gadolinium and terbium, the Ln3+ content limit varies from one REE to another.

The following compositions having the apatite structure were prepared by WANMAKER et al

with Me(II) = Ca, Sr, Ba, Mg, Zn or Cd and Me(III) = Y or La. Among these, there are several new compounds, e.g. Zn2La8(SiO4)6O2, BaMgY8(SiO4)6O2, Zn2Y8(SiO4)6O2, Cd2Y8(SiO4)6O2, Ca4La5(SiO4)5(PO4) and Ba4La5(SiO4)5(PO4). The crystallographic parameters were deter‐ mined and their luminescence was studied. The most efficient activator proved to be triva‐ lent antimony, especially in the compositions of type I. At 77°K, an emission band at about 400

The humidity-sensitivity of yttrium-substituted calcium oxyhydroxyapatites was studied by OWADA et al [141]. The logarithm of the electrical resistance of present sensors decreased linearly with increasing relative humidity (RH) from 30 to 65%. The resistance of [Ca9.0Y1.0] (PO4)6[O1.5□0.5] with the largest OH vacancy content was about one order of magnitude lower than that of calcium hydroxyapatite. It was found that the larger the ratio of surface hydrox‐ yl groups per unit surface area in the sample, the lower the resistance and the higher the

SiO MeO MeSiO 2 x + ® 2 x <sup>+</sup> (16)

) is formed as the oxidation product of silicon yttrium oxynitride (H-phase)

boundary constituents to form silicates:

**5.6. REE vanadocalcic apatite**

in the temperature range from 700 to 1400°C [137],[138].

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

It decreases when the REE ionic radius declines.

**5.7. Apatite-type yttrium phosphates**

nm was observed in many of these apatites.

amount of OH vacancies.

**a.** Me(II)2+xMe(III)8−x(SiO4)6−x(PO4)xO2, where 0 ≤ *x* ≤ 6; **b.** Me(II)3+xMeIII6−x(SiO4)6−x(PO4)x, where 0 ≤ *x* ≤ 1.5; **c.** Me(II)4+xMe(III)6−x(SiO4)6−x(PO4)xO, where 0≤ *x* ≤ 6.

(britholite phase2

[140]:

Brno University of Technology, Czech Republic
