**5.7. Apatite-type yttrium phosphates**

The following compositions having the apatite structure were prepared by WANMAKER et al [140]:

**a.** Me(II)2+xMe(III)8−x(SiO4)6−x(PO4)xO2, where 0 ≤ *x* ≤ 6;


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 nm was observed in many of these apatites.

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 amount of OH vacancies.

A ceramic proton conductor was obtained in the solid solutions of yttrium-substituted oxyhydroxyapatite (Ca10−xYx)(PO4)6((OH)2−x−2yOx+y□y) [142]. Using the hydrogen concentra‐ tion cells, it was confirmed that the specimens with the composition of *x* ≤ 0.65 have the protonic transference number (*t*<sup>i</sup> ) is equal to one, while the values of *t*<sup>i</sup> of specimens with 0.65 < *x* < 1 were smaller than one. The conduction properties were also dependent on the composition of apatites. At *x* = 0.65, the conductivity (*σ*) showed the maximum value (5·10−4S·cm−1 at 800°C) in the relationship between *σ* and *x*, while the activation energy was the lowest (about 1.0 eV) at corresponding *x*. The applicability of proton conductive apatite for a fuel cell was dis‐ cussed in **Section 10.4**.
