Complete and limited substitution of rare-earth elements in apatite silicates La_(9-x)Ln_x(SiO₄)₆O_1.5

The isomorphous substitution of smaller RE elements (Ln = Nd, Eu, Gd, Ho, Tm, and Yb) for lanthanum in the apatite silicate solid solutions La_(9-x)Ln_x(SiO₄)₆O_1.5 was studied by X-ray powder diffraction and the Rietveld structure refinement, scanning electron microscopy and energy-dispersive X-ray microanalysis. Single-phase samples were prepared by solid-state synthesis at a moderate temperature of 1200 C using an amorphous SiO₂ nanopowder (10-40 nm) as a reactant. As the atomic number of Ln increases, the complete solubility, 0 -(hash)<=-(hash) x -(hash)<=-(hash)9, found in the systems with Ln = Nd, Eu, Gd, and Ho changes to a limited one for Ln = Tm (0 -(hash)<=-(hash) x < 1.5) and Yb (0 -(hash)<=-(hash) x < 1). The distribution of La and smaller Ln over two structurally independent cationic sites is close to statistical. In both cationic polyhedra, Ln(1)O₉ and Ln(2)O₇, the bond lengths Ln-O decrease with x, except the longest bonds Ln(1) - O(3) and Ln(2) - O(1) which increase slightly. The experimental results on the substitution limits agree with the values of the mixing energy, and critical temperature of miscibility calculated in the approximation of a regular solid solution. (authors)