Isomorphous Replacements and Crystal Structure of Pb(-x)CaxNa2(VO4)6 Solid Solutions

Compounds with the apatite structure and general formula M10(EO4)6(X)2, where M = Ca2+, Sr2+, Ba2+, Eu3+, Na+ ets.; E = P5+, V5+, Si4+, As5+ etc.; and X = OH-, F-, Cl-, Br-, I-, O2- etc. find an application as bioactive, luminescent and laser materials, sensors, solid electrolytes, absorbents and catalysts. Compounds with apatite structure can easily accommodate a great variety of substitutions. Substitutions make not only changes in already existing properties but also cause the emergence of the new ones. Thus, synthesis and study of solid solutions is a way to produce novel functional materials. Among all apatites only Pb-containing compounds retain the apatite structure in which the Y ions are absent in the center of structural channels. Whenever the anions are removed from the channels, the apatite lattice collapses, except for those cases where the M2 sites are occupied by cations having 6s2 – lone pair (Pb2+). These stereochemically active lone pairs orient toward the channel and create a negative charge which, stabilizes the apatite structure. The solid solutions Pb(8-x)CaxNa2(VO4)6 were synthesized by the solid-phase method and studied by X-ray powder diffraction and scanning electron microscopy. The substitution of calcium for lead under the scheme: Pb2+ → Ca2+ was accompanied by a decrease of the unit cell parameter c, whereas a change in the parameter was found to be within an error of its determination. Such changes are due to the compression of the structural channel, which occurs as the substitution of lead for calcium takes place