Isomorphous Substitution of Rare-Earth Elements in Lacunary Apatite Pb8Na2(PO4)(6)

摘要

The substitution of rare-earth elements (REEs) for Pb in the lacunary apatite Pb8Na2(PO4)(6) with void structural channels was studied by means of powder X-ray diffraction (including the Rietveld refinement), scanning electron microscopy, energy-dispersive X-ray microanalysis, and IR spectroscopy and also measurements of the electrical conductivity. The substitution limits (x(max) in Pb(8-x)Ln(x)Na(2)(PO4)(6)O-x/2) at 800 degrees C were found to decrease with the atomic number of the REE from 1.40 for La to 0.12 for Yb with a rapid drop from light to heavy lanthanides (between Gd and Tb). The REE atoms substitute for Pb predominantly at Pb2 sites of the apatite structure according to the scheme 2Pb(2+) + square -> 2Ln(3+) + O2-, where square is a vacancy in the structural channel. The substitution in lacunary apatite produces quite different changes in the structural parameters compared with broadly studied alkaline-earth hydroxyapatites. In spite of the much lower ionic radii of REE than that of Pb2+, the mean distances < Pb1-O > somewhat increase, whereas the distances < Pb2-Pb2 > and < Pb2-O4 > do not change considerably with the degree of substitution. This implies control of the substitution by not only spatial and charge accommodation of REE ions but also the availability of a stereochemically active 6s(2) electron pair on Pb2+. The high-temperature electrical conductivity shows dependence on the degree of substitution with a minimum at x = 0.2 indicative of a possible change of the type of conductivity.