RARE-EARTH MIXED-VALENCE VANADIUM-OXIDES WITH SUPERLATTICE STRUCTURESOF CALCIUM FERRITE

New ternary ytterbium mixed valence vanadium oxides, alpha- and beta-Y bV4O8, were synthesized by solid-state reaction at 1473 and 1673 K, re spectively. The crystal structure of alpha-YbV4O8 was determined by si ngle-crystal X-ray diffraction; monoclinic, P12(1)/n1, alpha = 9.0648( 3) Angstrom, b = 10.6215(4) Angstrom, c = 5.7607(1) Angstrom, beta = 9 0.184(3)degrees, Z = 4, R = 0.030, and R(w) = 0.029. beta-YbV4O8 was s tudied by Weissenberg photography and powder X-ray diffraction; monocl inic, P2(1)/n11, a = 9.0625(7) Angstrom, b = 11.0086(9) Angstrom, c = 5.7655(5) Angstrom, alpha = 105.070(7)degrees. Its crystal structure w as rerefined, for the sake of comparison between alpha- and beta-form, adopting an A-centered pseudoorthorhombic cell dimension with relatio ns a(p-o) = a(m), b(p-o) = 2b(m) + c(m), and c(p-o) = c(m); monoclinic , A2(1)/d11, a = 9.030(5) Angstrom, b = 21.44(3) Angstrom, c = 5.752(2 ) Angstrom, alpha = 89.911(3)degrees, Z = 8, R = 0.029, and R(w) = 0.0 32. Their three-dimensional V-O frameworks are similar to that of Fe-O one in the well-known orthorhombic CaFe2O4. Yb3+ cations are located in tunnels running along the [001] direction as are Ca2+ cations in Ca Fe2O4. However, Yb3+ occupy half of the Ca2+ sites in ordered manners, resulting in superlattice structures of CaFe2O4. The shortest Yb-Yb d istances, 5.4996(2) Angstrom in alpha-YbV4O8 and 5.552(8) Angstrom in beta-YbV4O8, are much longer than the shortest Ln-Ln distances in well -known ternary rare-earth oxides such as perovskites, pyrochlores, LnV O(4) type phases, and garnets.