REDUCED MOLYBDENUM NIOBIUM OXIDES .1. SYNTHESIS, STRUCTURE, AND PHYSICAL-PROPERTIES OF K2MO4NB3O20

K2Mo4Nb3O20 crystallizes in the orthorhombic system, space group Pbcm (No. 57). The unit cell dimensions are a = 4.0176(7) angstrom, b = 17.891(4) angstrom, c = 21.825(3) angstrom, v = 1568.7(5) angstrom3, and Z = 4. Because of the inability to distinguish between the Mo and Nb atoms by X-ray diffraction, the structure of K2Mo4Nb3O20 was refined on the basis of K2Mo7O20. The final residual indexes are R = 0.035 and R(w) = 0.041, with 717 observed reflections (I > 3sigma(I)) and 98 variable parameters. The structure of K2Mo4Nb3O20 consists of pentagonal clusters of polyhedra that are linked together via bridging, corner-sharing octahedra in the bc plane. These layers stack along the a axis, forming pentagonal columns and octahedral strings. The pentagonal cluster of polyhedra contains an MO7 pentagonal bipyramid that shares equatorial edges with rive MO6 octahedra, forming a star-shaped cluster of polyhedra (M = Mo, Nb). The K ions are located in the one-dimensional S-shaped tunnels, running along the a axis, created by the network of polyhedra. K2MO4Nb3O20 is an electronic insulator and exhibits ionic conductivity at temperature above 150-degrees-C. The temperature dependence of the magnetic susceptibility of K2Mo4Nb3O20 does not follow Curie-Weiss behavior. A magnetic moment of 1.56 mu(B)/mol calculated at 250 K is consistent with one localized electron per formula unit estimated from the valence count.