Reduced molybdenum-niobium oxides. 1. Synthesis, structure, and physical properties of K2Mo4Nb3O20

Abstract

K[sub 2]Mo[sub 4]Nb[sub 3]O[sub 20] 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) [angstrom][sup 3], and Z = 4. Because of the inability to distinguish between the Mo and Nb atoms by X-ray diffraction, the structure of K[sub 2]Mo[sub 4]Nb[sub 3]O[sub 20] was refined on the basis of K[sub 2]Mo[sub 7]O[sub 2]. The final residual indexes are R = 0.035 and R[sub w] = 0.041, with 717 observed reflections (I > 3[sigma](I)) and 90 variable parameters. The structure of K[sub 2]Mo[sub 4]Nb[sub 3]O20 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 MO[sub 7] pentagonal bipyramid that shares equatorial edges with five MO[sub 6] 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. K[sub 2]Mo[sub 4]Nb[sub 3]O[sub 20] is an electronic insulator and exhibits ionic conductivitymore » at temperature above 150 [degrees]C. The temperature dependence of the magnetic susceptibility of K[sub 2]Mo[sub 4]Nb[sub 3]O[sub 20] does not follow Curie-Weiss law behavior. A magnetic moment of 1.56 [mu][sub B]/mol calculated at 250 K is consistent with one localized electron per formula unit estimated from the valence count.« less