Synthesis, crystal structures, and physical properties of the novel Ca(x)R(17-x)Mo19O46 (4 < or = x < or = 7; R = Ce, Pr, Sm, and Gd) compounds containing centered Mo19 nu2-octahedral clusters.
Résumé
The novel quaternary reduced molybdenum oxides Ca(x)R(17-x)Mo(19)O(46) (4 < or = x < or = 7; R = Ce, Pr, Sm, and Gd) have been synthesized by a solid-state reaction at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of Ca(5.24)R(11.76)Mo(19)O(46) by X-ray diffraction. This compound crystallizes in the monoclinic space group C2/m: a = 19.5192(4) A, b = 11.1244(3) A, c = 13.2589(5) A, beta = 132.055(1) degrees, V = 2137.7(1) A(3), Z = 2. Refinements yield R1(F(2)) = 0.0388 and wR2(F(2)) = 0.0792 for 5667 unique reflections. The structure is built up from alternating slabs made up of Mo forming centered Mo(19) nu(2)-octahedral clusters, Ca, Pr, and O atoms and slabs containing only Ca, Pr, and O atoms. The Mo(19) cluster, in which the Mo-Mo distances range from 2.7274(4) to 2.7940(7) A, results from a three-dimensional condensation of six Mo(6) octahedra. The Ca(2+) and Pr(3+) cations occupy seven crystallographically independent sites with coordination numbers in the O atoms varying from 6 to 8. Magnetic susceptibility measurements made on the Ca(x)Pr(17-x)Mo(19)O(46) (x = 4-7) compounds confirm the presence of Pr(3+) cations, and no magnetic ordering was observed down to 4.2 K. Electrical resistivity measurements on a single crystal of Ca(approximately 5)Pr(approximately 12)Mo(19)O(46) show a semiconducting behavior.