SYNTHESIS, CRYSTAL-STRUCTURE, AND PHYSICAL-PROPERTIES OF A NEW REDUCED LANTHANUM NIOBIUM OXIDE - LANB7O12

Single crystals of LaNb7O12 were obtained by annealing a mixture of La 2O3, Nb2O5, and Nb in 1:1:1 molar ratio with NH4Cl in a sealed quartz tube. LaNb7O12 crystallizes in the monoclinic space group P2(1)/c with cell parameters a 10.762(1), b = 9.192(1), c = 10.314(1) Angstrom, be ta = 104.25 degrees, and z = 4 formula units per cell. The structure w as determined from single-crystal X-ray diffraction data and refined t o a residual of R(F) = 0.037 and R(W)(F-2) = 0.077 for 119 variable pa rameters and a unique data set of 1400 observations [I > 2 sigma(I)]. The essential building units of the structure are Nb-6 and NbO6 octahe dra and La ions in eight coordination of oxygen. Two NbO6 octahedra co rner-share to form a pair and connect with Nb-6 octahedra via oxygen a toms. Nb-6 octahedra share both apical and inner oxygen atoms with nei ghboring Nb-6 to form columns of Nb6O12 clusters along the c axis. The Nb6O12 clusters exhibit 14 valence electrons in M-M bonding states. L aNb7O12 is a semiconductor with an activation energy of 0.08 eV. The m agnetic susceptibility data show weak temperature-independent paramagn etism. The Madelung part of the lattice energy has been calculated and is discussed. (C) 1996 Academic Press, Inc.