Synthesis, crystal structure, and physical properties of a new reduced lanthanum niobium oxide: LaNb7O12

Abstract

Single crystals of LaNb7O12 were obtained by annealing a mixture of La2O3, Nb2O5, and Nb in 1:1:1 molar ratio with NH4C1 in a sealed quartz tube. LaNb7O12 crystallizes in the monoclinic space group P21/c with cell parameters a = 10.762(1), b = 9.192(1), c = 10.314(1) Å, β = 104.25°, and z = 4 formula units per cell. The structure was determined from single-crystal X-ray diffraction data and refined to a residual of R(F) = 0.037 and Rw(F2) = 0.077 for 119 variable parameters and a unique data set of 1400 observations [I > 2σ(I)]. The essential building units of the structure are Nb6 and NbO6 octahedra and La ions in eight coordination of oxygen. Two NbO6 octahedra corner-share to form a pair and connect with Nb6 octahedra via oxygen atoms. Nb6 octahedra share both apical and inner oxygen atoms with neighboring Nb6 to form columns of Nb6O12 clusters along the c axis. The Nb6O12 clusters exhibit 14 valence electrons in M-M bonding states. LaNb7O12 is a semiconductor with an activation energy of 0.08 eV. The magnetic susceptibility data show weak temperature-independent paramagnetism. The Madelung part of the lattice energy has been calculated and is discussed. © 1996 Academic Press, Inc.