Local perturbation in Bi2CuO4: Hydrothermal synthesis, crystal structure, and characterization of the new Bi-2(Cu1-2xMx)O-4 (M = Bi, Pb)

Two new compounds Bi-2(CU1-2xMx)O-4 (M = Bi, Pb) have been hydrothermally s ynthesized and structurally characterized. Hydrothermal treatment of Bi6O5( NO3)(5)(OH)(3). 5H(2)O, Pb(NO3)(2), and CuO mixtures in NaOH solution at 18 0 degreesC for a week yielded single crystals having the nonstoichiometric compositions Bi2.08Cu0.84O4 and Bi2Pb0.04CU0.92O4 depending on the starting mixture. The two compounds crystallize tetragonally, P4/ncc, Z = 4 with a = 8.588(3) A, c = 5.799(2) Angstrom and a = 8.535(4) Angstrom, c = 5.812(3) Angstrom for Bi2.08Cu0.84O4 and Bi2Pb0.04Cu0.92O4, respectively. The struc ture refinements based on 157 and 155 independent reflections respectively converged to R1 = 0.036, wR2 = 0.069 and R1 = 0.049, wR2 = 0.121, respectiv ely. The parent compound Bi2CuO4 structure consists of CuO4 square-planar u nits stacked one on top of another in a staggered manner by Bi3+ ions along the c axis. The (CuO4)(8) columns are connected by Bi3+ ions. In these new compounds, two consecutive Cu2+ ions are randomly removed and an M ion (M = Bi3+, Pb2+) occupies the site at the middle of the two copper vacancies a nd are thus 8-fold-coordinated by oxygen atoms. This involves the presence of a mixed Cu2+/3+ valence state. Magnetic measurements indicated a low-tem perature behavior different from that of the solid-state-prepared Bi2CuO4, but very similar to the Bi2CuO4 samples previously hydrothermally synthesiz ed, which have the nonstoichiometric composition Bi-2(Cu1-2xBix)O-4 with an as yet undetected low x value. The present compounds are the first hydroth ermally prepared Bi2CuO4-derived oxides whose structures have been fully de termined.