Synthesis and Characterisation of Reduced Niobates CaMNb₂O₇, M=La-Ce, Sm, Gd, Dy-Lu and Y With a Pyrochlore Structure. S.Ya. Istomine, O.G. D'yachenko, G. Svensson^*, E.V. Antipov, Chemical Department, Moscow State University, 119899, Moscow, Russia, ^*Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden

Complex niobium oxides are considered as promising candidates to find new superconductors. The properties of such compounds are noticeably influenced by crystal structure type and formal valence of niobium. The first known reduced niobates with a pyrochlore structure, CaMNb₂O₇, M=La-Ce, Sm, Gd, Dy-Lu and Y, have been synthesized from stoichiometric mixtures of Ca₂Nb₂O₇, NbO₂ and appropriate M₂O₃ in sealed Nb ampoules heated at 1350-1500⁰C for 6-10h. The prepared compounds were characterized by EDX analysis, X-ray and neutron powder, electron diffraction and thermogravimetric analysis. The compounds have undistorted pyrochlore structure. The a parameter of the CaMNb₂O₇ increases concomitantly with increasing of M³⁺ cations radii. CaMNb₂O₇ phases have small homogeneity regions according to the formula Ca_2-xM_xNb₂O₇, e.g. for M=Nd, 0.9(x(1.0 and M=Y 1.0(x(1.1.

The crystal structures of CaNdNb₂O₇ and CaYNb₂O₇ compounds were refined using X-ray and neutron powder diffraction data. NPD study of CaNdNb₂O₇ (a=10.416(1)(, S.G.: Fd3m, R_I=0.023, R_P=0.056) showed the absence of oxygen nonstoichometry as was confirmed by a TG analysis also.

Resistivity measurements showed the semiconducting-like type of conductivity. The ((T) dependence for the CaMNb₂O₇ with a magnetic rare-earth cations obey the Curie-Weiss law while for M=La, Lu and Y nearly temperature independent paramagnetism was found.