Synthesis and characterization of reduced niobates CaLnNb2O7, Ln = Y, Nd with a pyrochlore structure

The first known reduced niobates with a pyrochlore structure, CaLnNb2O7Ln = Y, Nd, have been synthesized from Ca2Nb2O7, Ln2O3, and NbO2 in sealed Nb ampoules heated at 1400 °C for 6 h. Our studies show that these compounds have a small homogeneity region according to the formula Ca2 − x LnxNb2O7, Ln = Nd with x = 0.9, 1.0; and Ln = Y with x = 1.0, 1.1. The prepared compounds were characterized by EDX (energy dispersive X-ray) analysis, X-ray and electron diffraction, and thermogravimetric analysis. The crystal structures of CaNdNb2O7 and CaYNb2O7 were refined using X-ray and neutron powder diffraction data. CaNdNb2O7 and CaYNb2O7 have undistorted pyrochlore structures: space group Fd3m, a = 10.416(1) Å and 10.3159(8) Å, respectively. Resistivity measurements show that CaNdNb2O7 exhibits semiconducting behavior.