Synthesis, Structure, Magnetic Properties, and XANES Spectra of Reduced Niobate RbNaxCa2Nb3O10

Abstract

Reduced layered niobate RbNaxCa2Nb3O10 was synthesized via the reaction of RbCa2Nb3O10 with sodium vapor at 500 °C. The structure has been refined from X-ray powder diffraction data, revealing that this black compound crystallizes in the space group P42/ncm with a = 5.4790(3) A, c = 28.8264(9) A, and Z = 4. The NbO6 octahedra are slightly tilted, and the interlayer geometry between the Ca2Nb3O8 units is similar to those of Rb2La2Nb3O10 and β-NaCa2Nb3O10. Magnetization measurements show no indication of superconductivity down to 4 K and instead exhibit a typical paramagnetic behavior. The influence of reductive sodium insertion on the electronic structure is discussed on the basis of Nb K-edge and O K-edge XANES spectra of RbNaxCa2Nb3O10. In addition, we report the crystal structure of another layered niobate, β-NaCa2Nb3O10, which was synthesized by the ion-exchange reaction of RbCa2Nb3O10 in molten NaNO3 at 340 °C. Powder neutron diffraction reveals that the white compound also adopts a tetragonal symmetr...