Synthesis and structural, magnetic and electrical characterisation of the reduced oxoniobates BaNb8O14, EuNb8O14, Eu2Nb5O9 and Eu NbO3 (x = 0.7, 1.0)

Powder samples of the reduced oxoniobates Eu0.7NbO3, EuNbO3, Eu2Nb5O9, EuNb8O14 and BaNb8O14 have been synthesised. EuNbO3 and Eu0.7NbO3 adopt the perovskite type structure (space group Pm3m) with unit cell parameters a = 4.021(1) and 3.981(3) A , respectively. Eu2Nb5O9 exhibits an intergrowth structure which can be described as alternating slabs of NbO and EuNbO3. It crystallises in P4 mmm with a = 4.131(1) and c = 12.043(5) A . EuNb8O14 and the here synthesised modification of BaNb8O14 is isostructural with SrNb8O14. The structures of EuNb8O14 and BaNb8O14 have been refined from X-ray powder diffraction data with the Rietveld technique. They have the unit cell parameters a = 9.2272(1), b = 10.2647(2), c = 5.9381(1) and a = 9.3451(1), b = 10.2689(2), c = 5.9470(4) A , (space group Pbam), respectively. In all the studied compounds AO12 cuboctahedra (A = Eu, Ba) form either chains (in ANb8O14), double layers (in Eu2Nb5O9) or three-dimensional nets (in EuNbO3 and Eu0.7NbO3). Magnetic susceptibility measurements show temperature independent paramagnetism for BaNb8O14, while the susceptibilities of the Eu niobates follow Curie-Weiss laws with effective magnetic moments in good agreement with the expected μeff = 7.9 μB for Eu2+. Eu0.7NbO3 and Eu1.0NbO3 exhibit ferromagnetic ordering below 4 K. BaNb8O14 and EuNb8O14 are semiconductors, while Eu2Nb5O9, EuNbO3 and Eu0.7NbO3 are metallic.