SYNTHESIS AND STRUCTURAL, MAGNETIC AND ELECTRICAL CHARACTERIZATION OFTHE REDUCED OXONIOBATES BANB8O14, EUNB8O14, EU2NB5O9 AND EUXNBO3 (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 ado pt the perovskite type structure (space group Pm3m) with unit cell par ameters a = 4.021 (1) and 3.981 (3) Angstrom, respectively. Eu,Nb,Og e xhibits 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) Angstrom. EuNb8O14 and the here synthesised modifica tion of BaNb8O14 is isostructural with SrNb8O14. The structures of EuN b8O14 and BaNb8O14 have been refined from X-ray powder diffraction dat a 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 = 10.268 9(2), c = 5.9470(4) Angstrom, (space group Pbam), respectively. In all the studied compounds AO(12) cuboctahedra (A = Eu, Ba) form either ch ains (in ANb(8)O(14)), double layers (in Eu2Nb5O9) or three-dimensiona l nets (in EuNbO3 and Eu0.7NbO3). Magnetic susceptibility measurements show temperature independent paramagnetism for BaNb8O14, while the su sceptibilities of the Eu niobates follow Curie-Weiss laws with effecti ve magnetic moments in good agreement with the expected mu(eff)=7.9 mu (B) for EU(2+). Eu0.7NbO3 and Eu1.0NbO3 exhibit ferromagnetic ordering below 4 K. BaNb8O14 and EuNb8O14 are semiconductors, while Eu2Nb5O9, EuNbO3 and Eu0.7NbO3 are metallic.