FERRIMAGNETISM IN NI4NB2O9

For the Pbcn form of nickel diniobate Ni4Nb2O9 ferrimagnetic ordering is observed below Tc = 76.1(1) K. Two models for the magnetic structur e result in calculated intensities for the magnetic reflections, which are in satisfying agreement to the data observed in neutron powder di ffraction. In both cases the magnetic structure consists of two ferrom agnetic sublattices according to the two Ni sites, which are antiparal lel to each other. One model belongs to the magnetic symmetry group Pb 'cn' with the moments aligned parallel [010] and the other one to Pbc' n' with the moments parallel [100]. The absolute values of the magneti c moments, constrained to the same value for both sites, are determine d to \mu\ = 1.35(5)mu(B) (60 K) and 1.65(5)mu(B) (1.5 K) for both mode ls; A compensation temperature of T-comp = 33 K is observed, and the m agnetic properties can be explained by the competition between antifer romagnetic intrasite and intersite couplings using mean held approxima tion. In the paramagnetic region an averaged magnetic moment of 3.54(5 )mu(B) is derived from the Curie-Weiss constant. These values are in g ood agreement with the predictions by Hund's rules for Ni2+.