CRYSTAL AND MAGNETIC-STRUCTURES OF SRBI0.2TB0.8O3 VIA NEUTRON-DIFFRACTION

Crystal and magnetic structures of SrBi0.2T0.8O3 have been studied via neutron powder diffraction. As many other perovskites, SrBi0.2TD0.8O3 is orthorhombic, Pbnm space group, with a = 5.8948(2) Angstrom, b = 5 .9741(2) Angstrom, and c = 8.3793(3) Angstrom at T = 300 K. Magnetic-s usceptibility measurements yield antiferromagnetic order with the para magnetic Curie temperature theta(p) = -34.9 K and the effective magnet ic moment mu(eff) = 8.12 mu(B)/Tb. This proves that the substitution o f Bi for TD in SrTbO3 does not affect the valency of Tb which remains close to +4. Bismuth occurs as a mixture of Bi+3 and Bi+5 giving the e ffective valency also close to +4. Two magnetic transitions are observ ed: at T-N = 25.5 K and T-N' = 5.1 K. A collinear structure, with Tb m agnetic moments parallel to the b axis exists for T-N' less than or eq ual to T less than or equal to T-N. Below T-N', a small antiferromagne tic component develops parallel to the c axis, giving rise to the nonc ollinear antiferromagnetic structure. Both magnetic phases can be desc ribed within the Pbnm magnetic space group.