CRYSTAL AND MAGNETIC-STRUCTURE OF TBCO0.5NI0.5C2

The pseudo-ternary intermetallic dicarbide TbCo0.5Ni0.5C2 has been inv estigated by powder neutron diffraction in the temperature range from 300 K to 2 K. The crystallographic space group is Amm2 with Tb on 2a ( 0, 0, 0), Co and Ni statistically distributed on 2b (1/2, 0, 0.608(5)) and C on 4e (1/2,0.154(5)), 0.305(4)); room temperature lattice param eters are a = 3.558(2) A, b = 4.501(1) Angstrom and c = 6.045(2) Angst rom. C-C distances are 1.39 Angstrom. Below 13 K the Tb-moments order magnetically in a two-component structure: (1) an incommensurate compo nent along c with propagation vector [0.165(1), 0, 0] due to a sine-mo dulation of the moments (amplitude 6.82(2)mu(B) and (2) an additional commensurate ferromagnetic component of 2.5(2)mu(B) along a. The magne tic order of Tb in TbCo0.5Ni0.5O2 appears to be a compromise between T bNiC2, antiferromagnetic along c, and TbCoC2, ferromagnetic along a.