Magnetic order in the TbCo6Ge6 and TbCo2Ge2 compounds:: A neutron study

A structure determination using high-resolution neutron powder data has shown that the hexagonal compound TbCo6Ge6 (P6/mmm) has the YCo6Ge6-type structure. The R and the Gel atoms share the 1(a) (000) and the 2(e) (00z) sites while the Ge-2 and Co atoms are at 2(c) (1/3, 2/3, 0) and 3(g) (1/2, 0, 1/2), respectively. The refinement has shown that the Tb/Ge distribution is 0.36 Tb and 0.14 Ge-1 for the 1(a) site and 0.74 Gel and 0.26 Tb for the 2(e) site. This atomic distribution leads to different R-R interactions to those observed in the parent compounds TbMn6Ge6 and TbFe6Ge6. As the Co-Co interaction is very weak a possible TbCo6Ge6 ordering could be caused by the R-R interaction. However, no evidence of magnetic ordering in TbCo6Ge6 has been obtained down to 1.5 K. The coexisting tetragonal phase (14/mmm) of TbCo6Ge6 with ThCr2Si2 structure reported earlier to order antiferromagnetically at 31 K with a uniaxial moment arrangement along c and the wave vector (001) was found to undergo a magnetic phase transition at 22 K. Above this temperature the magnetic structure changes into a longitudinal sinewave incommensurate phase with the wave vector q=(0, 0, 1-q(z)), or q(z)=0.063(1) r.l.u when referring to the primitive cell. This wave vector corresponds to a 16xc cell enlargement and its length is temperature-dependent and at T(1)approximate to 22 K locks in to the (001) value. The Tb atoms related by the centering translation are coupled antiparallely over the entire ordered regime. The ordered moment value varies from 8.8(1)mu(B)/Tb atom at 1.5 K to 7.9(1) mu(B)/Tb at 22 K. The amplitude of the modulated structure is 9.3(2)mu(B) at 24 K. (C) 1998 Elsevier Science S.A.