The TbFe6Ge6-xGax compounds (0.5 less than or equal tox less than or equal to3.5) have been prepared and studied by X-ray powder diffraction and magnetization measurements. For 0 less than or equal tox less than or equal to1.0, a TbFe6Sn6-type solid solution is stable. The TbFe6Ge4Ga2 sample is characterized by a monoclinic cell [P2/m; a=30.90(2) Angstrom, b=8.266(5) Angstrom, c=18.32(5) Angstrom and beta =99.21(5)degrees]. This cell, 25-times larger than the HfFe6Ge6-subcell, is characterized by the presence of HfFe6Ge6 fragments diluted in a ScFe6Ga6 matrix. The TbFe6Ge4Ga2 sample is characterized by a monoclinic cell 12-times larger than the HfFe6Ge6-subcell [C2/m; a=20.29(3) Angstrom, b=8,283(7) Angstrom, c= 15.20(2) Angstrom and beta= 119.98(4)degrees]. The TbFe6Ge3.5Ga2.5 and TbFe6Ge3Ga3 samples are characterized by orthorhombic cells [Cmmm a=5.064(2) Angstrom, b=26.32(2) Angstrom, c=8.317(4) Angstrom and a=5.065(2) Angstrom, b=149.1(1) Angstrom, c=8.347(4) Angstrom, respectively]. Both are characterized by different stackings of HfFe6Ge6 and ScFe6Ga6 infinite slabs. The magnetization measurements indicate an antiferromagnetic ordering of the Fe sublattice above the room temperature. The Neel temperatures increase from 536 K for TbFe6Ge5.5Ga0.5 to 646 K for TbFe6Ge3.5Ga2.5. The low temperature transitions related to the ordering of the Th sublattice also increase with the Ga contents from 13 to 40 K. For 0.5 less than or equal tox less than or equal to2.0, a possible ferrimagnetic order of the Fe and Th sublattices is suggested by this study. (C) 2001 Elsevier Science B.V. All rights reserved.
