ANTIFERROMAGNETISM IN THE TERNARY CARBIDES Tb₂Cr₂C₃, Ho₂Cr₂C₃ and Er₂Cr₂C₃. By M. Reehuis, N. Stüßer, K. Zeppenfeld and W. Jeitschko, Hahn-Meitner-Institut, Berlin and Anorganisch-Chemisches Institut, Münster, Germany

The magnetic properties of the compounds R₂Cr₂C₃ (R = Y, Gd - Lu) with Ho₂Cr₂C₃ structure (C2/m) were determined with a SQUID magnetometer and by neutron powder diffraction. The chromium atoms do not carry a magnetic moment as is shown by the Pauli paramagnetism of the yttrium and lutetium compounds. The lanthanoid sublattices show antiferromagnetic order below the Néel temperatures T_N = 50 K (Tb₂Cr₂C₃), T_N = 14 K (Ho₂Cr₂C₃) and T_N = 7 K (Er₂Cr₂C₃) (K. Zeppenfeld, R. Pöttgen, M. Reehuis, W. Jeitschko and R. K. Behrens, J. Phys. Chem. Solids 54(1993)257). In the 2 K neutron powder patterns of Tb₂Cr₂C₃ and Er₂Cr₂C₃ the magnetic reflections could be indexed with the primitive cell corresponding to the monoclinic C-centered nuclear cell with the propagation vector k = (1 0 0). The possible orientation of the moments of the four lanthanoid atoms in the unit cell (in the positions: (1) x, 0, z; (2) -x, 0, -z; (3) 1/2+x, 1/2, z; (4) 1/2-x, 1/2, -z) can be of the types F(++++), G(+-+-), C(++--) and A(+--+). For the terbium and the erbium compound the magnetic moments are aligned within the monoclinic ab-plane of the type [A_x, 0, A_z] and [A_x, 0, -A_z], respectively. The magnetic structure of the terbium compound is identical to that one found earlier for Dy₂Cr₂C₃ (T_N = 23 K) (Reehuis, K. Zeppenfeld, W. Jeitschko and E. Ressouche, J. Alloys Comp. 209(1994)217). For Er₂Cr₂C₃ a second set of magnetic reflections with the propagation vector k = (0 0 1/2) was found indicating that the magnetic cell is doubled along the c-axis. The moments in the unit cell are ordered of the type [G_x, 0 -G_z] and are oriented nearly parallel to those of the structure with k = (1 0 0). For the carbide Ho₂Cr₂C₃ a magnetic structure with the propagation vector k = (1 0 -1/2) corresponding to the C-centered monoclinic cell was found. Within the unit cell the magnetic order of the holmium atoms is of the type [A_x, 0, A_z]. In these carbibes the moments are aligned in such a way, that they form an angle with the c-axis, which is practically the same as that one formed by the two neighbouring centrosymmetric lanthanoid atoms.