Antiferromagnetic ordering in the KTb 3F 12 mixed-valence (III/IV) terbium fluoride studied by neutron diffraction and magnetic measurements

Abstract

The magnetic structure of the mixed-valence terbium (III/IV) fluoride KTb 3F 12 has been determined by means of neutron powder diffraction. This compound which crystallizes with the I4/m tetragonal space group (no. 87) and close lattice parameters, a=7.715(1) Å and c=7.530(1) Å (values from single-crystal X-ray diffraction at 298 K), exhibits a 3D antiferromagnetic order below T N=3.6±0.1 K. The collinear magnetic structure belongs to the I p4′/m magnetic Shubnikov space group with k=(1 0 0) and is characterized by ferromagnetic infinite chains of edge-sharing (TbF 8) 4− dodecahedra with antiferromagnetic ordering between the nearest chains arranged in a tetragonal packing. These (TbF 6) 2− chains of dodecahedra are further linked by (TbF 8) 5− distorted cubes surrounding the Tb 3+ ions and alternating along the c direction with K + ions in 12-coordination. The magnetic moment of the Tb 4+ions deduced from the Rietveld refinement ( R mag=0.033) is equal to 6.95(4) μ B at 1.4 K and aligned along the c-axis. The Tb 3+ ions lie in a special position with zero magnetic moments due to symmetry cancellation effect of the internal molecular field. In other words, Tb 3+ ions present a zero time-averaged magnetic moment in the ordered state (idle spin behavior) whereas T N corresponds to the long-range antiferromagnetic ordering of the moments of the Tb 4+ions. From the thermal variation of the reciprocal magnetic susceptibility, a molar Curie constant of 27.97(1) emu K mol −1has been obtained, in good agreement with the theoretical value of 27.64 corresponding to two Tb 4+ and one Tb 3+ free ions. The respective contribution of the Tb 4+and Tb 3+ions to the crystal magnetization are deduced from high-magnetic-field magnetic measurements.