Add time:09/10/2019         Source:sciencedirect.com

The magnetic structure of the mixed-valence terbium (III/IV) fluoride KTb3F12 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 TN=3.6±0.1 K. The collinear magnetic structure belongs to the Ip4′/m magnetic Shubnikov space group with k=(1 0 0) and is characterized by ferromagnetic infinite chains of edge-sharing (TbF8)4− dodecahedra with antiferromagnetic ordering between the nearest chains arranged in a tetragonal packing. These (TbF6)2− chains of dodecahedra are further linked by (TbF8)5− distorted cubes surrounding the Tb3+ ions and alternating along the c direction with K+ ions in 12-coordination. The magnetic moment of the Tb4+ions deduced from the Rietveld refinement (Rmag=0.033) is equal to 6.95(4) μB at 1.4 K and aligned along the c-axis. The Tb3+ ions lie in a special position with zero magnetic moments due to symmetry cancellation effect of the internal molecular field. In other words, Tb3+ ions present a zero time-averaged magnetic moment in the ordered state (idle spin behavior) whereas TN corresponds to the long-range antiferromagnetic ordering of the moments of the Tb4+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 Tb4+ and one Tb3+ free ions. The respective contribution of the Tb4+and Tb3+ions to the crystal magnetization are deduced from high-magnetic-field magnetic measurements.

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