Magnetic properties of a mixed-valence (III/IV) terbium fluoride KTb3F12

The mixed-valence terbium (III/IV) fluoride KTb3F12 crystallizes with the tetragonal symmetry, space group I4/m. Below TN=3.6±0.1 K this compound exhibits an antiferromagnetic collinear structure with k=0 characterized by ferromagnetic chains of edge-sharing Tb4+ polyhedra and an antiferromagnetic order between the nearest chains. The magnetic moments of the Tb4+ ions deduced from neutron diffraction are 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 a symmetry cancellation effect of the internal molecular field. In other words, Tb3+ ions present a zero time-averaged magnetic moments in the ordered state, 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−1 has been obtained, in good agreement with the theoretical value of 27.64 expected for two Tb4+ and one T...