Neutron diffraction study of the magnetic structures of one-dimensional M_2TbF_6 (M = Li, K, Rb) fluorides: frustration, incommensurability and magnetic interactions

摘要

The three title compounds crystallize in the monoclinic system, with unit cell parameters (at 4.95K): Li_2TbF_6: a = 7.5643(3) A, b = 4.9372(3) A, c = 11.0710(4) A, beta = 107.012(3) deg, space group P2_1/c(n deg 14),K_2TbF_6:a = 6.5909(2) A, b = 11.4135(4) A, c = 7.1235(2) A, beta = 90.502(2) deg, space group C2/c (n deg 15), and Rb_2TbF_6: a = 6.8298(2) A, b = 11.8385(5) A, c = 7.2013(2) A, beta = 91.358(2) deg, space group C2/c (n deg 15). Neutron diffraction study down to 1.4 K revealed an antiferromagnetic ordering for the three compounds, occurring at T_N = 2.02(1), 1.60(5) and 2.10(5) K for Li, K and Rb representatives, respectively. All the magnetic structures are incommensurate and square-modulated with extremely long periods at 1.40K. While the magnetic structure of Li_2TbF_6 is characterized by a propagation vector k = (0, 0.01617(7), 0), those of the potassium and rubidium representatives exhibit propagation vectors k = (0.0073(2), 0, 0) and k = (0.01039(8), 0, 0), respectively, these three propagation vectors corresponding to the direction of the shortest interchain distances. This singular magnetic behaviour will be correlated with the nuclear structures and the potential competition between magnetic interactions occurring in these M_2TbF_6 compounds and will be discussed in the light of a fine topological analysis.