Ordered magnetic frustration: VII. Na 2NiFeF 7: Reexamination of its crystal structure in the true space group after corrections from renninger effect and refinement of its frustrated magnetic structure at 4.2 and 55 K

Abstract

A new refinement of the crystal structure at 300 K and of the magnetic structure at 4.2 and 55 K of the ferrimagnetic weberite Na 2NiFeF 7 is undertaken in order to fully reveal both the true space group of this compound and its magnetically frustrated character. The reflections which previously obliged us to choose space group Imm2 are only due to Renninger effect. The true space group is Imma ( a = 7.2338(3)Å, b = 10.3050(3)Å, c = 7.4529(3)Å, Z = 4) at 300 K. The structure was refined from 1148 independent reflections to R = 0.025 ( R w = 0.030). The ferrimagnetic behavior is confirmed ( T c = 88(2) K). Neutron powder diffraction shows that the nuclear and magnetic cells are identical and that there is an accident in the thermal evolution of the intensity of some magnetic peaks. Among the different modes given by the Bertaut's macroscopic theory, the best fit is obtained for both temperatures with the modes −F x and F x,G z for Fe 3+ and Ni 2+ sublattices, respectively, instead of −F x and +F x in the solution previously proposed by Heger. The corresponding moments are 4.93(11) and 1.36(21) μ B at 4.2 K ( R mag = 0.045) and 4.34(12) and 0.97(22) μ B at 55 K ( R mag = 0.052). The slight anomaly in the thermal variation of the intensity of some magnetic reflections at 50 K is due to a significant change in the spin canting at this temperature, without any modification of the magnetic modes. A Mo¨ssbauer study confirms the anomaly from the thermal variation of the magnetic hyperfine field at the Fe nucleus.