CRYSTAL AND MAGNETIC-STRUCTURES OF NAMNF4

An X-ray single crystal structure determination (monoclinic, space group P2(1)/c, a = 573.6(2), b = 489.2(1), c = 574.8(2) pm, beta = 108.07(2)-degrees, Z = 2; wR = 0.038 for 380 reflections) shows that NaMnF4 crystallizes in the same layered structure type as LiMnF4. In the quadratic layers the Mn-F-Mn bridges are strongly asymmetric, due to the Jahn-Teller effect, leading to an antiferrodistortive order of elongated octahedra. The bridging angle is 138.4-degrees. The Na+ ions are 6-coordinated as well. The common structural arrangement of both [MnF6] and [NaF6] octahedra shows topological relation to the rutile structure. In the magnetic measurements performed on powder samples NaMnF4 behaves as an antiferromagnet with a weak ferromagnetic component below 13 K. By neutron diffraction on powder (4-70 K) a magnetic cell doubled along the a-axis is found and below a Neel temperature of T(N) = 13 K the magnetic structure shows colinear antiferromagnetic arrangement of the spins pointing slightly (16-degrees) out of the layer plane. The resulting magnetic moment is 3.52-mu-B.