Crystal structure and magnetic properties of a new form of NH4MnFeF6

The hydrothermal synthesis at 380°C, 200 MPa of NH₄MnFeF₆, NH₄MnCrF₆, and RbMnFeF₆ leads to a new AM^IIM^IIIF₆ structural type of orthorhombic symmetry with Z = 8. Lattice constants are found to be, respectively, a = 7.844 (4), b = 12.819 (8), c = 10.582 (6); a = 7.808 (5), b = 12.755 (9), c = 10.501 (7); and a = 7.913 (5), b = 12.858 (9), c = 10.619 (5). The structure was solved for NH₄MnFeF₆ from 755 X-Ray reflections and refined to R_ω = 0.029 in the space group Pb2nC⁶_2v. The network is built from edge-sharing MnFeF₁₀ bioctahedra connected to each other by their vertices. RbMnFeF₆ upon heating transforms irreversibly to the modified pyrochlore structure at 881 K. From magnetic and Mössbauer experiments, NH₄MnFeF₆ and NH₄MnCrF₆ are established to be antiferromagnetic with T_N = 117.7 ± 0.5 K and < 6 K, respectively.