A new mixed-valence iron (Fe2+/Fe3+) fluoride material with a layered perovskite-related structure has been synthesized and characterized. The material, K4Fe3F12 [K-4(Fe2+)-(Fe3+)(2)F-12], was synthesized using mild hydrothermal conditions. The material exhibits a layered perovskite structure consisting of alternating sheets of apex-linked Fe2+F6 and Fe3+F6 octahedra; thus, each layer of Fe2+F6 centers is sandwiched between two layers of Fe3+F6 centers. Magnetization and neutron powder diffraction data show that, upon cooling below 120 K, K4Fe3F12 adopts a magnetically ordered state in which the Fe3+ and Fe2+ spins are aligned in an approximately antiparallel manner to each other to yield a pseudoferrimagnetic structure with a net spontaneous moment of 5.41 mu B per formula unit at 10 K. Crystal data: K4Fe3F12, trigonal space group R (3) over barm (No. 166), a = b = 5.7649(9) angstrom, c = 28.086(9) angstrom, V= 808.36(3) angstrom(3), Z = 3, T = 296(2) K.