Triangular Exchange Interaction Patterns in K₃Fe₆F₁₉: An Iron Potassium Fluoride with a Complex Tungsten Bronze Related Structure

The synthesis and structural and magnetic characterizations of K₃Fe₆F₁₉, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K₃Fe₆F₁₉ compound has an orthorhombic structure (space group <i>Cmcm</i> (63), <i>a</i> = 7.6975(3) Å, <i>b</i> = 18.2843(7) Å, <i>c</i> = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality