Magnetic Properties of KMn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">F</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>. I. Crystallographic Studies

The lattice parameters for KMn${\mathrm{F}}_{3}$ have been determined by means of an x-ray rotation camera designed for temperatures down to 15\ifmmode^\circ\else\textdegree\fi{}K. The cubic room temperature perovskite structure transforms at 184\ifmmode^\circ\else\textdegree\fi{}K to an orthorhombic phase with ${{D}_{2h}}^{16}\ensuremath{-}Pbnm$ as the most probable space group with a unit cell containing 4 formula units; it has a tetragonal pseudocell with $\frac{c}{a}&gt;1$ in which the fluorine octahedra about the manganese remain essentially regular but till relative to the crystal axes. At 84\ifmmode^\circ\else\textdegree\fi{}K, just below the N\'eel temperature of 88\ifmmode^\circ\else\textdegree\fi{}K, the pure antiferromagnetic also has a tetragonal pseudocell, but with $\frac{c}{a}&lt;1$ and the still essentially regular octahedra rotated as well as twisted. Below a second magnetic transition at 81.5\ifmmode^\circ\else\textdegree\fi{}K, the 65\ifmmode^\circ\else\textdegree\fi{}K structure shows in addition a significant distortion of the regularity of the octahedra.

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