Magnetic Properties of KMnF3. I. Crystallographic Studies

Abstract

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°K. The cubic room temperature perovskite structure transforms at 184°K to an orthorhombic phase with ${D_{2h}}^{16}-Pbnm$ as the most probable space group with a unit cell containing 4 formula units; it has a tetragonal pseudocell with $\frac{c}{a}>1\mathrm{}$ in which the fluorine octahedra about the manganese remain essentially regular but till relative to the crystal axes. At 84°K, just below the Néel temperature of 88°K, the pure antiferromagnetic also has a tetragonal pseudocell, but with $\frac{c}{a}<1\mathrm{}$ and the still essentially regular octahedra rotated as well as twisted. Below a second magnetic transition at 81.5°K, the 65°K structure shows in addition a significant distortion of the regularity of the octahedra.