The Crystal Structures of KMnF₃, KFeF₃, KCoF₃, KNiF₃ and KCuF₃ above and below their Néel Temperatures

Abstract

The crystal structures of the antiferromagnets KMnF 3 , KFeF 3 KCoF 3 , KCNiF 3 and KCuF 3 have been determined above and below their Néel temperatures ( T N ) by X-ray diffraction using single crystals. At room temperature (above T N ) the structures of these compounds are of the ideal perovskite type (cubic) except for that of KCuF 3 Which crystallizes as a tetragonal modification ( a > c ) of the perovskite type. At 78°K (below T N ) the lattice symmetries of KMnF 3 , KFeF 3 and KCoF 3 are monoclinic, rhombohedral (α<90°) and tetragonal ( a > c ), respectively, while KNiF 3 and KCuF 3 retain their own symmetries at room temperature. The lattice constants have been determined between these temperatures and their changes near the transition temperature were closely followed. This temperature region was extended up to 670°K for KCuF 3 .