NUCLEAR AND MAGNETIC-STRUCTURES OF THE 2-DIMENSIONAL ANTIFERROMAGNET KFEF4

The nuclear and magnetic structures of the layered compound KFeF4 (tha t undergoes structural and magnetic phase transitions at T-c = 380 K a nd T-N = 137 K respectively) are investigated by neutron powder diffra ction over a large temperature range (673-2 K). At 673, 543 and 413 K, it is deduced that KFeF4 exhibits the ideal high symmetry structure ( aristotype) with Bmmb space group; there is no evidence for the phase transition observed by Hidaka el a[ at 563 K. The diffraction patterns collected below T-c (at 300, 170 K) are consistent with the Pcmn spac e group and the phase transition mainly arises from FeF6 octahedron ro tation around the [001] axis. The magnetic structure is reinvestigated in the framework of the new low temperature space group. It is deduce d that the moments are aligned normally to the FeF6 sheets with antife rromagnetic coupling in the planes. It seems that only the coexistence of two phases can describe the magnetic interactions between the plan es.