CRYSTAL AND MAGNETIC-STRUCTURES OF RBMNF4 AND KMNF4 INVESTIGATED BY NEUTRON POWDER DIFFRACTION - THE RELATIONSHIP BETWEEN STRUCTURE AND MAGNETIC-PROPERTIES IN THE MN3+ LAYERED PEROVSKITES AMNF4 (A = NA, K, RB, CS)

The crystal and magnetic structures of RbMnF4 and KMnF4 have been determined by neutron powder diffraction. The crystal symmetry of both compounds belongs to the layered perovskite structure and exhibits a pseudo-tetragonal unit cell, space group P2(1)/a. The [MnF2F4/2]- octahedra show a distortion induced by both steric and Jahn-Teller effects. They are also tilted by an angle which depends on the size of the alkali ion. KMnF4 orders as a non-collinear antiferromagnet below 5.2 +/- 0.1 K exhibiting four magnetic sublattices with an angle between the two spin directions of 17-degrees. RbMnF4 is a collinear antiferromagnet below 3.7 +/- 0.1 K. Interestingly enough and contrary to what is found for the K compound, there are two active irreducible representations in the magnetic structure of the Rb derivative. Moreover, the relationship between crystal structure and magnetic behaviour has been investigated in the AMnF4 (A = Na, K, Rb, Cs) series. The sign of the isotropic magnetic interaction is studied as a function of the superexchange angle Mn-F-Mn and the degree of distortion of the octahedra. The critical superexchange angle alpha(c), at which the crossover between ferromagnetic (alpha > alpha(c)) and antiferromagnetic (alpha < alpha(c)) isotropic interaction takes place in the AMnF4 family, is found to be alpha(c) congruent-to 147-degrees.