Dependence of exchange interactions on chemical bond angle in a structural series: cubic perovskite-rhombic orthoferrite-rhombohedral hematite

Mossbauer spectroscopy is used to study the hyperfine magnetic fields at tin Sn-119 ions introduced as an isomorphic impurity in the lattices of the orthoferrites RFeO3. The large reduction in the field H-hf(Sn) (4.2 K) observed when R is changed from La to Lu correlates with the drop in the Neel point and indicates that the exchange interactions are decreasing over this series. A crystal chemical analysis of the structural series with the general formula ABO(3) shows that the ideal structure of cubic perovskite can be converted to a rhombohedral hematite-corundum structure by simple rotation of the [BO6] octahedra if the B-O interionic distances remain unchanged. The rhombic distortions are associated with a reduction in the B-O-B bond angle from theta=180 degrees in perovskite to similar to 132 degrees in hematite. The rare earth orthoferrites RFeO3 follow the same mechanism for structural transformations and the LaFeO3-LuFeO3 series occupies an intermediate position (157 degrees>theta>142 degrees) between the extreme members of the series mentioned above. A reduction in the bond angle leads to weakening of the Fe-O-Fe exchange interaction, which shows up as a drop in the Neel temperature and in the hyperfine magnetic field at the nucleus. An analysis of theoretical models shows that for a suitable choice of the exchange and transfer parameters, the angular variation in the parameters of the exchange interaction is described fairly well by the Moskvin theory over a rather wide range of angles theta. The contributions to the fields H-hf(Sn) and H-hf(Fe) from the t(2g)- and e(g)-orbitals of neighboring paramagnetic ions in the orthoferrites and orthochromites are examined. (C) 1999 American Institute of Physics. [S1063-7761(99)02203-9].