Magnetically Induced Electric-Field Gradients in Octahedral Fe2+:RbFeF3

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Abstract
A theory for magnetically induced electric‐field gradients in ferrous compounds, where the Fe2+ ion occupies a site of octahedral symmetry, is outlined. From the sign of the quadrupole splitting, as determined from a Mössbauer measurement on a powder sample, one can, in this model, determine the direction of the magnetic axis in the crystal. Comparison of the theory with the experimental results on RbFeF3 in the antiferromagnetic region leads to the prediction that the magnetic axis should be parallel to a (111) direction. Recently, however, a small tetragonal distortion has been observed in RbFeF3 below TN, indicating that the proposed model cannot account for the appearance of the quadrupole splitting in this case. Below 85°K, RbFeF3 undergoes a second transition to a state with spontaneous magnetization, which is a crystallographic transition.