Crystal Structure and Charge Distribution of ErFeMnO4.

The structure of synthetic ErFeMnO4 has been refined by single-crystal X-ray diffraction. Space group is Rm, a=3.4831(1) A, c=25.609(4) A, V=269.07(5) A3, Z=3. Er is in octahedral coordination, whereas Fe and Mn occupy a single crystallographic type of distorted trigonal bipyramid, with one of the two apical bonds shorter and the other longer than the three basal bonds. Assuming Er in the centre of symmetry R1=0.031, but the Charge Distribution analysis suggests that the position of Er was not completely determined, and the Fourier difference indicates splitting of the Er cation around the centre of symmetry. Refinement in the split-atom model for Er lowers R1 to 0.016, reduces the displacement parameter of Er and removes the anomalies in the Fourier difference. A deviation from the mean structure in this kind of compounds seems regularly associated with the presence of distorted trigonal bipyramids.