Crystal structure and charge distribution of YbFeMnO4.

The structure of synthetic YbFeMnO(4) has been refined by single-crystal X-ray diffraction. Space group R3m, a = 3.4580 (1), c = 25.647 (3) A, V = 265.59 (3) A(3), Z = 3. Yb is in octahedral coordination, whereas Fe and Mn are disordered on a single crystallographic type of trigonal bipyramid, in which the cation is off-centred from the basal plane. Assuming perfect stoichiometry, R(1) = 0.0195, but the charge distribution (CD) analysis suggests incomplete occupation of the Yb site. Refinement of the occupancy lowers R(1) to 0.0175, resulting in s.o.f.(Yb) = 0. 963 (3), with a significant improvement of the Fourier difference. The electroneutrality is likely preserved through incomplete occupancy of one of the two oxygen sites: the compound is thus non-stoichiometric, with the formula Yb(0.963)FeMnO(3.945). Another mechanism for preserving the electroneutrality is the oxidation of a small amount of Mn(2+) to Mn(3+), which is, however, less probable because of the reduction conditions in which the sample was synthesized. Both models give a satisfactorily CD result, but they cannot be definitively distinguished by X-ray data.