STUDY OF THE BIPYRAMIDAL SITE IN MAGNETOPLUMBITE-LIKE COMPOUNDS, SRAL12O19, SRFE12O19, SRGA12O19

Structures of magnetoplumbite-type crystals SrM12O19 (M = Al, Fe, Ga) have been refined by the single-crystal X-ray diffraction method. The space group is P63/mmc with Z = 2. The cell dimensions are a = 5.5666(2) and c = 22.0018(8) Å for SrAl12O19, a = 5.8836(1) and c = 23.0376(9)Å for SrFe12O19, and a = 5.7929(1) and c = 22.8123(7) Å for SrGa12O19 at 22°C. Four structural models were tested with respect to the states of M atoms at trigonal bipyramidal sites: central atom and split atom models with and without anharmonic thermal vibrations. The simple split atom model gave lower Rw values than the central atom model with anharmonic thermal vibrations for SrAl12O19 and SrGa12O19, whereas it gave an Rw value approximately equal to that given by the central atom model with anharmonic vibration for SrFe12O19. The present study suggested that the potential around the M atom with trigonal bipyramidal coordination has double minima in all the magnetoplumbite-type crystals at lower temperatures. © 1990.