Abstract
a) The perovskite phase was not obtained by firing the compositions Pb(Cr1/2Nb1/2)O3 and Ba(Cr1/2Nb1/2)O3 in air, although the compounds Ca(Cr1/2Nb1/2)O3 and Sr(Cr1/2Nb1/2)O3 have been prepared with the perovskite structure. These results are attributed to the lower ionic potential of Ba2+ and Pb2+ ions.b) The compound Pb(Cr1/2Nb1/2)O3 was prepared with the cubic perovskite structure by firing in an inert gas. The lattice constant was given as a=3.975A.c) The compound Ba(Cr1/2Nb1/2)O3 was prepared with the hexagonal layer structure having two polytypes by firing in an inert gas. The compound with 6-layer hexagonal structure was obtained as a single phase in a low-temperature reaction. The lattice constants were determined as a=5.758A and c=14.108A. Then the compound with 8-layer hexagonal structure was obtained as a single phase in a high-temperature reaction. The lattice constants were determined as a=5.741A and c=18.778A. The 8-layer hexagonal structure has one hexagonal stacking out of four in the layer sequence, and differs from any of several types of layer structures which have been well known in the perovskite-type compounds.d) The compound Ba(Cr1/2Nb1/2)O3 was prepared with the cubic perovskite structure by high pressure reaction. The lattice constant was determined as a=4.026A.e) The X-ray densities for the compound Ba(Cr1/2Nb1/2)O3 obtained by different reaction conditions decrease in the order of the cubic perovskite structure, the 8-layer hexagonal structure and the 6-layer hexagonal structure. This result indicates that the volume of unit cell increases for the repulsive force between Cr3+ ions in the octahedra sharing faces.f) The barium compounds containing Cr3+ ion tend to have the hexagonal layer structure. This will be attributed to the weaker repulsive force between Cr3+ ions without eg electrons than that between the other transition metal ions with eg electrons.g) The same results as described above were obtained when Nb5+ ion was replaced by Ta5+ ion in the Nb-containing compounds.
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