STRUCTURAL-CHANGES IN BA(SR1 3TA2/3)O-3-TYPE PEROVSKITE COMPOUNDS UPON TILTING OF OXYGEN OCTAHEDRA/

Since the early study by Galasso and Pyle, the structure of complex pe rovskite compounds with 1 : 2 ordering of B-site ions, ordered perovsk ite compounds, has been thought to be Ba(Sr1/3Ta2/3)O-3-type pe hexago nal. Recently, a structural phase transformation has: been found in ii -site substituted ordered perovskite compounds, (Ba,Sr) (B'B-1/3 ''(2/ 3))O-3. The structural change due to the phase transformation has been connected with the tilting of oxygen octahedra in the perovskite cell . However, the structure of the transformed phase has not been clarifi ed because there has not been any model to treat the crystal structure of ordered perovskite compounds having tilled oxygen octahedra. In ti lls paper, Glazer's model for structural changes in cubic perovskite d ue to the tilting of oxygen octahedra has been expanded to the 1 : 2 o rdered perovskite. Based on the model, electron diffraction and X-ray diffraction of (Ba1-xSrx)(Mg1/3Ta2/3)O-3 are discussed. A monoclinic u nit cell in space group C-2h(6) with an angle gamma > 90 degrees, whic h is derived from antiphase tilting of oxygen octahedra, successfully explains the experimental results.