CRYSTAL-STRUCTURE AND DOMAIN-WALL ORIENTATIONS OF ANTIFERROELECTRIC PB(YB1 2NB1/2)O-3/

The crystal structure of antiferroelectric Pb(Yb1/2Nb1/2)O-3 with orde red complex perovskite structure was reexamined by TEM (transmission e lectron microscope). Its domain-wall orientations were investigated by the spontaneous strain method. From the CBED (convergent-beam electro n diffraction) patterns, its point group was determined to be orthorho mbic mmm. During transition from the high-temperature cubic to low-tem perature orthorhombic phase, the prototype cubic cell is monoclinicall y distorted. The monoclinic cell was found to be contracted along the direction of Pb2+ shifts. The reduced symmetry of the orthorhombic pha se transformed from the prototypic phase allows the existence of ten s pecies of domain walls, i.e, six W-walls and four W'-walls. The breaki ng of reflection symmetry during transition occurred at (1 0 0), (0 1 0), (0 1 1), (0 (1) over bar 1), ((1) over bar 0 1) and (1 0 1) planes of the cubic prototype. These high-symmetry planes correspond to the W-walls in the ferroelastic phase. Taking into account the domain geom etry, the (1 0 0)(m) and (0 1 0)(m) planes art: the 90 degrees domain- boundaries and the others can be both 60 degrees and 120 degrees ones. The orientations of W'-walls calculated from the strain components we re approximately ((2) over bar 3 3)(m), (2 (3) over bar 3)(m), ((3) ov er bar 2 3)(m) and (3 (2) over bar 3)(m) planes at room temperature. O n approaching the transition these W'-walls tend to orient themselves toward the direction of 60 degrees or 120 degrees domain W-walls.