CRYSTAL-STRUCTURE AND B-SITE ORDERING IN ANTIFERROELECTRIC PB(MG1 2W1/2)O3, PB(CO1/2W1/2)O3 AND PB(YB1/2NB1/2)O3/

The high-temperature phase of Pb(Mg1/2W1/2)O3, Pb(Co1/2W1/2)O3 and Pb( Yb1/2Nb1/2)O3 belongs to the space group Fm3m with the unit cell size of 2a(c) X 2a(c) X 2a(c) where a(c) is the lattice parameter of the cu bic perovskite unit cell. All these complex perovskites undergo phase transition into low-temperature orthorhombic structures with a shrunke n lattice parameter along the c-axis and the approximate square-root 2 a lattice parameters along the a0- and b0-axes, which are, respective ly, parallel to the prototype [110]c and [110BAR]c directions. The sym metry along the b0-axis of the perovskites is most prominently disting uished from that along the a0-axis in that the low-temperature structu re is modulated in the b0-axis direction. For Pb(Mg1/2W1/2)O3, a diffr action pattern detrimental to the crystal structure determination well below the Curie temperature is presented. At the same time, the cryst al structures of lock-in phases of the aforementioned oxides are compa red with the well-known structure of PbZrO3.