Positions of the morphotropic phase boundaries in solid solutions of lead titanate

The morphotropic phase boundary, which, separates the tetragonal and rhombohedral ferroelectric solid solutions in the (1 - x)PbTiO3 +xPbBO(3) and (1 - x)PbTiO3 + xPb(B', B")O-3 systems, is almost always close to x = 0.5, when the relation of numbers of the smaller Ti and larger B or (B', B") ions is near to 1: 1. Then, the smaller and larger ions can be ordered chequerwise. As the main cause of the morphotropic boundary existence, the paper considers growth of the B, B', B" ion sizes at increase of x. Nevertheless, it is assumed that its appearance can be triggered by the ion ordering, expected at x approximate to 0.5. Possibly, such an ordering could assist also to formation of some intermediate monoclinic ferroelectric phase.