Evolution of domain structures in Na1/2Bi1/2TiO

The domain structures of Na${}_{1/2}$Bi${}_{1/2}$TiO${}_{3}$-Ba${}_{x}$TiO${}_{3}$ (NBT-$x$%BT) crystals for $x$$=$0, 4.5, and 5.5 have been investigated by polarized light and piezoresponse force microscopies. The results show that BaTiO${}_{3}$ (i) refines the size of polar nanoregions and enhances their self-organization, and (ii) suppresses the formation of proper ferroelastic domains at high temperatures in the paraelectric state, and it rather favors the formation of improper ones that form below the ferroelectric Curie temperature and that elastically accommodate the ferroelectric ones.