Structural phase transition and domain structures in YMn _1-xTi_xO₃

We investigated changes in domain structures induced by a partial substitution of Ti for Mn in multiferroic YMnO₃ by transmission electron microscopy (TEM). In the ferroelectric phase of YMnO₃ at room temperature, there exist 180-degree ferroelectric (FE) domain structures of a micrometer size. On substituting Ti for Mn, the size of FE domains becomes fragile and, at approximately x = 0.175 in YMn_1-xTi_xO ₃, the size can be estimated to be 10-20nm. On further substitution, it is found that a structural phase transition from a hexagonal P6₃ cm structure to a new rhombohedral R3̄c one takes place at approximately x = 0.20. In YMn_1-xTi_xO₃ with 0.20 < x < 0.40, these two distinct structures (the hexagonal P6₃ cm and rhombohedral R3̄c structures) coexist and this coexistence should have some strong effect on the ferroelectric properties such as the magnetocapacitance effect found in Ti-substituted YMnO₃.