A new family of ferroelectric tetragonal tungsten bronze phases, Ba2MTi2X3O15

The compositions Ba2MTi2X3O15: M = La, Pr, Nd, Sm, Eu, Gd, Dy, Bi; X = Nb, Ta form a new family of ferroelectric phases. With larger M cations, La, Bi, the materials exhibit relaxor behaviour with sub-ambient Tc. With decreasing M size, a sharp first order ferroelectric phase transition is observed and Tc rises to a maximum at ∼330 °C for Gd. The phases are tetragonal tungsten bronzes with A-site order (BaM) and non-statistical B-site disorder (of Ti, Nb for M = La). All phases show a weak superstructure with doubled c and multiple periodicity parallel to [1 1 0] that is incommensurate for the relaxor ferroelectrics (M = La, Bi) but commensurate for the remainder. The phases exhibit complex non-stoichiometry with variable Ba:M and Ti:X ratios as well as variable oxygen content.