Studies of single crystalline tetragonal tungsten bronze type K₃Fe₅F₁₅, a potential magnetoelectric material

Two phase transitions of K₃Fe₅F₁₅, at about 290 and 120 K, have been identified by polarized light microscopy and magnetic measurements on single crystals grown from high temperature solution with subsequent quenching from 973 K. The former one at about 290 K is a new second order structural transition from the orthorhombic to a monoclinic phase, as shown by the appearance of new types of ferroelastic domain walls and a rotation of the optical indicatrix around the pseudo-tetragonal principal axis. The transition at about 120 K is of second order and magnetic nature, accompanied by anomalies of spontaneous linear birefringence and antiferromagnetic type magnetic susceptibility. The temperature dependence of the magnetization suggests that a spontaneous ferromagnetic component appears in the magnetically ordered phase below 120 K.