Optical birefringence imaging of the phase transition of K₂Mn₂(SO₄)₃

The present paper reports on the observation of the optical birefringence as a function of temperature of the langbeinite-type crystal K₂Mn₂(SO₄)₃ by using an imaging version of the so-called rotating-polarizer method. This method provides separate moving images of the orientation of the optical indicatrix and the magnitude of the optical anisotropy, showing the domain-wall behaviour during temperature changes. Some new results on the domain behaviour of the phase transition of K₂Mn₂(SO₄)₃ have been obtained. It is concluded that the sample single crystal consists of local regions, indicating the coexistence of two phases in this langbeinite-type crystal.