Birefringent Materials Are Used to Create Polarising Filters or Interference Colours

Birefringence, a property of a crystalline material describes how within the material there are two distinct indices of refraction. Birefringent materials are used to create polarising filters or interference colours.

Krystian Roleder, the Regional Editor for the Central and Eastern Europe edition of the multinational journal ‘Phase Transitions’ and Researcher at the Institute of Physics, University of Silesia, Poland, has been studying birefringence measurements for a number of years. Mr Roleder has been using the THMSG600 stage to study birefringence in several materials, including BaTiO3. He commented: “This Linkam stage is indispensable for my scientific work."

In this research, Roleder and his colleagues have focused on the materials properties above crystallisation temperature. BaTiO3 has been the focus of many research projects since its first recognition in the 1940s. Extensive studies have looked at the phase transitions and domain structure along with the optical, electrical and mechanical properties. The cubic to tetragonal phase change is described as the order-disorder type and is displacive in nature. It is widely accepted that the Ti ion becomes displaced from its position within the oxygen octahedron, and this is the cause of the polar region formation within the crystal. It was found that the BaTiO3 crystals were birefringent over a broad temperature range but disappeared over 160-170°C.

The group has demonstrated the temperature dependence of the birefringence above crystallisation temperature and the behaviour of anomalous birefringence can be attributed to the existence of polar clusters. These polar clusters originate due to the interdependence of the structural and polar soft modes. These polar clusters interrupt the cubic symmetry of the crystal and about crystallisation temperature the crystal become birefringent. These polar regions are connected to the movement of the Ti molecule within the oxygen cage.

Samples were tested using the THMSG600 stage on an Oxford Cryosystems Metripol Birefringence Imaging System and heated at a constant 0.2°C/min.

The THMS600 is one of the most widely used heating and freezing microscope stages on the market. With its excellent 0.1°C accuracy and stability, the THMS600 is used in many applications including birefringence research. The stage gives researchers the option to characterise their sample quickly by heating to within a few degrees of the required temperature at a rate of up to 150°C/min with no overshoot.

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