Phase transitions in In₂Se₃ as studied by electron microscopy and electron diffraction

Indium selenide undergoes several reversible inter‐ and intrapolytypic phase transitions, above and below room temperature. A number of superstructures of the hexagonal (or rhombohedral) room temperature phase have been discovered by means of electron diffraction. The transition at 200 °C which corresponds to the α → β transformation is accompanied by intense non‐radial diffuse scattering, which is attributed to anisotropic transverse acoustic phonons. On cooling, a one‐dimensional deformation modulated structure is formed, which is interpreted as the frozen‐in configuration of the predominant vibration mode, which becomes soft below the transition temperature in the range 60 to 200 °C. This transition shows a large hysteresis, which is also found in thermal expansion. A new low temperature phase, which is presumably orthorhombic, is discovered on cooling below −125 °C. The structure of this phase can alternatively be described as being the result of pairing of indium ions or as the frozen‐in configuration of a longitudinal mode. Both phases exhibit a domain structure, which in the case of the α‐phase can be revealed by lattice resolution. Several high temperature superstructures are observed by means of electron diffraction. A direct relationship with the β‐phase is established and in a number of cases the superperiods are directly imaged.