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.