Crystal Structure and Chemical Bonding of Layered α-In2Se3

Layered alpha-In2Se3 has found widespread applications in the electronic, optoelectronic, and thermoelectric fields. However, the crystal structure of alpha-In2Se3, which plays a fundamental role in understanding its diverse physical properties, remains poorly explored. In this study, we present a comprehensive analysis of the temperature-dependent evolution of lattice constants, fractional coordinates, and atomic displacement parameters of alpha(3R)-In2Se3 using high-resolution synchrotron powder X-ray diffraction. The temperature range of investigation spans from 114.2 to 472.2 K. From temperature-dependent cell parameters, the linear thermal expansion coefficients along the a-axis and c-axis at room temperature are determined as 0.71 x 10(-5) K-1 and 1.83 x 10(-5) K-1, respectively, giving rise to a marked anisotropy owing to the weak interlayer interactions along the c-axis. Through modeling of isotropic atomic displacement parameters, the Debye temperature is evaluated to be 173 K, showing good agreement with the result by fitting the low-temperature heat capacity data. Furthermore, we conduct chemical bonding analysis within the quantum theory of atoms in molecules and reveal that alpha(3R)-In2Se3 exhibits polar covalent intralayer bonds with weak van der Waals interlayer interaction.