Stability of layered In₂Se₃ phases from Diffusion Quantum Monte Carlo calculations

In₂Se₃ is a semiconductor material that can be found in different crystal structures, most of them forming two-dimensional layers stacked via van der Waals interactions. These layered crystal structures, composed of quintuple Se-In-Se-In-Se layers, differing on the stacking within the strongly bonded quintuple layer and across the van der Waals spacing, display electronic and optical properties that can be leveraged in a variety of device applications, including solar cells, photodetectors, and phase-change memory devices. However, the phase ordering and the transition between them remains unclear. Density functional theory and hybrid functional calculations show large variations in total energy differences between the different phases of In₂Se₃, dependent of the functional and the van der Waals correction used. Here we use diffusion Monte Carlo (DMC) calculations to determine the total energies and charge-density differences between the three most common layered structures of In₂Se₃, namely, α, α’, and β, which differ in bonding/coordination within the quintuple layer and stacking of quintuple layers. Effects of temperature were also included by calculating the Helmholtz free energy, where the temperature dependence is included through the vibrational entropy.