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Halide perovskites have emerged as a promising class of materials for photovoltaic applications. A challenge of these applications is preventing the crystal structure from degrading to photovoltaically inactive phases, which requires an understanding of the free energy landscape of these materials. Here, we uncover the free energy landscape of two prototypical halide perovskites, CsPbBr3and MAPbI3, via atomic-scale simulations using umbrella sampling and machine-learned potentials. For CsPbBr3, we find very small free energy differences and barriers close to the transition temperatures for both the tetragonal-to-cubic and orthorhombic-to-tetragonal transitions. For MAPbI3, however, the situation is more intricate. In particular, the orthorhombic-to-tetragonal transition exhibits a large free energy barrier, and there are several competing tetragonal phases. Using large-scale molecular dynamics simulations, we explore the character of these transitions and observe the latent heat and a discrete change in the structural parameters for the tetragonal-to-cubic phase transitions in both CsPbBr3and MAPbI3, indicating first-order transitions. We find that in MAPbI3, the orthorhombic phase has an extended metastability range, and we identify a second metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI3that includes potential metastable phases.

Intermetallic clathrates exhibit great variability with respect to elemental composition and distribution. While this provides a lot of flexibility for tuning properties, it also poses a challenge with regard to developing a comprehensive understanding of these systems. Here, we employ a combination of alloy cluster expansions and density functional theory calculations to exhaustively sample the compositional space with ab initio accuracy. We apply this methodology to study chemical ordering and related properties in the clathrate systems Ba8GaxGe46–x, Ba8GaxSi46–x, Ba8AlxGe46–x, and Ba8AlxSi46–xas a function of composition and temperature. We achieve very good agreement with the available experimental data for the site occupancy factors (SOFs) even for stoichiometries outside the composition range considered during construction of the cluster expansions. This validation enables us to reconcile the variations in the experimental data and explain nonmonotonic variations of the SOFs. In particular, we provide a rationale for the extreme SOF behavior with varying composition observed in Al-based clathrates. Furthermore, we quantify the effect of chemical ordering on both heat capacity and lattice expansion. Finally, we determine the effect of chemical disorder on the displacements of the guest species (Ba), which enables us to at least partially explain experimental observations of the nuclear density of Ba in different clathrates.