2 research outputs found
Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr<sub>3</sub> and MAPbI<sub>3</sub>
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, CsPbBr3 and 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 CsPbBr3 and 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 MAPbI3 that includes potential metastable phases
Efficient calculation of the lattice thermal conductivity by atomistic simulations with ab initio accuracy
High-order force constant expansions can provide accurate representations of the potential energy surface relevant to vibrational motion. They can be efficiently parametrized using quantum mechanical calculations and subsequently sampled at a fraction of the cost of the underlying reference calculations. Here, force constant expansions are combined via the hiphive package with GPU-accelerated molecular dynamics simulations via the GPUMD package to obtain an accurate, transferable, and efficient approach for sampling the dynamical properties of materials. The performance of this methodology is demonstrated by applying it both to materials with very low thermal conductivity (Ba8Ga16Ge30, SnSe) and a material with a relatively high lattice thermal conductivity (monolayer-MoS2). These cases cover both situations with weak (monolayer-MoS2, SnSe) and strong (Ba8Ga16Ge30) pho renormalization. The simulations also enable to access complementary information such as the spectral thermal conductivity, which allows to discriminate the contribution by different phonon modes while accounting for scattering to all orders. The software packages described here are made available to the scientific community as free and open-source software in order to encourage the more widespread use of these techniques as well as their evolution through continuous and collaborativeĀ development.</p