Ekaterina Spirande | Search arXiv

arXiv2026

Thermal expansion mechanisms, heat transport, and band-gap renormalization in strongly anharmonic Rb$_2$SO$_4$, Rb$_2$Ca$_2$(SO$_4$)$_3$, and Ba$_3$Lu(BO$_3$)$_3$ from machine-learned interatomic potentials

Rb$_2$SO$_4$, Rb$_2$Ca$_2$(SO$_4$)$_3$, and Ba$_3$Lu(BO$_3$)$_3$ are structurally complex materials whose thermal expansion has been interpreted, on the basis of high-temperature x-ray diffraction, in terms of specific polyhedral rearrangements. We study their lattice dynamics and its consequences for thermal and electronic properties using molecular dynamics with actively learned moment tensor potentials trained on density-functional theory data. The anharmonicity measure $σ^A$ (compares the anharmonic contribution to the interatomic forces with the total forces acting on atoms) reaches $\approx$0.3-0.5 for $β$-Rb$_2$SO$_4$, Rb$_2$Ca$_2$(SO$_4$)$_3$, and Ba$_3$Lu(BO$_3$)$_3$ and $\approx$0.75 for orientationally disordered $α$-Rb$_2$SO$_4$, and the imaginary harmonic modes of Rb$_2$Ca$_2$(SO$_4$)$_3$ and $α$-Rb$_2$SO$_4$ are stabilized at finite temperature. The simulations reproduce the measured thermal expansion and support the mechanisms proposed from diffraction: expansion along $c$ in $β$-Rb$_2$SO$_4$ arises from both elongation and straightening of corrugated Rb chains, BO$_3$ reorientation accompanies the anisotropic expansion of Ba$_3$Lu(BO$_3$)$_3$, and bending of polyhedral linkages limits expansion along the weakly expanding directions and in cubic Rb$_2$Ca$_2$(SO$_4$)$_3$. Green-Kubo lattice thermal conductivities differ by more than an order of magnitude between compounds with comparable $σ^A$, showing that the anharmonicity measure alone does not determine heat transport. Phonon-induced band-gap renormalization, including lattice thermal expansion, reaches 2.7 and 1.7 eV at 1250 K in Rb$_2$Ca$_2$(SO$_4$)$_3$ and Ba$_3$Lu(BO$_3$)$_3$ and 1.4 eV at 650 K in $β$-Rb$_2$SO$_4$, with zero-point renormalizations of 0.64, 0.70, and 0.31 eV for $β$-Rb$_2$SO$_4$, Rb$_2$Ca$_2$(SO$_4$)$_3$, and Ba$_3$Lu(BO$_3$)$_3$, respectively.

cond-mat.mtrl-sci↗

arXiv2025

Automated Prediction of Thermodynamic Properties via Bayesian Free-Energy Reconstruction from Molecular Dynamics

Accurate free-energy calculations are essential for predicting thermodynamic properties and phase stability, but existing methods are limited: phonon-based approaches neglect anharmonicity and liquids, while molecular dynamics (MD) is computationally demanding, neglects low-temperature quantum effects, and often requires manual planning and post-processing of simulations. We present a unified workflow that reconstructs the Helmholtz free-energy surface from MD data using Gaussian Process Regression (GPR), augmented with zero-point energy corrections from harmonic/quasi-harmonic theory. The framework propagates statistical uncertainties, mitigates finite-size effects, and employs active learning to optimize sampling in the volume-temperature space. It applies seamlessly to both crystalline and liquid phases. We demonstrate the methodology by computing heat capacities, thermal expansion, isothermal and adiabatic bulk moduli, and melting properties for nine elemental FCC and BCC metals using 20 classical and machine-learned interatomic potentials, with all predictions accompanied by quantified confidence intervals. Automated, general, and uncertainty-aware, the workflow advances high-throughput thermodynamics and provides a systematic benchmark for interatomic potentials.

cond-mat.mtrl-sci↗