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PreprintCC BY-NC-ND 4.0arXiv

Thermal expansion mechanisms, heat transport, and band-gap renormalization in strongly anharmonic Rb2SO4, Rb2Ca2(SO4)3, and Ba3Lu(BO3)3 from machine-learned interatomic potentials

  • Ekaterina Spirande
  • Rustam Arabov
  • Sofya Demina
  • Nikita Rybin
  • Alexander Shapeev
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Abstract

Rb2SO4, Rb2Ca2(SO4)3, and Ba3Lu(BO3)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 σA (compares the anharmonic contribution to the interatomic forces with the total forces acting on atoms) reaches ≈0.3-0.5 for β-Rb2SO4, Rb2Ca2(SO4)3, and Ba3Lu(BO3)3 and ≈0.75 for orientationally disordered α-Rb2SO4, and the imaginary harmonic modes of Rb2Ca2(SO4)3 and α-Rb2SO4 are stabilized at finite temperature. The simulations reproduce the measured thermal expansion and support the mechanisms proposed from diffraction: expansion along c in β-Rb2SO4 arises from both elongation and straightening of corrugated Rb chains, BO3 reorientation accompanies the anisotropic expansion of Ba3Lu(BO3)3, and bending of polyhedral linkages limits expansion along the weakly expanding directions and in cubic Rb2Ca2(SO4)3. Green-Kubo lattice thermal conductivities differ by more than an order of magnitude between compounds with comparable σA, 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 Rb2Ca2(SO4)3 and Ba3Lu(BO3)3 and 1.4 eV at 650 K in β-Rb2SO4, with zero-point renormalizations of 0.64, 0.70, and 0.31 eV for β-Rb2SO4, Rb2Ca2(SO4)3, and Ba3Lu(BO3)3, respectively.

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Cite

BibTeXRIS
BibTeX
@misc{spirande2026thermal,
  title = {{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}},
  author = {Ekaterina Spirande and Rustam Arabov and Sofya Demina and Nikita Rybin and Alexander Shapeev},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.30918},
  eprint = {2610.30918},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.30918},
  note = {CC BY-NC-ND 4.0},
}
RIS
TY  - UNPB
TI  - Thermal expansion mechanisms, heat transport, and band-gap renormalization in strongly anharmonic Rb_2SO_4, Rb_2Ca_2(SO_4)_3, and Ba_3Lu(BO_3)_3 from machine-learned interatomic potentials
AU  - Ekaterina Spirande
AU  - Rustam Arabov
AU  - Sofya Demina
AU  - Nikita Rybin
AU  - Alexander Shapeev
PY  - 2026
DA  - 2026/10/22
PB  - arXiv
DO  - 10.48550/arxiv.2610.30918
UR  - https://arxiv.org/abs/2610.30918
AB  - Rb_2SO_4, Rb_2Ca_2(SO_4)_3, and Ba_3Lu(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 ≈0.3-0.5 for β-Rb_2SO_4, Rb_2Ca_2(SO_4)_3, and Ba_3Lu(BO_3)_3 and ≈0.75 for orientationally disordered α-Rb_2SO_4, and the imaginary harmonic modes of Rb_2Ca_2(SO_4)_3 and α-Rb_2SO_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_2SO_4 arises from both elongation and straightening of corrugated Rb chains, BO_3 reorientation accompanies the anisotropic expansion of Ba_3Lu(BO_3)_3, and bending of polyhedral linkages limits expansion along the weakly expanding directions and in cubic Rb_2Ca_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_2Ca_2(SO_4)_3 and Ba_3Lu(BO_3)_3 and 1.4 eV at 650 K in β-Rb_2SO_4, with zero-point renormalizations of 0.64, 0.70, and 0.31 eV for β-Rb_2SO_4, Rb_2Ca_2(SO_4)_3, and Ba_3Lu(BO_3)_3, respectively.
ER  -