Lone-Pair Stereoactivity Overrides Mass Scaling of Thermal Expansion and Heat Conduction in Non-vander Waals Layered Bismuth Oxychalcogenides
Abstract
Heavier chalcogens are generally expected to soften isostructural semiconductor lattices, raising thermal expansion and lowering thermal conductivity. Layered bismuth oxychalcogenides, Bi2O2X (X = S, Se, Te), are mixed-anion crystals whose covalent slabs and ionic sheets are bound electrostatically rather than by van der Waals forces and whose Bi3+ 6s^2 lone pairs can become stereochemically active. Despite the rapid development of Bi2O2Se for fin and gate-all-around transistors, its fundamental thermal expansion and heat transport mechanisms remain poorly understood. Here, we show that lone-pair stereoactivity, not chalcogen mass, governs both properties. Path-integral molecular dynamics shows that the volumetric thermal expansion coefficient at 300 K decreases from 8.14 x 10^-5 K^-1 in Bi2O2S to 6.72 x 10^-5 and 6.61 x 10^-5 K^-1 in Bi2O2Se and Bi2O2Te, respectively, despite a 13% larger unit cell in the telluride, whereas substituting the S mass into Bi2O2Se changes its value by only 0.6%. Lone-pair-driven Bi off-centering in orthorhombic Bi2O2S makes b-axis expansion 1.63 times a-axis expansion yet leaves in-plane thermal conduction isotropic within 3%. Phonon transport calculations including four-phonon scattering and Wigner coherence predict a thermal conductivity maximum in Bi2O2Se (1.83 and 0.95 W m^-1 K^-1 in the in-plane and cross-plane directions, respectively, at 300 K) because Bi2O2S is limited by anharmonic scattering and Bi2O2Te by slow cross-plane phonons. Thermoreflectance measurements from 300 to 500 K and in situ X-ray diffraction on Bi2O2Se single crystals agree with these predictions within 6.4%. Phonon Boltzmann transport simulations of 12-nm-wide fins, which run hotter than Fourier diffusion predicts, show that Bi2O2Se minimizes the hotspot temperature and the thermal-mismatch strain at the gate dielectric.
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BibTeX
@misc{li2026lonepair,
title = {{Lone-Pair Stereoactivity Overrides Mass Scaling of Thermal Expansion and Heat Conduction in Non-vander Waals Layered Bismuth Oxychalcogenides}},
author = {Linzhen Li and Guangwu Zhang and Ziyan Qian and Wei Jing and Yang Lu and Jiaqi Gu and Kaifeng Huang and Yanguang Zhou and Qiye Zheng},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.39213},
eprint = {2610.39213},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.39213},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Lone-Pair Stereoactivity Overrides Mass Scaling of Thermal Expansion and Heat Conduction in Non-vander Waals Layered Bismuth Oxychalcogenides
AU - Linzhen Li
AU - Guangwu Zhang
AU - Ziyan Qian
AU - Wei Jing
AU - Yang Lu
AU - Jiaqi Gu
AU - Kaifeng Huang
AU - Yanguang Zhou
AU - Qiye Zheng
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.39213
UR - https://arxiv.org/abs/2610.39213
AB - Heavier chalcogens are generally expected to soften isostructural semiconductor lattices, raising thermal expansion and lowering thermal conductivity. Layered bismuth oxychalcogenides, Bi2O2X (X = S, Se, Te), are mixed-anion crystals whose covalent slabs and ionic sheets are bound electrostatically rather than by van der Waals forces and whose Bi3+ 6s^2 lone pairs can become stereochemically active. Despite the rapid development of Bi2O2Se for fin and gate-all-around transistors, its fundamental thermal expansion and heat transport mechanisms remain poorly understood. Here, we show that lone-pair stereoactivity, not chalcogen mass, governs both properties. Path-integral molecular dynamics shows that the volumetric thermal expansion coefficient at 300 K decreases from 8.14 x 10^-5 K^-1 in Bi2O2S to 6.72 x 10^-5 and 6.61 x 10^-5 K^-1 in Bi2O2Se and Bi2O2Te, respectively, despite a 13% larger unit cell in the telluride, whereas substituting the S mass into Bi2O2Se changes its value by only 0.6%. Lone-pair-driven Bi off-centering in orthorhombic Bi2O2S makes b-axis expansion 1.63 times a-axis expansion yet leaves in-plane thermal conduction isotropic within 3%. Phonon transport calculations including four-phonon scattering and Wigner coherence predict a thermal conductivity maximum in Bi2O2Se (1.83 and 0.95 W m^-1 K^-1 in the in-plane and cross-plane directions, respectively, at 300 K) because Bi2O2S is limited by anharmonic scattering and Bi2O2Te by slow cross-plane phonons. Thermoreflectance measurements from 300 to 500 K and in situ X-ray diffraction on Bi2O2Se single crystals agree with these predictions within 6.4%. Phonon Boltzmann transport simulations of 12-nm-wide fins, which run hotter than Fourier diffusion predicts, show that Bi2O2Se minimizes the hotspot temperature and the thermal-mismatch strain at the gate dielectric.
ER -