Lone-Pair Stereoactivity Overrides Mass Scaling of Thermal Expansion and Heat Conduction in Non-vander Waals Layered Bismuth Oxychalcogenides

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.