Competing bonding and anharmonicity control piezoelectricity and thermal transport in janus BrSbX monolayers

Abstract

We show how competing bonding strength and lattice anharmonicity govern piezoelectricity and thermal transport in Janus 1T-BrSbX monolayers (X = S, Se, Te). Using density-functional theory and density-functional perturbation theory, combined with machine-learning-accelerated third-order force constants and Boltzmann transport calculations, we map composition property relationships within a single symmetry family. A bonding analysis (COHP) reveals a monotonic reduction in Sb–X covalency from S → Te, which tracks the piezoelectric stress response e₁₁ and yields in-plane strain coefficients d₁₁ = 41.0, 22.1, 6.3 pm V⁻¹ for BrSbS, BrSbSe, and BrSbTe, respectively. In thermal transport, longitudinal-acoustic group velocities decrease from ∼3.41 to <2.78 km s⁻¹ (S → Te), but mode-averaged Grüneisen parameters diminish more strongly, so phonon lifetimes dominate the trend: κ_L(300 K) = 5.94, 9.39, 12.32 W m⁻¹ K⁻¹ for S, Se, and Te, respectively. All monolayers are mechanically and dynamically stable. Together, these results establish a practical design rule: strengthening directional Sb–X covalency enhances d₁₁, while reduced anharmonicity raises κ_L; the chalcogen thus provides a clean chemical knob to balance electromechanical coupling against heat transport in 2D Janus pnictogen chalcogenides.