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

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 e ₁₁ and yields in-plane strain coefficients d ₁₁ = 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 d ₁₁, 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.