Mixed-Anion Mischief: Lone Pairs Meet Ionic Bonds to Suppress Thermal Transport in Quasi-1D Bi3Se4Br.

Abstract

Understanding thermal transport requires exploring materials with unconventional atomic arrangements and emergent bonding motifs. Mixed-framework halogeno-chalcogenides, with their structural anisotropy and unusual bonding connectivity, provide a versatile platform for tuning lattice dynamics and transport properties. Here, we report the transport behavior of the halogeno-chalcogenide Bi3Se4Br, a member of the quasi-binary nM2Q3-mMX3 phase space (n/m = 4:1; M = Bi, Q = Se, X = Br). Its structure consists of edge-sharing Bi-Se polyhedra forming quasi-one-dimensional [010] ribbons, weakly coupled through Br atoms. Unlike Bi2Se3, where Bi adopts homoleptic Se coordination, Bi3Se4Br features heteroleptic Bi environments with hemidirected off-center geometries driven by stereochemically active 6s2 lone pairs. Interactions among these lone pairs within the ribbons amplify structural anisotropy, while antibonding Bi-Se and Bi···Br interactions soften the lattice, reducing elastic moduli and acoustic phonon cutoff frequencies. The interplay of active lone pairs and mixed covalent-ionic bonding generates moderate lattice anharmonicity, giving rise to low-energy optical phonons and enhanced phonon scattering. Density functional theory (DFT)-derived phonon density of states (DOS), neutron-weighted phonon DOS, and Raman measurements collectively confirm these low-lying vibrational modes. Inelastic neutron scattering measurements reveal predominantly quasi-harmonic phonon behavior with moderate temperature-induced softening, while also exposing anharmonic effects through selective peak broadening and nonuniform intensity changes. Consequently, Bi3Se4Br exhibits an ultralow lattice thermal conductivity of 0.64 and 0.39 W m-1 K-1 at 320 and 623 K, respectively, ∼70% lower than that of Bi2Se3 It also displays semiconducting n-type transport, highlighting Bi3Se4Br as a promising platform for combining ultralow thermal conductivity with tunable electronic functionality.