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

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.