Beyond Stoichiometry: Layer-Selective Cation Disorder Enables Ultralow Lattice Thermal Conductivity in GeBi ₄ X ₇ and GeBi ₆ X ₁₀ ( X = Se, Te)

The nominal compounds GeBi 4 X 7 and GeBi 6 X 10 ( X = Se, Te) exhibit pronounced cation disorder, posing challenges in achieving single-phase materials at their nominal stoichiometries and thereby complicating functional optimization. In this work, through controlled cation composition regulation, we successfully synthesize single-phase compounds derived from nominal GeBi 4 Te 7 and GeBi 6 Te 10, eliminating the secondary phases previously reported in these compounds. The actual stoichiometries of these single phases are determined to be Ge 0.75 Bi 4.25 Te 7 and Ge 0.65 Bi 6.35 Te 10, respectively. Additionally, two isostructural single-phase compounds are discovered in the Se-based counterparts, with actual compositions of Ge 0.72 Bi 4.28 Se 7 and Ge 0.62 Bi 6.38 Se 10 . Rietveld refinement analyses reveal that the cation layers adjacent to the van der Waals gaps are preferentially occupied by Bi, whereas cation disorder predominantly arises in the central mixed-cation layer within the GeBi 2 X 4 stack. This layer-selective preference, combined with cation disorder enabled by the nearly identical electronegativities of Ge and Bi, drives the overall stoichiometry toward the Bi-rich regime. With respect to thermoelectric properties, the complex-stacking single-phase materials derived from nominal GeBi 4 X 7 and GeBi 6 X 10 show distinct advantages over the simple-stacking GeBi 2 X 4 counterparts and conventional Bi 2 X 3 matrices. Experimental results indicate that the incorporation of additional Bi 2 X 3 stacks markedly enhances phonon scattering and reduces the lattice thermal conductivity, while exerting minimal impact on carrier mobility. Of particular significance, the single-phase material Ge 0.65 Bi 6.35 Te 10 derived from nominal GeBi 6 Te 10 achieves a remarkably low lattice thermal conductivity of ∼0.35 W m –1 K –1 and a dimensionless thermoelectric figure of merit of ∼0.62 at 525 K. Predictions based on the single parabolic band model indicate a substantial potential for improving the thermoelectric performance of Ge 0.65 Bi 6.35 Te 10 through the optimization of carrier concentration, suggesting that this compound represents a promising alternative to the traditional Bi 2 Te 3 matrix.