Enhancement of Thermoelectric Performance through Transport Properties Decorrelation in the Quaternary Pseudo-Hollandite Chalcogenide Rb 0.2 Ba 0.4 Cr 5 Se 8 .

The novel quaternary compound Rb _0.2 Ba _0.4 Cr ₅ Se ₈ was synthesized and characterized in both single crystal and polycrystalline forms. Crystallizing in the monoclinic crystal system (space group C 2/ m , cell parameters a = 18.7071(4) Å, b = 3.6030(1) Å, c = 8.9637(3) Å, β = 104.494(2)°) and isostructural to pseudo-hollandite compounds, it features mixed Rb and Ba occupancy within its one-dimensional channels. High-temperature X-ray diffraction revealed no decomposition up to 973 K, and the thermal expansion coefficient at 300 K was determined to be 2.6(1)·10 ^-5 K ^-1 . Spin-polarized density functional theory (DFT) calculations showed that the density of states for Rb _0.2 Ba _0.4 Cr ₅ Se ₈ is more polarized than that of Ba _0.5 Cr ₅ Se ₈ , resulting in a higher Seebeck coefficient, which was experimentally confirmed to reach a peak value of 400 μV·K ^-1 at 620 K. Resistivity measurements indicated a degenerate semiconducting behavior below 550 K, with a resistivity peak of 100 mΩ·cm at that temperature, leading to a maximum power factor of 0.21 mW·m ^-1 ·K ^-2 . Thermal conductivity measurements indicated low values around 0.8 W·m ^-1 ·K ^-1 in the 300-900 K range, resulting in a thermoelectric figure of merit of 0.22 at 873 K. Decorrelated transport properties observed in this double-inserted pseudo-hollandite compound make Rb _0.2 Ba _0.4 Cr ₅ Se ₈ a good example of beneficial synergistic effects for higher thermoelectric performance.