Enhancement of Thermoelectric Performance through Transport Properties Decorrelation in the Quaternary Pseudo-Hollandite Chalcogenide Rb_0.2Ba_0.4Cr₅Se₈

The novel quaternary compound Rb_0.2Ba_0.4Cr₅Se₈ was synthesized and characterized in both single crystal and polycrystalline forms. Crystallizing in the monoclinic crystal system (space group C2/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_0.2Ba_0.4Cr₅Se₈ is more polarized than that of Ba_0.5Cr₅Se₈, 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_0.2Ba_0.4Cr₅Se₈ a good example of beneficial synergistic effects for higher thermoelectric performance.