CsCu₅S₃: Promising Thermoelectric Material with Enhanced Phase Transition Temperature

Abstract

Cu2S, featuring low cost, nontoxicity, and earth abundance, has been recently recognized as a high efficiency thermoelectric (TE) material. However, before reaching the maximum of the figure of merit ( ZT ), Cu2S undergoes three phase transformations starting at 370 K, which give rise to severe problems, such as possible decomposition and low reliability. Herein, we discover CsCu5S3 with phase transformation at 823 K, which is significantly higher than the 370 K value of Cu2S. Single crystal diffraction data reveal that its two phases are constructed by the same Cu4S4 columnar building unit via propagating either at the opposite sides into a layered o-CsCu5S3, or at the four apexes into a 3D t-CsCu5S3, respectively. Interestingly, the o-to-t transformation is quick, but the reverse one is relatively slow. Theoretical studies reveal that the Cu4S4 column exhibits not only the most condensed atomic aggregation (D column) but also the lightest effective mass (m*), along which higher σ is realized. More interestingly, both phases exhibit remarkable ZT enhancements, 0.46 at 800 K for o-CsCu5S3, and 0.56 at 875 K for t-CsCu5S3, which are 170% and 175% that of Cu2S at the same temperature.

β‐CsCu5Se3: A Promising Thermoelectric Material going beyond Photovoltaic Application

“…As shown in Figure a, it is isostructural to the known thermoelectric material o ‐CsCu 5 S 3. [ 8 ] The β ‐CsCu 5 Se 3 compound is formed by combining the Cu 5 Se 3 layers with Cs + ions, and the interactions between the Cu 5 Se 3 layer and Cs + ions are weak van der Waals (vdW) interactions. The unit cell of β ‐CsCu 5 Se 3 consists of 2 Cs, 10 Cu, and 6 Se atoms with the lattice constants a = 10.00 Å, b = 4.05 Å and c = 8.98 Å, which are in good agreement with the experimental results ( a = 9.99 Å, b = 4.10 Å, and c = 9.01 Å).…”

“…By solving the phonon BTE, the lattice thermal conductivities at room temperature along the x , y , and z directions are calculated to be 0.24, 0.41, and 0.25 W mK −1 , respectively, while decrease rapidly with temperature increasing. At 800 K, the corresponding lattice thermal conductivities further reduce to 0.09, 0.15, and 0.09 W mK −1 , which are much lower than that of its isostructural materials o ‐CsCu 5 S 3 (0.30 W mK −1 at 800 K), [ 8 ] and lower than that of many other bulk copper‐based materials, including Cu 1.97 S (0.35 W mK −1 at 1000 K), [ 16 ] Cu 2‐δ Se (around 0.4–0.6 W mK −1 at high temperature), [ 6 ] CuSbS 2 (0.46 W mK −1 at 300 K), [ 43 ] and α ‐CuSCN (0.5 W mK −1 at 800 K). [ 44 ] The variation of lattice thermal conductivity with temperature is plotted in Figure a.…”

“…Owing to the energy crisis and environmental problems, thermoelectric materials that can directly convert waste heat to electricity have attracted extensive interests, [ 1,2 ] and especially much attention has been paid in recent years to the materials with low costs, nontoxicity, and earth abundance. [ 3–8 ] Copper chalcogenides are such materials. For instance, the binary copper chalcogenide compounds, Cu 2 X (X = S, Se), show good thermoelectric performances ( ZT ≈1.5) at 1000 K, [ 5,6 ] and the ternary chalcogenide semiconductors, Cu 3 SbS 4 and o ‐CsCu 5 S 3 , exhibit excellent thermoelectric properties because of their low lattice thermal conductivities and large Seebeck coefficient.…”

“…For instance, the binary copper chalcogenide compounds, Cu 2 X (X = S, Se), show good thermoelectric performances ( ZT ≈1.5) at 1000 K, [ 5,6 ] and the ternary chalcogenide semiconductors, Cu 3 SbS 4 and o ‐CsCu 5 S 3 , exhibit excellent thermoelectric properties because of their low lattice thermal conductivities and large Seebeck coefficient. [ 7,8 ] However, most of these materials possess low phase transformation temperature. It has been reported that both Cu 2 S and Cu 2 Se possess low phase transformation temperatures of 373 and 400 K, [ 9,10 ] respectively, and for Cu 3 SbS 4 , phase transformation occurs at about 460 K. [ 11 ] While phase transition would significantly affect the thermoelectric efficiency of thermoelectric materials, [ 12 ] and result in poor device reliability.…”

“…Recently, a new ternary chalcogenide compound, three‐dimensional (3D) layered β ‐CsCu 5 Se 3 , was successfully synthesized via solvothermal methods, which shows a high phase transition temperature of 923 K. [ 15 ] Since β ‐CsCu 5 Se 3 is isostructural to the thermoelectric material o ‐CsCu 5 S 3, [ 8 ] and selenium is heavier and less electronegative than sulfur, so it is natural to expect that β ‐CsCu 5 Se 3 would possess a lower lattice thermal conductivity, better electrical transport properties, and higher thermoelectric performance as compared to o ‐CsCu 5 S 3 . Such a phenomenon has also been observed in many classic thermoelectric materials, including PbX, Bi 2 X 3 , Cu 2 X, SnX (X = S, Se).…”