ACuZrQ₃ (A = Rb, Cs; Q = S, Se, Te): Direct Bandgap Semiconductors and Metals with Ultralow Thermal Conductivity

ACuZrQ3 (A = Rb, Cs; Q = S, Se, Te) were synthesized as black platelet crystals. RbCuZrS3, RbCuZrSe3, and CsCuZrS3 crystallize in the KCuZrSe3 structure type with space group Cmcm, and RbCuZrTe3 and CsCuZrTe3 crystallize in the lower symmetry space group Pnma. The tellurides exhibit a second-order Jahn–Teller distortion with off-centering of Zr in its octahedral environment. The magnitude of the distortion is larger in RbCuZrTe3 than in CsCuZrTe3. The structures of β-CsCuS4 and Rb2Cu5Te5 were also determined. CsCuZrS3 melts at 910 °C and exhibits partial decomposition upon heating at 275 °C, while CsCuZrTe3 melts incongruently. Our DFT calculations of RbCuZrQ3 (Q = S, Se) and CsCuZrS3 indicate direct gap semiconductors in agreement with experiments. ACuZrTe3 (A = Rb, Cs) were calculated to be metals which was confirmed for RbCuZrTe3 with variable temperature conductivity measurements and consistent with heat capacity measurements. Spectroscopic measurements found a bandgap and work function of 1.44(5) eV and 4.89(5) eV for RbCuZrS3 and 0.95(5) eV and 4.67(5) eV for RbCuZrSe3, respectively. RbCuZrTe3 did not exhibit an optical bandgap and has a work function of 4.64(5) eV. RbCuZrTe3 exhibits a low thermal conductivity under 0.5 W m–1 K–1 at room temperature.