Ultra-low thermal conductivity in Na/Sb chalcobismuthates: Synthesis, crystal structures, optical properties and 23Na NMR spectroscopy

Three new quaternary compounds Na2BiSbQ4 (Q = S, Se, Te) have been synthesized by the direct combination reaction between Bi, Sb, Q and the corresponding Na2Q. All phases, sulfide, selenide and telluride, are isostructural and crystallize in the NaCl-type cubic space group Fm3m (no. 225) with lattice constants of a = 5.760(7), 5.964(9) and 6.331(5) Å, respectively. The unique feature of these structures is the existence of only one crystallographic metal site in the unit cell. These mixed position sites provide the desirable lattice complexity with a random distribution of the Na, Bi and Sb atoms. Consequently, extremely low thermal conductivities at room temperature have been observed for the studied phases. All three compounds are semiconductors and their Uv/Vis spectra exhibit redshifts toward narrower bandgap energies from sulfide to selenide to telluride.