Na₂TeS₃, Na₂TeSe₃-mP24, and Na₂TeSe₃-mC48: Crystal Structures and Optical and Electrical Properties of Sodium Chalcogenidotellurates(IV)

Pure samples of Na₂TeS₃ and Na₂TeSe₃ were synthesized by the reactions of stoichiometric amounts of the elements Na, Te, and Q (Q = S, Se) in the ratio 2:1:3. Both compounds are highly air- and moisture-sensitive. The crystal structures were determined by single-crystal X-ray diffraction. Yellow Na₂TeS₃ crystallizes in the space group P2₁/c. Na₂TeSe₃ exists in a low-temperature modification (Na₂TeSe₃-mP24, space group P2₁/c) and a high-temperature modification (Na₂TeSe₃-mC48, space group C2/c); both modifications are red. Density functional theory calculations confirmed the coexistence of both modifications of Na₂TeSe₃ because they are very close in energy (ΔE = 0.18 kJ mol^–1). To the contrary, hypothetic Na₂TeS₃-mC48 is significantly less favored (ΔE = 1.8 kJ mol^–1) than the primitive modification. Na₂TeS₃ and Na₂TeSe₃-mP24 are isotypic to Li₂TeS₃, whereas Na₂TeSe₃-mC48 crystallizes in its own structure type, which was first described by Eisenmann and Zagler. The title compounds have two common structure motifs. Trigonal TeQ₃ pyramids form layers, and the Na atoms are surrounded by a distorted octahedral environment of chalcogen atoms. Raman spectra are dominated by the vibration modes of the TeQ₃ units. The activation energies of the total conductivity of the title compounds range between 0.68 eV (Na₂TeS₃) and 1.1 eV (Na₂TeSe₃). Direct principal band gaps of 1.20 and 1.72 eV were calculated for Na₂TeSe₃ and Na₂TeS₃, respectively. The optical band gaps are in the range from 1.38 eV for Li₂TeSe₃ to 2.35 eV for Na₂TeS₃.