Electrical properties of Na2US3,NaGdS2 and NaLaS2

The electrical properties of ternary mixed sulfides Na2US3, NaGdS2, and NaLaS2 were studied by measuring the electrical conductivity and Hall coefficient by the van der Pauw method in a temperature range of 17–300 K. These compounds have closely related crystal structures with nearly the same atom separations, but uranium is in a U4+ state in Na2US3 in contrast to Ln3+ ions in NaGdS2 and NaLaS2. The electrical conductivity was the highest for NaGdS2 (7.75 × 102 and 11.2 × 102 Sm−1 at 17 and 300 K, respectively) and the lowest for Na2US3 (0.98 × 102 and 1.14 × 102 Sm−1 at 17 and 300 K, respectively). They showed semiconductive behavior from the temperature dependence of the electrical conductivity. The Hall coefficient showed the dominant carriers to be electrons for NaGdS2 and holes for NaLaS2 and Na2US3. The carrier densities were not so apart in these compounds, i.e., 0.2–0.3 × 1025 m−3 for NaGdS2 and ∼0.1 × 1025 m−3 for Na2US3. The activation energies of conduction were very low for all three compounds, especially at low temperatures below 200 K.