  Inspired by the recent first-principles calculations showing the high
thermoelectric performance of layered pnictogen chalcogenides, we
experimentally characterise the crystal structure and high-temperature
transport properties of the layered antimony selenides REOSbSe2 (RE = La, Ce).
The crystal structure of REOSbSe2 was the tetragonal P4/nmm space group,
consisting of alternate stacks of SbSe2 and REO layers. These two compounds
were n-type semiconductors. The optical band gaps of LaOSbSe2 and CeOSbSe2 were
evaluated to be 1.0 and 0.6 eV, respectively. The room-temperature thermal
conductivity was 1.5 Wm-1K-1 for RE = La and 0.8 Wm-1K-1 for RE = Ce. These
relatively low thermal conductivities were comparable to those of isostructural
layered bismuth chalcogenides. We substituted O2- with F- ions to introduce
electrons as charged carriers to optimize the thermoelectric performance, but
increasing the electrical conductivity was still challenging.
