High-Temperature Formation Phases and Crystal Structure of Hot-Pressed Thermoelectric CuGaTe2 with Chalcopyrite-Type Structure
Polycrystalline CuGaTe2 with a chalcopyrite-type structure consolidated by hot-pressing is a potential candidate as a medium-temperature thermoelectric (TE) material. However, its high-temperature formation phases have rarely been reported to date. Here, we investigated the temperature-dependent formation phases and crystal structure at 300–800 K of hot-pressed CuGaTe2. From synchrotron x-ray diffraction data and crystal structure analysis of the heating and cooling processes, it was clarified that a certain amount of impurity phases, such as Te and CuTe, precipitated from the CuGaTe2 matrix when the temperature was increased (to 500–650 K). This is the temperature range where CuGaTe2 has been reported to show high TE performance. After CuGaTe2 was heated to 800 K, such impurity phases remained, even when cooled to room temperature. They also affected the tetragonal distortion and the x-coordinate of Te in the CuGaTe2 matrix, probably due to deficiencies of Cu and Te in the matrix. Our results reveal detailed information on the formation phases of CuGaTe2 at high temperature and thus provide insight for evaluation of its high-temperature stability and transport properties.