Thermoelectric properties of TmTe under pressure up to 20 GPa

The effect of quasi-hydrostatic pressure up to 20 GPa on thermopower of single-crystal samples of thulium monotelluride and monosulfide was studied. The pressure dependences of the thermopower of thulium monotelluride were found to have features in pressure regions of 2 and 6 GPa, which can be associated with phase transitions to the cation mixed-valence state and tetragonal structure, respectively. The upper pressure boundary of stability of the thulium ion mixed-valence state in monotelluride was evaluated by comparing the pressure dependences of the thermopower of TmTe and TmS.