Investigation of the thermopower of thulium monoselenide under a pressure to 24 GPa

The experimental data have been obtained on the influence of high pressure on the thermopower of single-crystal thulium selenide at room temperature. At pressures of 2.5–3.0 GPa, the pressure dependences of the thermopower exhibit peculiarities indicating a transformation of the electronic spectrum of TmSe, which causes a transition from one antiferromagnetic state (AF1) to another antiferromagnetic state (AF2) at low temperatures. It has been shown that the coincidence of the thermopowers of TmSe and TmS under pressure cannot be a criterion of the transition of Tm ions to the trivalent state, because thulium sulfide is a concentrated Kondo system and its thermopower itself changes significantly uniform compression. It has been assumed that Tm ions are trivalent at pressures higher than 19 GPa, i.e., this pressure is a boundary for the mixed-valence state.