Cascade of phase transitions in Tm1−xSmxS due to Tm and Sm valence changes

Studies of the physical properties of Tm1−xSmxS solid solutions have been carried out. We have measured the lattice constant aat 300 K, the resistivity ρ (4.2–300 K), the Hall constant RH(at 4.2, 77 and 300 K) and the X-ray line shifts (300 K). We have found two phase transitions at x= 0.84 and 0.75 which are related to changes in the Tm and Sm valence states. We argue that for 1 > x⩾ 0.84 the Tm3+ions form donor centres.