Electron mechanism of high-temperature phase transformations in SmS and Smsub(1-x)Gdsub(x)S

The electron mechanism of phase transformations and determination of the valency state variation of samarium ions at the transformations are investigated by the X-ray K line shift technique for SmS and Smsub(1-x)Gdsub(x)S samples. Experiments are carried out in a broad concentration and temperature range (0 (<=) x (<=) 0.4; 300 (<=) T (<=) 1000 K). A smooth variation of the valency of samarium with growth of x is observed at a high temperature; this contrasts with the well known discontinuous variation observed at room temperature. An increase of the valency of samarium in SmS and its decrease in Smsub(0.81)Gdsub(0.19)S with increasing temperature is also observed. It is shown that these effects are due to electron transitions from the 4f to the conductivity band. In the range 300 < or approximately T < or approximately 1000 the valency of samarium in SmS and Smsub(0.81)Gdsub(0.19)S changes from m=2 to m=Sm3+ transition energy on the gadolinium concentration 2.18+-0.02 and from m=2.42+-0.02 to m=2.27+-0.02, respectively. At T approximately 1000 K the valency of samarium in the 0 (<=) x (<=)0.40 range increases from m=2.18+-0.02 to m=2.62+-0.02. The dependence of the Sm2+ → Sm3+ transition energy on the gadolinium concentration and temperature are calculated on the assumption of thermal excitation of 4f electrons to the conductivity band