Positron annihilation studies of rare-earth mixed valence compounds

Abstract

Positron lifetime has been measured in SmS under external pressures from 0 to ~16 kbar, and in (Sm_0.8Gd_0.2)S at temperatures from 10 K to 300 K. An abrupt drop in the lifetime was observed at the valence transition at a pressure of 6.5 kbar in SmS and at a temperature of ~120 K in (Sm_0.8Gd_0.2)S. Measurement of the Doppler broadening of the annihilation photons was made in (Sm_0.8 Gd_0.2)S as a function of temperature. The line shape parameter of the Doppler broadened spectrum also showed an abrupt change at the transition. Angular correlation of annihilation photons was measured in the semiconducting phase of SmS and in the metallic phase of (Sm_0.8Tb_0.2)S. The correlation profile in the mixed valence (metallic) phase was broader than that in the semi-conducting phase. These results have been explained on the basis of the electronic structure of the semiconducting and the metallic phases of these materials.