Study of the valence transition in SmS induced by alloying, temperature, and pressure

Abstract

Lattice-parameter studies as a function of composition as well as temperature (4.7 to 900 °K) have been carried out on a number of trivalent rare-earth-substituted SmS (Sm_1-xR_x³⁺S where R=Ce, Pr, Nd, Gd, Tb, Dy, Ho, En, Tm, Lu) compounds. The data for the Sm_1-xGd_xS compounds show that the lattice parameter a decreases smoothly first and undergoes an abrupt decrease at a critical concentration of 15.5-at.% Gd. For x<0.15 the material is black and for x>0.16 it turns metallic golden yellow. The crystal structure remains NaCl type but the lattice contracts from 5.86 Å to a value of ~5.68 Å. At higher concentrations the change in a is smooth and small. This is typical of the behavior of the Sm_1-xR_x³⁺S compounds, with the exception of La-substituted ones. These effects are attributed to a change in the valence state of the Sm ion from 2⁺ towards the 3⁺ state and R³⁺ substitution is equivalent to application of pressure. The metallic-looking samples show a strong first-order transition on cooling (depending upon the composition) and exhibit anomalous thermal-expansion behaviors in the 300 to 900 °K temperature range, which are related to a change in the valence state, from 3⁺ to the 2⁺. Based on pressure and temperature data a phase-stability diagram in the P-T plane for SmS, and in the T-x plane for the Sm_1-xGd_xS system are presented. In the latter, three NaCl phases designated B', B, and M appear, of which the B (black) phase is the high-temperature phase, B' is stable at low temperature, and M is stable at intermediate temperatures. The phase-stability diagrams, the lattice-parameter variation with temperature, pressure, and alloying, and the intermediate valence state of Sm in these compounds are discussed in terms of the electronic structure of the phases involved. It is shown that the lattice-parameter data are extremely valuable and unique in providing clues as to what goes on in these materials on a microscopic scale.