The magnetic properties of all known sulfides, selenides, and tellurides of Pr and Tm (as representative of the light and heavy rare earths, respectively) have been studied. In most cases, Pr and Tm compounds exhibit Van Vleck paramgnetism at low temperatures owing to crystal-field singlet ground states. Splittings have been derived in several cases by specific-heat measurements. In the case of ${\mathrm{Pr}}_3{X}_4$ ($X=\mathrm{S},\mathrm{S}\mathrm{e},\mathrm{T}\mathrm{e}$) compounds, specific-heat and susceptibility measurements reveal exchange-induced ferromagnetism. This is further supported by the study of the magnetic phase diagrams of ${\mathrm{Pr}}_3$${\mathrm{Se}}_4$-${\mathrm{Pr}}_2$${\mathrm{Se}}_3$, ${\mathrm{La}}_3$${\mathrm{Se}}_4$-${\mathrm{Pr}}_3$${\mathrm{Se}}_4$, and of superconductivity in the ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_3({\mathrm{S}}_4, {\mathrm{Se}}_4, {\mathrm{Te}}_4)$ series. Superconductivity persists to very high Pr concentrations, as in the ${\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\mathrm{Se}$ system, indicating a nonmagnetic ground state of Pr. The higher chalcogenides appear to exhibit in general semimetallic or semiconducting behavior, while most of the metallic La compounds are usually superconductors.

• Received 18 April 1974

DOI:https://doi.org/10.1103/PhysRevB.11.500