Valence changes in TmSe by alloying with TmTe and EuSe

Abstract

The valence of the Tm ions in mixed-valent TmSe has been influenced by alloying with TmTe and EuSe in order to study the interaction between intermediate-valent rare-earth ions as a function of the degree of valence mixing. Magnetic, elastic, electrical, and optical properties have been measured on $\mathrm{Tm}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ crystals for seven distinct compositions. The degree of valence mixing is increased as Se is replaced by Te up to ∼ 20% and the material remains metallic. Whereas TmSe orders metamagnetically ($T_N=3\mathrm{}$ K), the Te-containing samples are spontaneously magnetized below 3-5 K. This increasing strength of the ferromagnetic interactions, accompanying the increase of valence mixing, is consistent with the model of a double exchange coupling of mixed-valent Tm ions. $\mathrm{Tm}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ with $x>~0.5$ is semiconducting, the Tm ions are divalent and the overall electronic structure is similar to the ones of the Sm, Eu, and Yb monochalcogenides. The only peculiarities are the narrow energy gaps (0.2 to 0.35 eV) which require pressures of some 20 to 30 kbar to be closed. The magnetic properties are dominated by the crystal-field-split ${}_{}{}^2{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$ ground state, leading to ordering temperatures well below 1 K. As Eu replaces Tm in TmSe, it is divalent because of its highly stable $4f^7$ shell, whereas the ${\mathrm{Tm}}^{2+}$ $4f^{13}$ and ${\mathrm{Tm}}^{3+}$ $4f^{12}$ $5d$ configurations are nearly degenerate: in ${\mathrm{Tm}}_{0.5}$ ${\mathrm{Eu}}_{0.5}$Se they are separated by an energy gap of only 0.1 eV, which can be driven to zero with moderate external pressure (15 kbar). In the metallic ${\mathrm{Tm}}_{0.83}$ ${\mathrm{Eu}}_{0.17}$Se, the Tm ions are of predominantly inhomogeneously mixed-valent character since the Eu ions, being larger in volume are thought to give rise to local deformations of the lattice.