Intermediate valence in alloys of SmS with SmP

Abstract

Results are presented for the lattice parameter and magnetic susceptibility as a function of temperature for a series of solid solutions of SmS with SmP. At room temperature the $\mathrm{Sm}{\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ alloys with $x>~0.06$ exhibit a metallic appearance and a much smaller lattice constant and magnetic susceptibility as compared with SmS. For $x>~0.15$ the susceptibilities do not show a Curie-law divergence or magnetic ordering. These properties are consistent with the Sm ions being in a homogeneous intermediate-valence state similar to that of SmS under pressure. The intermediate valence estimated from the lattice parameter is 2.8 at $x=0.06$ and increases toward the pure trivalent value with increasing phosphorous concentration. Also, the color of the alloys changes from gold to red to silver grey. The alloys with $0.03<x<0.06$ exhibit a varying mixture of two distinct phases, the gold intermediate-valence phase and the black phase characteristic of the $x<0.03$ alloys. The properties of the black phase are consistent with a mixture of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions where Sm ions with one or more P ions as nearest neighbors are trivalent. The $0.04<~x<~0.08$ alloys exhibit a transition from the gold phase to the black phase with decreasing temperature. This transition which is continuous at higher concentrations changes to a discontinuous one near $x=0.06$ as seen in the temperature variation of the lattice parameter.