Temperature-Induced Explosive First-Order Electronic Phase Transition in Gd-Doped SmS

We have found a reversible temperature-induced explosive first-order electronic phase transition, with decreasing temperature at atmospheric pressure, in ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x\mathrm{S}$ for $0.16<~x<\mathrm{}0.22\mathrm{}$. The shiny golden-yellow metallic crystal disintegrates to a black powder. The transition is isostructural, but the unit cell expands from $a\approx 5.68 \mathrm{to} 5.82$ Å. The Sm ion in the black phase is characterized by an intermediate valence of ∼2.45. We present a phase diagram showing the stability regions in the $T-x$ plane and discuss a plausible driving mechanism for the transition.