  Negative thermal expansion is an unusual phenomenon appearing in only a
handful of materials, but pursuit and mastery of the phenomenon holds great
promise for applications across disciplines and industries. Here we report use
of X-ray spectroscopy and diffraction to investigate the 4f-electronic
properties in Y-doped SmS and employ the Kondo volume collapse model to
interpret the results. Our measurements reveal an unparalleled decrease of the
bulk Sm valence by over 20% at low temperatures in the mixed-valent golden
phase, which we show is caused by a strong coupling between an emergent Kondo
lattice state and a large isotropic volume change. The amplitude and
temperature range of the negative thermal expansion appear strongly dependent
on the Y concentration and the associated chemical disorder, providing control
over the observed effect. This finding opens new avenues for the design of
Kondo lattice materials with tunable, giant and isotropic negative thermal
expansion.
