Valence fluctuations and giant isotropic negative thermal expansion in Sm1-xRxS (R = Y, La, Ce, Pr, Nd)

The valence fluctuations of Sm in samarium monosulfide (SmS) are known to induce possible large isotropic negative thermal expansion (NTE), but difficulty in crystal growth prevents establishment of the NTE function. For this study, we prepared Ce-doped and Nd-doped SmS polycrystalline samples using a simpler method with much lower reaction temperature than the existing method. Typically, Sm_0.80Ce_0.20S exhibits giant NTE with total volume change of 2.6% in the wide temperature range from 330 K to 100 K, the lowest covered here. Achievement of giant NTE in a low-cost polycrystalline form paves the way for practical applications of this class of sulfides as a thermal-expansion compensator.