Variable thermal expansion in CuInP2S6

Materials normally exhibit positive thermal expansion, and materials with negative thermal expansion coefficient are of great interest for both fundamental study and applications such as high-precision microscale actuators, thermal expansion compensators, etc. Here, the coefficients of linear thermal expansion (${\ensuremath{\alpha}}_L$) along the $c$ axis of the layered ferroelectric $\mathrm{CuIn}{\mathrm{P}}₂{\mathrm{S}}₆$ have been measured. In the single-crystalline bulk specimen, ${\ensuremath{\alpha}}_L$ exhibits large negative thermal expansion up to $\ensuremath{-}20.05\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in the temperature range below 150 K. In contrast to the bulk, exfoliated ultrathin $\mathrm{CuIn}{\mathrm{P}}₂{\mathrm{S}}₆$ flakes exhibit an overall positive ${\ensuremath{\alpha}}_L\ensuremath{\sim}26.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. The thickness-dependent thermal expansion of $\mathrm{CuIn}{\mathrm{P}}₂{\mathrm{S}}₆$ crystal may facilitate the design of van der Waals heterostructures and ferroelectric devices with tunable thermal responses.