Dilatation thermique des phases stables et métastable du naphtalènethiol-2

It is shown by a diffraction technique that 2-naphthalenethiol (C₁₀H₈S) undergoes a first-order phase transition which occurs at 313 ± 0.5 K. The low-temperature form III is ordered whereas the high-temperature one I is disordered and may be kept as metastable far below 313 K. Thermal expansion studies, performed on the stable form III and on both metastable and stable forms I, indicate that: form III exhibits a pronounced `pretransitional' effect; discontinuities of the lattice constants occur at the III →I transition; the lattice-constant thermal-expansion curves of form I do not show any inflexion at the metastable to stable transition; in every case a large thermal expansion anisotropy is observed. Curiously, the compacity of the ordered low-temperature form is lower than that of the disordered high-temperature form. The JCPDS Diffraction File No. of the low-temperature form III is 34–1990.