The Thermal Expansion of the Bi Lattice Between 25° and 530° Abs.

Abstract

The thermal expansion of the lattice of Bi crystals between 25\ifmmode^\circ\else\textdegree\fi{} and 530\ifmmode^\circ\else\textdegree\fi{} abs. is measured with a Bragg spectrometer equipped with a movable slit between crystal and ionization chamber. The 2d, 3d and 5th orders of $\mathrm{Mo} K\ensuremath{\alpha}$ reflections on (111) were used. ${\ensuremath{\alpha}}_{\mathrm{II}}$ thus obtained varies from 8.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ to 17.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ at 25\ifmmode^\circ\else\textdegree\fi{} abs. and 120\ifmmode^\circ\else\textdegree\fi{} abs., respectively, beyond the latter temperature it stays unchanged until the melting point. Only the range between -15\ifmmode^\circ\else\textdegree\fi{}C and +75\ifmmode^\circ\else\textdegree\fi{}C forms an exception where the expansion coefficient drops to 13.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. This seems to indicate the existence of a separate "phase," not characterized by a change of the lattice configuration and confirms certain previous findings of discontinuities at +75\ifmmode^\circ\else\textdegree\fi{}. A definite divergence between the lattice and the integral expansion beginning 30\ifmmode^\circ\else\textdegree\fi{} before the melting point was found. The Gr\"uneisen rule ($\frac{\ensuremath{\alpha}}{{c}_{p}}=\mathrm{const}$) is found to hold in first approximation for the whole temperature range. The relation of the ${\ensuremath{\alpha}}_{\mathrm{II}}$ values obtained to those of other authors is discussed.