Compressibility of the Alkali Halides

Abstract

Compressibility of eleven alkali halides and its variations with pressure and temperature have been determined by measurements by Bridgman's new method, up to 12,000 atm. for both 30\ifmmode^\circ\else\textdegree\fi{} C and 75\ifmmode^\circ\else\textdegree\fi{} C. The samples were all single simple cubic crystals each grown from the melt in a new way, described elsewhere. The error in the values of the compressibility at zero pressure, ${\ensuremath{\kappa}}_{0}$, is probably less than one per cent; in the values for variation with pressure, ${\ensuremath{\psi}}_{0}$, and temperature the error may be 5 and 20 per cent respectively. By extrapolation approximate values of ${\ensuremath{\kappa}}_{0}$ for absolute zero are found. Periodic relations. Both ${\ensuremath{\kappa}}_{0}$ and ${\ensuremath{\psi}}_{0}$ when plotted against the alkali ion for a series of salts of the same halogen ion, or vice-versa, show similar behavior. The curves break sharply at the ion similar to argon (K or Cl) the rate of increase suddenly decreasing. This behavior is also shown by the grating space as measured by Davey, and tends to corroborate Bohr's theory of atomic structure according to which there is a discontinuity in atomic formation at argon, additional electrons going into inner shells.Interatomic forces in cubic crystals.--- The crystal potential energy as a function of volume, at absolute zero is determined from the experimental results, and an empirical expression for it is developed which is extrapolated to a volume so large that the repulsive forces between the atoms are small. The remaining attractive force is found approximately equal to that given by the expression derived by Madelung assuming each ion is singly charged. Then assuming electrostatic cohesion, a series development is obtained for the repulsion betwe atoms. This is found not to vary as any single inverse power of the grating space, as demanded by the theory of Born for salts of Na, K and Rb; nor is there any discontinuity between Li and the other metals. Values for the energy of dissociation of the crystal into ions are given. As would be expected, the change of compressibility with temperature is related to the thermal expansion in such a way that an increase of temperature and decrease of pressure have the same effect on both volume and compressibility.