THE THERMAL EXPANSION OF ALKALI HALIDES

Abstract

Measurements have been made of the coefficient of linear thermal expansion of NaCl, Nal, KCl, and KBr over the temperature range from 7 °K to 300 °K. The measurements were made by monitoring the changes in length of a Fabry–Perot etalon, whose spacer was of the material being studied, as it slowly warmed from liquid-helium temperatures. The detectable change in length of the 2-in. specimen was about 2 Å. The results are analyzed by reducing them to values of the Grueneisen γ. The temperature variation of γ at high temperatures is used to obtain values of γ (2s), which are compared with new calculations on a rigid-ion model to give the best fit to the Born exponent for the non-Coulomb interionic potential. The data for low temperatures are not sufficiently accurate to justify detailed analysis; there is some indication that the apparent low-temperature limit of γ is, in fact, the value at a minimum.