Isothermal equations of state for lithium fluoride

The change in length of 1‐m‐long LiF single crystal has been determined as a function of hydrostatic pressure up to 7 kbar over the temperature range 28–41°C. The length change has been measured to an accuracy of less than 500 A by using a Fabry‐Perot type He‐Ne laser interferometer, and under a temperature‐controlled environment with temperature variations less than 0.002°C. Several isothermal equations of state, together with those derived from the Born model of ionic solids, have been used in analyzing the pressure‐volume data. The isothermal bulk modulus B_o and its pressure derivative B′_o at atmospheric pressure and 28.83±0.05 °C obtained from the two‐parameter (B_o and B′_o) equations of state are B_o=664.5±0.5 kbar and B′_o=5.40±0.18, respectively. The present results differ considerably from the static measurements of Bridgman and also of Vaidya and Kennedy, but are in excellent agreement with the values obtained by ultrasonic measurements. Using the two‐parameter equations of state, the pressure volume data is extrapolated to ∼5 Mbar in order to compare with the shock wave data and to examine the differences between the various equations of state, which are hardly distinguishable in the low‐pressure region. Although the present V/V_o measurements are sufficiently accurate to obtain accurate values of B^″_o, the present pressure measurements are not (although we believe they are as accurate as is possible at the present time).