Isothermal equations of state of potassium

Volume versus pressure data of 1‐m‐long polycrystalline potassium has been obtained as a function of hydrostatic pressure up to 7 kbars at 28.58 °C. The length change has been measured to an accuracy of less than 500 Å using a Fabry‐Perot type He‐Ne laser interferometer in a temperature‐controlled environment with temperature variations ±0.001 °C. The isothermal bulk modulus B₀ and its pressure derivative B^′₀ at atmospheric pressure and 28.58 °C are B₀=31.01±0.02 kbars and B^′₀ =3.91±0.01, when the first‐order Murnaghan equation is used; and B₀=30.84±0.02 kbars and B^′₀ =4.16±0.02, when the Keane equation is used. The Keane equation yields $B0″$ =−0.145 kbar⁻¹ in the applied pressure range. Various two‐ and three‐parameter equations of state have been used to fit the measured pressure‐volume data. The Keane and Birch equations represent the data most closely when these equations are extrapolated into a higher‐pressure region.