Thermal equations of state of B2-structured rubidium halides RbCl, RbBr, and RbI

In this study, we determined the thermal equations of state (EoS) for rubidium chloride (RbCl), rubidium bromide (RbBr), and rubidium iodide (RbI) in the B2 (CsCl-type) structure. We conducted in situ energy-dispersive x-ray diffraction measurements at high pressures (up to 26 GPa) and temperatures (up to 1800 K) using a large volume press. Pressures were calibrated using CsCl, Mo, and Pt in the same cell assemblies. For each B2-structured Rb halide, the parameter V0 (unit cell volume at room pressure) was estimated from additional diamond anvil cell experiments at 300 K. Using the third-order Birch–Murnaghan equation and the Mie–Grüneisen–Debye thermal model, we derived the thermoelastic parameters for each phase: RbCl: K0 = 19.89(8) GPa, K0′ = 5.00(2), γ0 = 1.96(4), q = 1.05(9), RbBr: K0 = 16.28(4) GPa, K0′ = 5.28(2), γ0 = 2.18(14), q = 1.52(24), RbI: K0 = 13.69(4) GPa, K0′ = 4.95(1), γ0 = 2.21(7), q = 1.42(10). These parameters represent the isothermal bulk modulus (K0), its pressure derivative (K0′), the Grüneisen parameter (γ0), and the logarithmic volume dependence of the Grüneisen parameter (q). The newly derived EoS for rubidium halides provides effective pressure markers above 0.5 GPa, as they remain stable across wide pressure and temperature ranges. Additionally, RbCl and RbBr offer improved x-ray transmission compared to CsCl. These EoS can be combined with a secondary metallic phase to estimate pressure and temperature in the absence of a thermocouple, taking advantage of the large differences in thermal expansion between halides and metals.