Phase transitions and equations of state for the sodium halides: NaF NaCl, NaBr, and NaI

Abstract

Phase changes and equations of state of four sodium halides, NaF, NaCl, NaBr, and NaI, have been studied up to ∼60 GPa at room temperature with the use of a diamond anvil cell. NaF and NaCl transform from the B1 phase to the B2 phase at ∼23 GPa and ∼29 GPa, respectively. At ∼33 GPa, NaBr transforms from the B1 phase to an unidentified phase with a lower symmetry than the B2 phase. NaI also transforms at ∼32 GPa to a new phase which seems to have the same structure as that of the high‐pressure phase of NaBr. The B2 phase of NaF has bulk modulus K 0 = 103 (±19) GPa and volume ratio at 1 atm V o2 / V o1 = 0.811 (±0.025), and the B2 phase of NaCl has K 0 = 121 (±23) GPa and V 02 / V 01 = 0.734 (±0.022). The volume change across the transition was estimated to be ΔV/ V 1 ≃ −10.4% for NaF and −5.8% for NaCl.