IR spectroscopy of alkali halides at very high pressures: Calculation of equations of state and of the response of bulk moduli to the phase transition

New infrared (IR) data on NaF, NaCl, KCl, KBr, and KI were obtained at pressures of up to 42 GPa. The large (Formula presented) drops in vibrational frequencies of alkali halides upon transformation of the (Formula presented) phase to (Formula presented) are due to the decrease in bond strength as ionic separation increases, and strongly suggest that the bulk modulus (Formula presented) generally decreases during the transition, rather than increases, as commonly accepted. Bulk moduli and equations of state for (Formula presented) phases are obtained from one initial volume (Formula presented) and our vibrational frequencies (Formula presented) using a semiempirical model (previous IR data are used for Rb halides). For substances with a cation radius that is greater than 0.6 times the anion radius, initial values (Formula presented) are within 0.4 to 5% of ultrasonic determinations: thus, this model is accurate for cases where quantum mechanical calculations falter. The converse holds for relatively small cations. Curvature of (Formula presented) with pressure matches the previous determinations even if (Formula presented) is not precisely predicted, which allows determination not only of (Formula presented) but also of (Formula presented) which is generally poorly constrained. Care must be taken in specifying the equation of state, as values for both (Formula presented) and (Formula presented) are affected by the format chosen. For the (Formula presented) phases, (Formula presented) and (Formula presented) are constrained through similar calculations which utilize the volume at the transition as the starting point. Our results are unaffected by shear stress, in contrast to previous x-ray determinations for (Formula presented) After transformation at 32 GPa, (Formula presented) of NaCl-(Formula presented) is (Formula presented) below that of (Formula presented) of (Formula presented) rises steadily ((Formula presented) is fairly large, (Formula presented)) resulting in a λ curve. Results derived for KCl, KBr, and KI are similar such that (Formula presented) of their (Formula presented) phases are better constrained than those of (Formula presented) due to larger stability fields. For the Rb halides, (Formula presented) is roughly constant across the phase change. The compositional dependence of the changes in frequency, (Formula presented) and (Formula presented) for alkali halides are compatible with a simple ball-and-spring model. The calculated (Formula presented) phase volumes of the Na halide are infinite at 1 atm, consistent with instability below 8 GPa, which suggests that theoretical calculations should avoid use of 1 atm starting points for the high-pressure (Formula presented) phases.