IR spectroscopy of alkali halides at very high pressures: Calculation of equations of state and of the response of bulk moduli to the B1-B2 phase transition

New infrared (IR) data on NaF, NaCl, KCl, KBr, and KI were obtained at pressures of up to 42 GPa. The large (similar to 20%) drops in vibrational frequencies of alkali halides upon transformation of the B1 phase to B2 are due to the decrease in bond strength as ionic separation increases, and strongly suggest that the bulk modulus K-T generally decreases during the transition, rather than increases, as commonly accepted. Bulk moduli and equations of state for B1 phases are obtained from one initial volume V-0 and our vibrational frequencies v(i)(P) 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 K-0 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 K-T With pressure matches the previous determinations even if K-0 is not precisely predicted, which allows determination not only of K-0' but also of K-0'', which is generally poorly constrained. Care must be taken in specifying the equation of state, as values for both K-0' and K-0'' are affected by the format chosen. For the B2 phases, V(P) and K-T(P) 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 B2. After transformation at 32 GPa, K-T of NaCl-B2 is 119+/-4 GPa, 16+/-3% below that of B1. K-T(P) of B2 rises steadily (K' is fairly large, 4.7+/-0.3) resulting in a lambda curve. Results derived for KCl, KBr, and KI are similar such that K(P) of their B2 phases are better constrained than those of B1 due to larger stability fields. For the Rb halides, K-T is roughly constant across the phase change. The compositional dependence of the changes in frequency, K-T, and V for alkali halides are compatible with a simple ball-and-spring model. The calculated B2 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 B2 phases.