IR spectroscopy of alkali halides at very high pressures: Calculation of equations of state and of the response of bulk moduli to theB1−B2phase transition

New infrared (IR) data on NaF, NaCl, KCl, KBr, and KI were obtained at pressures of up to 42 GPa. The large $(\sim 20\%)$ drops in vibrational frequencies of alkali halides upon transformation of the $B1\mathrm{}$ phase to $B2\mathrm{}$ 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 $B1\mathrm{}$ phases are obtained from one initial volume $V_0$ and our vibrational frequencies ${\nu }_i\mathrm{}\left(P\right)\mathrm{}$ 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 $K_0^{\prime },$ but also of $K_0^{″},$ which is generally poorly constrained. Care must be taken in specifying the equation of state, as values for both $K_0^{\prime }$ and $K_0^{″}$ are affected by the format chosen. For the $B2\mathrm{}$ phases, $V\left(P\right)\mathrm{}$ and $K_T\mathrm{}\left(P\right)\mathrm{}$ 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 $B2.\mathrm{}$ After transformation at 32 GPa, $K_T$ of NaCl-$B2\mathrm{}$ is $119\pm 4\mathrm{GPa},$ $16\pm 3\%$ below that of $B1.\mathrm{}$ $K_T\mathrm{}\left(P\right)\mathrm{}$ of $B2\mathrm{}$ rises steadily ($K^{\prime }$ is fairly large, $4.7\pm 0.3$) resulting in a λ curve. Results derived for KCl, KBr, and KI are similar such that $K\left(P\right)\mathrm{}$ of their $B2\mathrm{}$ phases are better constrained than those of $B1\mathrm{}$ 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 $B2\mathrm{}$ 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 $B2\mathrm{}$ phases.