High-Pressure Refractive Indices of NaCl, KCl, CaO, SrO, and MgO Reveal the Dependence of Anion Polarizability on Coordination Number and Bond Length
Several alkali chlorides and alkaline-earth oxides adopt the B1 structure at ambient conditions and transform to B2 under compression, allowing to change coordination number and cation-anion distance independently across chemically distinct hosts. The change in anion polarizability under high pressure dominates the optical response of these prototypical ionic solids, yet its dependence on local structure and cation chemistry remains experimentally unconstrained at high pressure. Here we report refractive indices of NaCl, KCl, CaO, and SrO to ~100, ~100, ~120, and ~60 GPa, each in both the B1 and B2 structures, complemented by published MgO-B1 indices to 140 GPa and DFT calculations. Lorentz-Lorenz analysis yields pressure-independent strain polarizability parameters $Λ$ ranging from 0.34$\pm$0.05 for KCl-B1 to 1.57$\pm$0.02 for MgO-B2. Our results establish coordination number and cation-anion distance as separable predictors of anion polarizability in both B1 and B2 structures. Increasing anion coordination from six to eight at fixed cation-anion distance lowers the polarizability of both $Cl^{-}$ and $O^{2-}$ by ~0.34-0.40 $\mathring{A}^3$ and ~0.38-0.47 $\mathring{A}^3$ respectively. Our results suggest that the polarizability of $O^{2-}$ is transferable to within ~0.06 $\mathring{A}^3$ between MgO-B1 and CaO-B1 at fixed cation-anion distance, and that this transferability possibly extends to SrO-B1 and the B2 oxides. In contrast, the polarizability of $Cl^{-}$ is not transferable between NaCl-B2 and KCl-B2: the difference between them varies with cation-anion distance, from ~0.2 $\mathring{A}^3$ at 2.65 $\mathring{A}$ to 0 at ~2.5 $\mathring{A}$. The results show that the change of the strain polarizability parameters across the B1-B2 transition is governed by the decreasing and increasing sensitivities of $O^{2-}$ and $Cl^{-}$ polarizabilities to densification, respectively.