Spectroscopic Observation of Surface Distortion in Ionic Crystals. I. Lithium and Sodium Nitrate

Attenuated‐total‐reflection (atr) infrared data for the out‐of‐plane nitrate ion mode in crystalline LiNO₃. NaNO₃, and KNO₃ are interpreted in terms of a distorted lattice surface structure. The surface distortion, which is manifested by atr curves which shift toward the higher‐frequency bulk‐transmission band position as one increases the penetration depth of the electric field of the totally reflected wave, apparently decays exponentially into the crystal. The atr data are fitted to a ${\nu }_2‐\mathrm{vs}\mathrm{‐Z}$ (depth) function such that the calculated surface frequency shift reduces to $e^{\text{−1}}$ of its surface value at a depth of $\text{1.0 μ}$ . This surface “depth” is orders of magnitude greater than that predicted theoretically for the alkali halides, but can be rationalized in terms of a cooperative phenomenon.