Structural aspects of nonlinear optics: Optical properties of KIO2F2 and its related iodates

We report our measurements on KIO₂F₂ and compare these results with the parent MIO₃ compounds where M denotes H, Li, etc. Our results show that the nine observed nonlinear coefficients (d's) in these four different crystal systems, composed of ${\mathrm{IO}}_3^{-}$ and ${\mathrm{IO}}_2{\mathrm{F}}_2^{-}$ ions, are geometrically related to only one microscopic bond polarizability (β), the ∥ and ⊥ components of which are (3.6±0.4)×10⁻³⁰ esu and (1.0±0.1)×10⁻³⁰ esu, respectively. We further report that the nonlinear polarizability of the iodine lone pair is equivalent to that of an iodine‐oxygen bond ∼1.85 Å. Microscopic theories relate the observed bulk nonlinear coefficients (d's) to the individual bond polarizabilities (β's) via ${\mathrm{[d\; =\; \Sigma (G}}^{\parallel }{\beta }^{\parallel }{\mathrm{+G}}^{\perp }{\beta }^{\perp }\mathrm{)]}$ , where ${\mathrm{(\beta }}^{\parallel }{\mathrm{,\beta }}^{\perp })$ represent the ∥ and ⊥ components of the bond polarizabilities, respectively, and ${\mathrm{(G}}^{\parallel }{\mathrm{,G}}^{\perp })$ are the appropriate geometrical factors, viz., unit cell volumes and direction cosines of the bonds. Current theories maintain that β^⊥ can be neglected since ${\beta }^{\parallel }{\mathrm{\gg \beta }}^{\perp }$ . We find that while β^⊥ is indeed smaller than β^∥, it is not negligible, i.e., ${\beta }^{\parallel }{\text{∼ 3.6β}}^{\perp }$ for iodine‐oxygen bonds. We further find that in certain cases $G^{\perp }{\text{∼ 10G}}^{\parallel }$ . These two observations, which show that $G^{\parallel }{\beta }^{\parallel }{\mathrm{\sim \; G}}^{\perp }{\beta }^{\perp }$ , being previous calculations of β's and d's based on setting ${\beta }^{\perp }\text{=0}$ into serious question.