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 IO3− and IO2F2− 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 [d = Σ(G∥β∥+G⊥β⊥)], where (β∥,β⊥) represent the ∥ and ⊥ components of the bond polarizabilities, respectively, and (G∥,G⊥) are the appropriate geometrical factors, viz., unit cell volumes and direction cosines of the bonds. Current theories maintain that β⊥ can be neglected since β∥≫β⊥. We find that while β⊥ is indeed smaller than β∥, it is not negligible, i.e., β∥∼ 3.6β⊥ for iodine-oxygen bonds. We further find that in certain cases G⊥∼ 10G∥. These two observations, which show that G∥β∥ ∼ G⊥β⊥, being previous calculations of β's and d's based on setting β⊥=0 into serious question.