Ionic crystals, nonlinear optical

The most investigated crystals from the polariton point of view are ionic crystals, studied by IR spectroscopy.156 (Other applications of polariton theory are time spectroscopy and nonlinear optics in semiconductors182 and semiconductor-doped glasses.183) The mixed crystals are of the type K(C1, Br)... 

Solid phosphates show a huge variety of crystal structures, and it is not practical to classify them in terms of structural types as is done with simple oxides, halides, etc. However, some general classes of metal phosphate structures will be considered three-dimensional frameworks of linked phosphate tetrahedra and tetrahedrally or octahedrally coordinated cations, layered phosphates, and phosphate glasses. In all of these materials the size and topology of pores within the structure are of importance, as these determine the ability of ions and molecules to move within the structure, giving rise to useful ion exchange, ionic condnction, or catalytic properties. Ion exchange can also be nsed to modify the properties of the host network, for example, the nonlinear optical behavior of potassium titanyl phosphate (KTP) derivatives. 

E. Westin, Calculations of Linear and Nonlinear Optical Properties of Ionic Crystal Surfaces and Fullerenes Thesis Chalmers University of Technology, Goteborg, 1995. 

Ionic chromophores in which the packing is mainly driven by the stronger Coulomb interactions among ions are an approach [25-28]. Relevant results have been obtained with Af-methyl-stilbazolium salts or IV-phenyl-piridinium salts. In particular, DAST (4-dimethylamino-A-methylstilbazolium 4-toluene-sulphonate) is presently known as one of the best organic nonlinear optical crystal (SHG efficiency 1,000 times urea). 

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