2, Faculty of Mathematics and Physics, Charles University, Prague, , Czechia
3, Institute of Physics, The Academy of Sciences of Czech Republic, Prague, , Czechia
The family of hydrogen-bonded molecular crystals can be considered as a promising result of crystal engineering of novel materials for nonlinear optics (NLO). These crystalline materials (i.e. salts and co-crystals) are based on properly arranged organic molecules with highly delocalized π-electron systems acting as carriers of NLO properties. The energy of formed hydrogen bonds counteracts the natural tendencies of the organic molecules (ions) to form centrosymmetric pairs. In addition, the formed hydrogen-bonded structures frequently gain advantageous chemical and physical properties.
Several molecular crystals exhibit exceptional NLO properties based on χ(2)- and χ(3)- nonlinearities. Examples of these properties that can be employed for technical applications include harmonic generation (e.g. second harmonic generation – SHG), sum- and difference-generation, intensity dependence of the complex refractive index, light-by-light scattering, and stimulated light scattering. A very recent application of hydrogen-bonded salts of organic molecules (e.g. guanylurea hydrogen phosphite ) is based on stimulated Raman scattering (SRS). This χ(3) NLO phenomenon is used for the development of compact and efficient frequency converters of the one-micron laser emission based on neodymium or ytterbium lasants. 
The most of physical properties (including the optical ones) of crystalline materials are intimately related to the symmetry of their crystal structures, and the eventual phase transitions are frequently accompanied by the changes of their physical properties. In addition to the ordinary phase transitions (e.g. first and second order transitions), other rarely observed effects, such as glass transitions and isostructural phase transitions , can occur in the family of molecular hydrogen-bonded crystals.
This contribution deals with preparation and characterization of molecular crystals based on inorganic and organic salts of aminopyrimidines. Particular attention will be focused on 2,4,6-triaminopyrimidinium phosphates and dicarboxylates (novel prospective SHG materials) and their low temperature phase transformations. The detailed explanation of the mechanism of the observed transitions associated with low-temperature proton transfer between cations and anions in the crystal structure is based on combination of experimental (i.e. vibrational spectroscopy, XRD and calorimetry) and theoretical (solid state quantum-chemical calculations) methods.
 A.A. Kaminskii, P. Becker, H. Ree, O. Lux, A. Kaltenbach, H.J. Eichler, A. Shirakawa, H. Yoneda, I. Nemec, M. Fridrichova, L. Bohaty, Physica Status Solidi B 250(2013) 1837.
 A.A. Kaminskii, Laser and Photonics Reviews 1 (2007) 93.
 D. Chernyshov, M. Hostettler, K.W. Tornroos, H.B. Burgi, Angewandte Chemie- International Edition 42 (2003) 3825.
Financial support from the CUCAM project (project No. CZ.02.1.01/0.0/0.0/15_003/0000417) is gratefully acknowledged.