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Accurate x-ray data sets were collected at different temperatures for mixed crystals of , where x = 0.9, 0.8, 0.5, 0.4 and 0.0. The phase transition temperatures of and were observed at K and K respectively. The vibration amplitudes are found to be larger for concentrations corresponding to the glassy phases. These large vibrations result from the static part of the mean square displacement which may be due to a static disorder due to a large difference between cell parameters of pure phases, or to fluctuations of the concentration throughout the crystal. In the range 0.23 < x < 0.75, competition between the two configurations occurs (competition between the ferroelectric and antiferroelectric states): this leads to the observed static disorder of groups which prevents the phase transition from occurring.

The relations are established which allow the calculation of double-refraction walk-off angle for type I and type II processes of second harmonic generation. As an example these angles are calculated numerically in the case of second harmonic generation of 1064 nm from a Nd:YAG laser under phase-matched conditions for eight orthorhombic non-linear crystals: iodic acid, ammonium oxalate, lithium formate, potassium titanyl phosphate, calcium tartrate tetrahydrate, lithium gallium oxide, barium sodium niobate and potassium niobate.

The infrared reflectivity spectra polarized along the c-axis of potassium dihydrogen phosphate are studied for both, the ferro-and paraelectric phase, over a wide temperature range: 10 t o 420 K.The reflectivity spectra are fitted to the factorized form of the dielectric function. The temperature dependence of the optical mode parameters: frequencies, damping, and oscillator strengths are reported for both the phases. The analysis shows that besides the soft mode, a relaxational process of the very low-frequency region plays an important role near the phase-transition temperature.Nous avons 6tudiB les spectres de reflexion dans I'infrarouge lointain polarises suivant l'axe c du phosphate diacide de potassium, dansles phases ferro-et paraelectrique, sur un vaste domaine de temphrature: 10 B 420 K. Les spectres de reflexion sont ajustes B I'aide de la forme factorisbe de la fonction diklectrique. La variation des p3ramhtres des modes optiques avec la temperature: frkquences, amortissements et forces d'oscillateur, est Btudiee dsns les deux phases. L'analyse que nous prbsentons ici montre qu'en plus du mode mou, u n processus de type relaxation, joue un rale important au voisinage de la temperature de transition de phase.

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