KTiOPO4 precipitation from potassium titanium phosphate glasses, producing second harmonic generation

Glass formation in the K2O–TiO2–P2O5–SiO2 system, and the structure and crystallization behavior of glasses having compositions near the stoichiometry of KTiOPO4 (KTP) have been examined by differential thermal analysis (DTA), X-ray diffraction (XRD), fast Fourier transform infrared (FTIR) spectroscopy and second harmonic generation (SHG). Pure glasses may be obtained adding an appropriate amount of P2O5 as well as of SiO2 to the KTP composition. Transparent and homogeneous glasses were synthesized for the 56(K2O·2TiO2·P2O5)·44P2O5 (KTP–44P) and the 90(K2O·2TiO2·P2O5)·10SiO2 (KTP–10Si) molar compositions. In contrast, for the 80(K2O·2TiO2·P2O5)·20SiO2 (KTP–20Si) and for higher SiO2 contents, the obtained glasses were partially opaque and phase separated. KTP glasses with lower P2O5 content cannot be obtained as they crystallize during quenching. The KTP–10Si glass shows the only exothermic peak connected to KTP phase crystallization whereas the KTP–44P and the KTP–20Si glasses devitrify in two steps. Firstly the same unidentified phase is formed in both these glasses and the KTP phase is formed only at higher temperatures. Transparent and opalescent KTP glasses exhibiting SHG activity may be produced by careful heat treatments at temperatures just above Tg. The origin of SHG in transparent glasses is supposed to be connected with either precipitation of KTiOPO4 nanocrystallites or very early stages of liquid-type phase separation.