EVOLUTION TOWARDS CENTROSYMMETRY OF THE NONLINEAR-OPTICAL MATERIAL RBTIOPO4 IN THE TEMPERATURE-RANGE 293-973 K - ALKALINE DISPLACEMENTS ANDTITANYL DEFORMATIONS

The crystal structure of rubidium titanyl-phosphate, RbTiOPO4 (space g roup Pna2(1)), has been refined at room temperature, and at 473, 673, 873, and 973 K, by using single-crystal x-ray-diffraction techniques. The data show a large anharmonic motion of the rubidium ions increasin g with temperature. To describe the importance of this motion in the p hase-transition procedure and in the ionic conductivity phenomenon, tw o models are developed. The study of the deviation to the centrosymmet ric structure (space group Pnan) shows that the change towards centros ymmetry of the titanyl groups correlates directly with the observed va riation of the second-harmonic generating intensity I-2 omega Of the R bTiOPO4 crystal versus temperature.