Evolution towards centrosymmetry of the nonlinear-optical material RbTiOPO4 in the temperature range 293-973 K:: Alkaline displacements and titanyl deformations

The crystal structure of rubidium titanyl-phosphate, RbTiOPO4 (space group 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 increasing with temperature. To describe the importance of this motion in the phase-transition procedure and in the ionic conductivity phenomenon, two models are developed. The study of the deviation to the centrosymmetric structure (space group Pnan) shows that the change towards centrosymmetry of the titanyl groups correlates directly with the observed variation of the second-harmonic generating intensity I-2 omega Of the RbTiOPO4 crystal versus temperature.