Order-disorder evolution in solid solutions of the NLO material KTiOPO : K Rb TiOPO and K Rb TiOPO in the temperature range 293-973 K

Abstract:

K_0.88Rb_0.12TiOPO₄ and K_0.465Rb_0.535TiOPO₄ solid solutions of the potassium titanyl phosphate (KTiOPO₄, space group Pna2₁) family, are described at 293, 473, 673, 873 and at 973 K. Their high resolution structures are obtained by using accurate single-crystal X-ray diffraction techniques at high resolution, (sin Å^-1. Large anharmonic motion of alkaline ions increasing with temperature allows the evolution of the rubidium and potassium ions repartition in the two alkaline sites versus temperature. To describe this motion inducing ionic conductivity phenomenon and to determine accurately the order-disorder evolution, two alkaline site refinement models are developed and discussed. A thermodynamic model, which uses a barrier-potential model, is also developed to specify the different interactions operating in the order-disorder evolution, and more generally interactions between alkaline ions and the framework.