Order-disorder evolution in solid solutions of the NLO material KTiOPO4: K0.88Rb0.12TiOPO4 and K0.465Rb0.535TiOPO4 in the temperature range 293-973 K

K0.88Rb0.12TiOPO4 and K0.465Rb0.535TiOPO4 solid solutions of the potassium titanyl phosphate (KTiOPO4, space group Pna2(1)) family, are described at 2 93, 473, 673, 873 and at 973 K. Their high resolution structures are obtain ed by using accurate single-crystal X-ray diffraction techniques at high re solution, (sin theta/lambda)(max) = 1.36 Angstrom(-1). Large anharmonic mot ion of alkaline ions increasing with temperature allows the evolution of th e 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 sit e refinement models are developed and discussed. A thermodynamic model, whi ch uses a barrier-potential model, is also developed to specify the differe nt interactions operating in the order-disorder evolution, and more general ly interactions between alkaline ions and the framework.