ORDER-DISORDER PHENOMENA IN OXIDES WITH ROCK SALT STRUCTURES - SYSTEM LI2TIO3-MGO

Li2TiO3, which belongs to the family of rock salt superstructure phases, undergoes an order-disorder phase transition at 1213° C. It forms solid solutions with MgO and the temperature of the order-disorder change falls increasingly rapidly with increasing MgO content; the ordered solid solutions could not be prepared beyond ∼40 mol % MgO. Up to ∼20% MgO, the transition may be observed as a DTA heat effect and is probably first order. At ≳25% MgO the transition is second order; no DTA heat effect occurs and, by X-ray diffraction, the order-disorder change occurs, reversibly and continuously, over a range of several hundred ° C. For 25% MgO, the disorder, δ, is given by δ=exp[-m(Tc-T)] where Tc is the critical temperature, above which only short range order exists, and m is a constant. The order, δ, is determined from the relative intensity of the (0 0 2) superstructure reflection. The mechanisms of the order-disorder processes are discussed and a method of studying the kinetics is indicated. It is likely that many other oxide phases which have rock salt superstructures will exhibit order-disorder phenomena at high temperatures. © 1979 Chapman and Hall Ltd.