Structural transformation order-disorder transition

Fig. no. Phase diagram of the submonolayer In/Ge(l 11) system as determined using RHEED observations [8111]. Except for the order-disorder transitions to the high-temperature 1x1 structure, all transformations are irreversible, i.e. only structural changes from lower to upper structure are observed. The subsequent STM observations [96G4] confirmed the formation of all... 

When alloys exhibit an order-disorder transition or phase transformation with a rise of temperature, an anomalous temperature dependence of the elastic moduli is observed. Figure 13 shows the temperature dependence of the compliance constants ] s, and -s,2 for CU Au (LI2, cP4 structure) (Siegel, 1940). A discontinuity in the modulus versus temperature curves... 

CH de Novion, B Beuneu, T Priem, N Lorenzellii, A Finel. Defect structures and order-disorder transformations in transition metal carbides and nitrides. In R Freer, ed. The Physics and Chemistry of Carbides, Nitrides and Borides. Dordrecht Kluwer Academic Press, 1990, p 329. 

On warming to 300 K, the adlayer undergoes a disorder-order transition the Os states present at 120 K, together with the surface copper atoms, are highly mobile and can be considered to resemble a two-dimensional gas which at 300 K transforms into a structurally well-ordered immobile oxide adlayer.22 This is very similar to the model proposed from spectroscopic (XPS) studies and based on chemical reactivity evidence (see Chapter 2). 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

Theoretical treatment of order-disorder structural phase transitions in hydrogen-bonded ferroelectrics implies, in principle, the determination of the transition critical temperature Tc and accompanying crystal structure transformations. 

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