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Possible ordered distributions of carbon (C) and substituting dopant ( A) atoms over the sites in a graphene-type lattice are considered for C2 A- and C5 A-type (super)structures (corresponding to stoichiometries of 1/3 and 1/6, respectively). Relaxation model of atomic long-range order (LRO) in a graphene-based lattice is developed, and kinetic curves of the long-range order parameters are calculated for a wide range of values of temperature and energy parameters. The long-range order parameters of C2 A and C5 A structures, described by three long-range order parameters, can relax nonmonotonically. It is caused by the 'competition' of intra- and intersublattice interaction energies: the first ones predominate only at the early relaxation stage opposing the ordering, while the second ones predominate at the later relaxation stage and in the equilibrium contributing to the ordering. [ABSTRACT FROM AUTHOR]

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By using quantitative information about the radiation diffuse-scattering intensity of the disordered f.c.c. substitutional alloy Me′1-cMe″c (c—concentration) the Fourier component, w, upper limit ∼ k, of mixing energies of Me′ and Me″ atoms may be estimated. We have to use the measurement data of the diffuse-scattering intensities at the corresponding reciprocal-space points k of the disordered phase and then determine the parameter w, upper limit ∼ (k). The statistical thermodynamics of the non-ideal solid solution is determined by these energy parameters {w, upper limit ∼ (k)}. Therefore, one can obtain the configuration free energy of an alloy, F=U-TS (U—internal energy, S—entropy), and then determine its fundamental thermodynamic characteristics, including not only its phase diagram, but also the concentration-dependent order–disorder transformation temperature, temperature and concentration long-range order parameter dependences, chemical activity, heat capacity etc. Some thermodynamic properties are calculated within the framework of the statistical-thermodynamic approach for f.c.c.-Ni–Fe alloy. The diffuse-scattering intensity values are taken from data in the literature. [Copyright &y& Elsevier]