211 6792 465 This paper provides a comprehensive overview of state-ofthe-art computational techniques to thermodynamically model magnetic and chemical order-disorder transitions. Recent advances as well as limitations of various approaches to these so-called lambda transitions are examined in detail, focussing on calphad models and first-principles methods based on density functional theory (DFT). On the one hand empirical implementations -based on the Inden-Hillert-Jarl formalismare investigated, including a detailed interpretation of the relevant parameters, physical limiting cases and potential extensions. In addition, Bragg-Williams-based approaches as well as cluster-variation methods of chemical order-disorder transitions are discussed. On the other hand, it is shown how magnetic contributions can be introduced based on various microscopic model Hamiltonians (Hubbard model, Heisenberg model and beyond) in combination with DFT-computed parameters. As a result of the investigation we were able to indicate similarities between the treatment of chemical and magnetic degrees of freedom as well as the treatment within the calphad and DFT approaches. Potential synergy effects resulting from this overlap have been derived and alternative approaches have been suggested, in order to improve future thermodynamic modelling of lambda transitions.

We use the 1-bond→2-phonon percolation doublet of zincblende alloys as a 'mesoscope' for an unusual insight into their phonon behavior under pressure. We focus on (Zn,Be)Se and show by Raman scattering that the original Be-Se doublet at ambient pressure, of the stretching-bending type, turns into a pure-bending singlet at the approach of the high-pressure ZnSe-like rocksalt phase, an unnatural one for the Be-Se bonds. The 'freezing' of the Be-Se stretching mode is discussed within the scope of the percolation model (mesoscopic scale), with ab initio calculations in support (microscopic scale).

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ZenGen" is a script-tool which helps to automatically generate first-principles input files of all the ordered compounds of a given crystal structure in a given system. The complete set of heats of formation of each end-member can then easily be used in the thermodynamic phase modeling."ZenGen" is a free and open source code, which can be downloaded from http://zengen.cnrs.fr.In order to illustrate its possibilities, the quaternary system, Cr-Mo-Ni-Re, has been investigated.The binary solid solution parameters have been estimated from SQS calculations. The σ−phase has been considered according to its crystal structure, i.e. with a 5-sublattice model, by the DFT calculation of the 4 5 = 1024 different ordered quaternary configurations. Several tentative ab initio phase diagrams are presented.

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