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Phase-change materials are characterized by a unique property portfolio well suited for data storage applications. Here, a first treasure map for phase-change materials is presented on the basis of a fundamental understanding of the bonding characteristics. This map is spanned by two coordinates that can be calculated just from the composition, and represent the degree of ionicity and the tendency towards hybridization ('covalency') of the bonding. A small magnitude of both quantities is an inherent characteristic of phase-change materials. This coordinate scheme enables a prediction of trends for the physical properties on changing stoichiometry.

We propose a fully ab initio based integrated approach to determine the volume and temperature dependent free-energy surface of nonmagnetic crystalline solids up to the melting point. The approach is based on density-functional theory calculations with a controlled numerical accuracy of better than 1 meV/atom. It accounts for all relevant excitation mechanisms entering the free energy including electronic, quasiharmonic, anharmonic, and structural excitations such as vacancies. To achieve the desired accuracy of Ͻ1 meV/ atom for the anharmonic free-energy contribution without losing the ability to perform these calculations on standard present-day computer platforms, we develop a numerically highly efficient technique: we propose a hierarchical scheme-called upsampled thermodynamic integration using Langevin dynamics-which allows for a significant reduction in the number of computationally expensive ab initio configurations compared to a standard molecular dynamics scheme. As for the vacancy contribution, concentration-dependent pressure effects had to be included to achieve the desired accuracy. Applying the integrated approach gives us direct access to the free-energy surface F͑V , T͒ for aluminum and derived quantities such as the thermal expansion coefficient or the isobaric heat capacity and allows a direct comparison with experiment. A detailed analysis enables us to tackle the long-standing debate over which excitation mechanism ͑anharmonicity vs vacancies͒ is dominant close to the melting point.

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