Phase-change materials ͑PCMs͒ are the subject of considerable interest because they have been recognized as potential active layers for nonvolatile memory devices, known as phase-change random access memories. By analyzing first-principles molecular-dynamics simulations we develop a method for the enumeration of mechanical constraints in the amorphous phase and show that the phase diagram of the most popular system ͑Ge-Sb-Te͒ can be split into two compositional regions having a well-defined mechanical character: a tellurium rich flexible phase and a stressed rigid phase that encompasses the known PCMs. This sound atomic scale insight should open new avenues for the understanding of PCMs and other complex amorphous materials from the viewpoint of rigidity.

Decane has been used as swelling agent to enlarge the pore size of pure silica MCM-41 materials. The synthesis conditions such as the swelling agent/surfactant molar ratio, the adding sequence of swelling agent, etc., have been studied. The role of decane and the effect of crystallization time and temperature on the synthesis have also been discussed. Final compounds were intensively characterized by several techniques (XRD diffraction, SEM, TEM, and nitrogen adsorption-desorption analysis). The present work shows that decane is an effective expander to enlarge the pore size of mesoporous materials. Two possible mechanisms have been proposed to describe the swelling effect of decane molecules. The synthesis of mesoporous materials can be explained by different steps, which have been observed in the synthesis course of zeolites.

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Negative thermal expansion (NTE) in tellurium based liquid alloys (GeTe6 and GeTe12) is analyzed through the atomic vibrational properties. Using neutron inelastic scattering, we show that the structural evolution resulting in the NTE is due to a gain of vibrational entropy that cancels out the Peierls distortion. In the NTE temperature range, these competing effects give rise to noticeable changes in the vibrational density of states spectra. Additional first principles molecular dynamics simulations emphasize the role of the temperature dependance of the Ge atomic environment in this mechanism. For comparison, we extended our study to Ge2Sb2Te5 and Ge1Sb2Te4 phase-change materials.