Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

Amorphous Ge(x)Te(100-x) alloys were obtained over a broad composition range (12 ≤ x ≤ 44.6) by thermal co-evaporation. Their structure was investigated by x-ray diffraction and extended x-ray absorption fine structure measurements. Experimental datasets were fitted simultaneously by the reverse Monte Carlo simulation technique. It is concluded that Te is mostly twofold coordinated and the majority of Ge atoms have four neighbours. The number of Ge-Ge and Te-Te bonds evolves monotonically with composition. Ge-Ge bonding can be observed already at x = 24 while Te-Te bonds can be found even in Ge44.6Te55.4. The models obtained by simulation show that the structure of compositions with x > 24 should be considered as a random covalent network but there is chemical ordering for x ≤ 24, exactly in the composition range where glasses can be obtained from the melt by fast quenching. The composition dependences of some physical properties also point to the connection between chemical short range order and the stability of the amorphous phase: while the glass transition temperature and microhardness increase monotonically with the composition, the thermal stability of the amorphous films goes through a maximum around x = 20-24.

Neutron and x-ray weighted total scattering structure factors of liquid carbon, silicon, germanium, and tin tetrachlorides, CCl(4), SiCl(4), GeCl(4), and SnCl(4), have been interpreted by means of reverse Monte Carlo modeling. For each material the two sets of diffraction data were modeled simultaneously, thus providing sets of particle coordinates that were consistent with two experimental structure factors within errors. From these particle configurations, partial radial distribution functions, as well as correlation functions characterizing mutual orientations of molecules as a function of distance between molecular centers were calculated. Via comparison with reference systems, obtained by hard sphere Monte Carlo simulations, we demonstrate that orientational correlations characterizing these liquids are much longer ranged than expected, particularly in carbon tetrachloride.

Phase-change materials exhibit fast and reversible transitions between an amorphous and a crystalline state at high temperature. The two states display resistivity contrast, which is exploited in phase-change memory devices. The technologically most important family of phase-change materials consists of Ge-Sb-Te alloys. In this work, we investigate the structural, electronic and kinetic properties of liquid Ge2Sb2Te5 as a function of temperature by a combined experimental and computational approach. Understanding the properties of this phase is important to clarify the amorphization and crystallization processes. We show that the structural properties of the models obtained from ab initio and reverse Monte Carlo simulations are in good agreement with neutron and X-ray diffraction experiments. We extract the kinetic coefficients from the molecular dynamics trajectories and determine the activation energy for viscosity. The obtained value is shown to be fully compatible with our viscosity measurements.