Among the many possible phase‐change materials that can be used in digital memories, Ge‐rich GeSbTe (GGST) alloys are of special interest due to their much higher thermal stability, i.e., the higher crystallization temperature, they offer. However, in contrast to congruent materials which may transit from the amorphous to the crystalline state while keeping the same homogeneous chemical composition, GGST crystallization is obtained through the successive formation of the Ge and GST‐225 phases. For this reason, they show distinct properties and characteristics from those found in the canonical GST‐225 and GeTe alloys. Herein, some of these characteristics, their crystallization kinetics, the effect of N doping and oxidation, and their electrical properties are reviewed and highlighted.

Among the phase-change materials, Ge-rich GeSbTe (GST) alloys are of considerable interest as they offer a much higher thermal stability than their congruent contenders, a desirable characteristic for embedded digital memories and neuromorphic devices. Up to now, the mechanisms by which such alloys crystallize and progressively switch from one resistivity state to the other remain unclear and very controversial. Using in situ synchrotron X-ray diffraction during isothermal annealing and advanced transmission electron microscopy techniques, we solve this riddle and unveil the mechanisms leading to the overall crystallization of such alloys. During annealing at 310 °C, the initially homogeneous and amorphous material undergoes a progressive phase separation, leading to the formation of Ge-rich regions of different compositions. During this decomposition, the first formed GeTe embryos crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. As the phase separation proceeds, these embryos dissolve and the Ge phase gradually builds up through the nucleation of small grains. Only when this Ge cubic phase is largely formed, the remaining amorphous matrix may locally reach the Ge 2 Sb 2 Te 5 composition at which it can crystallize as large grains. Our density functional theory calculations confirm that the quite exotic Pnma GeTe structure we have experimentally identified is more stable than the regular R3m structure at nanometric sizes.

We report the influence of N concentration on the crystallization kinetics, microstructural evolution, and composition of Ge-rich GeSbTe (GST) alloys during thermal annealing by using X-ray diffraction and scanning and transmission electron microscopy. We show that the incorporation of N in Ge-rich GST tends to slow down the phase separation, crystallization, and growth processes during annealing. This can be attributed to the reduced diffusivity of Ge, which interacts and quickly bonds with N. Technological advantages of N doping are also discussed, considering the increased stability of the amorphous phase with respect to its parent crystalline phase, finer microstructure, flatness of the GST films after crystallization, and disappearance of the low-resistivity hexagonal phase at high temperature.

Synthesis, crystal structure, vibrational and dielectric properties of [C7H18N2]2ClBiCl6.H2O are reported. The compound crystallizes at room temperature in the orthorhombic system, space group P212121, with the following unit cell parameters : a = 7.5500(6) Å, b = 18.3780(2) Å, c = 19.8980(13) Å, V = 2760.9(4) Å3 and four molecules per unit cell. The structure has been solved by three-dimensional Patterson synthesis and refined by least-squares analysis (R1 = 0.0463, wR2 = 0.0764). The crystal structure of the title compound, [C7H18N2]2ClBiCl6.H2O consists of 2-(2-Aminoethyl)-1-methylpyrrolidinium cations, [BiCl6]3- anions, Cl- anions and free water molecules. The Bi(III) cation is coordinated by six Cl- anions in slightly distorsed octahedral geometry. In the crystal, extensive intermolecular N-H…Cl hydrogen bonds occur. The charge-transfer (CT) interactions between 2-(2-Aminoethyl)-1-methylpyrrolidinium cation and the anionic hosts have been revealed by structural analysis and UV-vis spectroscopy. The dielectric properties have been investigated at temperature range from 100 to 300 K at various frequencies (1 KHz – 1 MHz). The evolution of dielectric constant as a function of temperature and frequency of pellet has been investigated in order to determine some related parameters.