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Littérature scientifique sur le sujet « Phase change materials, phase change memories, first principles simulations, molecular dynamics »
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Articles de revues sur le sujet "Phase change materials, phase change memories, first principles simulations, molecular dynamics"
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Bernasconi, M. "Atomistic Simulations of Phase Change Materials for Electronic Memories." International Journal of Nanoscience 18, no. 03n04 (2019): 1940082. http://dx.doi.org/10.1142/s0219581x19400829.
Texte intégralRésumé :
We review our results on large-scale atomistic simulations of the phase change compound GeTe of interest for applications in nonvolatile electronic memories. The simulations are based on an interatomic potential with an accuracy close to that of the density functional theory (DFT). The potential was generated by fitting a DFT database by means of an artificial neural network method. This methodological advance allowed us to perform molecular dynamics simulations with several thousand atoms for several ns that provided useful insights on several properties of interest for the operation of phase change memories, including the crystallization kinetics, the dynamics of the supercooled liquid, the structural relaxation in the glass and the properties of nanowires.
2
Wang, Jiong, Dongyu Cui, Yi Kong, and Luming Shen. "Unusual Force Constants Guided Distortion-Triggered Loss of Long-Range Order in Phase Change Materials." Materials 14, no. 13 (2021): 3514. http://dx.doi.org/10.3390/ma14133514.
Texte intégralRésumé :
Unusual force constants originating from the local charge distribution in crystalline GeTe and Sb2Te3 are observed by using the first-principles calculations. The calculated stretching force constants of the second nearest-neighbor Sb-Te and Ge-Te bonds are 0.372 and −0.085 eV/Å2, respectively, which are much lower than 1.933 eV/Å2 of the first nearest-neighbor bonds although their lengths are only 0.17 Å and 0.33 Å longer as compared to the corresponding first nearest-neighbor bonds. Moreover, the bending force constants of the first and second nearest-neighbor Ge-Ge and Sb-Sb bonds exhibit large negative values. Our first-principles molecular dynamic simulations also reveal the possible amorphization of Sb2Te3 through local distortions of the bonds with weak and strong force constants, while the crystalline structure remains by the X-ray diffraction simulation. By identifying the low or negative force constants, these weak atomic interactions are found to be responsible for triggering the collapse of the long-range order. This finding can be utilized to guide the design of functional components and devices based on phase change materials with lower energy consumption.
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Noé, Pierre, Anthonin Verdy, Francesco d’Acapito, et al. "Toward ultimate nonvolatile resistive memories: The mechanism behind ovonic threshold switching revealed." Science Advances 6, no. 9 (2020): eaay2830. http://dx.doi.org/10.1126/sciadv.aay2830.
Texte intégralRésumé :
Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real technological breakthrough in the field of data storage memories. This breakthrough was achieved because of the successful 3D integration of so-called OTS selector devices with innovative phase-change memories, both based on chalcogenide materials. This paves the way for storage class memories as well as neuromorphic circuits. We elucidate the mechanism behind OTS switching by new state-of-the-art materials using electrical, optical, and x-ray absorption experiments, as well as ab initio molecular dynamics simulations. The model explaining the switching mechanism occurring in amorphous OTS materials under electric field involves the metastable formation of newly introduced metavalent bonds. This model opens the way for design of improved OTS materials and for future types of applications such as brain-inspired computing.
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Kojima, Takashi, and Masataka Koishi. "Mechanisms of Mechanical Behavior of Filled Rubber by Coarse-Grained Molecular Dynamics Simulations." Tire Science and Technology 48, no. 2 (2020): 78–106. http://dx.doi.org/10.2346/tire.20.160117.
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ABSTRACT: We reproduced mechanical behaviors, such as the reinforcement effect, hysteresis, and stress softening, of filled rubber under cyclic deformations using coarse-grained molecular dynamics simulations. We measured polymer density distribution in the nonload equilibrium state and conformational changes in polymer chains during deformation for dispersed and aggregated filler structures. We found that the polymer–filler attractive interactions increase the polymer density in the vicinity of fillers and decrease the polymer density in the other regions. The polymer bonds that connect polymer particles away from fillers are extended when the polymer density decreases. This alteration increases the modulus of the polymer phase, and the reinforcement effect appears. For aggregated filler structures, the polymer chains interacting with adjacent fillers act as a bridge between these fillers and increase the modulus, especially when the strain is low. To test the mechanisms of hysteresis and stress softening, we measured the changes in the polymer paths. A polymer path is the minimal path of polymer networks between two fillers; in other words, it is the “bridge” that connects two fillers. We found that the polymer paths increase in length, especially during primary loading, because of polymer adsorption/desorption on the filler surface to adjust the change of filler positions. It was also found that the influence of the filler structure diminishes in the first loading. During subsequent unloading, a long path does not become a short path again but will be folded even though the filler distance reduces. Hence, the change in the polymer paths in the second cycle is smaller than that in the first cycle because the polymer path is just unfolded. We confirmed the hysteresis and stress-softening result from these conformational changes. In this article, we also discuss the recovery mechanism for stress softening and the history dependence.
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Del Tatto, Vittorio, Paolo Raiteri, Mattia Bernetti, and Giovanni Bussi. "Molecular Dynamics of Solids at Constant Pressure and Stress Using Anisotropic Stochastic Cell Rescaling." Applied Sciences 12, no. 3 (2022): 1139. http://dx.doi.org/10.3390/app12031139.
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Molecular dynamics simulations of solids are often performed using anisotropic barostats that allow the shape and volume of the periodic cell to change during the simulation. Most existing schemes are based on a second-order differential equation that might lead to undesired oscillatory behaviors and should not be used in the equilibration phase. We recently introduced stochastic cell rescaling, a first-order stochastic barostat that can be used for both the equilibration and production phases. Only the isotropic and semi-isotropic variants have been formulated and implemented so far. In this paper, we develop and implement the equations of motion of the fully anisotropic variant and test them on Lennard-Jones solids, ice, gypsum, and gold. The algorithm has a single parameter that controls the relaxation time of the volume, results in the exponential decay of correlation functions, and can be effectively applied to a wide range of systems.
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Cui, Xiandai, Xiaomin Cheng, Hong Xu, Bei Li, and Jiaoqun Zhu. "Enhancement of thermophysical coefficients in nanofluids: A simulation study." International Journal of Modern Physics B 34, no. 25 (2020): 2050222. http://dx.doi.org/10.1142/s0217979220502227.
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Molten salts constitute one kind of PCMs (Phase Change Materials) widely used in concentrating solar power facilities for heat storage and heat transfer. This paper aims to simulate nanofluid PCMs with molecular dynamics method. Concretely, the thermophysical properties of a nanofluid of KNO3 doped with SiO2 nanoparticle are investigated by equilibrium and nonequilibrium molecular dynamics simulations. For the first time, these properties of a nanofluid in the family of PCMs are calculated. The density, thermal expansion coefficient, specific heat capacity, thermal conductivity, and viscosity are characterized as functions of the SiO2 nanoparticle concentration. The effect of the SiO2 nanoparticle size on the nanofluid’s properties is also investigated. The simulation results present an enhancement of the thermophysical properties, especially for the specific heat capacity, in good agreement with the existing experimental results on a representative nanofluid PCM, and open prospects for the understanding of microscopic mechanism leading to such enhancements.
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Shintaku, Ryoya, Tomoyuki Tamura, Shogo Nogami, and Takakazu Hirose. "First-Principles Study on Lithiation Process of Sio Anode for Li-Ion Batteries." ECS Meeting Abstracts MA2024-02, no. 5 (2024): 616. https://doi.org/10.1149/ma2024-025616mtgabs.
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Lithium-ion secondary batteries (LIB) with a high energy density have been developed mainly for use in small mobile devices. In recent years, the high capacity of LIB has become important for automobile applications. One possible high-energy density solution is the use of high-capacity negative electrodes fabricated from tin, silicon, or other materials, and a-SiO materials have been already commercialized. However, a-SiO materials has the issue of capacity degradation during charge-discharge cycles. In particular, it is speculated that the structural change during charging process causes the capacity degradation, but the detail has not been clarified yet. Therefore, in this study, we clarify the local structural changes in a-SiO up to full charge using first-principles calculations. In our previous study [1], a-SiO models were generated using classical molecular dynamics (MD) simulations with neural network potentials. Using one of these models, we explore the lithiation process of a-SiO using first-principles calculations with SIESTA code [2]. The simplest method to obtain a-Li x SiO model is to melt an initial structure in which Li atoms are randomly arranged at high temperature and then cool it. But it was found that this process could not reproduce the actual charge process and Li atoms should be placed at stable sites. Therefore, we developed a computational code with which stable sites of Li atoms can be searched efficiently using Bayesian optimization. Fig. 1 shows Li-inserted models at (a) 0%, (b) 55% and (c) 100% SoC (State of Charge). Li atoms are gradually inserted into the SiOx phase in the initial state of charging, and inserted into the Si phase over 20% SoC. These results are consistent with previous experimental results [3]. One Li6O octahedron can be seen in the model at 55% SoC, and the number of Li6O octahedra increases to five in the model at 100% SoC. The formation of Li6O octahedra during charging is irreversible, which is considered to cause the capacity degradation. To prevent capacity degradation, it is necessary to suppress the formation of these structures. References [1] H. Hirai et al., Phys. Rev. Materials 6, 115601 (2022). [2] J. Solar et al., J. Phys. Cond. Matt. 14, 2745 (2000). [3] T. Hirose et al., Solid State Inonics 304, 1 (2017). Figure 1
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Ren Qing-yong, Wang Jian-li, Li Bing, Ma Jie, and Tong Xin. "Neutron scattering studies of complex lattice dynamics in energy materials." Acta Physica Sinica 74, no. 1 (2025): 0. http://dx.doi.org/10.7498/aps.74.20241178.
Texte intégralRésumé :
Lattice dynamics play a crucial role in understanding the physical mechanisms of cutting-edge energy materials. Many excellent energy materials have complex multiple-sublattice structures, and their lattice dynamics are intricate and the underlying mechanisms are difficult to understand. Neutron scattering technologies, known for their high energy and momentum resolution, are powerful tools for simultaneously characterizing material structure and complex lattice dynamics. In recent years, neutron scattering techniques have significantly contributed to the study of energy materials, shedding light on their physical mechanisms. Starting from the basic properties of neutrons and double differential scattering cross sections, this paper introduces in detail the working principles, spectrometer structures, and comparisons with other technologies of several neutron scattering techniques commonly used in energy material research, including neutron diffraction and neutron total scattering to characterize material structure, quasi-elastic neutron scattering and inelastic neutron scattering to characterize lattice dynamics. Then, this article showcases significant research advancements in the field of energy materials utilizing neutron scattering as a primary characterization method:<br>1. In the case of Ag<sub>8</sub>SnSe<sub>6</sub> superionic thermoelectric materials, single crystal inelastic neutron scattering experiments debunk the "liquid-like phonon model" as the primary contributor to ultra-low lattice thermal conductivity. Instead, extreme phonon anharmonic scattering is identified as the key factor based on the special temperature dependence of phonon linewidth.<br>2. Analysis of quasi-elastic and inelastic neutron scattering spectra reveals changes in the correlation between framework and Ag<sup>+</sup> sublattices during the superionic phase transition of Ag<sub>8</sub>SnSe<sub>6</sub> compounds. Further investigations using neutron diffraction and molecular dynamics simulations unveil a new superionic phase transition and ion diffusion mechanism, primarily governed by weakly bonded Se atoms.<br>3. Research on NH<sub>4</sub>I compounds demonstrates a strong coupling between molecular orientation rotation and lattice vibration, and the strengthening of phonon anharmonicity with temperature can decouple this interaction and induce plastic phase transition. This phenomenon results in a significant configuration entropy change, showing potential applications in barocaloric refrigeration.<br>4. In the CsPbBr<sub>3</sub> perovskite photovoltaic materials, inelastic neutron scattering uncovers low-energy phonon damping of the [PbBr<sub>6</sub>] sublattice, influencing electron-phonon coupling and the band edge electronic state. This special anharmonic vibration of the [PbBr<sub>6</sub>] sublattice prolongs the lifetime of hot carriers, impacting the material's electronic properties.<br>5. In MnCoGe magnetic refrigeration materials, in-situ neutron diffraction experiments highlight the role of valence electron transfer between sublattices in altering crystal structural stability and magnetic interactions. This process triggers a transformation from a ferromagnetic to an incommensurate spiral antiferromagnetic structure, expanding our understanding of magnetic phase transition regulation.<br>These examples underscore the interconnected nature of lattice dynamics with other degrees of freedom, such as sublattices, charge, and spin, in energy conversion and storage materials. Through these typical examples, this article aims to provide a reference for further exploration and understanding of energy materials and lattice dynamics.
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Wei, Benxiang, Joseph M. Flitcroft, and Jonathan M. Skelton. "Structural Dynamics, Phonon Spectra and Thermal Transport in the Silicon Clathrates." Molecules 27, no. 19 (2022): 6431. http://dx.doi.org/10.3390/molecules27196431.
Texte intégralRésumé :
The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in “phonon-glass electron-crystal” materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower κlatt than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low κlatt for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.
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Nakamura, Tetsuya, Kaito Mori, Shogo Fukushima, et al. "One Million Atoms Large-Scale Reactive Molecular Dynamics Simulations for Design of Cathode Catalyst Layer in Polymer Electrolyte Fuel Cell Toward Boosting Its Performance." ECS Meeting Abstracts MA2024-02, no. 41 (2024): 2637. https://doi.org/10.1149/ma2024-02412637mtgabs.
Texte intégralRésumé :
Boosting polymer electrolyte fuel cell (PEFC) performance is required in transportation. PEFC performance depends on the electrode reaction activity related to proton conductivity and oxygen diffusivity in catalyst layer (CL), consisting of Pt nanoparticles (Pt NPs), ionomer, water, and carbon supports. To achieve high proton conductivity and oxygen diffusivity, the optimization of CL structures such as Pt composition, ionomer/water distribution, and carbon support structures is essential by computational approaches, such as first-principles molecular dynamics (MD) and classical MD methods. The first-principles MD can describe chemical reactions, however can only simulate a small part of CL structures, consisting of a few hundred atoms. In contrast, classical MD can perform over 1 million atoms simulation to calculate whole CL structures, while the simple inter-atomic potential which enables 1 million atoms calculation, can not describe chemical reactions. Then, developing our original MD simulator "Laich", implementing MPI and OpenMP with ReaxFF inter-atomic potential, enabled us to calculate million atoms system reproducing whole CL structures and to simulate chemical reactions in whole CL structures. In this work, to design the higher-performance CL, we performed reactive MD simulations using a 1 million atoms CL model. A CL structure model was constructed with ionomers, water, Pt NPs, and carbon support. The carbon support composed of six meso pores with a size of 6 nm was constructed. To improve the hydrophilicity, hydroxyl groups and hydrogen terminated 32% and 31% of its surface carbon, respectively. Pt NPs were put on the carbon support at the exterior and in the interior of the meso pores. The Pt-supported carbon was coated with ionomers and water. Protons and oxygen were introduced into the system. Hereafter, we refer to this structure as a catalyst particle (CP) model (Fig. 1). To assess the influence of the carbon support structures on the electrode reactions, the oxygen diffusivity was evaluated from the trajectories of oxygen at the exterior and in the interior of the meso pores. Ideally, oxygen should be supplied to the Pt NPs without hindering its diffusion by obstacles. At the exterior of the meso pore, oxygen could not approach the Pt NPs, because ionomers and water were obstacles to the oxygen diffusion (Figs. 2(a)). Conversely, in the interior of the meso pores, oxygen could approach the Pt NPs via the gas phase since ionomers and water did not inhibit oxygen diffusion (Figs. 2(b)). From this analysis, the oxygen diffusivity showed higher value in the interior of the meso pores than at the exterior of the meso pores. Furtherly, it is reported that oxygen diffusivity in ionomers varies with temperature [1]. However, the influence of temperatures on oxygen diffusivity at the exterior and in the interior of the meso pores has not been analyzed. In this study, we calculated the mean squared displacement (MSD) and diffusion coefficients at 300 K and 365 K. At 300 K, oxygen diffusion coefficients (the linear regressions of MSD) at the exterior (Dexterior ) and in the interior of the meso pores (Dinterior ) were 1.67×10-10 m2/s and 3.02×10-6 m2/s, respectively (Fig. 3(a)). At 365 K, Dexterior and Dinterior were 12.3×10-10 m2/s and 2.59×10-6 m2/s, respectively (Fig. 3(b)). Therefore, Dexterior and Dinterior respectively increased 7.37 and 0.86 times with increasing temperature. Next, to investigate whether the change of Dexterior was caused only by temperatures, we calculated the oxygen diffusion coefficient at 1 atm (D1atm ) in an environment consisting of oxygen atoms only. As a result, D1atm increased 1.05 times from 300 K to 365 K. By rising temperature, the increase of the oxygen diffusion coefficient showed 1.05 time at D1atm , while it showed 7.37 time at D exterior. Therefore, other factors besides temperature affect the oxygen diffusivity. To identify the factors contributing to D exterior , we focused on the size of the water cluster, one of the obstacles to oxygen diffusion. Water clusters are defined as groups of water with an inter-molecular distance of less than 3.5 Å. The cluster size is defined as the number of water molecules forming the water cluster. Fig. 4 depicts the sizes of the 10 largest water clusters. As the temperature rose, the size of the largest water cluster decreased, while the size of other water clusters increased. These analyses indicate that the morphology of water clusters changed from continuous to discontinuous clusters with increasing temperature. Therefore, as water cluster sizes decrease by increasing temperature, oxygen diffuses through the gas phase. These results indicate that the control of the water cluster size improves the oxygen diffusivity. [1] K. Kudo et. al., Electrochim. Acta, 209, 682-690 (2016). Figure 1
Thèses sur le sujet "Phase change materials, phase change memories, first principles simulations, molecular dynamics"
1
GABARDI, SILVIA. "First principles simulations of phase change materials for data storage." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2015. http://hdl.handle.net/10281/76292.
Texte intégralRésumé :
I materiali a cambiamento di fase sono calcogenuri a base di tellurio di notevole interesse tecnologico per la realizzazione di memorie ottiche (DVD) e di memorie elettroniche non volatili di nuova concezione, le memorie a cambiamento di fase o PCM. Questi dispositivi si basano su una veloce (50 ns) e reversibile transizione di fase amorfo-cristallo indotta per riscaldamento. Le due fasi corrispondono ai due stati di memoria che possono essere distinti grazie alla grande differenza tra le proprietà ottiche ed elettroniche dell'amorfo e quelle del cristallo. Nonostante il Ge2Sb2Te5 (GST) sia il materiale attualmente usato nelle PCM, si stanno studiando nuovi materiali con una temperatura di cristallizzazione più alta per aumentare la stabilità termica delle PCM. A questo proposito in questa tesi sono state studiate, attraverso simulazioni di dinamica molecolare ab-initio, diverse leghe ad alta temperatura di cristallizzazione con composizione In3Sb1Te2, In13Sb11Te3 e Ga4Sb6Te3. Queste leghe sono state studiate sperimentalmente e proposte come sostituti del GST, ma le proprietà strutturali e l'origine microscopica dell'elevata temperatura di cristallizzazione della fase amorfa di questi composti non è ancora del tutto chiara. Sono stati, quindi, generati modelli di qualche centinaio di atomi della fase amorfa raffreddando dal liquido in centinaia di ps allo scopo di trovare una relazione tra la struttura dell'amorfo e l'alta temperatura di cristallizzazione di queste leghe. La topologia di legame dei modelli amorfi risulta principalmente tetraedrica, molto diversa dalla geometria della fase cristallina che presenta invece intorni ottaedrici. La presenza di strutture tetraedriche nell'amorfo, assenti invece nella fase cristallina, può quindi costituire un ostacolo alla cristallizzazione con l'effetto di innalzare la temperatura di cristallizzazione rispetto al GST che presenta una geometria di legame prevalentemente ottaedrica sia nell'amorfo che nel cristallo.
Nella seconda parte di questo lavoro è stato affrontato il problema del drift, che consiste in un aumento della resistenza elettrica
della fase amorfa con il tempo. Questo fenomeno rappresenta un problema nelle celle PCM in quanto modifica le caratteristiche elettriche del dispositivo; tuttavia, manca ancora una spiegazione completa del meccanismo microscopico alla base di questo processo. Il drift sembra però legato al fenomeno del rilassamento strutturale che si verifica nei semiconduttori amorfi e che modifica nel tempo gli stati di difetto in prossimità degli edge delle bande di valenza e di conduzione, da cui dipende la conduzione nella fase amorfa. Per studiare il fenomeno del drift sono stati generati modelli di grandi dimensioni (circa duemila atomi) di GeTe amorfo raffreddando dal liquido in 100 ps attraverso simulazioni di dinamica molecolare classica con un potenziale Neural-Network. Una volta rilassati ab-initio, i modelli presentano diversi stati nel gap localizzati su catene di atomi di Ge. Dopo aver riscaldato i modelli a 500 K in modo da accelerare il processo di drift, si osserva una riduzione del numero di catene di Ge e di legami omopolari Ge-Ge con un conseguente allargamento del gap e riduzione dell'ampiezza delle code di Urbach che possono giustificare un aumento della resistenza. Si propone quindi che il drift sia dovuto al rilassamento strutturale della fase amorfa che porta alla riduzione delle catene di legami omopolari di Ge.<br>Phase change materials based on chalcogenide alloys are of great technological importance because of their use in optical data storage
devices (DVDs) and electronic non-volatile memories of new concept, the Phase Change Memory cell (PCM). These applications rely on a
fast (50 ns) and reversible change between the crystalline and the amorphous phases upon heating. The two phases correspond to the two
states of the memory that can be discriminated thanks to a large difference in their optical and electronic properties. Although Ge2Sb2Te5
(GST) is the compound presently used as active layer in PCMs, alternative materials with a higher crystallization temperature are under scrutiny in order to increase the thermal stability of the PCM devices. In this respect, we analysed, by means of ab-initio molecular dynamics simulations, different high crystallization temperature alloys with composition In3Sb1Te2, In13Sb11Te3 and Ga4Sb6Te3, which have been experimentally proposed as substitute of GST. However, the structural properties and the microscopical reason of the high thermal stability of the amorphous phases of these compounds is still unclear. We, thus, generated models of the amorphous phase of few hundreds of atoms by quenching from the melt in few hundreds of ps aiming at finding out a relation between the structural properties of the amorphous phase and the high crystallization temperature of these alloys. The topology of our amorphous models turned out to be mostly tetrahedral which differs from the octahedral-like geometry of the crystalline phases. The presence of tetrahedral structures in the amorphous which are absent in the crystalline phase, probably hinders the crystallization process resulting in a higher crystallization temperature with respect to GST which display a mostly octahedral-like structures in both amorphous and the crystalline phase.
In the second part of this work we addressed the issue of the resistance drift phenomenon, which consists of an increase of the electrical
resistance of the amorphous phase with time. This effect is detrimental in PCMs since it changes the electrical characteristics of the devices.
This process is believed to be due to an aging of the amorphous phase which modifies during time the defect states in the proximity
of the valence and conduction band edges which control the electrical conductivity. The microscopic origin of the structural relaxations leading to the drift is still unknown. To address this problem, we generated large models (about two thousand atoms) of amorphous GeTe by quenching from the melt in 100 ps with classical molecular dynamics simulations by using a neural-network potential. Once relaxed by first principles, the models showed the presence of several in-gap states localized on chains of Ge atoms. After an annealing at 500 K, performed to accelerate the drift process, Ge chains and homopolar Ge-Ge bonds reduce in number resulting in a band gap widening and a reduction of the Urbach tails at the band edges which can account for the increase of the resistance. We thus propose that the resistance drift originates from structural relaxations leading to the removal of Ge chains.