Tuning the Crystallization Mechanism by Composition Vacancy in Phase Change Materials.

Interface-influenced crystallization is crucial to understanding the nucleation- and growth-dominated crystallization mechanisms in phase-change materials (PCMs), but little is known. Here, we find that composition vacancy can reduce the interface energy by decreasing the coordinate number (CN) at the interface. Compared to growth-dominated GeTe, nucleation-dominated Ge2Sb2Te5 (GST) exhibits composition vacancies in the (111) interface to saturate or stabilize the Te-terminated plane. Together, the experimental and computational results provide evidence that GST prefers (111) with reduced CN. Furthermore, the (8 - n) bonding rule, rather than CN6, in the nuclei of both GeTe and GST results in lower interface energy, allowing crystallization to be observed at the simulation time in general PCMs. In comparison to GeTe, the reduced CN in the GST nuclei further decreases the interface energy, promoting faster nucleation. Our findings provide an approach to designing ultrafast phase-change memory through vacancy-stabilized interfaces.

Rules of hierarchical melt and coordinate bond to design crystallization in doped phase change materials.

While alloy design has practically shown an efficient strategy to mediate two seemingly conflicted performances of writing speed and data retention in phase-change memory, the detailed kinetic pathway of alloy-tuned crystallization is still unclear. Here, we propose hierarchical melt and coordinate bond strategies to solve them, where the former stabilizes a medium-range crystal-like region and the latter provides a rule to stabilize amorphous. The Er0.52Sb2Te3 compound we designed achieves writing speed of 3.2 ns and ten-year data retention of 161 °C. We provide a direct atomic-level evidence that two neighbor Er atoms stabilize a medium-range crystal-like region, acting as a precursor to accelerate crystallization; meanwhile, the stabilized amorphous originates from the formation of coordinate bonds by sharing lone-pair electrons of chalcogenide atoms with the empty 5d orbitals of Er atoms. The two rules pave the way for the development of storage-class memory with comprehensive performance to achieve next technological node.

Inherent Simple Cubic Lattice Being Responsible for Ultrafast Solid-Phase Change of Ge2Sb2Te5.

Crystallization of solid is generally slow in kinetics for atoms trapped in solids. Phase-change materials (PCMs) challenge current theory on its ultrafast reversible amorphous-to-crystal transition. Here by using the stochastic surface walking global optimization method, we establish the first global potential energy surface (PES) for Ge2Sb2Te5. By analyzing all structures on the global PES, we show that an inherent structural pattern of simple cubic lattice is present universally in low-energy structures, either globally in a newly found metastable simple cubic crystal phase or locally in the amorphous structures. Our solid-to-solid reaction pathway sampling reveals that this simple cubic lattice plays a critical role in the rapid amorphous-to-crystal transition, which occurs via dynamic vacancy creation/annihilation, Martensitic-type {100} shearing, and diffusionless local relaxation. This knowledge from global PES allows the prediction of PCMs by linking the phase-change kinetics with the geometry of metastable phases.

Mechanisms of metastable states in CuZr systems with glass-like structures.

The local structural inhomogeneity of glasses, as evidenced from broad bond-length distributions (BLDs), has been widely observed. However, the relationship between this particular structural feature and metastable states of glassy solids is poorly understood. It is important to understand the main problems of glassy solids, such as the plastic deformation mechanisms and glass-forming ability. The former is related to ß-relaxation, the relaxation of a system from a subbasin to another in the potential energy landscape (PEL). The latter represents the stability of a metastable state in the PEL. Here, we explain the main reason why CuZr systems with glass-like structures exist in metastable states: a large strain energy. The calculation results obtained in this study indicate that a system with broad BLD has a large strain energy because of the nonlinear and asymmetric strain energy of bonds. Unstable polyhedra have larger volumes and more short and long bonds than stable polyhedra, which are most prone to form deformation units. The driving force for pure metal crystallization was also elucidated to be the decrease in strain energy. The results obtained in this study, which are verified by a series of calculations as well as molecular dynamics simulations, indicate the presence of metastable states in amorphous materials and elucidate the mechanisms of plastic deformation and the driving force for crystallization without chemical bonding.

Effects of partitioned enthalpy of mixing on glass-forming ability.

We explore the inherent reason at atomic level for the glass-forming ability of alloys by molecular simulation, in which the effect of partitioned enthalpy of mixing is studied. Based on Morse potential, we divide the enthalpy of mixing into three parts: the chemical part (ΔEnn), strain part (ΔEstrain), and non-bond part (ΔEnnn). We find that a large negative ΔEnn value represents strong AB chemical bonding in AB alloy and is the driving force to form a local ordered structure, meanwhile the transformed local ordered structure needs to satisfy the condition (ΔEnn/2 + ΔEstrain) < 0 to be stabilized. Understanding the chemical and strain parts of enthalpy of mixing is helpful to design a new metallic glass with a good glass forming ability. Moreover, two types of metallic glasses (i.e., "strain dominant" and "chemical dominant") are classified according to the relative importance between chemical effect and strain effect, which enriches our knowledge of the forming mechanism of metallic glass. Finally, a soft sphere model is established, different from the common hard sphere model.

Spin polarization gives rise to Cu precipitation in Fe-matrix.

This article tries to uncover the physical reason of Cu precipitation from an Fe matrix at the electronic level. The general rule is obtained that the more bonds among Cu atoms, the more stable the system is. It was shown that Cu would precipitate from the matrix with Fe spin-polarization but not without spin-polarization. The partial density of states (PDOS) analysis illustrated that the d states of Fe near the Fermi level potentially have strong interaction with other atoms, but Cu d states below the Fermi level lack this potential, which results in weak covalent d orbital interaction between Fe and Cu. Furthermore, the charge density difference also confirmed the weaker bond between Fe and Cu with spin-polarization compared to without spin-polarization, due to the decreased charge between them. In addition, the {110} interface energy between Fe and Cu, estimated by the "dangling bond", is 676.3 mJ m(-2), which agrees with the DFT calculation, 414.2 mJ m(-2). Finally, this study also revealed that Ni atoms can reduce the "dangling bond" when it locates at the interface and separates Fe and Cu.