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Origin of short- and medium-range order in supercooled liquid Ge <sub/>3</sub> Sb <sub/>2</sub> Te <sub/>6</sub> using <i>ab initio</i> molecular dynamics simulations
摘要： Phase-change materials such as Ge-Sb-Te compounds have attracted much attention due to the potential value for electrical data storage. In contrast to the amorphous and crystalline phases, supercooled liquid is far from to be deeply understood despite of the inevitable role in both amorphization and crystallization process. To this end, we have studied the dynamics property and structural characteristic of liquid and supercooled liquid Ge3Sb2Te6 during the fast cooling process. As the temperature decreases, chemical bonds become more homogeneous, but coordination numbers of Ge, Sb and Te atoms change very little. Meanwhile, the structural order of short-range configuration is obvious enhanced. Further studies suggest that Ge-centered, Sb-centered and Te-centered configurations change to the more ordered defective octahedrons mainly by adjusting the bond-angle relation and bond length, rather than just changing the coordination environment. It is the more ordered octahedrons that promote the formation of medium-range order. Our findings provide a deep insight on the origin of local structural order in supercooled liquid Ge3Sb2Te6, which are of great importance for the comprehensive understanding of amorphization and crystallization processes.
关键词： molecular dynamics,phase-change material,supercooled liquid,short-range order
更新于2025-09-23 15:19:57
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Improved optoelectronic properties in CdSexTe1?x through controlled composition and short-range order
摘要： We employ ?rst principles methods based on density functional theory and beyond to study CdSexTe1?x alloys in the zincblende and wurtzite structures. From the cluster expansion formalism, we provide a detailed phase diagram showing a consolute temperature of 325 K, above which miscibility may be achieved. In the random solid solution, a zincblende-to-wurtzite phase boundary is found to range from Se concentrations of x = 0.5–0.6, in agreement with experiment, owing to increasing ionic character of the Cd-anion bonds. Disordered CdSexTe1?x con?gurations are modeled using special quasirandom structures, for which optoelectronic properties are computed with the hybrid HSE06 functional. Alloying is shown to cause strong bowing e?ects in the band gap and e?ective electron/hole masses, which we attribute to local structural distortions as illustrated by analysis of bond length distributions. Downward bowing in the band gap and e?ective hole mass of the zincblende structure is highlighted for its potential bene?ts in photovoltaics through increased net photocurrent. Absorption coe?cients and re?ectivity are also reported, showing promising results in zincblende CdSexTe1?x as indicated by substantial optical absorption throughout all Se concentrations. Lastly, we identify the presence of short-range order in CdSexTe1?x characterized by clustering among like atoms in order to minimize strain. The degree of clustering, which may be tuned by temperature, also controls the magnitude of the band gap. Therefore, we propose both composition and short-range order as e?ective tools to be utilized in the design and synthesis of improved solar cell absorber layer materials.
关键词： Phase diagram,Optical properties,Bandgap-bowing,Short range order,CdSexTe1?x alloys
更新于2025-09-16 10:30:52
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short-range order on the activation energies of translational and rotational viscosities of nematic liquid crystals with highly polar molecules
摘要： We report experimental studies on the temperature-dependent translational and rotational viscosities of a few nematic liquid crystals made of highly polar molecules. Cyanobiphenyl (nCB) with longitudinal and cyanobicyclohexane molecules (CCN-mn) with transverse dipole moments were chosen and the measurements were made in the nematic phase. The translational viscosity (shear viscosity) is measured by using a rheometer and the rotational viscosity is measured from the dynamics of the Freedericksz transition by measuring the optical phase-retardation decay of the sample. The associated activation energies are obtained from the ?tting of the appropriate equation in the nematic phase. Only 8CB and CCN-47 exhibits N to Sm-A phase transition and their activation energies of translational and rotational viscosities are larger compared to the shortest homologs in the respective series. Both activation energies of 8CB are signi?cantly larger than that of CCN-47. The temperature-dependent steady states of the director distribution under shear are comprehensibly re?ected in the rheodielectric measurements of 8CB than that of CCN-47. The results are explained based on the differences in the Sm-A short-range orders and the critical slowing down of the order parameter ?uctuations. We present a simple physical model of molecular association that gives rise to different Sm-A short-range orders in both classes of compounds.
关键词： nematic liquid crystals,rotational viscosity,activation energies,Sm-A short-range order,translational viscosity
更新于2025-09-04 15:30:14