Chalcogenide alloys are materials of interest for optical recording and non-volatile memories. We perform ab-initio molecular dynamics simulations aiming at shading light onto the structure of amorphous Ge2Sb2Te5 (GST), the prototypical material in this class. First principles simulations show that amorphous GST obtained by quenching from the liquid phase displays two types of short range order. One third of Ge atoms are in a tetrahedral environment while the remaining Ge, Sb and Te atoms display a defective octahedral environment, reminiscent of cubic crystalline GST.
A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 10 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and tetrahedral sites of the face-centered cubic lattice and calculating their energy at different theoretical levels from first principles, we determine that metastable structures with partial or full oc…
This paper reports the presence of extended-range ordering in the atomic pair-correlation function of amorphous silicon ($a$-Si) using ultra-large atomistic models obtained from Monte Carlo and molecular-dynamics simulations. The extended-range order manifests itself in the form of radial oscillations, on the length scale of 20-40 angstrom, which are examined by directly analyzing the radial distribution of atoms in distant coordination shells and comparing the same with those from a class of partially-ordered networks of Si atoms and disordered configurations of crystalline silicon from an in…
We demonstrate that the amorphous material PAF-1, C[(C6H4)2]2, forms a continuous random network in which tetrahedral carbon sites are connected by 4,4'-biphenyl linkers. Experimental neutron total scattering measurements on deuterated, hydrogenous, and null-scattering samples agree with molecular dynamics simulations based on this model. From the MD model, we are able for the first time to interrogate the atomistic structure. The small-angle scattering is consistent with Porod scattering from particle surfaces, of the form Q^{-4}, where Q is the scattering vector. We measure a distinct peak i…
Using density functional calculations, we examine insertion/extraction of Mg ions in Mg3Bi2, an interesting Mg-ion battery anode. We found that a (1 1 0) facet is the most stable termination. Vacating a Mg2+ ion from the octahedral site is more favourable for both surface and bulk regions of the material. However, the diffusion barriers among the tetrahedral sites are around 3 times smaller than those among octahedral sites. Consequently, during the magnesiation/demagnesiation process, Mg ions first vacate the octahedral sites and then diffuse through the tetrahedral sites. The spin-orbit inte…
Aluminum and Gallium Distribution in the Lu3(Al5-xGax)O12:Ce Multicomponent Garnet Scintillators Investigated by the Solid-State NMR and DFT calculations
用~（27）Al和~（71）Ga MAS NMR结合单晶~（71）Ga NMR研究了混合晶体Lu_3Al_（5-x）Ga_xO_（12）中Al和Ga原子在石榴石结构中四面体和八面体位置上的分布。在实验研究的基础上，采用密度泛函理论进行了理论计算，预测了混合石榴石中Ga取代Al的趋势。实验和理论结果表明，在石榴石结构中的四面体和八面体位置上的Al和Ga的不均匀分布，具有比Al更大的离子半径的Ga强烈偏好占据石榴石结构中具有较小体积的四面体位置。在密度泛函理论的框架下，计算了所有化合物的Al和Ga核的四极耦合常数和化学位移参数，并计算了Al和Ga核的电场梯度.
The remarkable mechanical properties of high entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment…
Two-mode valence electron configuration of carbon atoms lays the foundation of the unique two-mode amorphous state of the monoatomic carbon solid. From the fundamentals of solid-state physics, sp$^3$ and sp$^2$ amorphous carbons are two different amorphous species characterized by conceptually different short-range orders, namely, groups of tetrahedrally bonded sp3 configured atoms and size-restricted sp$^2$ graphene domains framed with heteroatom necklaces. Molecular character of sp$^2$ amorphics is coherent with the reaction mechanism of the solid amorphicity in due course of the forced frag…
Disproportionate influence of site disorder on the evolution of magnetic phases in anti-Heusler alloy Al$_2$MnFe
Saha Institute of Nuclear Physics（萨哈核物理研究所）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Maulana Azad National Institute of Technology（莫拉纳·阿扎德国立理工学院）；Indian Institute of Science Education and Research (IISER) Pune（印度科学教育研究学院（普纳））；UGC-DAE Consortium for Scientific Research（UGC-DAE科学研究联盟）
Polycrystalline CuMo$_{1-x}$W$_x$O$_4$ solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L$_3$-edge to follow a variation of the tungsten local atomic and electronic structures across thermochromic phase transition as a function of sample composition and temperature. The experimental results were interpreted using ab initio calculations. The crystal-field splitting parameter for the 5d(W)-states was obtained from the analysis of the RXES plane and was used to evaluate the coordination of tungsten atoms. Temperature-dependent RXES measurements were successfull…
Enhanced Magnetization and Conductivity in NiFe2O4
利用密度泛函方法研究了NiFe 2 O 4的不同磁序和阳离子分布的组态，并考虑了在位库仑相互作用U的影响。其最低能态为反尖晶石结构，四面体空间群P4322，Ni位于八面体位（B位），Fe均匀分布在四面体位（A位）和八面体位，A位和B位阳离子间存在反平行磁矩。然而，四重增强的磁化强度获得的结构具有相同的阳离子分布的最低能量状态，即仍然是一个反尖晶石结构，但与Ni和Fe阳离子之间的反平行的时刻。它的能量比基态高约0.4eV/式。两相都是绝缘的。另一种具有增强磁化强度的相是正常尖晶石结构，Ni和Fe分别位于A和B位，即对应于阳离子反转结构。它的电子结构显示出导电性和接近半金属的行为。这两个相可能是实验观察到的NiFe 2 O 4薄膜具有显著增强的磁化强度，但不同的电性能，即一个绝缘和另一个导电的自旋极化电流的可能的候选人。
The size-dependent phase stability of gamma-Al2O3 was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the gamma-Al2O3 crystal, a bulk transformation to alpha-Al2O3 by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical gamma-Al2O3 NPs (with sizes of 6 and 10 nm) undergo a crystalli…
