Fourfold Rings Research Articles

Investigation of thermal stability improvement in Nb doped Sb2Te3

The phase change material Sb2Te3 (ST) exhibits a fast-switching speed and poor thermal stability because the four-fold rings are dominant and homo-polar bonds are absent in the amorphous state. Many studies have been performed on the improvement of phase change properties. Chemical doping methods have been widely used because additional chemical bonds are formed between the doped element and host atoms, leading to the modification of the local structure and electronic band structure. Previous reports have indicated the importance of the bond length and lattice mismatch in element doped ST. In this study, the electronegativity of the doped elements was investigated because it determines the strength of the bond via charge transfer between two atoms. A strong correlation between the electronegativity of the doped element and the phase change properties of the doped ST was observed. An ideal electronegativity range, which corresponds to some elements doped ST with excellent thermal stability, was found in this investigation. Nb atoms were selected as the optimal dopant to stabilize the amorphous phase of ST because Nb was in the ideal range. Additional homo-polar bonds and Nb-Te bonds can stabilize the amorphous phase because additional energy is required for ring reconstruction during crystallization. Consequently, the thermal stability and resistance difference of the Nb doped ST were significantly improved. In addition, the phase change speed of ST was not affected by Nb doping. This study highlights a new approach for pursuing better properties of phase change materials by chemical doping methods.

Simultaneously introducing tetrahedral and pseudo-octahedron by single-element doping enables faster and more stable phase change memory

Phase change memory (PCM) is one of the most promising candidates for the next generation non-volatile memory, but how to balance the contradiction between thermal stability and speed remains the bottleneck problem that limit its application as a universal memory. Here, we innovatively introducing the tetrahedral cluster and the pseudo-octahedron composed of fourfold abab rings at the same time into Sb2Te3 amorphous matrix by elemental Cu doping. The increased Cu tetrahedral clusters bring about enhanced amorphous stability in Sb2Te3 and raise its crystallization temperature from 153.3 ℃ to 203.7 ℃. Besides, the reduced number of the lone-pair electrons, the stronger bond strength and the greater Peierls-like distortions also play crucial roles. Simultaneously, crystallization process simulation results show the fourfold abab ring structure, as a crystalline precursor, accelerates the crystallization speed remarkably. Experimentally, 14 ns SET speed is achieved in Cu0.2(Sb2Te3)0.8 based PCM device, which is four times faster than in pure Sb2Te3 device. Therefore, by a simple single-element doping process, both amorphous stability and crystallization speed are improved at the same time.

Effect of temperature on structural, dynamical, and electronic properties of Sc2Te3 from first-principles calculations.

The compounds Sc2Te3 and Sb2Te3 have the same crystal structure. Ge-Sb-Te alloys are also the most common prototype phase change memory (PCM) compounds in the GeTe-Sb2Te3 pseudo-binary combination. Recently, alloying Sc atoms into Sb2Te3 has enabled sub-nanosecond switching in large conventional phase-change random access memory (PCRAM) devices. However, prior study on the electronic structure and dynamic properties of the Sc2Te3 system is very limited. In this work, we investigate the effect of temperature on the structural, dynamic, and electronic properties of the Sc2Te3 compound through ab initio molecular dynamics simulations. We show that the distorted-octahedral clusters are connected by four-fold rings even at higher temperatures. Moreover, our results clearly illustrate a liquid-to-glass transition temperature, which is between approximately 773 K and 950 K. The effect of temperature changes on the electronic properties of the system manifests as a metal-to-semiconductor transition. The band gap obtained using the mBJLDA functional is twice the value obtained using the PBE functional. Our studies provide useful insight into the local structure and dynamic and electronic properties of the Sc2Te3 system at the atomic level. We hope that this work could stimulate more theoretical work on the development of cache-type phase-change memory and broaden its application in the field of PCM.

Hybrid Xerogels: Study of the Sol-Gel Process and Local Structure by Vibrational Spectroscopy.

The properties of hybrid silica xerogels obtained by the sol-gel method are highly dependent on the precursor and the synthesis conditions. This study examines the influence of organic substituents of the precursor on the sol-gel process and determines the structure of the final materials in xerogels containing tetraethyl orthosilicate (TEOS) and alkyltriethoxysilane or chloroalkyltriethoxysilane at different molar percentages (RTEOS and ClRTEOS, R = methyl [M], ethyl [E], or propyl [P]). The intermolecular forces exerted by the organic moiety and the chlorine atom of the precursors were elucidated by comparing the sol-gel process between alkyl and chloroalkyl series. The microstructure of the resulting xerogels was explored in a structural theoretical study using Fourier transformed infrared spectroscopy and deconvolution methods, revealing the distribution of (SiO)4 and (SiO)6 rings in the silicon matrix of the hybrid xerogels. The results demonstrate that the alkyl chain and the chlorine atom of the precursor in these materials determines their inductive and steric effects on the sol-gel process and, therefore, their gelation times. Furthermore, the distribution of (SiO)4 and (SiO)6 rings was found to be consistent with the data from the X-ray diffraction spectra, which confirm that the local periodicity associated with four-fold rings increases with higher percentage of precursor. Both the sol-gel process and the ordered domains formed determine the final structure of these hybrid materials and, therefore, their properties and potential applications.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Crystallization of amorphous GeTe simulated by neural network potential addressing medium-range order

For the last decade, crystallization kinetics of phase change materials has been intensively investigated by molecular dynamics simulations. In particular, recent machine-learning potentials have advanced microscopic understanding of crystallization behavior of phase change materials by overcoming the computational limit of the density functional theory (DFT). Here we develop neural network potentials (NNP) for GeTe, an archetypal phase change material, and study the crystallization of amorphous GeTe. Consistently with the previous literature, we find that NNP results in a very short incubation time and the crystallization completes within a few nanoseconds, which is at variance with experimental measurement as well as DFT simulations. We show that such deficiencies of NNP originate from overly flat fourfold rings in the amorphous structure. By including explicitly relaxation paths from flat to puckered fourfold rings, we generate a modified NNP, which produces medium-range orders that are more consistent with DFT. Using the modified NNP, crystallization simulations are performed at two densities that represent partially or fully amorphized devices, and temperatures ranging from 500 to 650 K. At both densities, finite incubation times are clearly observed. In particular, the incubation time under the partially amorphized device condition is found to be 7 or 17 ns, which is consistent with experiments. By proposing a method to develop NNPs with correct medium-range order, this work will contribute to simulating phase change materials more accurately and realistically.

Structural signature and transition dynamics of Sb2Te3 melt upon fast cooling.

Crystalline Sb2Te3 is widely studied due to its important applications in memory materials and topological insulators. The liquid and amorphous structures of this compound as well as the dynamics upon quenching, however, are yet to be fully understood. In this work, we have systematically studied the dynamical properties and local structure of Sb2Te3 at different temperatures using ab initio molecular dynamics simulations. The calculated structure factors agree well with the experimental results. The atomic number density and mean-squared displacement as a function of temperature clearly indicate three states as the temperature decreases, namely, melt, undercooled liquid and glass state, respectively. By analyzing the chemical environments and bond-angle distribution functions, we demonstrate that the most probable short-range motifs in the Sb2Te3 system are defective octahedrons, and they are connected with each other via four-fold rings. This interesting structural feature may be responsible for the high fragility and easy phase transition upon glass forming that is applied in memory devices.

Towards accurate models for amorphous GeTe: Crucial effect of dispersive van der Waals corrections on the structural properties involved in the phase-change mechanism

The effect of van der Waals dispersion correction in combination with density functional theory is investigated on a canonical amorphous phase-change material. Density functional theory (DFT), using the generalized gradient approximation, usually fails to reproduce the structure of amorphous tellurides, which manifests by an overestimation of the interatomic bond distances, and particularly the Ge-Te one involved in local geometries (tetrahedral or defect octahedral). Here, we take into account dispersion forces in a semiempirical way and apply such DFT simulations to amorphous GeTe. We obtain a substantial improvement of the simulated structure factor and pair-correlation function, which now reproduce the experimental counterparts with an unprecedented accuracy, including on a recent partial contribution from anomalous x-ray scattering and from x-ray absorption. A detailed analysis of the corresponding structures indicates that the dispersion correction reduces the Ge-Te bond length, increases the fraction of tetrahedral germanium, and reduces the presence of heteropolar so-called fourfold ABAB rings. Given that these structural features have been stressed to be central for the understanding of the phase-change mechanism, the present results challenge our current understanding of the crystal to amorphous transformation at play.

New Data For Chiavennite and Ferrochiavennite

Abstract Chiavennite (Mn endmember) and ferrochiavennite (Fe endmember) are beryllium silicate zeolites with chemical compositions close to Ca 1–2 (Mn,Fe)Si 5 Be 2 O 13 (OH) 2 ·2H 2 O. Five chiavennite crystals from four different localities, Chiavenna, Italy; Telemark, Norway; Uto, Sweden; and Ambatovita, Madagascar, were chosen for crystal-structure analysis. Initially all had been reported to be orthorhombic, but all are monoclinic, with a β angle near 90°, as is ferrochiavennite from Blafjell, Norway. Twinning on {100} is common. Chiavennite and ferrochiavennite are isostructural. The zeolite structure consists of intersecting channels of nine-, six-, five-, and four-fold rings. All five structures have one Ca site in common while chiavennite from Uto and ferrochiavennite from Blafjell have a second, partially filled, Ca site. Hydrogen atoms were located in all five structures. Hydrogen bonding is similar but not identical in all five structures. The H atoms associated with BeO 4 tetrahedra act as terminations to a polymerization sequence causing chiavennite and ferrochiavennite to be interrupted zeolite structures.

Regioselective Functionalization of Core‐Persubstituted Perylene Diimides

Regioselective functionalization of core per-substituted perylene diimides has been achieved efficiently based on a new versatile building block, named tetrabromotetrachloro-perylene-3,4:9,10-tetracarboxylic acid dianhydride (Br4Cl4-PTCDA), which affords a series of novel chromophores with impressive optoelectronic properties. Direct palladium-catalyzed fourfold intramolecular ring fusion affords successfully unique propeller-shaped biscarbazole[2,3-b]carbazole diimides with six annulated rings.

FERROCHIAVENNITE, A NEW BERYLLIUM SILICATE ZEOLITE FROM SYENITE PEGMATITES IN THE LARVIK PLUTONIC COMPLEX, OSLO REGION, SOUTHERN NORWAY

Ferrochiavennite is a new beryllium silicate zeolite with chemical composition close to Ca1–2FeSi5Be2O13(OH)2·2H2O. It is described from two syenite pegmatite localities in Norway: Blafjell, Langangen, Telemark, and the AS Granit larvikite quarry, Tvedalen, Vestfold. The mineral is monoclinic, P 21/ c , with a 8.759(5), b 4.864(2), c 31.258(6) A, β 90.31(6)°, V 1331.7(6) A3, and Z = 4. The crystal structure was refined to R 1 = 0.048 for 3651 observed reflections. The zeolite structure is isostructural with chiavennite, consisting of intersecting channels of nine-, six-, five-, and four-fold rings. The strongest eight reflections of the X-ray powder-diffraction pattern [ d (obs.) in A ( I ) ( hkl )] are: 15.555 (100) (002), 4.104 (29) (112, 112), 3.938 (36) (113, 113), 3.909 (60) (008), 3.820 (30) (204, 204), 3.251 (66) (017, 210, 211), 3.186 (27) (212, 212), 2.884 (64) (215, 215). The mineral is biaxial (+) with refractive indices α 1.583(1), β 1.589(1), γ 1.602(1), measured at 590 nm. 2 V (meas.) = 62(4)° from extinction curves 2 V = 76(5)°; 2 V (calc.) = 69°. The optical orientation is X ~ a , Y ~ c , and Z ~ b . The Mohs hardness is ~ 3; D (meas.) = 2.67(2) and D (calc.) = 2.709 g/cm3.

Mechanisms of network collapse in GeO2 glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO2 is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density ρ/ρ0 ≳ 1.16, where ρ0 is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO5 units versus octahedral GeO6 units as they replace tetrahedral GeO4 units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when ρ/ρ0 = 1 and fourfold rings when ρ/ρ0 = 1.64.

Observing the amorphous‐to‐crystalline phase transition in Ge2Sb2Te5 non‐volatile memory materials from ab initio molecular‐dynamics simulations

AbstractPhase‐change memory is a promising candidate for the next generation of non‐volatile memory devices. This technology utilizes reversible phase transitions between amorphous and crystalline phases of a recording material, and has been successfully used in rewritable optical data storage, revealing its feasibility. In spite of the importance of understanding the nucleation and growth processes that play a critical role in the phase transition, this understanding is still incomplete. Here, we present observations of the early stages of crystallization in Ge2Sb2Te5 materials through ab initio molecular‐dynamics simulations. Planar structures, including fourfold rings and planes, play an important role in the formation and growth of crystalline clusters in the amorphous matrix. At the same time, vacancies facilitate crystallization by providing space at the glass–crystalline interface for atomic diffusion, which results in fast crystal growth, as observed in simulations and experiments. The microscopic mechanism of crystallization presented here may deepen our understanding of the phase transition occurring in real devices, providing an opportunity to optimize the memory performance of phase‐change materials.

Glass structural studies for borosilicates of the KBSi2O6—RbBSi2O6system from X-ray diffraction data

Borosilicate / Glass structure / Thermal behaviour Abstract. For technologically important borosilicate glass and crystalline phases, the structure of K1� xRbxBSi2O6 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions has been investigated employing X-ray diffraction data. Glass sam- ples were prepared by a melt quenching from 1250- 1300 � C for 0.5-1 hour. For glass samples glass transition (Tg), crystallization (Tcryst) and melting (Tm) temperatures were determined by differential scanning calorimetry. To investigate the glass structure, angle-dispersive X-ray dif- fractometry was applied by using an energy-resolving semiconductor detector. The radial distribution functions (RDFs) were calculated. Comparing correlation distances of glass RDFs and mean interatomic distances calculated for polycrystalline samples, the conclusion is drawn that in the glass state not only TO4 (T ¼ Si, B) tetrahedra as stable structure-units but two corner sharing tetrahedra and apparently fourfold rings from tetrahedra exist.

Time for Change

Materials Science Phase-change materials, such as Ge2Sb2Te5 (GST), show exceptionally fast phase transitions, which renders them useful for optical data storage and nonvolatile memory applications. Ab initio molecular dynamics simulations can explore the early stages of nucleation and crystallization in such materials. Lee and Elliott used models with 180 atoms that were held at constant volume to simulate capped films and annealed from a quenched glassy state. They defined the smallest structural element in the metastable rocksalt phase of GST as a fourfold ring (or square), six of which are required to make a cube. Both these elements are observed in the amorphous phases as transient structures. By tracking the number of rings and cubes, they identified four stages in the crystallization process, with a critical nucleus size of 5 to 10 GST cubes. At the interface between the nucleus and the amorphous phase, they observed a series of quasi-ordered rings, an arrangement facilitated by the near 90° bond angles found in GST. They propose that it is the similarity between the bonds found in the amorphous state and the distorted rocksalt structure, common to most phase-change materials, that allows such rapid crystallization. Phys. Rev. Lett. 107 , 145702 (2011).

Preparation of High Compression Strength Silicon Hybrid Material at Low Temperature through Sol-Gel

In this work, high compression strength silicon hybrid material was prepared through a sol-gel process. Structure of high compression strength hybrid material was characterized through FTIR spectra. The molar ratios of MTES to TEOS, mass ratio of siloxane to modified silicate sand were optimized to enhance compression strength. The results showed that linear Si-O-Si network and fourfold siloxane rings were produced in the sol-gel process which were necessary to obtain high compression strength material. When the volume ratio of MTES to TEOS was 3.0 and mass ratio of siloxane to sand was in the range of 0.64~0.70, the hybrid material showed high compression strength of 5.49 MPa.

Effects of pressure on atomic and electronic structure and crystallization dynamics of amorphousGe2Sb2Te5

We investigate the effects of pressure on atomic and electronic structures of amorphous ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ using first-principles methods. The supercell volume is varied from 0.94 to 1.13 times the nominal amorphous volume in melt-quench simulations. When the simulation cell is compressed, we find that the energy gap is reduced and the number of localized in-gap states is increased. These results indicate that pressurized amorphous ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ exhibits better conduction than ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ formed under stress-free conditions. We also find that the positive pressure increases the densities of fourfold rings in amorphous ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$, shifting the local order toward the crystalline phase. Consistently, fast crystallization is identified for compressed amorphous ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$.

Simultaneous normal and parallel incidence planar left-handed metamaterial

We investigate numerically the negative refraction of a simultaneous normal and parallel incidence planar left-handed metamaterial (LHM) in this paper. This LHM is comprised of fourfold C-shaped rings, which are printed on both sides of the substrates symmetrically, and it can exhibit left-handed properties with electromagnetic wave incident in three different directions. The retrieved result and the simulated result verify the left-handed properties of the fourfold C-shaped metamaterial very well. Then the different electric responses of the normal and parallel incidence cases to the incident electromagnetic wave are discussed, and it is due to the different distribution of the induced currents in the metallic wires.

Theoretical determination of stable fourfold coordinated vacancy clusters in silicon

We have identified stable fourfold coordinated vacancy clusters (${V}_{n}$ $3\ensuremath{\le}n\ensuremath{\le}18$) in Si using a combination of metropolis Monte Carlo, tight-binding molecular-dynamics, and density-functional theory calculations. Our calculations show that the small vacancy defects exclusively favor fourfold coordination thermodynamically rather than hexagonal ringlike structure formation, which has widely been adapted to explain the behavior and properties of vacancy defects. Among those examined, the fourfold ${V}_{12}$ cluster with ${S}_{22}$ symmetry is identified to be the most stable, yielding a formation energy of 1.16 eV per vacancy. The fourfold ${V}_{12}$ structure is about 4 eV more favorable than the conventional hexagonal ring structure. We also discuss how the relative stability between the fourfold and hexagonal ring configurations will change as the cluster size increases to greater than a few tens of vacancies.

Co-ordination and intermediate-range order alterations in densified germania

We analyze the structure of a-GeO 2, pressure-densified from a starting density of 3660 kg/m 3 to a final density of 6000 kg/m 3, using a combination of molecular dynamics simulation and topological analysis employing efficient local cluster ring counting algorithms. The initial modeled configuration is dominated by fourfold germanium co-ordination and rings of six or seven germanium atoms (as in cristobalite- or tridymite-like a-SiO 2). The first response to increasing density is a change to larger ring (e.g. 8-ring) configurations (as in quartz-like a-SiO 2) that pack more compactly. At still higher pressure, an intermediate fivefold germanium co-ordination appears that progressively converts into almost entirely sixfold (octahedral) germanium at higher pressures, accompanied by growth of the 3- and 4-rings that characterize the rutile-GeO 2 crystalline polymorph. The topology of the final densified structure does not entirely resemble a rutile-like glass analogue, however, because of the retention of 6-rings and almost 10% 5-co-ordinated Ge. Overall, the present study shows how pressure (or density) affects the structure of the glass at a range of length scales.