Octahedral Vacancies Research Articles

Octahedral distortion and oxygen vacancies induced band-gap narrowing and enhanced visible light absorption of Co/Fe co-doped Bi3.25Nd0.75Ti3O12 ferroelectrics for photovoltaic applications

Reducing bandgap energy in ferroelectric materials has become an important commitment to improve the performance of the photovoltaic solar cell. This study reports the effect of the transition metals Co and Fe ions co-doping on the structure and bandgap of neodymium-modified Bi4Ti3O12 based oxide. The quantitative structural analyses by XRD confirms well-crystallized samples with orthorhombic crystal structure based on B2cb space group. Optical properties were measured by a UV–vis spectrometer. The oxygen vacancies presence and the Fe and Co valence states estimation were carried out by electron spin resonance. The incorporation of Co and Fe ions into Bi3.25Nd0.75Ti3O12 significantly modified the bandgap structure, promoting a red-shift and lower energies absorption which was related to the changes in tilting angles, bond lengths, octahedral distortions and oxygen vacancies formation, still maintaining a ferroelectric spontaneous polarization. To our knowledge, it is a new approach to link quantitively the octahedra distortion to the bandgap decreasing for Aurivillius compounds providing a newly available method of manipulating the bandgap tuning on the ferroelectric oxide’s materials.

Double the Capacity of Manganese Spinel for Lithium‐Ion Storage by Suppression of Cooperative Jahn–Teller Distortion

AbstractThe relatively low capacity and capacity fade of spinel LiMn2O4 (LMO) limit its application as a cathode material for lithium‐ion batteries. Extending the potential window of LMO below 3 V to access double capacity would be fantastic but hard to be realized, as it will lead to fast capacity loss due to the serious Jahn–Teller distortion. Here using experiments combined with extensive ab initio calculations, it is proved that there is a cooperative effect among individual Jahn–Teller distortions of Mn3+O6 octahedrons in LMO, named as cooperative Jahn–Teller distortion (CJTD) in the text, which is the difficulty to access the capacity beyond one lithium intercalation. It is further proposed that the cationic disordering (excess Li at Mn sites and Li/Mn exchange) can intrinsically suppress the CJTD of Mn3+O6 octahedrons. The cationic disordering can break the symmetry of Mn3+ arrangements to disrupt the correlation of distortions arising from individual JT centers and prevent the Mn3+O bonds distorting along one direction. Interestingly, with the suppressed CJTD, the original octahedral vacancies in spinel LMO are activated and can serve as extra Li‐ion storage sites to access the double capacity with good reversible cycling stability in microsized LMO.

Engineering covalently bonded 2D layered materials by self-intercalation.

Two-dimensional (2D) materials1-5 offer a unique platform from which to explore the physics of topology and many-body phenomena. New properties can be generated by filling the van der Waals gap of 2D materials with intercalants6,7; however, post-growth intercalation has usually been limited to alkali metals8-10. Here we show that the self-intercalation of native atoms11,12 into bilayer transition metal dichalcogenides during growth generates a class of ultrathin, covalently bonded materials, which we name ic-2D. The stoichiometry of these materials is defined by periodic occupancy patterns of the octahedral vacancy sites in the van der Waals gap, and their properties can be tuned by varying the coverage and the spatial arrangement of the filled sites7,13. By performing growth under high metal chemical potential14,15 we can access a range of tantalum-intercalated TaS(Se)y, including 25% Ta-intercalated Ta9S16, 33.3% Ta-intercalated Ta7S12, 50% Ta-intercalated Ta10S16, 66.7% Ta-intercalated Ta8Se12 (which forms a Kagome lattice) and 100% Ta-intercalated Ta9Se12. Ferromagnetic order was detected in some of these intercalated phases. We also demonstrate that self-intercalated V11S16, In11Se16 and FexTey can be grown under metal-rich conditions. Our work establishes self-intercalation as an approach through which to grow a new class of 2D materials with stoichiometry- or composition-dependent properties.

Classical mechanical simulations of layer- and tunnel-structured manganese oxide minerals

Tecto- and phyllo-manganate minerals serve as important controls in the geochemical cycle of many elements in soils and sediments due to their high surface areas and chemical reactivity. These nanoscale Mn oxides are mainly composed of Mn octahedra in edge- and corner-sharing configurations that produce a range of tunnel and layer structures. Some fundamental aspects of their atomic structures, however, are difficult to characterize unambiguously because of dynamic structural transformations and a wide range of the structural and chemical compositions. Molecular dynamics (MD) simulations based on interatomic pair potentials provide a perspective on nanoscale Mn-oxides that can be inaccessible to experiment or density functional theory (DFT) calculations. We recently introduced a set of interatomic potentials for the atomistic simulation of Mn-oxides (MnFn) that reproduced many of the subtle phyllomanganate structural variations associated with pH, vacancy content, and Mn oxidation state. In this manuscript, we expand the roster of simulated Mn-oxide minerals to include hydrous and anhydrous tunnel structures and δ-MnO2 with octahedral vacancy contents up to 20%. The MnFn potentials reproduced the experimental tectomanganate lattice parameters and interatomic distances to within 5% and the relative energetics of tectomanganates as predicted by a recent DFT study. We also provide new insights into the Mg site occupancy in todorokite and the Zn coordination in vernadite through large-scale atomistic simulations with MnFn that are challenging to simulate with DFT. In the 3 × 3 tunnel structures of todorokite, the central tunnel site is the predominant site for fully-hydrated Mg2+·6H2O with a minority of partially-hydrated Mg2+ cations binding to corner sites at decreased water/cation ratios. The occurrence of tetrahedral Zn complexes is dependent upon not only the Mn(IV) vacancy content but also the layer spacing and sheet registry. These atomistic simulation results demonstrate the ability of the MnFn potentials to reproduce the basic structures of nanophase Mn-oxides and the capability to perform large-scale modeling that is required to address the subtle structural variations associated with changes in environmental conditions.

Towards ultra-stable lithium metal batteries: Interfacial ionic flux regulated through LiAl LDH-modified polypropylene separator

Lithium metal batteries have attracted remarkable attention because of their high theoretic specific capacity and low redox potential. However, the issues, including uncontrollable dendritic formation and the highly reactive nature of Li anodes hinder the practical application of lithium metal batteries. Herein, LiAl layered double hydroxide (LiAl LDH) modified separators (LiAl LDH@PP) are prepared by simply coating [LiAl2(OH)6]Cl nanoflakes onto polypropylene (PP) membranes. LiAl LDH nanoflakes with plenty of octahedral vacancies and abundant lithium ion diffusion pathways improve the ionic conductivity, surface electrolyte wettability and mechanical robustness of the separators. The modified separator promotes the interfacial lithium ionic flux, suppressing the dendrite formation and consequently increasing the cycling stability of lithium metal batteries. Even at an ultrahigh current density of 20 mA cm−2, the Li|Li symmetric batteries with the LiAl LDH@PP separators can be charged/discharged for more than 3000 h, demonstrating the remarkably high cycling stability. This work provides a feasible and efficient strategy for enhancing the cycling performance and safety of lithium metal batteries.

Promoting spinel formation and growth for preparation of refractory materials from ferronickel slag

AbstractThe study provides a method for improving the quality of the refractory material prepared from ferronickel slag by promoting the spinel formation and growth in the slag which was sintered with sintered magnesia and chromium oxide in a broad sintering temperature range from 1200°C to 1500°C. According to the thermodynamic analysis, except for forsterite due to the addition of sintered magnesia, a number of high‐melting point spinel phases can also be formed in the presence of chromium oxide and this trend becomes more apparent with increasing sintering temperature, along with declined presence of low‐melting point clinopyroxene, mainly enstatite. This expectation was verified by conversion of a part of the original phase of ferronickel slag, olivine, to two main spinel phases, including magnesium aluminate spinel and donathite which was produced by the replacement of nontoxic Cr3+ ions with Fe3+ ions in the octahedral vacancies of magnesium chromate spinel. The formation and growth of these spinel phases were promoted by elevating temperature from 1200°C to 1500°C, which accelerated the transition of initially generated enstatite to a glassy phase, in favor of densification. The formation and growth of spinel during sintering contributed to high refractoriness and compressive strength of the resulting refractory materials

Trapping sites of Li atom in the rare gas crystals Ar, Kr, and Xe: Analysis of stability and manifestation in the EPR spectra

The classical model of an ideal crystal, parametrized according to non-empirical calculations, is used to determine the structure and geometry of the atomic lithium trapping sites in solid inert gases RG = Ar, Kr, and Xe, and to define their thermodynamic stability. The diversity of the observed stable sites reduces to four highly symmetric structures corresponding to the interstitial introduction of an Li atom, its substitution of an inert gas atom, or its incorporation into tetrahedral and octahedral vacancies formed by the removal of four and six inert gas atoms from the crystal lattice. The non-empirically calculated dependences of the isotropic hyperfine coupling constant tensor on the distance in the diatomic Li@RG complex are used to estimate the shifts of the electron paramagnetic resonance signals in the predicted thermodynamically stable sites. A comparison with published data does not contradict the assignment of the observed multiple signals to certain types of stable sites, taking into account the spectral features of samples that were prepared by thermal deposition and laser ablation in three different inert gases.

Tuning Oxygen Redox Reaction through the Inductive Effect with Proton Insertion in Li-Rich Oxides.

As a parent compound of Li-rich electrodes, Li2MnO3 exhibits high capacity during the initial charge; however, it suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities toward reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in the Li2MnO3 system with its original structural integrity maintained. Combining structural probes, theoretical calculations, and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g-1 in the protonated Li2MnO3 electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

One-pot synthesis of the reduced-charge montmorillonite via molten salts treatment

Layer charge is a critical factor determining the structural characteristics and physicochemical properties of clay minerals, and fixing small cations (e.g., Li+) into the tetrahedral/octahedral sheets of clay minerals is a feasible way for reducing their layer charges. However, conventional methods for preparing reduced-charge clay minerals are usually quite complex and time-consuming. Herein, a facile one-pot treatment method for preparing reduced-charge clay minerals was successfully developed using molten salts. In particular, montmorillonite (Mt) was mixed with mono LiNO3 or LiNO3-NaNO3 mixture, which was then heated above the melting points of salts. The obtained results showed that the cation exchange capacity (CEC) of Mt decreased evidently after being treated with molten LiNO3, and the CEC value was reversely related to the heating time. The X-ray diffraction results demonstrated that Mt layers collapsed upon treatment with molten LiNO3. On the other hand, the partial replacement of LiNO3 by NaNO3 could restrict the collapse of Mt layers, as the larger ionic radius of Na+ (than Li+) helped in maintaining the interlayer spaces of Mt during the process of charge reduction. As indicated by the Fourier transform infrared spectroscopy, the structural OH stretching vibration shifted to a higher wavenumber (from 3623 to 3632 cm−1) after the molten LiNO3 treatment, which suggested that some Li+ ions migrated into the hexagonal cavities in the tetrahedral sheets of Mt, and the new band at 3670 cm−1 indicated that some Li+ ions even further migrated into the octahedral vacancies of Mt. The locations of fixed Li+ ions were further confirmed by the X-ray photoelectron spectroscopy characterization. Due to the migration of Li+ ions into the layers of Mt, the Li1s peak shifted to lower binding energy. The above results showed that the molten salt treatment might be a cost-effective and environmentally friendly approach for synthesizing reduced-charge clay minerals, and using mixed molten salts can help in maintaining the interlayer structure.

Calculating biotite formula from electron microprobe analysis data using a machine learning method based on principal components regression

We present a new machine learning method for calculating biotite (sensu lato) structural formula from electron microprobe analysis (EMPA) data, which is based on principal components regression (PCR) of a dataset consisting of 155 fully analyzed biotite references that have chemistry and crystal structure refinement. The dataset is randomly grouped into a training set (75% in amount) and a test set (25% in amount). The training set is used to implement the structural formula and the test set is used to evaluate the performance of the model. The resulting linear regression coefficient matrix is then applied to calculate mole proportions of cations and anions of biotite samples using their compositional data from EMPA. Through this method, the distribution of the different cations and anions in the different sites can be calculated, including the tetrahedral Fe3+, octahedral Fe2+, octahedral Fe3+, OH and WO2− at the O(4) site. The O(4) site is assumed to be occupied by anions with a relation of 2 = F + Cl + OH + WO2−. Octahedral and interlayer vacancies could also be estimated in this model. The prediction quality for major elements is perfect with R2 > 0.95. The absolute errors in the estimated octahedral Fe2+, octahedral Al and OH at O(4) site are determined to be ±0.2 apfu (atom per formula unit based on 11O + 2(F, Cl, OH, O)), while those in total Fe3+ and WO2− at O(4) site are approximately ±0.3 apfu. A funnel-shaped relationship between absolute error in Fe3+/ΣFe ratio and FeOT wt% is observed, with the majority falling in the range of ±20%. Compared to previous normalization schemes, our model shows significant improvements in estimating Fe3+/ΣFe and WO2− at O(4) site. Our model is capable for calculating mineral formulae of common igneous and hydrothermal biotites, but not suitable for those that have been modified in a post-formation oxidation or reduction process. A supplementary Excel spreadsheet is provided that can be easily used for performing calculation from EMPA data.

Anomalous thermal expansion behavior in Er1-xCaxMnO3

Perovskite Er1-xCaxMnO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) was synthesized using a solid-state method. Thermal expansion behavior was tested using a thermal dilatometer and high-temperature X-ray diffraction (XRD). The experimental results indicated the doping contents of Ca (x) in the Er1-xCaxMnO3 have a dramatic effect on their thermal expansion behavior. The samples of Er1-xCaxMnO3 (x = 0.1,0.2 and 0.25) exhibit positive thermal expansion (PTE) characteristics while Er0.7Ca0.3MnO3 (x = 0.3) exhibits a negative thermal expansion (NTE) property with a thermal expansion coefficient of −3.1 × 10−6 K−1 in room temperature (RT) −750 K. In addition, Er0.6Ca0.4MnO3 (x = 0.4) exhibits NTE properties only at RT–500 K, and Er0.5Ca0.5MnO3 (x = 0.5) exhibits PTE properties at RT–750 K. The thermal shrinkage mechanism is the Jahn–Teller effect of the Mn3+ ions and the double exchange of Mn3+–O–Mn4+ in Er0.7Ca0.3MnO3. This phenomenon causes Mn–O octahedral distortion and oxygen vacancy, causing Er0.7Ca0.3MnO3 to become anisotropic. This feature results in the elastic deformation of Er0.7Ca0.3MnO3 during heating, which consumes the void and displays NTE at macro level.

Low-pressure–temperature stability of pyrope + quartz relative to orthopyroxene + kyanite: a new model for aluminous orthopyroxene with vacancies

Determination of the stability of pyrope with quartz could provide theoretical pressure–temperature (P–T) constraints on silica-saturated eclogites as well as near end-member pyrope-bearing terrains. The lower P–T stability of pyrope in the presence of quartz in the system MgO–Al2O3–SiO2 can be modeled by the reaction: 3 enstatite + 2 kyanite = 2 pyrope + 2 quartz. Reaction reversals were experimentally obtained using a ½-in.-diameter piston-cylinder press and a 1000-t multi-anvil press. Besides natural quartz, all other starting materials, including orthopyroxene, pyrope, and kyanite were synthesized. The reaction has been bracketed at 1.50 GPa at 1040–1080 °C; 1.69–1.79 GPa at 1000 °C; 2.26 GPa at 920–940 °C; 2.90 GPa at 840–860 °C. The bracketed boundary is higher in temperature and pressure than that calculated using the newest dataset of Holland et al. (J Petrol 59:881–900, https://doi.org/10.1093/petrology/egy048, 2018), particularly at higher pressures. Aluminum is usually thought to be taken into orthopyroxene in the form of Mg-Tschermak’s pyroxene (MgTs; MgAl2SiO6) under these experimental P–T conditions. In contrast, the observed orthopyroxene composition indicated the involvement of a non-stoichiometric component Mg-Eskola pyroxene (MgEs; Mg0.5☐0.5AlSi2O6), an aluminous orthopyroxene with octahedral vacancies (☐). End-member thermodynamic properties for MgEs as well as a mixing model for the ternary enstatite–MgTs–MgEs solid solution have been derived. The thermodynamic calculations, employing the mixing model, are in good agreement with the experimental brackets. The new model has potential applications to constrain P–T conditions of tectonic settings such as ultrahigh-pressure and ultrahigh-temperature metamorphisms.

Calcium amphibole exsolution lamellae in chromite from the Semail ophiolite: Evidence for a high-pressure origin

We report here for the first time aluminous Ca-amphibole exsolution lamellae in an anhydrous chromite host of podiform chromitite from the Nakhl massif of the Semail Ophiolite. Transmission electron microscopy observations show that the Ca-amphibole lamellae have a crystallographic topotaxy and d-spacing relationship with the host chromite, i.e., [103]am//[118_]chr, (020)am//(22_0)chr and d060am ≈ d220chr, that indicate an exsolution origin. The presence of Ca-amphibole exsolution implies the incorporation of Si4+ and Ca2+ cations and hydroxyl in the host precursor chromite. The reconstructed precursor chromite had SiO2 concentrations of 0.33–0.58 wt%, suggesting upwelling from a minimum pressure of 8 GPa (depth of ~250 km). Hydroxyl groups occupy octahedral vacancies, which can be introduced in high-pressure chromite by substituting 4 Cr3+ by 3 Si4+ and a vacancy. During upwelling, vacancies decrease when Si4+ cations exsolve stepwise from chromite. It leads to a coupled decrease of OH occupied vacancies and exsolution of Ca-amphibole lamellae at pressures below 3 GPa. Our results provide new evidence for a deep origin of the chromitite-bearing mantle section of the Semail ophiolite.

A XANES and EPMA study of Fe3+ in chlorite: Importance of oxychlorite and implications for cation site distribution and thermobarometry

Chlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other phyllosilicates of petrological interest, the oxidation state of iron in chlorite may differ from the usually assumed divalent state. However, the crystal chemistry of trivalent iron in chlorite remains poorly known, and the thermodynamic properties of ferric chlorite are missing from databases used for petrological modeling. As part of an attempt to fill this gap, we present results from in situ, micrometer-scale measurements of the oxidation state of iron in various chlorite-bearing samples. X-ray absorption near-edge spectroscopy (XANES) was combined with electron probe microanalysis (EPMA) on the same crystals. Results show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ~25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe2O3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ~30 wt% Fe2O3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe3+ substitution. In Fe-rich samples, Fe3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O10(OH)8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI(Mg, Fe2+) = VI☐ + 2 VIFe3+, as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe3+ content, up to ~ 2 H+ pfu in the Fe3+-rich chlorite samples, corresponding to a net exchange vector of the type R2+ + H+ = Fe3+. These results do not support substitutions toward di-trioctahedral ferric end-members, and highlight the need for considering substitution toward an “oxychlorite” (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O12(OH)6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored. They are deterring if H deficiency is ignored but, given the sensitivity of most thermometers to octahedral vacancy, the assumption FeTotal = Fe2+ is still safer than using high measured Fe3+ contents and the standard 28 charge basis, which artificially increases vacancies. In such ferric chlorites, EPMA measurement of oxygen allows a fair estimate of H content if Fe3+/Fe2+ is known; it should be more systematically implemented. For the same reasons, literature data reporting Fe3+-rich chlorite with vacancy content along the possibly artificial di-trioctahedral-type substitution should be verified. With the help of constraints from thermodynamic models, charge balance, crystal symmetry, and proton loss, a new cation site distribution is proposed for di-tri- to tri-trioctahedral chlorites in the Fe2+-Fe3+-Mg-Al-Si-O-H system, allowing a more realistic thermodynamic handling of their solid solutions.

Structure, morphology and magnetodielectric investigations of BaTi1−xFexO3−δ ceramics

Ferromagnetism is successfully imposed on a robust ferroelectric material by Fe-substitution and magnetodielectric investigation of the ceramics leads us to propose a sensor for magnetic field measurement. Tetragonal to hexagonal structural phase transformation is unambiguously identified from Rietveld refinement of the structure. Coexistence of tetragonal and hexagonal phase is also identified from Raman spectra and SAED. Structural details of the two phases and phase percentage are correlated to the physical properties obtained from P–E loop, M–H loop and magnetodielectric measurements. Ferroelectric loops are diminished due to pinning of domain wall motion by oxygen vacancies and an increase of the non-ferroelectric hexagonal phase percentage. The decrease of remnant magnetization is ascribed to the occupancy of Fe in pentahedral–octahedral sites and oxygen vacancies. Magnetodielectric dispersion below 104 Hz is predominantly due to extrinsic origin through the combined effect of space charge polarization and magnetoresistance. The intrinsic magnetodielectric effect above 104 Hz is attributed to strain mediated magnetoelectric coupling.

First-principles calculations on effects of Al and Ga dopants on atomic and electronic structures of amorphous Ge2Sb2Te5

Effects of post-transition metal dopants M (M = Al, Zn, and Ga) on structural and electronic properties of amorphous Ge2Sb2Te5 (a-GST) are investigated through first-principles calculations based on the density functional theory. The doped a-GST is generated through the melt-quench procedure using molecular dynamics simulations. It is found that the three dopants behave similarly in a-GST, and they are mostly coordinated by Te atoms in tetrahedral geometry, which is similar to those in crystalline MxTey. This is in contrast with crystalline GST wherein the most stable position of dopant M is the octahedral vacancy site. The number of wrong bonds such as Ge–Ge, Ge–Sb, or Sb–Sb increases as dopant atoms predominantly bond with Te atoms. The number of 4-fold ring structures, especially ABAB-type, decreases significantly, explaining the enhanced thermal stability of doped a-GST in the experiment. The bandgap estimated from density of states and the optical gap obtained from Tauc plot increase upon doping, which is also in good agreement with the experiment. By successfully relating the experimental doping effects and changes in the atomic structure, we believe that the present work can serve as a key to offer better retention and lower power consumption in phase-change memory.

An atomic-scale understanding of the initial stage of nucleation of heavy metal cations on clay edges

Abstract For the purpose of revealing the microscopic mechanism of the early stage in the nucleation of heavy metals on clay edges, we performed first principles molecular dynamics (FPMD) simulations systematically to characterize the chemistry of the incipient clustering processes by taking Pb2+ and Ni2+ as the model cations. We found that Pb2+ and Ni2+ have different complexation mechanisms: Pb2+ cations are adsorbed onto different sites with the similar probability whereas Ni2+ cations prefer octahedral vacancy to the other sites. The reason is that Pb2+ does not enter the vacancy due to the large ionic size. pKa calculations indicate that Ni2+ adions can provide available complexing sites (i.e., OH− groups) for the subsequent cations through hydrolysis and free energy calculations show that the complexation of subsequent cations is thermodynamically favored, which can thus result in nucleation and precipitation eventually. In contrast, the first adsorbed Pb2+ cannot provide available complexing sites due to the extremely high pKa, thus making nucleation impossible. Our findings, consistent with the experiment, reveal the role of the surface in heterogeneous nucleation, which should also apply to other similar systems. The results derived in this study are directly useful for understanding the mobility and fate of heavy metals in surficial environments.

Ion Conductivity Enhancement in Anti‐Spinel Li3OBr with Intrinsic Vacancies

AbstractSolid electrolytes are expected to play an important role in modern electrochemical energy storage technologies. However, it is still very challenging to achieve a high Li‐ion conductivity in solid electrolytes. In this study, an intrinsic vacancy induced ion conductivity enhancement in a recently synthesized anti‐spinel structure of Li3OBr compared with its anti‐perovskite phase is reported. With a high stability, the intrinsic defective structure of anti‐spinel Li3OBr contains a high concentration of octahedral vacancies. Ab initio molecular dynamic simulations confirm that octahedral vacancies can lead to an extraordinarily high Li mobility (0.136 mS cm−1 at room temperature) in anti‐spinel Li3OBr. Such an ion conductivity enhancement mechanism opens a new avenue in solid electrolyte material design.

From octahedral structure motif to sub-nanosecond phase transitions in phase change materials for data storage

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.

Molecular simulations of hydrated phyllomanganates

Hydrated phyllomanganates are layered Mn-oxide minerals with interlayers that can possess variable water contents and react strongly with trace metals due to octahedral vacancies in the layer. The unique properties of phyllomanganates afford them a significant role in many environmental phenomena that affect soil and water composition mainly via cation exchange and adsorption of trace metals. Slight variations in the structural and chemical composition often result in a dramatic difference in the chemical reactivity of the minerals. Molecular simulations at the classical mechanical level of theory, which uses a simplified description of the interatomic potential energies, can provide an atomistic perspective of the relationship between the chemical composition and the bulk and interlayer structures. We introduce a set of interatomic potentials for hydrated phyllomanganates with variable vacancy and Mn3+ content and report the classical mechanical simulation results performed at standard temperature and pressure. The potentials we introduce provide not only a reasonable reproduction of the experimentally determined atomic structures of the chalcophanite group, but also new insights on similar phyllomanganate minerals with hexagonal symmetry and a range of vacancy contents. When a vacancy is protonated, Mn3+ is unstable in the hexagonal birnessite layer and occupies the interlayer as a cap on the associated vacancy. When a vacancy was charge-balanced by Mn3+ and Na+, considerable amounts of Mn3+ were incorporated into the hexagonal birnessite layer, but only at total Mn(III) contents greater than ∼10% and with disordered layer stacking. The potentials also predicted a vacancy-free, triclinic Na-birnessite structure with Mn(III)-rich rows in the layer which were arranged parallel to the b-axis and separated by two rows of Mn(IV) octahedra. The dominant interlayer Na complex at a water content ≥0.7 H2O/MnO2 was an octahedrally-coordinated, split interlayer site with two birnessite O atoms in the axial positions and four interlayer H2O in the equatorial positions. Other interlayer Na complexes including some edge- and face-sharing complexes existed in trace amounts (<10%). These classical mechanical simulations represent a successful first test of the introduced interatomic potentials, which can be used to further explore the interlayer and surface complexes of phyllomanganate and birnessite-group minerals.