Octahedral Volume Research Articles

Seeded Growth of Size-Tunable Au@Ag Core-Shell Nano-Octahedra and Their Yolk-Shell Derivatives for Near Infrared Photothermal Conversion.

Gold-based nanostructures with well-defined morphologies and hollow interiors have significant potential as a versatile platform for various plasmonic applications including biomedical diagnostics and sensing. In this study, we report the synthesis of Au@Ag core-shell nanocrystals with perfect octahedral shapes and tunable edge lengths via seeded growth. These nanocrystals were then oxidatively carved into yolk-shell nanocages with a retained octahedral morphology. The increase in octahedral edge length and volume of the interior hollow cavity synergistically leads to a red-shift of the LSPR peak. As a result, the optimized Au@AuAg yolk-shell octahedral nanocages showed a remarkable temperature increase of 23 °C upon 15 min irradiation of an 808 nm laser at a power density of 1 W cm-2. This study provides a feasible strategy for creating octahedral AuAg nanostructures with tunable sizes and hollow interiors and validates their promising use in NIR photothermal conversion.

Optimization radiation shielding properties of aluminum-based spinel minerals through the crystal tetrahedral and octahedral voids and their ions composition

In this research paper, we delve into the influence of crystal structure, void volumes and their ionic composition along with the atomic packing factor on the radiation attenuation properties of Al-based spinel compounds. The study investigates various Al-spinel chemical compositions explores their applications in radiation shielding. Analyzing mass attenuation coefficient (MAC) of six spinel minerals across different energy regions, we observe specific trends in scattering processes with the following sequential order: MACPleonaste > MACGahnite > MACHercynite > MACGalaxite > MACCeylonite > MACSpinel. The results demonstrated that the MAC values were influenced by the spinel chemical composition, crystal structure, and atomic arrangement in the lattice. Furthermore, the correlation between MAC values and the tetrahedral volume (Vtet), octahedral volume (Vocta), and the ratio of void volumes to the unit cell volume (R) was examined. Optimal void volumes and divalent and trivalent ion compositions were determined for different spinel minerals, leading to enhanced radiation shielding efficiency. The findings offer valuable perspectives for refining and optimization of spinel materials for use in radiation applications.

Structure Evolution of Ge-Doped CaTiO3 (CTG) at High Pressure: Search for the First 2:4 Locked-Tilt Perovskite by Synchrotron X-ray Diffraction and DFT Calculations.

This research investigates the high-pressure behavior of the Ca(Ti0.95Ge0.05)O3 perovskite, a candidate of the locked-tilt perovskite family (orthorhombic compounds characterized by the absence of changes in the octahedral tilt and volume reduction under pressure controlled solely by isotropic compression). The study combines experimental high-pressure synchrotron diffraction data with density functional theory (DFT) calculations, complemented by the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), to understand the structural evolution of the perovskite under pressure. The results show that CTG undergoes nearly isotropic compression with the same compressibility along all three unit-cell axes (i.e., Ka0 = Kb0 = Kc0, giving a normalized cell distortion factor with pressure dnorm(P) = 1). However, a modest increase in octahedral tilting with pressure is revealed by DFT calculations, qualifying CTG as a new type of GdFeO3-type perovskite that exhibits both isotropic compression and nonlocked tilting. This finding complements two existing types: perovskites with anisotropic compression and tilting changes and those with isotropic compression and locked tilting. The multimethod approach provides valuable insights into the structural evolution of locked-tilt perovskites under high pressure and establishes a protocol for the efficient study of complex high-pressure systems. The results have implications for the design of new functional materials with desirable properties.

Highly Quantum Efficient and Thermally Stable Near‐Infrared‐Emitting K‐β‐Al2O3:Cr3+ Phosphor

AbstractNear‐infrared (NIR) phosphors are enablers for NIR phosphor‐converted light‐emitting diodes (pc‐LEDs). However, fewer NIR‐emitting phosphors with both high internal/external quantum efficiency (IQE/EQE) and thermal stability are discovered, which obstructs the promotion of NIR pc‐LEDs. Herein, by partially replacing Al3+ in K‐β‐Al2O3:2Cr3+ with Ga3+, the photoluminescence (PL) intensity of the solid solution K1+δ(Al0.4Ga0.6)11O17:2Cr3+, (KA0.4G0.6O:Cr) phosphor is increased 2.75 and 1.25 times that of end‐members K1+δAl11O17:2Cr3+ (KAO:Cr) and K1+δGa11O17:2Cr3+ (KGO:Cr). The IQE/EQE of optimal KA0.4G0.6O:Cr reaches 88.9%/50.8% with high thermal stability (77.4%@150 °C). The PL intensity enhancement is due to the Al/Ga‐6O octahedral volume and distortion variation caused by the substitution of Ga3+ for Al3+ in K1+δ(Al1‐y,Gay)11O17:2Cr3+ (KA1‐yGyO:Cr), which leads to the forbidden d–d transition being broken and crystal field strength varied. Finally, a NIR pc‐LED device fabricated based on KA0.4G0.6O:Cr NIR‐emitting phosphor and blue chip reaches an electro‐optical efficiency of 16.3% under a drive current of 100 mA. Meanwhile, non‐destructive detection and plant germination applications of the NIR pc‐LED are demonstrated. These results prove that KA0.4G0.6O:Cr is a promising NIR phosphor for diverse applications.

The effects of lanthanum ions substitution on properties and effective absorption bandwidth (EAB) of zinc ferrite

A comprehensive study was conducted to examine the effect of lanthanum (La3+) ions substitution on zinc ferrite crystallography parameters, microstructure, and related physical properties. In this work, ZnLaxFe(2-x)O4 (0.01 ≤ x ≤ 0.04) compositional series were prepared by co-precipitation method. Rietveld refinement method combined with Raman spectra analysis reveal that all compositions are only crystallized into zinc ferrite (ZnFe2O4) after high-temperature treatment at 1000 °C for 5 h La3+ ions preferentially reside in BO6 sub-lattice, which corresponds to the expanding unit cell and octahedral volume of ZnFe2O4. The elongation of cation-anion bonds at the BO6 sub-lattice (∼0.02 Å) and the movement of oxygen toward the AO4 sub-lattice play an important role in weakening super-exchange interaction (SE) between sub-lattices and raising the frustration of magnetic structure as confirmed by the electron density (ED) mapping analysis. The particle size of the sample was decreased from ∼1400 nm for x = 0.01–∼600 nm for x = 0.04. These factors were found to cause the decrease of the magnetization values (Ms and Mr) and the increase of field coercivity due to the increase of La3+ ions substitution. The substitution of La3+ ions are found effective to improve the microwave absorbing (MA) ability of zinc ferrite. The trends of MA ability are related with the different spin-orbit coupling of La–Fe, impedance matching characteristic, natural magnetic resonance, and particle size distribution. The substitution of La3+ ions in the composition of x = 0.04 (ZnLa0.04Fe1, 96O4) affected the full coverage of the EAB across the entire X-band frequency range, making it an excellent microwave absorbing materials (MAMs) candidate.

Effect of aliovalent substitution on the crystal distortion and negative thermal expansion properties of Zr2SP2O12

We evaluated the thermal expansion of newly synthesized Zr2-xMxSyP2O12-δ, with zirconium (IV) ion (Zr4+) partially substituted by five aliovalent ions (aluminum (III) ion (Al3+), iron (III) ion (Fe3+), yttrium (III) ion (Y3+), niobium (V) ion (Nb5+), and gadolinium (III) (Gd3+)) ion. High-temperature X-ray diffraction patterns showed that Zr2-xMxSyP2O12-δ had the same negative thermal expansion as Zr2SP2O12 and that the volumetric coefficient of thermal expansion (αv) could be controlled by ion substitution. The lowest αv value obtained for Zr1.7Y0.3SP2O12-δ at 373–453 K was approximately −117 ppm/K. Rietveld's analysis showed that aliovalent substitution could make the crystal structure of Zr2SP2O12 at 303–373 K distorted and the lattice volume larger. In addition, multiregression analysis demonstrated that ZrO6 octahedral volume and certain bond angles in the crystal structure were related to the lattice volume.

Murphyite, Pb(TeO4), the Te-Analogue of Raspite, a New Mineral from Tombstone, Arizona, USA

Abstract A new mineral species, murphyite (IMA 2021-107), ideally Pb(TeO4), has been found from the Grand Central mine, Tombstone, Arizona, USA. It occurs as bladed or prismatic crystals on top of a quartz matrix. Associated minerals include chlorargyrite, emmonsite, ottoite, stolzite, scheelite, schieffelinite, quartz, and jarosite. Individual crystals of murphyite are up to 0.20 × 0.05 × 0.05 mm in size. Twinning is common on {100}. Murphyite is colorless to very pale yellow in transmitted light, transparent with white streak and adamantine luster. It is brittle and has a Mohs hardness of ∼3½, with perfect cleavage on {100}. The calculated density is 7.579 g/cm3. Murphyite is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded the empirical formula (based on 4 O apfu): (Pb0.96Fe0.03Mn0.02)Σ1.01[(Te0.61W0.38)Σ0.99O4], which can be simplified to Pb[(Te,W)O4]. Murphyite is the Te-analogue of raspite, Pb(WO4), and represents the first mineral with Te6+ substituting for W6+ over 50%. It is monoclinic with space group P21/a and unit-cell parameters a = 13.6089(3), b = 5.01750(10), c = 5.5767(2) Å, β = 107.9280(10)°, V = 362.302(17) Å3, and Z = 4. Its crystal structure consists of distorted MO6 (M = Te + W) octahedra sharing edges to form zigzag chains running parallel to [010]. These chains are linked together by PbO7 polyhedra. Compared to raspite, the substitution of W6+ by Te6+, which has a smaller ionic radius, results in a noticeable structural change: a significant decrease in MO6 octahedral angle distortion, with a concomitant increase in both MO6 octahedral volume and average Pb–O bond length. The unit-cell volume increases from 358.72(4) Å3 for raspite to 362.302(17) Å3 for murphyite. Raman spectroscopic data show that the major peak ascribable to M–O symmetrical stretching vibrations within the MO6 octahedron is centered at 870 cm−1 for raspite but at 881 cm−1 for murphyite.

The effect of vacancy defects on the electronic properties of β-Ga2O3

In this paper, the characteristics of oxygen vacancies (VO) and gallium vacancies (VGa) in β-Ga2O3 are studied using density functional theory. The performed relaxation analysis indicates that the relaxation of VGaⅡ increases its octahedral volume. Compared with the tetrahedron, the Ga atom of the octahedron surrounding VO is more stable. The formation energy results suggest also that VGa0 is expected to be most favorable in p-type β-Ga2O3, while VGa-3 is preferred in the n-type. The −3 charge state acts as a compensating acceptor, which reduces electron conductivity. VO+2, which acts as the compensation donor center, can easily form in p-type β-Ga2O3, while the VO0 forms readily in n-type β-Ga2O3. VGa at high density can exist in β-Ga2O3 in an oxygen-rich environment, while VO can exist under oxygen-poor conditions. The defect charge-state transition level reveals that VGa is a deep acceptor and VO is a deep donor. For VGa, the transition levels ε (0/-1), ε (-2/-1), and ε (-3/-2) can appear in the thermodynamic equilibrium system. However, for VOⅠ and VOIII, which are used as negative-U centers, only the transition level ε (2+/0) can appear in thermodynamic equilibrium. Furthermore, the calculated bandgap properties indicate that VGa is capable of introducing ferromagnetism in β-Ga2O3, while VO cannot. VGa introduces both acceptor levels and donor levels within the bandgap, which compensate each other, and they, thus, determine the acceptor characteristics of the system. The appearance of VO-introduced donor levels into the bandgap increases the overall electron concentration of the system. A Bader charge analysis shows that the vacancy defect can have a strong effect on the charge distribution of the crystal and alter the electronic characteristics of the crystal. The research results in this paper show that the electronic and magnetic properties of β-Ga2O3 can be changed significantly by introducing different types of vacancy defects with different charge states during the preparation process of the device. This can be done to meet specific performance requirements of a commercial electronic component.

BEM evaluation of surface octahedral strains and internal strain gradients in 3D-printed scaffolds used for bone tissue regeneration

Most of the mechnoregulatory computational models appearing so far in tissue engineering for bone healing predictions, utilize as regulators for cell differentiation mainly the octahedral volume strains and the interstitial fluid velocity calculated at any point of the fractured bone area and controlled by empirical constants concerning these two parameters. Other stimuli like the electrical and chemical signaling of bone constituents are covered by those two regulatory fields. It is apparent that the application of the same mechnoregulatory computational models for bone healing predictions in scaffold-aided regeneration is questionable since the material of a scaffold disturbs the signaling pathways developed in the environment of bone fracture. Thus, the goal of the present work is to evaluate numerically two fields developed in the body of two different compressed scaffolds, which seem to be proper for facilitating cell sensing and improving cell viability and cell seeding efficiency. These two fields concern the surface octahedral strains that the cells attached to the scaffold can experience and the internal strain gradients that create electrical pathways due to flexoelectric phenomenon. Both fields are evaluated with the aid of the Boundary Element Method (BEM), which is ideal for evaluating with high accuracy surface strains and stresses as well as strain gradients appearing throughout the analyzed elastic domain.

Effect of chemical oxidation of spinel-type LiNi0.5Mn1.3Ti0.2O4 by soaking in HNO3, HCl and H2SO4

A new chemical oxidation method was proposed to mimic the crystalline structure change of cathode materials during charge-discharge process by chemical treatment as an efficient experimental data collection method for exploring cathode materials for lithium batteries by informatics such as machine learning. LiNi0.5Mn1.3Ti0.2O4 prepared by the solid-state reaction method was soaked in 2 ​mol/L HNO3 for 10 days, and the chemical delithiation about 53% was confirmed. Furthermore, from the information obtained by synchrotron XRD and XAFS measurements, it could be predicted that the chemical oxidation reaction was accompanied by delithiation. In HCl and H2SO4, which are different acids, the lattice parameter decreased with decreasing Li composition as in the case of HNO3. However, from the value of the metal-oxygen octahedral volume calculated from the crystal data obtained by structure refinement, it was concluded that the Li+ to H+ ion-exchange reaction rather than the oxidation reaction proceeded in HCl.

Structural change from Pbnm to R3̅c phase with varying Fe/Mn content in (1-x) LaFeO3.xLaMnO3 solid solution leading to modifications in octahedral tilt and valence states

A theoretically supported experimental study of the (1-x)LaFeO3.xLaMnO3 (LFO-LMO) solid solution is being reported for the first time which reveals a structural change from the Pbnm and R3̅c phase at a chemical composition of x = 0.625. Correlation of octahedral distortion and structural change was extensively investigated using x-ray photoelectron spectroscopy (XPS), Raman and x-ray diffraction (XRD) measurements and density functional theory (DFT) calculation. A detailed study of the structural lattice parameters, bond lengths, bond angles have been done, supported by valence state and electronic properties studies. All the above parameters show a correlated modification to the structural change. The distortion and tilting of the BO6 octahedra has been studied as a function of different Fe:Mn content and expressed by Glazer representation from the refined Crystallographic Information Files (CIF). The angle of tilting from the central non-tilted position also shows a correlated modification with the structural change. The valence state and size of cations influences the octahedral tilting. Octahedral volume is reduced as the entire perovskite structure is relatively flattened with increasing Mn-content implying a flattening of both the BO6 octahedra and the La8O6 cage. The vibrational properties were studied experimentally and supported by DFT phonon calculations, detailing the displacement pattern (eigen vectors) revealing considerable insight into the lattice dynamics of the compounds. The optoelectronic modifications in the band properties were studied experimentally and supported with theory. Hence, this manuscript is an in-depth analysis of the structure-correlated structural change of the LFO-LMO solid solution.

Magnetoelastic coupling, negative thermal expansion, and two-dimensional magnetic excitations in FeAs

We present a temperature-dependent investigation of the local structure and magnetic dynamics of the FeAs binary. The magnetic susceptibility $\ensuremath{\chi}(T)$ result shows an anomalous broad feature up to 550 K, with \ensuremath{\chi} continuing to increase above the N\'eel antiferromagnetic ordering temperature (${T}_{\mathrm{N}}=70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), peaking at \ensuremath{\sim}250 K, then decreasing gently above. It is remarkable that this peak susceptibility temperature corresponds to the onset of anisotropic negative thermal expansion in both the $a$ and $c$ axes, suggesting that magnetic interactions are affecting the structure even well above the N\'eel point. A systematic investigation into local bonding correlations, from time-of-flight neutron pair distribution function analyses, shows the octahedral volume around each Fe site growing monotonically while adjacent octahedra tilt toward one another before relaxing away past this peak in the anomalous magnetic susceptibility. We use inelastic-neutron scattering to map spin-wave excitations in FeAs at temperatures above and below the ${T}_{\mathrm{N}}$. We find magnetic excitations near ${T}_{\mathrm{N}}$ to be very different from the excitations in the ground state at 1.5 K. Spin waves measured at 1.5 K are three dimensional (3D), however, in the vicinity of the magnetic transition, the magnetic fluctuations clearly indicate two-dimensional (2D) character in this intrinsically 3D crystal structure. Unlike the undoped 2D parents of iron-arsenide superconductors, where the magnetic correlations are considerably weaker along the $c$ axis than in the $\mathit{ab}$ plane, inelastic neutron scattering here shows that the spin fluctuations in the 3D FeAs binary are nearly 2D in the $\mathit{bc}$ plane at 90 K. These results demonstrate the importance of short-range correlations in understanding the magnetic properties of transition-metal binaries, and suggest how 2D excitations, even in a 3D structure, can potentially become a breeding ground for unconventional superconductivity.

Pressure induced lattice expansion and phonon softening in layered ReS2

We report high pressure x-ray diffraction and systematic Raman measurements on a ReS2 sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted 1T to distorted 1T′ in a triclinic structure. The same is corroborated by the appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to Re atom vibrations is observed in the distorted 1T′ phase in the pressure range of 15–25 GPa. In the same pressure range, the anomalous change in the volume is found to be induced by the lattice expansion. The volume expansion is related to the sliding of layers leading to octahedral distortion and an increase in octahedral volume. The sample is found to be very incompressible above 25 GPa with respect to below 15 GPa data. The same is also reflected in the Raman mode shifts with pressure.

Crystal Structure, Thermal Expansivity and High-Temperature Vibrational Spectra on Natural Hydrous Rutile

A natural rutle sample was measured by in situ high-temperature X-ray diffraction (XRD) patterns, as well as Raman and Fourier transform infrared (FTIR). Crystal structure is refined on the sample with 1.4 mol.% Fe and 510±120 ppmw. H2O. The unit-cell and TiO6 octahedral volumes are expanded by 0.7%–0.8°/o for Fe3+ incorporation, as compared with the reported Ti-pure samples. The volumetric thermal expansion coefficient (α, K−1) could be approximated as a linear function of T (K): 4.95(3)×10−9×T+21.54(5)×10−6, with the averaged value α0=30.48(5)×10−6 K−1, in the temperature range of 300–1 500 K. The internal Ti-O stretching (A1g and B2g) and O-Ti-O bending (Eg) modes show ‘red shift’, whereas the multi-phonon process exhibits ‘blue shift’ at elevated temperature. The rotational mode (B1g) for TiO6 octahedra is nearly insensitive to temperature variations. The OH-stretching bands at 3 279 and 3 297 cm−1 are measured by high-temperature spectroscopy experiments. Both the IR-active and Raman-active OH-stretching modes shift to lower frequencies at higher temperature, with the signal intensities decreasing. And after quenching, we expect about 43% dehydration around 873 K, and 85% dehydration at 1 273 K for this hydrous sample.

Structural, electronic, optical and lattice dynamic properties of the different WO3 phases: First-principle calculation

First-principle calculations were performed to investigate the structural, electronic, optical, and lattice dynamic properties of the cubic (c), hexagonal (h), tetragonal (α), orthorhombic (β), monoclinic (γ) and triclinic (δ) WO3 phases. Based on the analysis of structures and relative energies, we noted that the asymmetric distortion of WO6 octahedron was mainly responsible for the enhanced crystal stability of α-, β-, γ- and δ-WO3, while the titling of WO6 octahedron played a secondary effect. The results of the calculated dipole moment indicated that the phase transformation from β-to γ-WO3 induced the change of antiferroelectricity-ferroelectricity, which was contributed to the octahedral distortion along x and y directions. The band gaps calculated by PBE0 hybrid function were 1.562, 1.943 and 2.244eV for c-, h- and α-WO3. The increasing of the band gaps were contributed to the decreasing valence band width due to the expanded octahedral volume. Importantly, the distortion of the WO6 octahedron lifted the level of 5dxy states at bottommost of the conduction band, which much enlarged the band gaps of β-, γ- and δ-WO3 to 2.974, 2.881 and 2.813eV. The calculated optical properties gave the decreasing static dielectric constants ε1(0) of 8.9, 7.6, 7.3, 5.6, 5.3 and 5.1 for c-, h-, α-, β-, γ-, and δ-WO3, respectively, which were agreement with the band gaps. The lattice dynamic properties presented unstable structures of c-, h- and α-WO3, and stable structures of β-, γ-, and δ-WO3. The instability mainly originated from the stretching vibrations of OWO bonds with few contributions by the blending vibrations of WO bonds. IR and Raman spectra were discussed and compared with experimental data, which confirmed the reasonability of the calculated results. The analyzed vibrational modes with different frequencies at Gamma point noted that the grid of WO3 presented “soft” characteristic and the WO6 octahedron was “rigid”.

Theoretical study of the electronic and optical properties of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb)-doped VO2 nanoparticles

Vanadium dioxide (VO2) is an important thermochromic material due to its reversible metal-insulator transition at 340 K. However, the high phase transition temperature of VO2 prevents its practical application in smart windows. Here, we use first-principles calculations to study the effect of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb) dopants on the variation of the local octahedral volumes, lattice distortion of VO2 and dimerization of the V-V chain in VO2. Interestingly, Tb and La dopants with a concentration of 3.1 at.% decreased the phase transition temperature of VO2 to 206 K and 225 K, respectively. Moreover, three factors, including the V-V chain change, distortion of the REO6 octahedron, and formation of impurity states contributed by RE-f states, assist the phase transition of VO2. Additionally, the incorporation of an RE dopant scaled the band gap of VO2 and further tuned its near-IR absorption. These findings provide novel insights and guidance on chemically tailoring the phase transition of VO2.

Effect of interstitial nitrogen in Fe18Cr6Mn8 austenitic alloys from density functional theory

Based on the supercell structures of Fe-Cr-Mn alloys having the composition Fe18Cr6Mn8, the structural, electronic and magnetic properties of the alloys have been investigated using first-principle calculations, and the effects of dissolved nitrogen on the properties of Fe18Cr6Mn8 alloys have been further discussed. The Fe18Cr6Mn8 alloys were considered to have two different structures, Cr6Fe6Mn8Fe12 and Fe6Cr6Mn8Fe12. The lattice constants of cells Cr6Fe6Mn8Fe12 and Fe6Cr6Mn8Fe12 are similar to each other, however the octahedral volumes in the center of the cell (VO) differ. After nitrogen solid solution, the lattice constants and cell volumes of Fe18Cr6Mn8 alloys slightly increase, while the octahedral volume sees a large increase: 11.6% for Cr6Fe6Mn8Fe12 and 18.2% for Fe6Cr6Mn8Fe12. The Fe18Cr6Mn8 (N) alloys are of high stability. Solute atoms (N) increase the stability of the Fe18Cr6Mn8 system, and Cr6Fe6Mn8Fe12-N is more stable than Fe6Cr6Mn8Fe12-N. Besides N atoms with negative charges (obtaining electrons), Fe atoms in different sites also display different electronegativity. After nitrogen solid solution, the magnetic moment of Cr6Fe6Mn8Fe12 decreases about 20%, and that of Fe6Cr6Mn8Fe12 decreases about 79%. The ratio of bulk modulus B to shear modulus G (B/G ratio), representative of the ductility of the alloy, of Cr6Fe6Mn8Fe12 decreases about 15%, and that of Fe6Cr6Mn8Fe12 decreases about 30%. The addition of nitrogen decreases the Zener anisotropy ratios of Fe18Cr6Mn8, which shows the elastic anisotropy of Fe18Cr6Mn8N is lower than that of Fe18Cr6Mn8.

Crystal structure and elasticity of Al-bearing phase H under high pressure

Al has significant effect on properties of minerals. We reported crystal structure and elasticity of phase H, an important potential water reservoir in the mantle, which contains different Al using first principles simulations for understanding the effect of Al on the phase H. The crystal and elastic properties of Al end-member phase H (Al2O4H2) are very different from Mg end-member (MgSiO4H2) phase H and two aluminous phase H (Mg0.875Si0.875Al0.25O4H2 (12.5at%Al) and Mg0.75Si0.75Al0.5O4H2 (25at% Al)). However differences between Mg end-member phase H and aluminous phase H are slight except for the O-H bond length and octahedron volume. Al located at different crystal positions (original Mg or Si position) of aluminous phase H has different AlO6 octahedral volumes. For three Al-bearing phase H, bulk modulus (K), shear modulus (G), compressional wave velocity (Vp) and shear wave velocity (Vs) increase with increasing Al content. Under high pressure, density of phase H increases with increasing Al content. The Al content affects the symmetry of the phase H and then affects the density and elastic constants of phase H. The total ground energy of phase H also increases with increasing Al content. So an energy barrier for the formation of solid solution of phase H with δ-phase AlOOH is expected. However, if the phase H with δ-phase AlOOH solid solution does exit in the mantle, it may become an important component of the mantle or leads to a low velocity layer at the mantle.

Interfacial strain effects on lithium diffusion pathways in the spinel solid electrolyte Li-doped MgAl2O4

(Li,Al)-co-doped magnesium spinel (Li$_x$Mg$_{1-2x}$Al$_{2+x}$O$_4$) is a solid lithium-ion electrolyte with potential use in all-solid-state lithium-ion batteries. Interfaces with spinel electrodes, such as Li$_y$Mn$_2$O$_4$ and Li$_{4+3z}$Ti$_5$O$_{12}$, may be lattice-matched, with potentially low interfacial resistances. Small lattice parameter differences across a lattice-matched interface are unavoidable, causing residual epitaxial strain. This strain potentially modifies lithium diffusion near the interface, contributing to interfacial resistance. Here we report a density functional theory study of strain effects on lithium diffusion pathways for (Li,Al)-co-doped magnesium spinel, for $x_\mathrm{Li} = 0.25$ and $x_\mathrm{Li} = 0.5$. We have calculated diffusion profiles for the un-strained materials, and for isotropic and biaxial tensile strains of up to 6%, corresponding to {100} epitaxial interfaces with Li$_y$Mn$_2$O$_4$ and Li$_{4+3z}$Ti$_5$O$_{12}$. We find that isotropic tensile strain reduces lithium diffusion barriers by as much as 0.32 eV, with typical barriers reduced by ~0.1 eV. This effect is associated with increased volumes of transitional octahedral sites, and broadly follows local electrostatic potentials. For biaxial (epitaxial) strain changes in octahedral site volumes and in lithium diffusion barriers are much smaller than under isotropic strain. Typical barriers are reduced by only ~ 0.05 eV. Individual effects, however, depend on the pathway considered and the relative strain orientation. These results predict that isotropic strain strongly affects ionic conductivities in (Li,Al)-co-doped magnesium spinel electrolytes, and that tensile strain is a potential route to enhanced lithium transport. For a lattice-matched interface with candidate spinel-structured electrodes epitaxial strain has a small, but complex, effect on lithium diffusion barriers.

The structure and elasticity of phase B silicates under high pressure by first principles simulation**Project supported by the Science Fund from the Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration (Grant No. 2016IES010104) and the National Natural Science Foundation of China (Grant Nos. 41174071, 41273073, 41373060, and 41573121).

The structures and elasticities of phase B silicates with different water and iron (Fe) content are obtained by first-principles simulation to understand the effects of water and Fe on their properties under high pressure. The lattice constants a and b decrease with increasing water content. On the contrary, c increases with increasing water content. On the other hand, the b and c decrease with increasing Fe content while a increases with increasing Fe content. The decrease of M (metal)–O octahedral volume is greater than the decrease of SiO polyhedral volume over the same pressure range. The density, bulk modulus and shear modulus of phase B increase with increasing Fe content and decrease with increasing water content. The compressional wave velocity (Vp) and shear wave velocity (Vs) of phase B decrease with increasing water and Fe content. The comparisons of density and wave velocity between phase B silicate and the Earth typical structure provide the evidence for understanding the formation of the X-discontinuity zone of the mantle.