Interstitial Charge Research Articles

Ferromagnetic ordering in Fe: TiO2 solid solutions: A visual electron density mapping by maximum entropy method using powder X-ray data

Magnetic semiconducting FeTiO2 solid solutions with Fe compositions of 6 %, 9 %, and 12 % were analyzed using multi-phase Rietveld refinement analysis for average structure and maximum entropy for electronic structure. VSM analysis showed that the 9 % and 12 % Fe doped TiO2 compositions had a maximum magnetization of 0.3128 emu/g and coercivity of 574 Oe and 568 Oe, respectively. This confirms ferromagnetic tuning with Fe doping at or above 9 %. The MEM electron density at the bond critical points (BCP) of the apical and equatorial (Ti/Fe)–O bonds in 12 % Fe-doped TiO2 is 1.088 e/Å3 and 0.8652 e/Å3, indicating polar covalent bond. This work primarily examines a new empirical correlation between MEM electron density and magnetic parameters. Fe: TiO2 solid solutions have high magnetization and coercivity due to strong and identical (Ti/Fe) – O apical and equatorial polar covalent bonding, with low interstitial charge buildup at non-regular tetrahedral rutile lattice sites.

Effects of loosely bound electrons and electron–phonon interaction on the thermoelectric properties of electrenes

The interstitial charge in 2D HfI2 exhibits high mobility due to weak lattice perturbation, and the high mobility induces a high power factor and ZT value.

Empirical correlation of ferromagnetism and electronic structure in Fe and Ni co-doped SnO2 solid solutions: An X-ray analysis using maximum entropy method

The phase-pure magnetic semiconducting Fex/2Nix/2Sn1-xO2 solid solutions with x = 0.03, 0.06, 0.09, and 0.12, were prepared by a cost-effective room-temperature one-step co-precipitation method. XRD peak red-shift reveals lattice compression due to the doping of low/similar ionic radii of Fe3+, Ni3+, and Ni2+ in the Sn4+ lattice, which is also confirmed by the EPR results. Maximum entropy method (MEM)-based electronic structure confirmed interstitial charge accumulation of 6.79 % and 6.05%, forming non-nuclear maxima in 3% and 12% doping systems, respectively, which reduces considerably the ferromagnetism. In addition, pure polar covalent apical and equatorial (Fe/Ni/Sn) – O bonding without interstitial charge accumulation is a key factor for maximum ferromagnetism. A perfect-fitting of the pair distribution function (PDF) and its correlation to charge accumulation at interstitial and regular lattice points in comparison with MEM results provides hope in PDF using low-Q XRD data. The samples with 9% and 6% dopants showed soft ferromagnetism with magnetization (0.0416 and 0.0372) emu/g respectively, which are helpful for magnetic semiconducting applications. The ferromagnetic and antiferromagnetic coupling between interstitial charges' local magnetic domains causes positive exchange anisotropy (exchange bias) in 3% and 12% compositions. The novel and empirical correlation between magnetism and the MEM-based electronic structure is the highlight of this study.

On the Electride Nature of Na-hP4.

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

On the Electride Nature of Na‐hP4

AbstractEarly quantum mechanical models suggested that pressure drives solids towards free‐electron metal behavior where the ions are locked into simple close‐packed structures. The prediction and subsequent discovery of high‐pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na‐hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na‐pd hybrid orbitals point towards the center of an 11‐membered penta‐capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na‐hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

Al and H incorporation and Al-diffusion in natural rutile and its high-pressure polymorph TiO 2 (II)

Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3 ) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

Structure, Electron Density Distribution using Maximum Entropy Method, Optical and Magnetic Characteristics of Fe Doped SnS2

AbstractTwo dimensional layered magnetic materials like Fe‐doped SnS2 are cheap, abundant, and biocompatible, used for spintronics applications. The hydrothermal synthesized hexagonal nano disc structure of the sample is seen in the SEM images at room temperature. Pure SnS2, which is diamagnetic with magnetization 0.02067 emu/g becomes soft ferromagnetic by 2.5 % Fe doping with 0.672 emug−1 saturation magnetization and 0.222 kOe coercivity. A ultraviolet‐visible spectrometer measurement of the energy bandgap reveals a drop in energy from 2.35 to 2.2 eV, and the PL spectrum displays intense blue emission at a wavelength of 482–484 nm. Maximum entropy method (MEM), confirmed the ionic to covalent conversion upon Fe doping, due to the residual charge accumulation at the intermediate regions. The electron densities in Fe doped system in the plane (023) at Bond Critical Point of Sn─S and S─S are 0.327 and 0.354 e Å−3, respectively, with negative total energy density values confirming covalent bonding. A high electron density of 0.856 e Å−3 at the Sn─Sn region confirms interstitial charge accumulation. This introduces new intermediate energy levels in the forbidden region, reducing the energy bandgap and making the substance more semiconducting, making it useful for optical, opto‐electronic, and half‐metal applications.

High temperature stability and transport characteristics of hydrogen in alumina via multiscale computation

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic Hi+1 interstitial is the most stable defect species for most values of the electronic bandgap in both α and amorphous alumina (Al2O3). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being ∼340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (Hi0 and Hi−1) and published experimental literature indicate that Hi+1 diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between Hi+1 diffusivity calculated in α-Al2O3 from DFT: 5.05 × 10−3 exp(-0.81 eV/kB/T) cm2/s and reported experiment: 9.7 ×10−4 exp(-0.83 eV/kB/T) cm2/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of Hi+1 in amorphous Al2O3 in the 1000–2000 K range is calculated to be 2.53 × 10−2 exp(-0.89 eV/k B/T), just one order of magnitude higher than the corresponding value in α-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of Hi+1 in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.

Superconducting incommensurate host-guest phases in compressed elemental sulfur

We use Density Functional Theory (DFT) and structure searching methods to show that the ground state of elemental sulfur is a Barium-IVa type incommensurate host-guest (HG) phase between $386$-$679$ GPa, becoming the first group-VI element predicted to possess such a structure. Within the HG phase, sulfur undergoes a series of transitions in which adjacent guest chains are not aligned but rather offset by different amounts, which can be described as a rearrangement of the stacking order of the chains. We show that these chain rearrangements are intimately coupled to modulations of the host and guest atoms which prove crucial to stabilising the HG structure. Unlike the high-pressure HG phases of other elements, sulfur does not exhibit interstitial charge localisation, and instead features strongly localised `voids' that are depleted of electronic charge. Prior to adopting the HG structure, we predict that sulfur possesses an orthorhombic structure of $Fdd2$ symmetry. We calculate the superconducting critical temperatures of these newly discovered phases, and show that $T_c$ is expected to peak between $24.8$ and $28.2$ K at $271$ GPa.

La2Ga3O7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt

The La1+xAE1-xGa3O7+x/2 melilite family (AE = Ca, Sr, Ba, and 0 0.65 with very high Oint concentrations (referred to here as compositions) have the potential to support correspondingly high ionic conductivities, but have never before been accessed due to the limitations of conventional solid-state ceramic synthesis. Here we report that fully-substituted La2Ga3O7.5 (x = 1) melilite ceramics can be synthesized by direct crys-tallization of an under-cooled melt, demonstrating that super-excess compositions are accessible under suitable nonequilibrium reaction conditions. La2Ga3O7.5 is stable up to 830 °C and exhibits an ionic conductivity of 0.01 S.cm-1 at 800 °C, three orders of magnitude higher than the corresponding x = 0 end-member LaSrGa3O7, and close to the range exhibited by the current best-in-class La1.54Sr0.46Ga3O7.23 (0.1 S.cm-1). It crystallizes in an orthorhombic √2a x √2a x 2c expansion of the parent melilite cell, in space group Ima2, with full long-range ordering of Oint into chains within the [Ga3O7.5] layers. The emergence of this chain-like (1D) ordering within the 2D melilite framework, which appears to be an incipient feature of previously reported partially-ordered melilites, is explained in terms of the underlying hexagonal topology of the structure. These results will enable the exploration of extended com-positional ranges for the development of new solid oxide ion electrolytes with high concentrations of interstitial oxide charge carriers.

Interstitial Oxide Ion Conductivity in the Langasite Structure: Carrier Trapping by Formation of (Ga,Ge)2O8 Units in La3Ga5-x Ge1+x O14+x/2 (0 < x ≤ 1.5).

Framework oxides with the capacity to host mobile interstitial oxide anions are of interest as electrolytes in intermediate temperature solid oxide fuel cells (SOFCs). High performance materials of this type are currently limited to the anisotropic oxyapatite and melilite structure types. The langasite structure is based on a corner-shared tetrahedral network similar to that in melilite but is three-dimensionally connected by additional octahedral sites that bridge the layers by corner sharing. Using low-temperature synthesis, we introduce interstitial oxide charge carriers into the La3Ga5–xGe1+xO14+x/2 langasites, attaining a higher defect content than reported in the lower dimensional oxyapatite and melilite systems in La3Ga3.5Ge2.5O14.75 (x = 1.5). Neutron diffraction and multinuclear solid state 17O and 71Ga NMR, supported by DFT calculations, show that the excess oxygen is accommodated by the formation of a (Ge,Ga)2O8 structural unit, formed from a pair of edge-sharing five-coordinated Ga/Ge square-based pyramidal sites bridged by the interstitial oxide and a strongly displaced framework oxide. This leads to more substantial local deformations of the structure than observed in the interstitial-doped melilite, enabled by the octahedral site whose primary coordination environment is little changed by formation of the pair of square-based pyramids from the originally tetrahedral sites. AC impedance spectroscopy on spark plasma sintered pellets showed that, despite its higher interstitial oxide content, the ionic conductivity of the La3Ga5–xGe1+xO14+x/2 langasite family is lower than that of the corresponding melilites La1+ySr1–yGa3O7+y/2. The cooperative structural relaxation that forms the interstitial-based (Ga,Ge)2O8 units stabilizes higher defect concentrations than the single-site GaO5 trigonal bipyramids found in melilite but effectively traps the charge carriers. This highlights the importance of controlling local structural relaxation in the design of new framework electrolytes and suggests that the propensity of a framework to form extended units around defects will influence its ability to generate high mobility interstitial carriers.

Pressure induced multi-centre bonding and metal–insulator transition in PtAl2

A study on pressure induced metallization or metal-insulator transition (MIT) of materials is of enormous importance in high pressure research not only in view of various possible device applications but also for basic physical insight into their mechanism. Though the former transition (metallization) is quite common, the latter one (MIT) is comparatively rare, and only a few simple metals (Na, Al, etc.) are predicted to be an insulator at megabar pressure. In this work, we have shown that the binary intermetallic compound (PtAl2) exhibits a pressure induced metal-insulator transition comparatively at low pressure, near ∼28 GPa. By means of first principles calculations, we have established that a unique multi-centre bonding is developed under pressure which causes charge-density-ordering in the system and finally leads to MIT and structural phase transition involving unit cell reduction. In search of some exotic properties, we have found that the system has a good thermoelectric figure-of-merit in its insulating phase.

Manipulation of the magneto-optical properties of a Co/C heterostructure under an applied voltage

In this work, electrical manipulation of the magneto-optical properties of a cobalt (Co)/carbon (C) heterostructure is demonstrated via reversible voltage (V)-dependent magnetic circular dichroism (MCD) measurements to provide information on its spin-polarized density of states. Complex impedance spectroscopy revealed the occurrence of capacitance effects between Co and C. Suppression of the radial distribution functions of the X-ray absorption spectra and changes in the Raman spectra reveal that charge injection under an applied V results in increased interstitial charge densities. Angle-dependent changes in MCD under an applied V reveal that variations in C–Co hybridization due to charge accumulation are responsible for the observed phenomena.

Interstitial charge transfer pathways in a TiO2/CdIn2S4 heterojunction photocatalyst for direct conversion of sunlight into fuel

The synergistic interaction between CdIn2S4 and TiO2 matrices leads to alteration in the electronic band structure of TiO2 and creates the energy levels between the conduction and valence bands for interstitial charge transfer.

Photocatalytic Applications of Colloidal Heterostructured Nanocrystals: What's Next?

Recent progress in the colloidal synthesis of inorganic nanocrystals has led to the realization of complex, multidomain nanoparticle morphologies that give rise to advanced optoelectronic properties. Such nanocomposites are particularly appealing for photocatalytic applications where tunable absorption, extensive charge separation, and large surface-to-volume ratios are important. To date, heterostructured nanocrystals featuring a metal catalyst and a semiconductor "chromophore" component have shown compelling efficiencies in photoreduction reactions, including sacrificial hydrogen production. Time-resolved optical studies have attributed their success to a near-complete separation of photoinduced charges across dissimilar nanoparticle domains. The spectroscopy approach has also identified the key performance-limiting factors of nanocrystal catalysts that arise from inefficient extraction of photoinduced charges to catalytic sites. Along these lines, the main scope of present-day efforts targets the improvement of interstitial charge transfer pathways across the chromophore-catalyst assembly through the design of high-quality stoichiometric interfaces.

Investigation of exotic stable calcium carbides using theory and experiment.

It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions using the USPEX code. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3 and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance is the base-centred monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semi-metallic behaviour.

Identification of bulk oxide defects in an electrochemical environment.

We discuss how electronic-structure calculations can be used to identify the dominant point defects that control the growth and dissolution of the oxide barrier layer formed if a metal comes into contact with a corrosive environment. Using the example of the Zn/ZnO/H(2)O interface we develop and apply a theoretical approach that is firmly based on ab initio computed defect formation energies and that unifies concepts of semiconductor defect chemistry with electrochemical concepts. Employing this approach we find that the commonly invoked and chemically intuitive defects such as the doubly negatively charged oxygen vacancy in electrochemically formed ZnO films may not be present. Rather, hitherto not discussed defects such as the oxygen interstitial or unexpected charge states, such as the neutral oxygen vacancy, are found. These new defect types will be shown to critically impact our understanding of fundamental corrosion mechanisms and to provide new insight into strategies to develop alloys with higher corrosion resistance.

Stable compositions and structures in the Na-Bi system.

At P = 1 atm, the only stable compounds in the Na-Bi binary system are Na3Bi and NaBi, which have recently been discovered to exhibit intriguing electronic behaviour as a 3D topological Dirac semimetal and a topological metal, respectively. By means of first-principles calculations coupled with evolutionary structural searches, we have systematically investigated the phase stabilities, the crystal structures and the corresponding electronic properties of the binary Na-Bi system. At ambient pressure, our calculations have reproduced well the experimentally observed compositions and structures of Na3Bi and NaBi. At high pressures, we have found that Na3Bi is transformed from the ground-state hexagonal hP24 phase to a cubic cF16 phase above 0.8 GPa, confirming previous experiments, and then to a conventional band-insulating oC16 phase above 118 GPa. The cubic cF16 phase would exhibit novel topological band ordering similar to that in HgTe. The topological metal NaBi has also been found to undergo a structural phase transition from the ambient tetragonal tP2 to a cubic cP2 structure above 36 GPa. Four compounds never before reported, Na6Bi, Na4Bi, Na2Bi and NaBi2, with new compositions, have been predicted to be experimentally synthesizable over a wide range of pressures starting at 142.5 GPa, 105 GPa, 38 GPa and 171 GPa, respectively. Moreover, a common charge transfer from Na to Bi has been observed for all compounds, but substantial interstitial charge localization in Na atomic cages has been noticed only in two compounds, Na6Bi and Na4Bi, and may be associated with close-packed Na environments.

The origin of the site preference of H adsorption on Pd(100)

Spin-polarized density functional theory (DFT) calculations are carried out to determine the site preference of H adsorption on Pd(100) surface and subsurface. We carefully scrutinize the energy difference between different patterns at θ=0.50 ML and confirm the LEED observation that surface adsorption can form c(2×2) ordering structure. On the contrary, we disclose that p(2×1) structure become more favorable than c(2×2) for subsurface adsorption. These site preferences are rationalized via an analysis of the layer and orbital resolved density of states. Furthermore, we propose that the interstitial charge as a key factor determining the preferred H adsorbed site.

Bonding, structures, and band gap closure of hydrogen at high pressures

We have studied dense hydrogen and deuterium experimentally up to 320 GPa and using ab initio molecular dynamic (MD) simulations up to 370 GPa between 250 and 300 K. Raman and optical absorption spectra show significant anharmonic and quantum effects in mixed atomic and molecular dense phase IV of hydrogen. In agreement with these observations, ab initio MD simulations near 300 K show extremely large atomic motions, which include molecular rotations, hopping and even pair fluctuations suggesting that phase IV may not have a well-defined crystalline structure. The structurally diverse layers (molecular and graphene-like) are strongly coupled thus opening an indirect band gap; moreover, at 300 GPa we find fast synchronized intralayer structural fluctuations. At 370 GPa the mixed structure collapses to form a metallic molecular Cmca-4 phase, which exhibit a new interstitial valence charge bonding scheme.