Low-symmetry Distortions Research Articles

Energetic disorder dominates optical properties and recombination dynamics in tin-lead perovskite nanocrystals

Tin-lead alloyed perovskite nanocrystals (PNCs) offer a promising pathway toward low-toxicity and air-stable light-emitting devices. However, substantial energetic disorder has thus far hindered their lighting applications compared to pure lead-based PNCs. A fundamental understanding of this disorder and its impact on optical properties is crucial for overcoming this limitation. Here, using temperature-dependent static and transient absorption spectroscopy, we meticulously distinguish the contributions of static disorder (including defects, impurities, etc.) and dynamic disorder (carrier–phonon interactions). We reveal how these disorders shape band-tail structure and ultimately influence inter-band carrier recombination behaviors. Surprisingly, we find that static and dynamic disorder primarily control band-tail defect states and bandgap renormalization, respectively, which together modulate fast carrier trapping and slow band-band recombination rates. Furthermore, we link these disorders to the tin-induced symmetry-lowering distortions in tin-lead alloyed PNCs. These findings illuminate critical design principles for highly luminescent, low-toxicity tin-lead PNCs, accelerating their adoption in optoelectronic applications.

Hypersensitivity and nephelauxetic effect of Er3+ in bismuth tellurite glass system

The optical absorption and emission properties of 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag glass system had been analyzed to investigate the hypersensitivity shift and its mechanism. According to the Covalent model, the hypersensitive shift indicated by the drop of F2 at x = 5 mol% can be attributed to charge transfer from O2– ligands to Er3+ ions meanwhile according to the Dielectric screening model, drop of F2 may due to Er3+ ions contraction. The dynamic coupling mechanism was used to explain hypersensitivity transition probabilities in non-centrosymmetric systems. The addition of Bi2O3 may modify the site symmetry of Er3+ ions and oxygen to a high asymmetry τ2, resulting in an asymmetrical electron distribution, thus increasing Er-O covalency, as shown by the maximum Ω2 value at x = 5 mol%. The Er3+ ions site symmetry was investigated using Hamiltonian crystal field fitting in the frame of the D4 point symmetry model, which yielded maximum crystal field strength Nv at x = 5 mol%, indicating low point symmetry distortion of the Er3+ ions site symmetry.

Magnetic-field-induced structural phase transition and giant magnetoresistance in La0.85Ce0.15Fe12B6

Magnetoelastic coupling, structural, magnetic, electronic transport, and magnetotransport properties of ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ have been studied by a combination of macroscopic [magnetization, electrical resistivity, and magnetoresistance (MR)] and microscopic temperature- and magnetic-field-dependent x-ray powder diffraction measurements. The itinerant-electron system ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ exhibits an antiferromagnetic (AFM) ground state and multiple magnetic transitions, AFM-ferromagnetic (FM) and FM-paramagnetic (PM), triggered by changes in both temperature and magnetic fields. At low temperatures, the field-induced first-order AFM-FM metamagnetic phase transition is discontinuous, manifesting itself by extremely sharp steps in magnetization as well as in MR and is accompanied by large magnetic hysteresis. A remarkably large negative MR of \ensuremath{-}73% was discovered. In addition, the time evolution of the electrical resistivity displays a colossal spontaneous jump when both the applied magnetic field and temperature are constant. Diffraction data reveal a magnetic-field-induced structural phase transition associated with the AFM-FM and PM-FM transformations. The lattice distortion is driven by magnetoelastic coupling and converts the crystal structure from rhombohedral ($R\overline{3}m$) to monoclinic $(C2/m)$. The AFM and PM states are related to the rhombohedral structure, whereas the FM order develops in the monoclinic symmetry. A huge volume magnetostriction of \ensuremath{\sim}0.9% accompanies this symmetry-lowering lattice distortion. Meanwhile, a highly anisotropic thermal expansion involving giant negative thermal expansion with an average volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}195\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ was observed. The consistency seen in these different experimental data constitutes direct evidence of the strong correlations between charge, magnetic, and crystallographic degrees of freedom in this material.

Emergence of topological superconductivity in doped topological Dirac semimetals under symmetry-lowering lattice distortions

Recently, unconventional superconductivity having a zero-bias conductance peak is reported in doped topological Dirac semimetal (DSM) with lattice distortion. Motivated by the experiments, we theoretically study the possible symmetry-lowering lattice distortions and their effects on the emergence of unconventional superconductivity in doped topological DSM. We find four types of symmetry-lowering lattice distortions that reproduce the crystal symmetries relevant to experiments from the group-theoretical analysis. Considering inter-orbital and intra-orbital electron density-density interactions, we calculate superconducting phase diagrams. We find that the lattice distortions can induce unconventional superconductivity hosting gapless surface Andreev bound states (SABS). Depending on the lattice distortions and superconducting pairing interactions, the unconventional inversion-odd-parity superconductivity can be either topological nodal superconductivity hosting a flat SABS or topological crystalline superconductivity hosting a gapless SABS. Remarkably, the lattice distortions increase the superconducting critical temperature, which is consistent with the experiments. Our work opens a pathway to explore and control pressure-induced topological superconductivity in doped topological semimetals.

Ansatz for the Jahn-Teller triplet instability.

A threefold degenerate electronic state is Jahn-Teller unstable with respect to symmetry lowering distortions, which transform as the five quadrupolar modes. The solution of the corresponding vibronic Hamiltonian is constructed using the analytical method introduced by Bargmann, as an alternative to existing group-theoretical techniques based on coefficients of fractional parentage. It involves the construction of an ansatz that incorporates SO(5) to SO(3) symmetry breaking. The resulting Jahn-Teller equations are derived and solved in terms of radial polynomials and Gegenbauer functions.

Large structural heterogeneity in submicrometer BaTiO3 revealed via Eu+3 photoluminescence study

Apart from being a model ferroelectric, BaTiO3 and its derivatives are materials of considerable technological importance in the capacitor industry and as a potential Pb-free piezoelectric. Here, we have examined BaTiO3 for its propensity to stabilize local low symmetry distortions and induce structural heterogeneity for two different situations, namely, (1) by introducing inhomogeneous lattice strain and (2) by restricting the average grain size to the submicrometer regime. We have introduced Eu+3 ions in BaTiO3 in a very dilute concentration to act as local probes. Our strategy relies on exploiting the great sensitivity of the 5D0 → 7F2 transition of Eu+3 with regard to the variation in the asymmetric distribution of the ligand field around itself to probe the structural heterogeneity developed in BaTiO3. The Eu+3 photoluminescence signal revealed a remarkable increase in the local structural heterogeneity in submicrometer (∼0.4 μm) sized BaTiO3. This manifests as an ∼170% increase in the intensity of the 7F2 band with respect to the structure insensitive 7F1 band. Although to a lesser extent, a similar scenario was observed when the large grain BaTiO3 develops residual strain. The structural insights presented here can be helpful in explaining the properties of BaTiO3-based multilayered ceramics wherein the two conditions of submicrometer size and residual strain is a common occurrence.

Spin crossover properties of Fe(III) complexes in [Fe (bzacen)(tvp)]BPh4 ·nSolv chain structures: EPR study.

Two types of Fe(III) polynuclear iron(III) 1D-chain coordination compounds of the general formula [Fe (L)(tvp)]BPh4 nSolv, where L = dianion of N,N'-ethylenebis (benzoylacetylacetone)2,2'-imine (bzacen), tvp = 1,2-di(4-pyridyl)ethylene were synthesized and studied by the electron paramagnetic resonance (EPR) and magnetic susceptibility methods in the temperature range (100-300) К. Two types of spin-variable complexes are formed depending on the time of precipitation of the complexes from the same solution leading to differently solvated species. They have different characteristics of the local ligand field and the spin transition behavior. The thermodynamic parameters of spin transitions were determined from the temperature dependence of the EPR signals integral intensity. The energy levels splitting values obtained by analyzing g-factors of low-spin Fe(III) centers evidenced not only on the crucial role of low-symmetry distortions on the principal possibility of spin-crossover processes, but also on the temperature peculiarities of spin transitions.

Potential Solid Electrolytes for Multivalent-Ions Based on the Anti-Perovskite Structure

Li-ion batteries (LIBs) are the most widely used energy storage devices. Beyond Li, Na and K ion systems are being explored for grid energy storage. Additionally, multivalent (MV) ions like Mg, Ca, and Al have the potential to increase the volumetric capacity of batteries. Currently, additional improvements in battery technologies are being driven by demands for longer-lasting batteries and safety concerns. Safety hazards in LIBs originate from the use of liquid electrolytes, which are volatile and flammable. In principle, these safety problems can be circumvented by the use of a solid electrolyte (SE). Furthermore, SEs may enable the use of metallic anodes which significantly increase the energy density.Recently, Li and Na anti-perovskites (AP) have been suggested as SEs. APs adopt the formula X3AB, where X is a monovalent cation and A and B are anions with respective charges of -2 and -1. Using a series of model APs with X = Li, Na, or K, A = O, S, or Se, and B = F, Cl, Br, or I, our previous study revealed that ‘lattice distortion’ can enhance ion mobility. We found that symmetry-lowering lattice distortions correlate with lower percolating migration barriers and reduced thermodynamic stability. Therefore, ‘distortion tuning’ can result in SE materials by balancing a mobility/stability tradeoff.Thus far, few MV SEs have been reported, and the discovery of MV SEs remains an important pursuit to advance the prospects for MV solid-state batteries. Previous literature showed that most alkaline earth metals, 3d transition metals, and lanthanides can reside in the X site of the AP system. These materials exhibit a wide range of electronic, magnetic, and thermal properties, and have been exploited in applications ranging from thermoelectrics, memory devices, sensors, and optical devices.Among the many possible MV APs, here we focus on ten non-metallic Mg and Ca APs for MV SEs, i.e., Mg3NB, Ca3NB, Ca3PSb, and Ca3AsSb (where B = P, As, Sb, or Bi). First-principles calculations were used to evaluate suitability for SE applications by predicting equilibrium structures, band gaps, elastic moduli, percolating migration barriers, defect formation energies, and thermodynamic (electro)chemical stability. All compounds are predicted to be thermodynamically stable. Ca and Mg APs follow a similar trend to that observed in monovalent APs, where lattice distortions decrease the barriers for percolating ion migration. These MV APs are also predicted to have high defect formation energies, implying that defect concentrations need to be frozen in during synthesis at high temperatures.We found that Mg3NAs, Ca3NAs, and Ca3PSb have relatively low percolating barriers of less than 500 meV for vacancy migration and less than 200 meV for dumbbell migration. Band gaps are predicted to be larger than 2 eV. Also, these systems are stable against the respective metal anode and expected to have moderate oxidative stability up to 1.2, 1.7, and 1.4 V, respectively, implying that coating materials may be needed to prevent oxidation by the cathode and increase the operating voltage.

Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

The local atomic and magnetic structures of the compounds $A$MnO$_2$ ($A$ = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO$_2$ is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by N\'eel order. In NaMnO$_2$, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order.

Sr2FeIrO4: Square-Planar Ir(II) in an Extended Oxide.

Topochemical reduction of the double-perovskite oxide Sr2FeIrO6 under dilute hydrogen leads to the formation of Sr2FeIrO4. This phase consists of ordered infinite sheets of apex-linked Fe2+O4 and Ir2+O4 squares stacked with Sr2+ cations and is the first report of Ir2+ in an extended oxide phase. Plane-wave density functional theory calculations indicate high-spin Fe2+ (d6, S = 2) and low-spin Ir2+ (d7, S = 1/2) configurations for the metals and confirm that both ions have a doubly occupied d z2 orbital, a configuration that is emerging as a consistent feature of all layered oxide phases of this type. The stability and double occupation of d z2 in the Ir2+ ions invites a somewhat unexpected analogy to the extensively studied Ir4+ ion as both ions share a common near-degenerate (d xy/ xz/ yz)5 valence configuration. On cooling below 115 K, Sr2FeIrO4 enters a magnetically ordered state in which the Ir and Fe sublattices adopt type II antiferromagnetically coupled networks which interpenetrate each other, leading to frustration in the nearest-neighbor Fe-O-Ir couplings, half of which are ferromagnetic and half antiferromagnetic. The spin frustration drives a symmetry-lowering structural distortion in which the four equivalent Ir-O and Fe-O distances of the tetragonal I4/ mmm lattice split into two mutually trans pairs in a lattice with monoclinic I112/ m symmetry. This strong magneto-lattice coupling arises from the novel local electronic configurations of the Fe2+ and Ir2+ cations and their cation-ordered arrangement in a distorted perovskite lattice.

Polymorphous band structure model of gapping in the antiferromagnetic and paramagnetic phases of the Mott insulators MnO, FeO, CoO, and NiO

A band structure description of the observed large band gaps and moments in both the antiferromagnetic (AFM) and paramagnetic (PM) phases of the classic NaCl-structure Mott insulators MnO, FeO, CoO, and NiO is provided by ordinary, single-determinant density functional theory method. As noted by previous authors, the ordered AFM phases already show in band theory significant band gaps. However, for the disordered PM phases the commonly used band model has been to assume the macroscopically observed, averaged NaCl structure, where all transition metal sites are forced to be symmetry-equivalent (a monomorphous description); for the PM phase this forces zero moment on an atom by atom basis, thus producing a gapless PM state, in sharp conflict with experiment. Instead, we allow larger NaCl-type supercells where each TM site can have different local bonding and spin environments (a polymorphous description) and thus the geometric flexibility to acquire symmetry-lowering distortions that lower the total energy and can break the symmetry of the d orbitals. The existence of a distribution of different local spin and bonding environments allows large on-site magnetic moments to develop spontaneously in the DFT+U calculations leading to significant (1-3 eV) band gaps in the AFM, FM, and PM phases of the classic Mott insulators MnO, FeO, CoO, and NiO. We adapt to the spin disordered configurations in the PM phases the "special quasi-random structure" whereby supercell approximants which represent the best random configuration average for finite supercells of a given lattice symmetry are constructed. Thus, avoiding a monomorphous description of the disordered magnetic phases allows even ordinary DFT+U, which represents the N electron system with a single-determinant wavefunction, to describe the gapping not only in the AFM phases but also in the PM phases of MnO, FeO, CoO, and NiO.

Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers.

A series of mixed-metal oxide structures based on the stacking of α-U3O8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; ; , 1.5, 2), and the edge-shared BO7 pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO7 layers and the local coordination environment of the "A" site cations. Temperature dependent polymorphism has been investigated for structures containing single layered ( ) monovalent ( ) "A" site cations (e.g., Ag2Nb4O11, Na2Nb4O11, and Cu2Ta4O11). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag2Ta4O11 to yield PbTa4O11. Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting.

Unveiling the Local Structure of Cu2+ Ions from d-Orbital Splitting. Application to K2ZnF4:Cu2+ and KZnF3:Cu2+

This work investigates why Jahn-Teller (JT) transition-metal ions like Cu2+ (d9) or Mn3+ (d4) with octahedral coordination exhibit low-symmetry distortions of the CuF64- (or MnF63-) octahedron, when Cu2+ replaces Zn2+ either in the cubic perovskite KZnF3 or in the layer perovskite K2ZnF4. The aim is to establish correlations between the splitting Δe of the parent Oh eg(x2 - y2, 3z2 - r2) d orbitals and the low-symmetry local distortion ρ, allowing us to unravel the local structure of Cu2+ introduced as an impurity into such perovskites. However, the question of whether tetragonal a1g - b1g splitting Δe of Cu2+ is proportional to ρ or Δe contains additional contributions from the rest of the lattice (crystal anisotropy) is controversial. Here we show that Δe depends linearly on ρ. High-pressure experiments and compound series involving JT ions give support for the proposed scenario and provide correlations relating ρ and Δe in Cu2+ and Mn3+ systems, from which the electron-vibration coupling for the E⊗e JT effect is obtained experimentally.

Free energy calculation of mechanically unstable but dynamically stabilized bcc titanium

The phase diagram of numerous materials of technological importance features high-symmetry high-temperature phases that exhibit phonon instabilities. Leading examples include shape-memory alloys, as well as ferroelectric, refractory, and structural materials. The thermodynamics of these phases have proven challenging to handle by atomistic computational thermodynamic techniques, due to the occurrence of constant anharmonicity-driven hopping between local low-symmetry distortions, while maintaining a high-symmetry time-averaged structure. To compute the free energy in such phases, we propose to explore the system's potential-energy surface by discrete sampling of local minima by means of a lattice gas Monte Carlo approach and by continuous sampling by means of a lattice dynamics approach in the vicinity of each local minimum. Given the proximity of the local minima, it is necessary to carefully partition phase space by using a Voronoi tessellation to constrain the domain of integration of the partition function, in order to avoid double-counting artifacts and enable an accurate harmonic treatment near each local minima. We consider the bcc phase of titanium as a prototypical examples to illustrate our approach.

Internal structure of hexagonal skyrmion lattices in cubic helimagnets

We report the most precise observations to date concerning the spin structure of magnetic skyrmions in a nanowedge specimen of cubic B20 structured FeGe. Enabled by our development of advanced differential phase contrast (DPC) imaging (in a scanning transmission electron microscope (STEM)) we have obtained high spatial resolution quantitative measurements of skyrmion internal spin profile. For hexagonal skyrmion lattice cells, stabilised by an out-plane applied magnetic field, mapping of the in-plane component of magnetic induction has revealed precise spin profiles and that the internal structure possesses intrinsic six-fold symmetry. With increasing field strength, the diameter of skyrmion cores was measured to decrease and accompanied by a nonlinear variation of the lattice periodicity. Variations in structure for individual skyrmions across an area of the lattice were also studied utilising a new increased sensitivity DPC detection scheme and a variety of symmetry lowering distortions were observed. To provide insight into fundamental energetics we have constructed a phenomenological model, with which our experimental observations of spin profiles and field induced core diameter variation are in good agreement with predicted structure in the middle of the nanowedge crystal. In the vicinity of the crystal surfaces, our model predicts the existence of in-plane twisting distortions which our current experimental observations were not sensitive to. As an alternative to the requirement for as yet unidentified sources of magnetic anisotropy, we demonstrate that surface states could provide the energetic stabilisation needed for predomination over the conical magnetic phase.

Single- and Double-Site Substitutions in Mixed-Metal Oxides: Adjusting the Band Edges Toward the Water Redox Couples

New mixed-metal oxide solid solutions, i.e., the single-metal substituted Na2Ta4–yNbyO11 (0 ≤ y ≤ 4) and the double-metal substituted Na2–2xSnxTa4–yNbyO11 (0 ≤ y ≤ 4; 0 ≤ x ≤ 0.35), were investigated and used to probe the impact of composition on their crystalline structures, optical band gaps, band energies, and photocatalytic properties. The Na2Ta4O11 (y = 1) phase was prepared by flux-mediated synthesis, while the members of the Na2Ta4–yNbyO11 solid solution (1 ≤ y ≤ 4) were prepared by traditional high-temperature reactions. The Sn(II)-containing Na2–2xSnxTa4–yNbyO11 (0 ≤ y ≤ 4) solid solutions were prepared by flux-mediated ion-exchange reactions of the Na2Ta4–yNbyO11 solid solutions within a SnCl2 flux. The crystalline structures of both solid solutions are based on the parent Na2B4O11 (B = Nb, Ta) phases and consist of layers of edge-shared BO7 pentagonal bipyramids that alternate with layers of isolated BO6 octahedra surrounded by Na(I) cations. Rietveld refinements of the Na2Ta4–yNbyO11 solid solution showed that Nb(V) cations were disordered equally over both the BO7 and BO6 atomic sites, with a symmetry-lowering distortion from R3̅c to C2/c occurring at ∼67–75% Nb (y = ∼2.7–3.0). A red-shift in the optical band gaps from ∼4.3 to ∼3.6 eV is observed owing to a new conduction band edge that arises from the introduction of the lower-energy Nb 4d-orbitals. Reactions of these phases within a SnCl2 flux yielded the new Na2–2xSnxTa4–yNbyO11 solid solution with Sn-content varying from ∼11% to ∼21%. However, significant red-shifting of the band gap is found with increasing Nb-content, down to ∼2.3 eV for Na1.4Sn0.3Nb4O11, because of the higher energy valence band edge upon incorporation of Sn(II) into the structure. Aqueous suspensions of the particles irradiated at ultraviolet–visible energies yielded the highest photocatalytic hydrogen production rates for Na1.3Sn0.35Ta1.2Nb2.8O11 (∼124 μmol H2·g–1·h–1) and Na1.4Sn0.3Ta3NbO11 (∼105 μmol H2·g–1·h–1), i.e., for the compositions with the highest Sn(II)-content. Further, polycrystalline films show n-type anodic photocurrents under ultraviolet–visible light irradiation. These results show that the valence and conduction band energies can be raised and lowered, respectively, using single-metal and double-metal substituted solid solutions. Thus, a novel approach is revealed for achieving smaller visible-light bandgap sizes and a closer bracketing of the water redox couples in order to drive total water splitting reactions that are critical for efficient solar energy conversion.

Unusual ferroelectricity induced by the Jahn-Teller effect: A case study on lacunar spinel compounds

The Jahn-Teller effect refers to the symmetry-lowering geometrical distortion in a crystal (or non-linear molecule) due to the presence of a degenerate electronic state. Usually, the Jahn-Teller distortion is not polar. Recently, GaV4S8 with the lacunar spinel structure was found to undergoes a Jahn-Teller distortion from cubic to ferroelectric rhombohedral structure at TJT = 38K. In this work, we carry out a general group theory analysis to show how and when the Jahn-Teller effect gives rise to ferroelectricity. On the basis of this theory, we find that the ferroelectric Jahn-Teller distort in GaV4S8 is due to the non-centrosymmetric nature of the parent phase and strong electron-phonon interaction related to two low-energy T2 phonon modes. Interestingly, GaV4S8 is not only ferroelectric, but also ferromagnetic with the magnetic easy axis along the ferroelectric direction. This suggests that GaV4S8 is a multiferroic in which an external electric field may control its magnetization direction. Our study not only explains the Jahn-Teller physics in GaV4S8, but also paves a new way for searching and designing new ferroelectrics and multiferroics.

Flux-mediated syntheses, structural characterization and low-temperature polymorphism of the p-type semiconductor Cu2Ta4O11

A new low-temperature polymorph of the copper(I)-tantalate, α-Cu2Ta4O11, has been synthesized in a molten CuCl-flux reaction at 665°C for 1h and characterized by powder X-ray diffraction Rietveld refinements (space group Cc (#9), a=10.734(1)Å, b=6.2506(3)Å, c=12.887(1)Å, β=106.070(4)°). The α-Cu2Ta4O11 phase is a lower-symmetry monoclinic polymorph of the rhombohedral Cu2Ta4O11 structure (i.e., β-Cu2Ta4O11 space group R3̅c (#167), a=6.2190(2)Å, c=37.107(1)Å), and related crystallographically by ahex=amono/√3, bhex=bmono, and chex=3cmonosinβmono. Its structure is similar to the rhombohedral β-Cu2Ta4O11 and is composed of single layers of highly-distorted and edge-shared TaO7 and TaO6 polyhedra alternating with layers of nearly linearly-coordinated Cu(I) cations and isolated TaO6 octahedra. Temperature dependent powder X-ray diffraction data show the α-Cu2Ta4O11 phase is relatively stable under vacuum at 223K and 298K, but reversibly transforms to β-Cu2Ta4O11 by at least 523K and higher temperatures. The symmetry-lowering distortions from β-Cu2Ta4O11 to α-Cu2Ta4O11 arise from the out-of-center displacements of the Ta 5d0 cations in the TaO7 pentagonal bipyramids. The UV–vis diffuse reflectance spectrum of the monoclinic α-Cu2Ta4O11 shows an indirect bandgap transition of ∼2.6eV, with the higher-energy direct transitions starting at ∼2.7eV. Photoelectrochemical measurements on polycrystalline films of α-Cu2Ta4O11 show strong cathodic photocurrents of ∼1.5mA/cm2 under AM 1.5G solar irradiation.

Structural and electronic investigations of PbTa4O11 and BiTa7O19 constructed from α-U3O8 types of layers

The PbTa4O11 and BiTa7O19 phases were prepared by ion-exchange and solid-state methods, respectively, and their structures were characterized by neutron time-of-flight diffraction and Rietveld refinement methods (PbTa4O11, R3 (No. 146), a=6.23700(2)Å, c=36.8613(1)Å; BiTa7O19, P6¯c2 (No. 188), a=6.2197(2)Å, c=20.02981(9)Å). Their structures are comprised of layers of TaO6 octahedra surrounded by three 7-coordinate Pb(II) cations or two 8-coordinate Bi(III) cations. These layers alternate down the c-axis with α-U3O8 types of single and double TaO7 pentagonal bipyramid layers. In contrast to earlier studies, both phases are found to crystallize in noncentrosymmetric structures. Symmetry-lowering structural distortions within PbTa4O11, i.e. R3¯c→R3, are found to be a result of the displacement of the Ta atoms within the TaO7 and TaO6 polyhedra, towards the apical and facial oxygen atoms, respectively. In BiTa7O19, relatively lower reaction temperatures leads to an ordering of the Bi/Ta cations within a lower-symmetry structure, i.e., P63/mcm→P6¯c2. In the absence of Bi/Ta site disorder, the Ta–O–Ta bond angles decrease and the Ta–O bond distances increase within the TaO7 double layers. Scanning electron microscopy images reveal two particle morphologies for PbTa4O11, hexagonal rods and finer irregularly-shaped particles, while BiTa7O19 forms as aggregates of irregularly-shaped particles. Electronic-structure calculations confirm the highest-energy valence band states are comprised of O 2p-orbitals and the respective Pb 6s-orbital and Bi 6s-orbital contributions. The lowest-energy conduction band states are composed of Ta 5d-orbital contributions that are delocalized over the TaO6 octahedra and layers of TaO7 pentagonal bipyramids. The symmetry-lowering distortions in the PbTa4O11 structure, and the resulting effects on its electronic structure, lead to its relatively higher photocatalytic activity compared to similar structures without these distortions.

Enhanced magnetic anisotropies of single transition-metal adatoms on a defective MoS2 monolayer.

Single magnetic atoms absorbed on an atomically thin layer represent the ultimate limit of bit miniaturization for data storage. To approach the limit, a critical step is to find an appropriate material system with high chemical stability and large magnetic anisotropic energy. Here, on the basis of first-principles calculations and the spin-orbit coupling theory, it is elucidated that the transition-metal Mn and Fe atoms absorbed on disulfur vacancies of MoS2 monolayers are very promising candidates. It is analysed that these absorption systems are of not only high chemical stabilities but also much enhanced magnetic anisotropies and particularly the easy magnetization axis is changed from the in-plane one for Mn to the out-of-plane one for Fe by a symmetry-lowering Jahn-Teller distortion. The results point out a promising direction to achieve the ultimate goal of single adatomic magnets with utilizing the defective atomically thin layers.