Anion Displacement Research Articles

Organic ferroelastic enantiomers with high Tc and large dielectric switching ratio triggered by order-disorder and displacive phase transition

Molecular-based ferroelastics with dielectric switching properties are highly desirable for their applications on microelectronic dielectric switches, sensors, data storage, and so on. However, the current reports mostly focus on organic-inorganic hybrids containing toxic heavy metal atoms, and the relatively low phase transition temperature limits their application. In this paper, low-toxic organic salt ferroelastic enantiomers (R/S)-4-fluoro-1-azabicyclo[3.2.1]octonium chloride [(R/S)-F-321] were designed and synthesized under the introducing chirality strategy. They undergo a 432F422-type ferroelastic phase transition with a high Curie temperature (Tc) of 470 K, simultaneously exhibiting excellent dielectric switching characteristics. In addition to the ordered-disordered movement of cations, the significant displacement of anions is also responsible for such high Tc and large dielectric switching ratios, which is very rare in molecular-based switching materials. This work enriches the development of molecular ferroelastic switching materials and gives inspiration for the exploration of environmentally friendly high Tc organic salt ferroelastics with prominent switching performances.

Peculiar bond length dependence in (Ag,Cu)GaSe2 alloys and its impact on the bandgap bowing

Contrary to other semiconductor alloys, incorporation of Ag into CuGaSe2 increases the bandgap, even though the lattice expands, and the Ga–Se bond length is theoretically predicted to decrease rather than increase. Herein, we experimentally confirm this peculiar bond length dependence of (Ag,Cu)GaSe2 using x-ray absorption spectroscopy. We further model the different anion displacements and estimate that their combined contribution to the bandgap bowing is close to zero. These findings differ from those for Cu(In,Ga)Se2 and demonstrate the diversity of chalcopyrite alloys and their properties.

Electrochemical Exploration of Layered Cu-Based Oxychalcogenides Obtained Topochemically via Anionic Redox and Displacement Reaction

Li-rich oxides, operating partially on high-voltage anionic redox, permit energy densities larger than electrode materials relying solely on transition-metal redox. On the contrary, Li-rich sulfides present a lower hysteresis but operate at a lower voltage. Oxysulfide electrodes, if adequately designed, can bridge this gap and offer high energy density with a minor penalty of hysteresis or capacity. Tetragonal-layered CuMOX (M = Bi/La, X = S/Se) chalcogenides are explored in this direction, and we found that Cu can be extracted at room temperature both electrochemically and chemically with different outcomes, some of which lead to new layered monoclinic bismuth “oxyperchalcogenide” oxidized phases, BiOX*, with dimerized S/Se. Moreover, we show that these new BiOX* phases can insert lithium to form new tetragonal-layered LiBiOX phases, isostructural to pristine CuBiOX. Both the CuBiOX and BiOX* phases are shown to insert four lithium ions via conversion and intercalation–conversion reactions, respectively. Further, we demonstrate the importance of the host cation on the above insertion reactivity by replacing Bi with La to produce a new layered tetragonal phase Cu1–xLixLaOS. The encountered different reaction pathways are rationalized by theoretical calculations. This work shows that Cu-based layered oxychalcogenides can offer rich hunting grounds for new metastable electrochemically active layered materials.

Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ-Al2O3 Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges

The size-dependent phase stability of γ-Al2O3 was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al2O3 crystal, a bulk transformation to α-Al2O3 by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al2O3 nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites.

Electronic excitation driven structural evolution in Ce0.8Zr0.2O2

The cubic phasic CeO2–ZrO2 system with the composition Ce0.8Zr0.2O2 has been synthesized at room temperature. Highly dense pellets were synthesized by standard solid-state route. Fluorite structured cubic phase was probed by X-ray diffraction along with a qualitative analysis through Rietveld refinement. A ‘pseudocubic’ phase of pristine material was noted experimentally by Raman vibrational modes corresponding to anionic displacement in cubic phase and theoretically explained as the tetragonal anionic arrangement (P42/nmc) at nanoscale embedded in framework of cubic structure (Fm3‾m). Electron microscopy accompanied by density measurement was employed to conclude that the synthesized ceramic has a very compact surface morphology with a density ∼95% of its theoretical value. The as-prepared pellets were irradiated with 100 MeV I7+ ions to mimic energy loss by fission fragment during reactor operation. A comprehensive investigation of structural damage was done through the analysis of diffraction patterns of irradiated samples. We carefully estimated the damaged regions with diameter as ∼3.18 nm and ∼2.54 nm from irradiation induced peak broadening and lattice swelling, respectively. These results are fully capable to conceptualize a model about the damaged cylindrical region along ion trajectory. A central region (∼2.54 nm) consists of extended defects revealed by lattice swelling. Moreover, an annular region (width ∼0.32 nm) consists of vacancy type simple defects accommodated in as-synthesized lattice structure. These results reveal excellent structural properties of CeO2–ZrO2 modelled system comparable to PuO2−ZrO2.

High-Throughput screening quaternary kesterite Cu2ABS4 (A = Divalent metals; B = Tetravalent metals) photovoltaic materials

Quaternary kesterite Cu2ZnSnS4 has attracted more and more attention in the field of thin film solar cells. However, its conversion efficiency is limited by its intrinsic properties, which has promoted researchers to actively seek more efficient categories. In this work, 84 kesterite Cu2ABS4 (A = Divalent metals; B = Tetravalent metals) compounds were compared and screened in detail by First-principles high-throughput calculations. The structural parameters, stability, and band gap of these 84 Cu2ABS4 compounds exhibit periodic changes as the atomic number increases. Among them, Cu2BaPbS4 and Cu2NiZrS4 have the lowest values of anion displacement and tetragonal distortion, resulting in their lowest binding energy and good stability. The atomic interaction in the [Cu2ABS] tetrahedra is the key factor to determine the band gap of kesterite Cu2ABS4 compounds. Based on these calculated results, Cu2NiTiS4, Cu2BaTiS4, Cu2HgTiS4 and Cu2HgZrS4 have been screened as potential efficient photovoltaic materials, due to the appropriate band gap.

Photoinduced evolution of lattice orthorhombicity and conceivably enhanced ferromagnetism in LaMnO3 membranes

Ultrashort laser pulses have been utilized to dynamically drive phase transitions in correlated quantum materials. Of particular interest is whether phases not achievable in thermal equilibrium can be induced in complex oxides with intricately coupled lattice, electron and spin degrees of freedom. Here, we tracked atomic motions in LaMnO3 following photoexcitation with MeV ultrafast electron diffraction (MeV-UED) technique. We found that the light excited state exhibits numerous signatures different from thermal equilibrium ones, including nearly conserved Bragg intensities, strongly suppressed La cation and oxygen anion displacements, and the long-range lattice orthorhombicity evolution. Furthermore, using first-principles calculations, we predict that the ferromagnetic ordering and conductivity are both enhanced upon laser excitation due to the reduction of the lattice orthorhombicity. This work benefits from recent advance in fabrication of membrane films with high epitaxial quality and in MeV-UED with large momentum space access and high temporal resolution.

Structural and electronic properties of Mn substituted ZnIn2Se4 chalcopyrite type semiconductor

The structural and electronic properties of ZnMnIn2Se4 chalcopyrite semiconductors are carried out using the density functional theory (DFT) considering the effect of strong electron correlation (DFT + U study). The structural parameters such as lattice parameters (a and c) and tetragonal distortion (η) for both DFT and DFT + U study are generated from the relaxed structure. The bond lengths are calculated, which show that the system undergoes anion displacement from its ideal zinc blende position. The electronic structure calculation shows that the system is an n-type and direct band gap semiconductor. Considering the on-site Coulomb interaction, the lattice parameters and the band gap increase, whereas, the change in tetragonal distortion is negligible.

Improved electronic structure prediction of chalcopyrite semiconductors from a semilocal density functional based on Pauli kinetic energy enhancement factor

The correct treatment of d electrons is of prime importance in order to predict the electronic properties of the prototype chalcopyrite semiconductors. The effect of d states is linked with the anion displacement parameter u, which in turn influences the bandgap of these systems. Semilocal exchange–correlation functionals which yield good structural properties of semiconductors and insulators often fail to predict reasonable u because of the underestimation of the bandgaps arising from the strong interplay between d electrons. In the present study, we show that the meta-generalized gradient approximation (meta-GGA) obtained from the cuspless hydrogen density (MGGAC) (2019 Phys. Rev. B 100 155140) performs in an improved manner in apprehending the key features of the electronic properties of chalcopyrites, and its bandgaps are comparative to that obtained using state-of-art hybrid methods. Moreover, the present assessment also shows the importance of the Pauli kinetic energy enhancement factor, α = (τ − τ W )/τ unif in describing the d electrons in chalcopyrites. The present study strongly suggests that the MGGAC functional within semilocal approximations can be a better and preferred choice to study the chalcopyrites and other solid-state systems due to its superior performance and significantly low computational cost.

Atomic-scale understanding of enhanced polarization of highly strained nanoscale columnar PbTiO3

Spontaneous polarization in displacive ferroelectric oxides originates from the separation of positive charge centers from negative charge centers, which is induced by the off-center displacements of cations and anions in unit cells. For hugely strained ferroelectric oxides, understanding the correlation between the off-center displacements and the strain level is a prerequisite for understanding the polarization behavior. In the present work, the off-center displacements of atoms in nanometer columnar ${\mathrm{PbTiO}}_{3}$ under a strong tensile strain of 13% along the $c$ -axis direction are quantified by quantitative high-resolution transmission electron microscopy. The measured off-center displacement ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Ti}\ensuremath{-}\mathrm{O}}$ (0.052 nm) is about 60% larger than the value of unstrained bulk ${\mathrm{PbTiO}}_{3}$. The experimental results are confirmed by first-principle calculations, leading to a polarization of $122\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$, providing the basis for understanding the enhanced spontaneous polarization of highly strained displacive ferroelectric oxides.

X-ray diffraction study of the atomic interactions, anharmonic displacements and inner-crystal field in orthorhombic KNbO3

An X-ray diffraction study aimed at establishing the subtle details of the electron density and anharmonicity of the atomic vibrations in a stoichiometric monodomain single crystal of potassium niobate, KNbO3, has been conducted at room temperature (orthorhombic ferroelectric phase Amm2). The cation and anion displacements obtained from the experiment are weakly anharmonic without any manifestation of structural disorder. The chemical bond and interatomic interactions inside and between crystal substructures at the balance of intracrystalline forces are characterized in detail. The role of each of the ions in the formation of the ferroelectric phase was studied and the features of the electron-density deformation in the niobium and oxygen substructures, and the role of each of them in the occurrence of spontaneous polarization are established. The position-space distribution of electrostatic and quantum forces in KNbO3 is restored. It is emphasized that for the completeness of the analysis of the nature of the ferroelectric properties it is necessary to consider both static and kinetic electronic factors, which are of a quantum origin. The experimental results and theoretical estimations by the Kohn–Sham calculation with periodic boundary conditions are in reasonable agreement, thus indicating the physical significance of the findings of this study.

A Metal-Free Molecular Antiferroelectric Material Showing High Phase Transition Temperatures and Large Electrocaloric Effects.

Antiferroelectric (AFE) materials, featuring an antiparallel alignment of electric dipoles in the adjacent sublattices, are keeping a great promise toward solid-state refrigeration applications on account of their electrocaloric (EC) effects. Although extensive studies have been performed on inorganic oxide counterparts (e.g., PbZrO3 and AgNbO3), metal-free molecular AFE alternatives with the above-room-temperature EC activities are quite scarce but urgently demanded in terms of environmental issues. Herein, we present a new metal-free molecular AFE, cyclohexylmethylammonium bromide (CMB), which exhibits the unusual antiferroelectric-ferroelectric-paraelectric phase transitions around 364 and 368 K upon heating. The phase transition temperatures are much higher than the majority of known molecular AFE materials. The practical utilization level of electric polarization (∼6 μC/cm2) is clearly evidenced by the typical double polarization-electric field hysteresis loops. Strikingly, large positive and negative EC responses with the temperature changes (ΔT) of 4.2 and -3 K are achieved under an electric field of 20 kV/cm. The origin of its antiferroelectricity and EC properties is elucidated by the antipolar reorientation of cations along with displacement of bromine anions, being distinct from the known mechanism of inorganic oxides. Such intriguing AFE behaviors, including large polarization and EC effects, reveal great potentials of CMB for the solid-state refrigeration. This study sheds light on further exploration of new AFE candidates toward environmentally friendly solid-state cooling devices.

Structural properties of MeF2 (Me=Ca, Sr, Ba) crystals doped with rare-earth ions RE3+/ RE2+: ab initio route

In this paper, we simulated the structural properties of the cubic impurity centers RE3+/RE2+ (RE = Rare earth) in fluorites crystals MeF2 (Me = Ca, Sr, Ba) by ab initio route. Doping fluorite crystals with the rare-earth ion RE3+/RE2+, this replaces the cation Me2+ and determines distortions of the MeF2 crystal lattice near the impurity center. These distortions characterized by radial and angular displacements of anions were investigated by ab initio method. We have employed the MO LCAO approach, in the frame of DFT method as it is implemented in the CRYSTAL17 software for periodic systems, using the hybrid functional PBE0. Since the main contribution to the crystal field on the rare-earth impurity ions comes from its nearest neighbors, the distance “rare-earth ion – ligand” and the radial and angular coordinates of the RE3+/RE2+ ions at several coordination spheres of anions near the impurity centers are calculated. Also, the changes in the local compressibility of the cationic and anionic sublattice next to the impurity ion are determined. The obtained results were discussed, compared with available experimental data and other theoretical results, and the agreement is quite satisfactory.

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed‐Anion Lanthanide Oxyhalides

AbstractA challenge in anion control in periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post‐synthetic modification routes that place cations or anions in non‐equilibrium configurations are promising; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non‐hydrolytic sol‐gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard–soft acid–base principles using post‐synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes, preserving the tetragonal matlockite structure. Energy‐variant X‐ray excited optical luminesce signatures of alloyed Tb3+‐ions is a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed-Anion Lanthanide Oxyhalides.

A challenge in anion control in periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations are promising; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes, preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+ -ions is a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

Displacements in the Cationic Motif of Nonstoichiometric Fluorite Phases Ba1−xRxF2+x as a Result of the Formation of {Ba8[R6F68–69]} Clusters: III. Defect Cluster Structure of the Nonstoichiometric Phase Ba0.69La0.31F2.31 and Its Dependence on Heat Treatment

The defect structure of Ba0.69La0.31F2.31 single crystals in as-grown state and after annealing at 1173 K for 336 h was studied by X-ray diffraction analysis. Both crystals belong to the CaF2 structure type (sp. gr. Fm3¯m). They have vacancies in the main anion motif and interstitial fluorine anions in Wyckoff positions 48i and 4b. Relaxation (static displacement of some main anions to Wyckoff position 32f) is observed in the annealed crystal. It was established that annealing leads to a change in the type of displacement of the main anions in Wyckoff positions 8c from dynamic to static. Displacement of La3+ cations to Wyckoff position 32f is observed in both crystals. A model of the defect structure of Ba0.69La0.31F2.31 is proposed, according to which interstitial fluorine anions and La3+ cations are aggregated into [Ba14−nLanF64+n] clusters with the cuboctahedral anionic core formed by interstitial fluorine anions in Wyckoff positions 48i. Ba2+ cations are located in the cluster in the centers of the faces, and the La3+ cations are shifted by 0.24 Å from the vertices of the cluster along the three-fold axis towards the center of the cluster. The study establishes the relationship between the defect structure of crystals and their structurally sensitive properties, and to develop approaches to their management.

Theoretical study on the tunable electronic band structure of Cs2PbI2Cl2/CsPbBr3 halide perovskite heterostructure driven by ferroelectric polarization modulation

Ferroelectric polarizationhas been considered to be an key factor to tune the structural and photoelectric properties of perovskites and their heterostructures. While there has been growing researches made in the novel phenomena originating from interface formed between oxide perovskites, the effects of ferroelectric polarization on the electronic properties of halide perovskites and their heterostructures are rarely studied. Herein, by using first-principles calculations, all-inorganic halide perovskite heterostructure composed of 3D perovskite tetragonal CsPbBr3 and 2D Ruddlesden-Popper (RP) perovskite Cs2PbI2Cl2 is constructed for disclosing the relationship between the intrinsic polarization of tetragonal CsPbBr3 and electronic band structure of heterostructure. Cs atoms and Pb atoms of tetragonal CsPbBr3 in heterostructure are artificially moved away from the equivalent centers to simulate increased polarization. Our results show that with the spontaneous polarization of tetragonal CsPbBr3 increasing, the bandgap of heterostructure decreases, and the band alignment switches from staggered type-II to broken-gap type-III. Moreover, large cation–anion displacements along z-direction in tetragonal CsPbBr3 can be observed when tensile strains (≥5%) are applied, indicating a increased ferroelectric polarization, which also facilitates the decreasing of bandgap in heterostructure and the type-II-type-III transition of band alignment. Our study suggests that control over the polarization of ferroelectric materials is of great importance to tune the photoelectric properties of perovskite-based devices.

Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests*.

Cube‐like double four‐ring (d4r) cages are among the most frequent building units of zeolites and zeotypes. In materials synthesised in fluoride‐containing media, the fluoride anions are preferentially incorporated in these cages. In order to study the impact of framework composition and organic structure‐directing agents (OSDAs) on the possible occurrence of local distortions of fluoride‐containing d4r cages, density functional theory (DFT) calculations and DFT‐based molecular dynamics simulations were performed for AST‐type zeotypes, considering four different compositions (SiO2, GeO2, AlPO4, GaPO4) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO2‐AST show significant deformations, with a pyritohedron‐like distortion of the d4r cages occurring in GeO2‐ and GaPO4‐AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO4‐ and GaPO4‐AST. While the distortions occur at random in TMA‐containing zeotypes, they exhibit a preferential orientation in systems that incorporate QNU cations. In addition to providing detailed understanding of the local structure of a complex host‐guest system on the picosecond timescale, this work indicates the possibility to stabilise ordered distortions through a judicious choice of the OSDA, which might enable a tuning of the material's properties.

Structural signatures of the insulator-to-metal transition in BaCo1−xNixS2

The solid solution $\mathrm{Ba}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{S}}_{2}$ exhibits an insulator-to-metal transition close to $x=0.21$. Questions of whether this transition is coupled with structural changes remain open. Here we follow the structural evolution as a function of the Ni content $x$ using synchrotron powder x-ray diffraction and pair distribution function analyses to reveal significant basal sulfide anion displacements occurring preferentially along the ${\mathrm{CoS}}_{5}$ pyramidal edges comprising the edge-connected bond network in $\mathrm{Ba}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{S}}_{2}$. These displacements decrease in magnitude as $x$ increases and are nearly quenched in $x=1\phantom{\rule{4pt}{0ex}}{\mathrm{BaNiS}}_{2}$. Density-functional-theory-based electronic structure calculations on $x=0\phantom{\rule{4pt}{0ex}}{\mathrm{BaCoS}}_{2}$ suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate ${\mathrm{Co}}^{2+}\phantom{\rule{4pt}{0ex}}{d}_{xz}$ and ${d}_{yz}$ orbitals, leading to local structural symmetry breaking in the $xy$-plane of the Co-rich phases. The Jahn-Teller instability is associated with the opening of a band gap that is further strengthened by electronic correlation. The Jahn-Teller effect is reduced upon increased electron filling as $x\ensuremath{\rightarrow}1$, indicating that the local structure and band filling cooperatively result in the observed insulator-to-metal transition.

Nanostructured Crystals of Fluorite Phases Sr1–xRxF2+x (R Are Rare-Earth Elements) and Their Ordering. 16: Defect Structure of the Nonstoichiometric Phases Sr1–xRxF2+x (R = Pr, Tb–Yb) As Grown

The defect structure of as grown Sr0.72Pr0.28F2.28, Sr0.87Tb0.13F2.13, Sr0.88Dy0.12F2.12, Sr0.89Ho0.11F2.11, and Sr0.9R0.1F2.1 single crystals (R = Y, Er, Tm, Yb) prepared under identical conditions has been studied by X-ray diffraction analysis. All crystals belong to the CaF2 structure type (sp. gr. $$Fm\bar {3}m$$ ) and have vacancies in the main anion motif and interstitial fluorine anions at site 32f in the crystal with R = Pr and site 48i for the other R. Interstitial fluorine anions are found near site 4b in the crystals with R = Tb and Dy (sites 32f and 24e, respectively) and at site 4b in the crystal with R = Er. Relaxation (static displacement of some main anions to site 32f) is observed in all crystals (except for R = Tb). Displacements of some Sr2+ cations to sites 32f and 24e is simultaneously observed in crystals with R = Tb, Dy, Tm, and Yb. In crystals with R = Ho, Y, and Er, some Sr2+ cations are displaced only to site 32f; no cation displacement is observed for R = Pr. A model of the defect structure of the phases is proposed based on the structure of crystals studied, according to which interstitial fluorine anions and R3+ cations are aggregated into [Sr4–nRnF26] clusters of tetrahedral configuration for the phase with R = Pr or [Sr14–nRnF64+n] clusters of octacubic configuration for other R.