Local Structural Distortions Research Articles

Phonon signatures for polaron formation in an anharmonic semiconductor

Mechanistic studies on lead halide perovskites (LHPs) in recent years have suggested charge carrier screening as partially responsible for long carrier diffusion lengths and lifetimes that are key to superior optoelectronic properties. These findings have led to the ferroelectric large polaron proposal, which attributes efficient charge carrier screening to the extended ordering of dipoles from symmetry-breaking unit cells that undergo local structural distortion and break inversion symmetry. It remains an open question whether this proposal applies in general to semiconductors with LHP-like anharmonic and dynamically disordered phonons. Here, we study electron-phonon coupling in Bi2O2Se, a semiconductor which bears resemblance to LHPs in ionic bonding, spin-orbit coupling, band transport with long carrier diffusion lengths and lifetimes, and phonon disorder as revealed by temperature-dependent Raman spectroscopy. Using coherent phonon spectroscopy, we show the strong coupling of an anharmonic phonon mode at 1.50 THz to photo-excited charge carriers, while the Raman excitation of this mode is symmetry-forbidden in the ground-state. Density functional theory calculations show that this mode, originating from the A1g phonon of out-of-plane Bi/Se motion, gains oscillator strength from symmetry-lowering in polaron formation. Specifically, lattice distortion upon ultrafast charge localization results in extended ordering of symmetry-breaking unit cells and a planar polaron wavefunction, namely a two-dimensional polaron in a three-dimensional lattice. This study provides experimental and theoretical insights into charge interaction with anharmonic phonons in Bi2O2Se and suggests ferroelectric polaron formation may be a general principle for efficient charge carrier screening and for defect-tolerant semiconductors.

Magnetocaloric effect in Y-doped La0.6Ca0.4MnO3 enhanced by Griffiths phase and re-entrance of first-order phase transition

It has been known that bulk La0.6Ca0.4MnO3 is an intermediate material of the first- and second-order characters with the tricritical-point exponents, and the doping of a metal ion in it usually causes a continuous second-order transition. The present work reports the re-entrance of a discontinuous first-order transition in orthorhombic La0.6-xYxCa0.4MnO3 (x = 0.03–0.09) compounds. This enhances the magnetocaloric effect. For the field H = 30 kOe, the maximum magnetic-entropy change (|ΔSmax|) and relative cooling power (RCP) have been evaluated being about 5.45–6.3 J/kg·K and 130–185 J/kg, respectively. If combining these compounds as refrigerant blocks in a rotary ring model, a magnetic cooling device can operate at temperatures T = 85–280 K, with |ΔSmax| ≈ 5.5 J/kg⋅K and RCP ≈ 1073 J/kg. Aside from the re-entranced first-order phase transition, the magnetization and structural analyses have proved the enhanced magnetocaloric effect in La0.6-xYxCa0.4MnO3 related to a Griffiths singularity, and local Jahn-Teller distortions of the perovskite structure (since the Mn3+/Mn4+ ratio and orthorhombic structural phase are unchanged vs. x).

Metal Site Substitution and Role of the Dimer on Symmetry Breaking in FePS3 and CrPS4 under Pressure

We combine infrared absorption, Raman scattering, and diamond anvil cell techniques to explore the properties of FePS3 and CrPS4 under pressure, comparing our findings with a symmetry analysis, lattice dynamics calculations, and an examination of the energy landscape. Although these complex chalcogenides are considered to be members of the same family of materials, they display remarkably different phase progressions on account of the metal center orbital filling, character of the P–P linkage, layer corrugation, and differing size of the van der Waals gap. We discuss the space group progressions, structure–property relations, and development of pressure-induced metallicity in terms of the competition between local and long-range symmetry transformations and structural distortion pathways. These findings place the properties of FePS3 and CrPS4 on a firm foundation for work under strain control and in the single layer limit.

Unraveling the doping preference of zinc ions in sulfoaluminate cement clinker minerals: A study combining experiments and DFT calculations

Combining X-ray diffraction, elemental distribution spectroscopy, and density functional theory simulations, we systematically studied the doping preference of zinc ions in sulfoaluminate cement (SAC) clinker. Our results show that Zn atoms prefer to enter into C4AF while hardly into the other two minerals, C4A3S‾ and C2S. Quantitative analyses of the doping behaviours of Zn in C4AF were also performed with Rietveld refinements. The Zn atoms as solid solute tend to occupy the sites of Fe, which is well supported by the low formation energy, small local structural distortions, and bond order-bond length distribution overlaps between the host and the guest ions. The difference in electron contributions of spatial distributions between Zn–O and Fe–O may be the main reason determining the solid solution of Zn. The deep insights on these fundamental issues should provide an efficient way to improve the solid solution of Zn by increasing the C4AF content, thus should be helpful in guiding the synthesis of SAC clinker by utilizing Zn-bearing solid waste.

Impact of Overlithiation and Al doping on the battery performance of Li-rich layered oxide materials

Lithium rich layered oxides (LRLOs) are one of the best alternatives for the next generation positive electrodes materials for Li-ion batteries. However, LRLOs suffer a remarkable voltage decay upon cycling that prevents stable and prolonged electrochemical performances and contains large quantities of cobalt in the transition metal blend. Here, we demonstrate the performance in batteries of a series of innovative materials with general formula Li1.2+xMn0.54Ni0.13Co0.13-x-yAlyO2 (where 0.03≤x ≤ 0.08 and 0.03≤y ≤ 0.05) capable to supply large reversible specific capacities (around 200 mAh g−1), stable cycling performance with reduced voltage decay. This communication sheds light on the interplay between structural and electronic disorder induced by Al/Li co-doping in LRLO and the corresponding functional properties in batteries. The impact of substitution of cobalt by Li and Al co-doping on the structural and morphological properties has been examined by Synchrotron X-ray diffraction (XRD), microwave plasma atomic emission spectrometer (MP-AES) and scanning electron microscopy (SEM). Furthermore, to better understand the structure-function interplay of these over-lithiated materials, ex situ analyses are here reported coupling Synchrotron X-Ray diffraction and Raman Spectroscopy. Based on the obtained results, we proved that coupling the over-lithiation with the aluminum doping, is an effective strategy to reduce the cobalt content into the LRLOs structure maintaining high electrochemical performance. Beside the values of specific capacities obtained in lithium cells, these materials exhibit excellent capacity retention and voltage retention, in particular for the material with the smallest content of cobalt, i.e. Li1.28Mn0.54Ni0.13Co0.02Al0.03O2. Furthermore, thanks to the ex-situ analysis of the latter, we explained the structural evolution of the sample upon cycling that showed the formation of a secondary trigonal phase and the occurrence of limited local distortions of layered structure.

Local structure distortion enhanced the emission intensity of KBaYSi2O7:Eu2+ phosphor for full-visible-spectrum LED lighting

Due to Eu2+ and Eu3+ ions often co-exist in the crystal lattices, the luminous intensity of some phosphors is limited greatly. In this work, we propose a crystal-site engineering approach to reduce Eu3+ to Eu2+ in the KBaYSi2O7 lattice through substitution of Ba2+ by Ca2+ or Sr2+. The mechanism between the local environmental regulation of the crystal structure around Eu2+ as well as the luminous intensity and thermal stability of the phosphor was systematically explored. The results of XRD, Rietveld refinement and EDX confirmed that Ca2+ and Sr2+ are dissolved into the crystal lattice effectively without changing the main phase structure. As a result, the luminous intensity of samples doped Ca2+ and Sr2+ increased by 39.7% and 41.2%, respectively. The mechanisms for such improvement is ascribed to the reduction of Eu3+ into Eu2+ under the lager volume of [EuO9]2 polyhedron. The treated samples exhibited relatively superior thermal stability with I150°C > 70%, indicating that doping with Ca2+ or Sr2+ have no effect on the thermal stability. A pc-WLED device fabricated by combining (Ba,Sr)2SiO4:Eu2+, (Ca,Sr)AlSiN3:Eu2+, BaSi2O2N2:Eu2+ and KBa0.86Ca0.1YSi2O7:Eu2+ with 380 nm chip exhibits CCT (5010 K) and high CRI (96.6). These results indicate that this phosphor has potential for applications in full-spectrum warm white lighting.

Understanding the surface segregation behavior of bimetallic CoCu toward HMF oxidation reaction

Surface segregation is ubiquitous in multi-component materials and is of great important for catalysis but little is known on the surface structure under graphene encapsulation. Here, we show that the graphene encapsulated CoCu performs well for the electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) with the onset potential before 1.23 VRHE and a nearly 100% selectivity of FDCA under 1.4 VRHE. From the experimental results, the unprecedented catalytic performance was attributed to local structural distortion and sub-nanometer lattice composition of the CoCu surface. We accurately show the dispersed Cu doped Co3O4 nano-islands with a lot of edge sites on the bimetallic Co-Cu surface. While, the gradient components effectively facilitate the establishment of built-in electric field and accelerate the charge transfer. Theoretical and experimental results reveal that the surface Co and neighbouring Cu atoms in sub-nanometer lattice synergistically promote the catalysis of HMF. This work offers new insights into surface segregation in tuning the element spatial distribution for catalysis.

Optoelectronic Properties of Mixed Iodide-Bromide Perovskites from First-Principles Computational Modeling and Experiment.

Halogen mixing in lead-halide perovskites is an effective route for tuning the band gap in light emission and multijunction solar cell applications. Here we report the effect of halogen mixing on the optoelectronic properties of lead-halide perovskites from theory and experiment. We applied the virtual crystal approximation within density functional theory, the GW approximation, and the Bethe–Salpeter equation to calculate structural, vibrational, and optoelectronic properties for a series of mixed halide perovskites. We separately perform spectroscopic measurements of these properties and analyze the impact of halogen mixing on quasiparticle band gaps, effective masses, absorption coefficients, charge-carrier mobilities, and exciton binding energies. Our joint theoretical–experimental study demonstrates that iodide–bromide mixed-halide perovskites can be modeled as homovalent alloys, and local structural distortions do not play a significant role for the properties of these mixed species. Our study outlines a general theoretical–experimental framework for future investigations of novel chemically mixed systems.

Magnetism and the Trimeron Bond.

A review of progress in understanding the Verwey transition in magnetite (Fe3O4) over the past decade is presented. This electronic and structural transition at TV ≈ 125 K was reported in 1939 and has since been a contentious issue in magnetism. Long range Fe2+/Fe3+ charge ordering has been confirmed below the transition from crystal structure refinement, and Fe2+ orbital ordering and formation of trimerons through weak bonding of Fe2+ states to two Fe neighbors has been discovered. This model has accounted for many spectroscopic observations such as the 57Fe NMR frequencies. The trimeron lifetime has been measured, and trimeron soft modes have been observed. The origin of the first to second order crossover of Verwey transitions in doped magnetites has been revealed by a nanoparticle study. Electronic and structural fluctuations are found to persist to temperatures far above TV and local structural distortions track the bulk magnetization, disappearing at the 850 K Curie transition. New binary mixed-valent iron oxides discovered at high pressure are found to have electronic transitions and orbital molecule ground states similar to those of magnetite.

Modeling polarons in density functional theory: lessons learned from TiO2

Density functional theory (DFT) is nowadays one of the most broadly used and successful techniques to study the properties of polarons and their effects in materials. Here, we systematically analyze the aspects of the theoretical calculations that are crucial to obtain reliable predictions in agreement with the experimental observations. We focus on rutile TiO2, a prototypical polaronic compound, and compare the formation of polarons on the (110) surface and subsurface atomic layers. As expected, the parameter U used to correct the electronic correlation in the DFT + U formalism affects the resulting charge localization, local structural distortions and electronic properties of polarons. Moreover, the polaron localization can be driven to different sites by strain: due to different local environments, surface and subsurface polarons show different responses to the applied strain, with impact on the relative energy stability. An accurate description of the properties of polarons is key to understand their impact on complex phenomena and applications: as an example, we show the effects of lattice strain on the interaction between polarons and CO adsorbates.

Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet

With the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba3CoSb2O9, a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba2.87Sr0.13CoSb2O9 with Sr doping on non-magnetic Ba2+ ion sites. The results show that Ba2.87Sr0.13CoSb2O9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120 degree spin structure at zero field with reduced ordered moment as 1.24 μ B/Co; (iii) a series of spin state transitions for both H∥ab-plane and H∥c-axis. For H∥ab-plane, the magnetization plateau feature related to the up–up–down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba3CoSb2O9, which demonstrates that the non-magnetic ion site disorder (the Sr doping) plays a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.

Influence of local structural distortion on the magnetism of Na2IrO3 compounds

First-principles calculations are conducted to investigate the magnetic properties of the ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ compounds. We reveal that the ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$'s local structural distortions are essential for an accurate description of the magnetism of such systems. They provide a feasible explanation for the experimentally observed antiferromagnetic zigzag magnetic ground state. We demonstrate that the underlying competition between the spin-orbit coupling and the crystal-field splitting rules the crystal structure and profoundly influences the strength of the magnetic exchange interactions. We unambiguously identify that the ${D}_{4h}$-type distortions, due to the Jahn-Teller effect, are disclosed in an elongation along the ${\mathrm{IrO}}_{6}$ polyhedra's apical axis in the $ac$ plane. On the other hand, the Ir atoms off-centering in the basal plane (perpendicular to the local ${D}_{4h}$ elongation axis) arises owing to an inhomogeneous cationic charge distribution of ${\mathrm{Ir}}^{4+}/{\mathrm{Na}}^{+}$ in the transition-metal layer resulting in both effects being decisive in controlling the magnetism of ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ together with the spin-orbit interactions.

High Piezoelectric Performance in Pb(Ni1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3 Ternary System Featuring Small Structural Distortion and Heterogeneous Domain Configuration.

Ternary/polynary perovskite solid solutions based on binary systems are well-known for their high piezoelectric performance. In this work, a series of Pb(Ni1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3 compositions with the particularly high piezoelectric coefficient of d33* > 1000 pm/V and d33 > 700 pC/N have been developed. The optimal performance was achieved in the 0.52PNN-0.14PSN-0.34PT composition (d33* = 1120 pm/V, d33 = 804 pC/N, and Tm = 109 °C). The high piezoelectric performance of this system is reported and is superior to those of most lead-based ternary/polynary ceramics. By a combination of in situ high-energy synchrotron diffraction with transmission electron microscopy (TEM), the origin of the high piezoelectric response has been unambiguously revealed. Upon application of an external electric field, synchrotron diffraction profiles show no splitting but prominent shifting, indicating that the large intrinsic lattice strain arising from the reduced crystal anisotropy and facilitated polarization variation is associated with the high piezoelectric response. Furthermore, microscopic studies by TEM highlight a heterogeneous ferroelectric domain configuration generated by a small local structural distortion, which is also beneficial for the high piezoelectric performance in the proposed ternary piezoelectric systems. The design process of ternary perovskite solid solutions with a wide morphotropic phase boundary region and small structural distortion may be enlightening for the exploration of other high-performance polynary piezoelectrics.

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids.

Understanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat ) in crystalline materials. Herein, we demonstrate local cationic off-centring within the global rock-salt structure of AgSbSe2 by using synchrotron X-ray pair distribution function analysis and unravel the origin of its ultralow κlat ≈0.4 W mK-1 at 300 K. The cations are locally off-centered along the crystallographic direction by about ≈0.2 Å, which averages out as the rock-salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off-centering distortions within an anharmonic double-well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next-generation crystalline materials with tailored thermal properties.

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids

AbstractUnderstanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat) in crystalline materials. Herein, we demonstrate local cationic off‐centring within the global rock‐salt structure of AgSbSe2 by using synchrotron X‐ray pair distribution function analysis and unravel the origin of its ultralow κlat≈0.4 W mK−1 at 300 K. The cations are locally off‐centered along the crystallographic direction by about ≈0.2 Å, which averages out as the rock‐salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off‐centering distortions within an anharmonic double‐well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next‐generation crystalline materials with tailored thermal properties.

Fourier ptychographic dark field x-ray microscopy.

Dark-field x-ray microscopy (DFXM) is an x-ray imaging technique for mapping three-dimensional (3D) lattice strain and rotation in bulk crystalline materials. At present, these maps of local structural distortions are derived from the raw intensity images using an incoherent analysis framework. In this work, we describe a coherent, Fourier ptychographic approach that requires little change in terms of instrumentation and acquisition strategy, and may be implemented on existing DFXM instruments. We demonstrate the method experimentally and are able to achieve quantitative phase reconstructions of thin film samples and maps of the aberrations in the objective lens. The method holds particular promise for the characterization of crystalline materials containing weak structural contrast.

Correlation between phase transition characteristics and hydrogen irradiation-induced Frenkel defect formations in FeRh films

FeRh films exhibit an extraordinary magnetic phase transition accompanied by structural and electrical variations. This study intentionally modified the local atomic environment using hydrogen irradiation to elucidate the transition characteristics. It was observed that with the increase in hydrogen irradiation, the initial residual-ferromagnetic state in the antiferromagnetic region (i.e., below the magnetic phase transition temperature) increased. In addition, the transition temperature decreased. Structural and chemical analysis revealed that the increased hydrogen irradiation enhanced local structural distortion and Fe-Frenkel defect formation. Furthermore, density functional theory calculations confirmed that the Fe-Frenkel defect affected the breakdown of hybridization between Fe and Rh. Therefore, it caused the increased residual ferromagnetic state below the transition temperature and decreased the transition temperature.

Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite

Pair-distribution-function analysis of X-ray total-scattering data for CuFe1−xGexS2 reveals a local structural distortion induced by the lone-pair of Ge2+. The resulting strain reduces thermal conductivity and improves thermoelectric performance.

Enhancing electrochemical performance by triggering a local structure distortion in lithium vanadium phosphate cathode for Li ion batteries

Local structural distortion caused by Ti-doping has the effect of expanding the diffusion path of Li ion.

Structure Preservation and Seam Optimization for Parallax-Tolerant Image Stitching

Parallax processing has long been a significant and challenging task in image stitching. In this paper, we study a new hybrid warping model based on multi-homography and structure preservation to achieve accurate alignment of regions at different depths while preserving local and global image structures. The homographies of different depth regions are estimated by dividing matching feature pairs into multiple layers. Then, layered warping is performed by determining the spatial relationships between the image mesh and these multi-homography, and then refining the local and global structural distortions through mesh optimization. Four constraints are considered during the local optimization process, including the local alignment error, global alignment error, and similarity error. In addition, we explore and introduce collinear structures into an objective function as a constraint for mesh optimization warping, which can preserve salient line structures while alleviating distortions in nonoverlapping areas. Furthermore, we develop an optimal seam search method based on seam error evaluation to improve the quality of the seams. Experimental results demonstrate that compared to existing methods, the proposed algorithm presents more accurate stitching results for images with large parallax and preserves salient image structures, and outperforming the existing methods both qualitatively and quantitatively.