Weak Distortion Research Articles

Visualizing highly bright and uniform cellular ultrastructure by expansion-microscopy with tetrahedral DNA nanostructures

Expansion Microscopy (ExM) is a widely used super-resolution technique that enables imaging of structures beyond the diffraction limit of light. However, ExM suffers from weak labeling signals and expansion distortions, limiting its applicability. Here, we present an innovative approach called Tetrahedral DNA nanostructure Expansion Microscopy (TDN-ExM), addressing these limitations by using tetrahedral DNA nanostructures (TDNs) for fluorescence labeling. Our approach demonstrates a 3- to 10-fold signal amplification due to the multivertex nature of TDNs, allowing the modification of multiple dyes. Previous studies have confirmed minimal distortion on a large scale, and our strategy can reduce the distortion at the ultrastructural level in samples because it does not rely on anchoring agents and is not affected by digestion. This results in a brighter fluorescence, better uniformity, and compatibility with different labeling strategies and optical super-resolution technologies. We validated the utility of TDN-ExM by imaging various biological structures with improved resolutions and signal-to-noise ratios.

Effects of weak intramolecular interactions and distortions from trigonal prismatic coordination on the magnetic properties of zero-field Co(ii) single-ion magnets

The change of counter anions significantly affects the environment around Co(ii) ions, leading to two zero-field single-ion magnets possessing different uniaxial anisotropies and dynamic magnetic properties.

Inducing weak and negative Jahn-Teller distortions to alleviate structural deformations for stable sodium storage.

In the quest for efficient supercapacitor materials, manganese-based layered oxide cathodes stand out for their cost-effectiveness and high theoretical capacity. However, their progress is hindered by the Jahn-Teller (J-T) distortion due to the unavoidable Mn4+ to Mn3+ reduction during ion storage processes. Our study addresses this challenge by stabilizing the K0.5MnO2 cathode through strategic Mg2+ substitution. This substitution leads to an altered Mn3+ electronic configuration, effectively mitigating the strong J-T distortion during ion storage processes. We provide a comprehensive analysis combining experimental evidence and theoretical insights, highlighting the emergence of the weak and negative J-T effects with reduced structural deformation during electrochemical cycling. Our findings reveal that the K0.5Mn0.85Mg0.15O2 cathode exhibits remarkable durability, retaining 96.0% of initial capacitance after 8000 cycles. This improvement is attributed to the specific electronic configurations of Mn3+ ions, which play a crucial role in minimizing volumetric changes and counteracting structural deformation typically induced by the strong J-T distortion. Our study not only advances the understanding of managing J-T distortion in manganese-based cathodes but also opens new avenues for designing high-stability supercapacitors and other energy storage devices by tailoring electrode materials based on their electronic configurations.

The Plumb-Line Matching Algorithm for UAV Oblique Photographic Photos

Building facades has always been a challenge for feature matching in oblique photogrammetry due to weak textures, non-Lambertian objects, severe occlusion, and distortion. Plumb lines are essential building geometry structural feature lines in building facades, which show strong spatial relevance to these problems. Achieving plumb line matching has great application potential for optimizing the process and products of oblique photogrammetry. Thus, we proposed a novel matching algorithm for plumb lines based on spatial and color hybrid constraints according to its central projection imaging characteristics. Firstly, based on vanishing point theory, the plumb lines from photos were back-calculated to determine the matching target set; secondly, the property of its large elevation ranges was exploited to calculate the homonymous points as spatial constraints by projecting plumb lines onto the stratified spatial planes; thirdly, the neighboring primary colors on both sides of the plumb lines were extracted as feature descriptors and compared by colorimetry; then, the greedy strategy was employed to successively filter out the locally optimal solutions satisfying the spatial and color hybrid constraints to complete the initial matching; finally, the intersection-over-union analysis of the solution plane and the verticalness evaluation of the matching results were implemented to eliminate errors. The results show that the proposed algorithm can achieve an average accuracy of 97.29% and 78.41% in the forward and lateral overlap experiments from multi-scenes, respectively, displaying a strong adaptability to poor texture, inconsistency, and distortion. In conclusion, thanks to the plumb-line-oriented matching strategy, this algorithm owns inherent advantages in theory and computational complexity. It is suitable for all building-oriented oblique photogrammetry tasks and is highly worthy of promotion and application.

Interplay of electronic structure and magnetism in Fe2 - and Rh2 -based Heusler alloys

In this work, we present a comparative study of the structural, elastic, electronic, magnetic, and transport properties of Heusler alloys ${\mathrm{Fe}}_{2}\mathrm{Rh}Z$ and ${\mathrm{Rh}}_{2}\mathrm{Fe}Z$ with $Z=\mathrm{Al}$, Si, Ga, Ge, In, Sn using the density functional theory. The strongly constrained and appropriately normed functional is considered for the exchange-correlation functional. The newly Heusler T-type pseudocubic structures, ${\mathrm{T}}^{p}$ and ${\mathrm{T}}^{c}$, with alternating layers and columns of Fe and Rh along the [001] direction being $\ensuremath{\approx}26$ meV/atom lower in energy than the inverse XA structure are predicted as ground states for the ${\mathrm{Fe}}_{2}\mathrm{Rh}Z$ family. According to the convex hull analysis, all the considered alloys are thermodynamically and mechanically stable or meta-stable. Hybridization schemes and their role in the formation of half-metallicity depending on the weak tetragonal distortion are discussed. XA-${\mathrm{Fe}}_{2}\mathrm{RhSi}$ is only predicted to be half-metallic in the minority channel with $100%$ spin polarization and integer magnetic moment of $5{\ensuremath{\mu}}_{B}$, while the ground state T structure with layered ordering of Fe and Rh is characterized by the $73%$ polarization due to a pseudogap around Fermi level. Additionally, a large Seebeck coefficient and figure of merit for the spin-down channel of XA-${\mathrm{Fe}}_{2}\mathrm{RhSi}$ is predicted.

Fabrication of outstanding mechanical performance engineered poly (lactic acid)/thermoplastic poly(ester)urethane in-situ nanofiber composites with a large-scale industrial innovation methodology

As one of the most promising biodegradable materials, poly(lactic acid) (PLA) is seriously restricted by its notorious brittleness and weak heat distortion resistance. Hitherto, the possibility of effectively toughening PLA while simultaneously maintaining high strength and stiffness has hardly been realized on an industrial scale. In this work, an innovative industrial methodology was applied to manufacture high-performance engineered (95-x) poly(l-lactic acid)/5poly(d-lactic acid)/xthermoplastic poly(ester)urethane ((95-x)L/5D/xT) nanofiber composites. The in-situ formed TPU nanofibers (TNFs) combined with sterocomplex crystals (SCs) generating synergistic effects significantly improve the crystallization behavior and elasticity of melt. The compatibility between PLA matrix and TNFs in (95-x)L/5D/xT nanofiber composites was significantly reinforced by constructing bundle structures. The bundle structure is composed of rigid PLA hybrid crystals tightly bundled with a handle of oriented TNFs with good interfacial compatibility, bundle structures interlink with each other forming 3D strengthening-toughening bundle structures contributing to the mechanical performance. The notched Izod impact strength of 80L/5D/15T and 75L/5D/20T nanofiber composites prodigiously increase to 74.8 and 98.7 kJ/m2, which is 27.7 and 35.6 times higher than that of neat PLLA. Meanwhile, the nanofiber composites can maintain balanced tensile yield strength, good Young's modulus, and obtain advantaged heat distortion resistance of 1132 MPa at 100 °C. Compared with reported PLA/elastomer blends, neat biodegradable plastics, general-purpose and engineering plastics, the efficient industrial-scale manufactured 80L/5D/15T and 75L/5D/20T nanofiber composites possessing outstanding mechanical performance of super toughness, balanced strength, good Young’s modulus and advantaged heat distortion resistance. It has immense potential to substitute non-degradable petroleum-based engineering plastics as structural materials in harsh environments, thereby reducing CO2 emissions and reducing pollution caused by discarded non-degradable petroleum-based plastics.

Time-Delay Estimation of Microseismic DAS Data Using Band-Limited Phase-Only Correlation

Time-delay estimation is a critical step in many geophysical applications. Conventional approaches are mainly based on the cross correlation of waveforms in time domain but show strong distortions with multiple oscillations and side lobes. We propose a band-limited phase-only correlation (POC) algorithm for time-delay estimation. The algorithm involves the following key steps: 1) transforming 1-D waveform into 2-D time–frequency spectra using S-transform; 2) calculating the band-limited POC function of the transformed spectra; and 3) measuring time delays by analyzing POC coefficient. We demonstrate the effectiveness of the proposed algorithm using synthetic and real microseismic fiber-optic distributed acoustic sensing (DAS) datasets. Results show that POC can effectively and accurately estimate the time delays of waveforms. Compared with the performance of the conventional cross correlation method in time domain, the proposed method has three main advantages: 1) better identification of event and noise waveforms; 2) lower uncertainty of narrow correlation peaks; and 3) weaker distortions with small oscillations and side lobes.

Localization of nuclear wave functions of lithium in [Li+@C60]PF6-: molecular insights into two-site disorder-order transition.

Lithium endohedral fullerene, Li+@C60, is a porous system ideal for studying the quantized translational motion of the Li+ nucleus under subnanoscale confinement. The quantized nuclear motion strongly depends on the anharmonic and polarizable adsorbent potential within the C60 cage, which can be perturbed by cage distortion and/or exterior ions. In our recent paper, H. Ando and Y. Nakao, Phys. Chem. Chem. Phys., 2021, 23, 9785-9803, we focused on a [Li+@C60]PF6- salt and theoretically investigated how the Li+ ion in each C60 cage is simultaneously localized at two equivalent disordered sites in 24 K < T ≪ 100 K. At 24 K, the salt exhibits a disorder-order transition, whereby every Li+ ion becomes mostly localized at one of the two disordered sites below that temperature. Herein we discuss the origin of this transition with special attention to the local structural distortion and intermolecular interactions. Using the Fourier grid Hamiltonian method and a model function that fits a post-Hartree-Fock potential energy surface, we obtained hundreds of low-energy nuclear wave functions of Li+ confined within the cage. The weak distortions of the C60 cage and the PF6- coordination sphere below 24 K and concurrent inversion-symmetry breaking affect intermolecular interactions, thus making the wave functions of the nuclear ground state and several low-energy excited states localized around the experimental high-occupancy disordered site. This demonstrates that the distortions correlate closely with the two-site disorder-order transition. Finally, we reveal that two absorption peaks in the terahertz frequency range show substantial blueshifts upon cooling below 24 K, which serve as fingerprints of the transition.

Organosulfonate‐Modulated Spin‐Crossover Behavior in Three Halogen‐Functionalized Cobalt(II) Complexes

AbstractWe present here the syntheses, crystal structures, and spin crossover (SCO) properties of a series of halogen‐functionalized cobalt(II) complexes, [Co(Brphtpy)2](OTf)2 ⋅ DMF ⋅ 2H2O (1), [Co(Brphtpy)2](HBS)2 ⋅ H2O (2), [Co(Brphtpy)2](MQ)2 ⋅ 2MeCN ⋅ 3H2O (3) (Brphtpy=4′–(4‐Bromophenyl)–2,2′:6′,2′′‐terpyridine; OTf−=trifluoromethanesulfonate; HBS−=hydroxybenzenesulfonate dihydrate; MQ−=methyl orange). Variable‐temperature single‐crystal X‐ray analyses revealed mononuclear compounds of 1–3 consisted of [Co(Brphtpy)2]2+ SCO active units and organosulfonate anions and no structural phase transformation happened in measured high‐low temperature. The packing structures of these complexes were tuned by varying organosulfonates. However, no notable supramolecular interactions can be found, in turn leading to gradual, incomplete, and non‐hysteretic SCO behaviors. Interestingly, the SCO behaviors of these three complexes were significantly modified after the removal of lattice solvents. Combined structural and magnetic investigations revealed the non‐cooperative supramolecular packing structures, guest internal pressure, and the small structural distortions of the SCO units should be responsible for the worse SCO properties of 1–3. The foregoing results show that to achieve high‐performance Co2+ SCO, both the weak interactions, internal pressure, and structural distortion should be considered during the design and construction of SCO complexes.

Magnetic and crystal structure of the antiferromagnetic skyrmion candidate GdSb0.71Te1.22

GdSb0.46Te1.48, a nonsymmorphic Dirac semimetal with Dirac nodes at the Fermi level, has a rich magnetic phase diagram with one of the phases predicted to be an antiferromagnetic skyrmion state. In the current work, we investigate GdSb0.71Te1.22 through bulk magnetization measurements, single-crystal, and powder synchrotron X-ray diffraction, as well as single-crystal hot-neutron diffraction. We resolve a weak orthorhombic distortion with respect to the tetragonal structure and charge density wave (CDW) satellites due to incommensurate modulations of the crystal structure. At 2 K the magnetic structure is modulated with two propagation vectors, kI = (0.45 0 0.45) and kII = (0.4 0 0), with all their arms visible. While kI persists up to the transition to the paramagnetic state at TN = 11.9 K, kII disappears above an intermediate magnetic transition at T1 = 5 K. Whereas magnetic field applied along the c-axis has only a weak effect on the intensity of antiferromagnetic reflections, it is effective in inducing an additional ferromagnetic component on Gd atoms. We refine possible magnetic structures of GdSb0.71Te1.22 and discuss the possibility of hosting magnetic textures with non-trivial 3D+ 2 topologies in the GdSb1−xTe1+x series.

Constructing synergistically strengthening-toughening 3D network bundle structures by stereocomplex crystals for manufacturing high-performance thermoplastic polyurethane nanofibers reinforced poly(lactic acid) composites

As one of the most promising biodegradable materials, poly(lactic acid) (PLA) is seriously restricted by its notorious inherent brittleness and weak heat distortion resistance. In this work, a novel and simple methodology is proposed using an eccentric rotor extruder (ERE) with a predominantly elongational flow field to manufacture high-performance engineered poly(L-lactic acid)/poly(D-lactic acid)/thermoplastic polyurethane nanofiber composites on an industrial scale. The oriented tough TPU nanofibers (TNFs), rigid integrated PLA hybrid crystals and their good interface compatibility produce a strengthening-toughening bundle structure. When the PDLA content reached 3 wt%, the bundle structures interlinked with each other to form a strengthening-toughening 3D network. This resulted in a high-performance engineered 80L/5D/15T nanofiber composite with super toughness of 74.1 kJ/m2, high strength of 47.3 MPa and superior Young's modulus of 1378 MPa, which are 28.6 times, 85% and 92.7%, respectively, compared with neat PLLA. Compared with those published literatures, the 80L/5D/15T nanofiber composite not only possesses super toughness (more than 28 times) but also maintains good strength (maintain 85%), additionally good heat distortion resistance was also obtained. The performance of 80L/5D/15T nanofiber composite was significantly superior in comparison with biodegradable plastics as well as petroleum-based non-degradable general-purpose plastics and engineering plastics, demonstrating its enormous potential as a substitute structural material in harsh environments. It is believed that the novel and industrial scale methodology opens new perspectives for manufacturing other high-performance engineered biodegradable materials.

On the Paramagnetic-Like Susceptibility Peaks at Zero Magnetic Field in hbox{WSe}_{2-x}hbox{Te}_{x} Single Crystals

A weakly temperature-dependent paramagnetic-like susceptibility peak at zero magnetic field is observed in hbox{WSe}_{2-x}hbox{Te}_{x} with only marginal amount of ferromagnetic impurities. The ferromagnetic hysteresis loop and the magnetic moment splitting between zero-field-cooled and field-cooled processes indicate ferromagnetism in the samples. The paramagnetic-like susceptibility peak height is proportional to the remanent magnetic moment of hysteresis loops. High-resolution transmission electron microscope image supports that the observed ferromagnetic feature originates from lattice distortion. These results imply that the weakly temperature-dependent paramagnetic-like susceptibility peak originates from weak lattice distortion and/or superparamagnetism.

Local Visual and Global Deep Features Based Blind Stitched Panoramic Image Quality Evaluation Using Ensemble Learning

Image stitching technology aims to generate a Stitched Panoramic Image (SPI) by stitching multiple narrow-view images containing over-lapping regions. However, heterogeneous artifacts may be introduced during the process of image stitching, which affects the visual perception. To automatically and accurately evaluate the perceptual quality of SPIs, a novel local visual and global deep features based blind SPI quality evaluation method is proposed in this paper. To be specific, with the consideration that image stitching mainly destroys structure, texture and color information, we first design a color structure-texture joint dictionary trained from the constructed stitched-specific image patches dataset. Given an input SPI, its local visual and global deep features are extracted to characterize the stitched-specific distortions. For local visual features, the trained dictionary is employed to capture structure, texture and color distortions by sparse features extraction. Then, considering that sparse features are insensitive to weak structural distortions, weighted local binary pattern features are extracted to measure various weak distortions. For global perceptual features, deep features are extracted via a pre-trained convolutional neural network model to represent the high-level semantics. Finally, considering the diversity of the extracted features, an ensemble learning strategy is adopted to promote the generalization performance and prediction accuracy of the proposed model. Experimental results show that, compared with the conventional 2D and SPI quality measurement methods, the proposed method can measure the stitched-specific distortions more accurately, and is more coincident with subjective ratings.

New highly precise weak gravitational lensing flexions measurement method based on ERA method

ABSTRACT Weak gravitational lensing flexions are a kind of weak lensing distortion that are defined as the spin 1 and spin 3 combinations of the third order derivatives of gravitational lensing potential. Since the shear has spin 2 combination of the second-order derivative, the flexion signal gives partly independent information from shear signal and is more sensitive to the local mass distribution than shear signal. Thus its measurement is expected to play important roles in observational cosmology. However, since the weakness of the flexion signal, as well as the complicatedness of its intrinsic noise, made its accurate observation very difficult. We propose a new method of measuring the flexion signal using ERA method which is a method to measure weak lensing shear without any approximation. We find two particular combinations of the flexions which provide the quantities with only lensing information and free of intrinsic noise when taken average. It is confirmed by simple numerical simulation that the statistical average of these combinations do not in fact depend on the strength of the intrinsic distortion. Then, we introduce a method which measures flexions with PSF correction. This method is developed by applying the ERA method for flexion distortions and we call this method the FIRE method. It uses the expansion technique with an assumption of weak flexion, and we show by using typical examples of 1st and 2nd flexion images that the estimated errors become less than 1 per cent in most cases with the lowest order of the expansion. Finally, we apply the method for real data to measure flexion components in real galaxy images.

Spin glass and magnetoelectric effect in BiFeO3-Bi0.5K0.5TiO3-Bi0.5Na0.5TiO3 single crystals

We report a cluster spin glass behavior, multiferroicity and magnetoelectric (ME) effects in the single crystals of 0.46BiFeO3-0.54Bi0.5(K0.31Na0.69)0.5TiO3. The crystals are found to possess a pseudo-cubic structure with a weak tetragonal distortion and show ferroelectricity at room temperature with a ferroelectric curie temperature of TC∼630.1 K. The cluster spin glass state in the crystal is evidenced by detailed dc and ac magnetic experiments, including thermo-remnant magnetization, aging effect, memory effect, etc. Magnetodielectric effects, poling enhanced magnetism, and electric field induced acceleration of magnetization relaxation in the spin glass state are observed and ascribed mainly to spin-lattice coupling. These results might suggest an effective route to improve ferromagnetism and ME effects by constructing a spin glass state in BiFeO3-based antiferromagnetic multiferroics.

Optimizing thermoelectric performance of CoSbS0.85Se0.15 by doping 3d transition metal ions M (M = Cr, Mn, Fe and Ni)

Transitional metal compound CoSbS attracted much attention as a thermoelectric material in the past decades. The configuration of the transition metal octahedra has a close relationship with the thermoelectric performance. This study reports the optimized thermoelectric performance of CoSbS through tuning Co octahedra distortion. The substitution of M (M ​= ​Cr, Mn, Fe and Ni) on Co sites could remarkably reduce the lattice thermal conductivity, and typically, Cr substitution endows the lowest lattice thermal conductivity owing to its strongest effect on the octahedra distortion, whereas Ni substitution induces to a relatively weak octahedra distortion at the same substitution level. Additionally, band structure calculations demonstrate that Ni-3d orbitals contribute shallow impurity levels near the conduction band minimum, strikingly increasing the carrier concentration and power factor. As a result, CoSbS0.85Se0.15 with Ni substitution on Co sites exhibits a much higher thermoelectric performance compared with the other counterparts. Furthermore, by optimizing Ni content, a maximum zT value of 0.52 ​at 876 ​K was achieved in Ni0.10Co0.90SbS0.85Se0.15. This study provides an exemplary approach to optimize the thermoelectric performance via polyhedral distortion.

Strong correlation between structure and magnetic ordering in tetragonally distorted off-stoichiometric spinels Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4

We report a systematic study on the structural and magnetic properties of off-stoichiometric polycrystalline bulk spinels ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ using neutron and x-ray diffraction, ferromagnetic resonance, and magnetic measurements. Both compounds show a weak tetragonal distortion with $c/a&lt;1$, where the crystal structure could be refined in the tetragonal space group $I{4}_{1}/amd$. Both ${\mathrm{Co}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ions are located at the tetrahedral $A$ site, and ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Co}}^{3+}$ at the octahedral $B$ site. Ferrimagnetic (FI) ordering of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ sets in below 184 and 164 K, respectively. Magnetic structure analysis revealed that the ferrimagnetically coupled ${A}^{2+}$- and ${B}^{3+}$-site moments are aligned parallel to the tetragonal $c$ axis. Additionally, a noncollinear antiferromagnetic order appears in the $ab$ plane, where the moments point along [110] and $[1\overline{1}0]$. The net magnetic moment [$2{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Mn}}_{B}$/${\mathrm{Co}}_{B})\ensuremath{-}{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Co}}_{A}$)] of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ obtained from neutron data varies between 0.88--1.08 ${\ensuremath{\mu}}_{B}$ which is in good agreement with $M=0.89\text{--}1.13$ ${\ensuremath{\mu}}_{B}$ as determined from magnetization measurements. However, for the Cu-containing compound a larger discrepancy in the magnetic moment was observed between the neutron data (1.89--1.92 ${\ensuremath{\mu}}_{B}$) and low-temperature ($T=1.9$ K) field-dependent ($H=90$ kOe) magnetization data ($0.97\text{--}1.21$ ${\ensuremath{\mu}}_{B}$). From the three-sublattice model we obtained canting angles ${28}^{\ensuremath{\circ}}$ and ${25}^{\ensuremath{\circ}}$ for ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$, respectively. Both the bulk systems exhibit high magnetocrystalline anisotropy (${K}_{\text{U}}\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and $7.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ erg/${\mathrm{cm}}^{3}$) and a field-induced transition (${H}_{\text{D}}$) across 4.0 kOe due to the domain reorientation. Temperature ($1.9\text{--}350$ K) and field ($\ifmmode\pm\else\textpm\fi{}90$ kOe) dependence of magnetization data confirms the high-spin ($S=\frac{3}{2}$ and $S=2$) ground-state configuration for both the divalent Co and trivalent Mn.

The Influence of the Elemental Composition, Crystal Structure, and Grain Structure of the Ferro-Piezoceramics of Various Degrees of the Ferro-Hardness on the Stability of the Polarized State

Ferro-piezoceramic materials (FPCM) with different degrees of ferrohardness were fabricated by double solid-phase synthesis followed by the sintering technique using hot pressing method. The X-ray studies carried out in a wide temperature range showed that with increasing temperature, each of the studied FPCM undergoes a series of phase transformations, accompanied by a change in the symmetry of the unit cell. In this case, near the phase transition to the nonpolar cubic phase, in each of the FPCM, the formation of a fuzzy symmetry region is observed, which is characterized by weak distortions and temperature–time instability of the crystal structure. The study of the piezoelectric modulus d33 in the quasi-static regime as a function of temperature made it possible to reveal the different nature of its behavior in materials of various degrees of ferrohardness. It was shown that the conservation of the state in ferrosoft materials above the Curie temperature is associated with the relaxation nature of the change in their properties, the existence of a region of fuzzy symmetry (noncubic phase) in them above the Curie temperature, and increased inertia of the system. The expediency of taking into account the presented results in the development of electromechanical converters based on FPCM of various degrees of ferrohardness, operated under temperature effects, including cyclic ones, was shown.

Temperature-Dependent Raman Studies of FAPbBr3 and MAPbBr3 Perovskites: Effect of Phase Transitions on Molecular Dynamics and Lattice Distortion

Three-dimensional hybrid organic–inorganic lead halide perovskites are promising photovoltaic and light-emitting materials. A key phenomenon relevant for their optoelectronic applications is electron–phonon coupling. Since it can be strongly modified by structural deformation and changes in the dynamics of molecular cations, it is of great importance to study the temperature dependence of phonon properties of hybrid perovskites. In this work, temperature-dependent Raman scattering studies of FAPbBr3 and MAPbBr3 single crystals are reported in the 1800–22 cm−1 and 300–90 K ranges. The Raman data of MAPbBr3 showed clear anomalies near 236, 155 and 148 K, which were attributed to Pm3¯m→I4/mcm→P4/mmm (or Imma)→Pnma phase transitions. They also provided strong evidence that crystal structure of the phase stable in the 155–148 K range is very similar to structure of the I4/mcm phase, not structure of the lowest-temperature Pmna phase, as suggested in some reports. Therefore, the symmetry of this phase seems to be more likely P4/mmm rather than Imma. An analysis of the temperature evolution of MAPbBr3 Raman modes revealed that the phase transitions near 236 and 155 K are associated with weak distortion of the inorganic subnetwork and changes in the dynamics of MA+ ions. Very pronounced changes in the lattice modes region and a narrowing of bands below 148 K indicated that the phase transition to the Pnma phase is triggered by a freezing of MA+ motions, which in turn leads to strong distortion of the inorganic subnetwork. Raman studies of FAPbBr3 showed that this material behaves in a very different way than MAPbBr3. First of all, the molecular dynamics of FA+ cations are not frozen even in the lowest-temperature Pnma phase. Moreover, the distortion of the inorganic subnetwork is small in the Pnma phase. The observation of weak anomalies in the lattice modes region confirmed, however, that the two crystallographically resolved phase transitions (Pm3¯m→P4/mbm near 260 K and P4/mbm→Pnma near 150 K) lead to weak distortion of the inorganic subnetwork. On the other hand, an analysis of FA+ internal modes indicated that these transitions, as well as two other crystallographically unresolved transitions near 120 and 180 K, are triggered by a change of FA+ dynamics.

Theoretical calculations of the effect of nitrogen substitution on the structural, vibrational, and electronic properties of wolframite-type ScTaO4 at ambient conditions.

In this study, the effect of nitrogen substitution in wolframite-type ScTaO4 was investigated using density-functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO4 were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO4. The results of our study show that the monoclinic structure of ScTaO4 is rigid and non-compressible. In addition, band-structure calculations show that ScTaO4 has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substituting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge-density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.