Tilt Pattern Research Articles

Synthesis and structural characterization of new perovskite phases, Ba2Bi0.572TeO6± δ and SrLa2NiFeNbO9

Ba2Bi0.572TeO6± δ and SrLa2NiFeNbO9 ceramics were prepared in polycrystalline form by conventional solid-state reaction techniques in air. The crystal structures of the title compounds were determined at room temperature from X-ray powder diffraction (XRPD) data using the Rietveld method. The Ba2Bi0.572TeO6± δ structure crystallizes in a triclinic space group I–1 with unit-cell parameters a = 6.0272(2) Å, b = 6.0367(1) Å, c = 8.5273(3) Å, α = 90.007(7)°, β = 90.061(2)°, and γ = 90.015(4)°. The tilt system of the BiO6 and TeO6 octahedra corresponds to the notation a–b–c–. The crystal structure of the SrLa2NiFeNbO9 compound adopts an orthorhombic Pbnm space group with lattice parameters a = 5.6038(5) Å, b = 5.5988(4) Å, and c = 7.9124(6) Å. The BO6 octahedra (B = Ni/Fe/Nb) sharing the corners in 3D. Along the c-axis, the octahedra are connected by O(1) atoms of (x,y,1/4) positions; while in the ab-plane, they are linked by O(2) atoms of (x,y,z) positions. The bond angle of B–O1–B is 168.7° and that of B–O2–B is 156.3°. The octahedral lattice corresponds to the tilt pattern a–a–c+; it indicates that the octahedra tilt out-of-phase along the a,b-axes and in phase along the c-axis.

Temperature-Dependent Structural and Optoelectronic Properties of the Layered Perovskite 2-Thiophenemethylammonium Lead Iodide.

Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden-Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV-visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.

‘Fraternal-twin’ ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles

In ferroelectric switching, an applied electric field switches the system between two polar symmetry-equivalent states. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped samarium nickelate (SNO) at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-equivalent states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO6 octahedron, a state unrelated by symmetry but equal in energy under a square epitaxial strain (a = b), resulting in a large change in polarization. We term this unconventional ferroelectric a ‘fraternal-twin’ ferroelectric.

Hydrothermal signature on episodic deflation/inflation ground tilt at Aso Volcano

Ground deformation in volcanic areas induced by geothermal fluid circulation can reveal useful information about the dynamical processes occurring in the subsurface hydrothermal system. In the present work, we investigate tiltmeter time-series recorded at Aso Volcano during 2011–2016, a time interval during which different phases of volcanic activity occurred. We performed polarization analysis of the data and identified peculiar long-lasting (hours) transients, defined as Very-Long-period Tilt Pulses. The transients were further characterized in terms of waveform cross-correlation, particle tilt pattern, energy, and time distributions. The analyses indicate that such signals, which appear like deflation–inflation (DI) events, are associated with a Poissonian process whose underlying dynamics evolves over time always driven by a Poissonian mechanism. The obtained results have been interpreted in light of the available geophysical, geochemical and volcanological information. In this framework, the Very-Long-period Tilt Pulses may be ascribed to the depressurization/pressurization of the shallow hydrothermal system according to a fault-valve mechanism, which was active with different efficiency throughout eruptive and inter-eruptive phases.Graphical

A shared aperture antenna with reconfigurable pattern by common and differential modes

AbstractIn this letter, a compact pattern‐reconfigurable (PR) antenna for indoor communication is proposed, which can provide one conical, one broadside, and two tilted patterns. The proposed antenna comprises a shared radiation patch with two highly isolated ports and a reconfigurable feeding network. By activating each port independently, a dipole mode with broadside pattern and a monopole mode with conical pattern are acquired, respectively. Meanwhile, inspired by the theory of common and differential modes, two tilted patterns toward and are realized by combining dipole and monopole modes. As a result, four‐type reconfigurable patterns can be obtained. To realize reconfigurability, a functional reconfigurable feeding network is designed with innovation. The proposed antenna is fabricated and measured. The measured overlapped −10 dB impedance bandwidth is 3.28–3.71 GHz (12.3%), and the peak radiation efficiency is superior to 82.7%. The experimental results indicate that the proposed PR antenna has a wide bandwidth and stable radiation patterns, which exhibits attractive potential for fifth‐generation indoor communication in sub‐6 GHz.

Proper and improper ferroelastics with perovskite-derived structures

Based on the combined group-theoretical and crystallographic approach, the possible perovskite-derived structures of proper and improper ferroelastics were established. The crystallographic analysis of the Wyckoff position splitting in the low-symmetry proper ferroelastic phases shows the absence of any non-ferroelastic degrees of freedom that reduces the possibilities of structural relaxation. Group-theoretical analysis reveals that tilts of anionic octahedra, as the most common type of structural distortions in perovskites, are closely related to ferroelastic strains, except for the phase with tilt pattern a + a + a +. For such kind of improper ferroelastics, the simplest classification was suggested based on the relationships between the coupled order parameter (tilts of octahedra) and the lattice strain components.

Embedding antennas with tilted beam patterns into parabolic wave equation‐based models for tunnel propagation

AbstractAccurate radio wave prediction models are necessary for the innovation of mobile communication technology and the deployment of wireless sensor networks in railways. The parabolic wave equation (PWE) method is one commonly used method to predict the electromagnetic characteristic of long railway tunnels. However, the advent of novel antennas and technologies (e.g., beam steering, beamforming) has posed new challenges to PWE‐based models such as effective means of taking tilted beam patterns into account. This study addresses such a challenge by using tilted Gaussian beams to embed antennas with oriented patterns or tilted beam directions into PWE‐based models. The proposed approach does not reduce the simulation efficiency of PWE itself. The validity and practicability of the technique are demonstrated by comparing against ray‐tracing as well as experimental measurements in various tunnel environments.

Atomic-scale insight into the epitaxial growth mechanism and interfacial coupling of BNT film prepared by hydrothermal synthesis

Revealing the epitaxial growth mechanism and the interfacial coupling effect between oxide films will help to build a “structure-property” bridge for the design of microelectronic devices. Here, the epitaxial growth mechanism and the interfacial coupling in Bi0.5Na0.5TiO3/SrTiO3 (BNT/STO) heterointerfaces are investigated by the aberration-corrected scanning transmission electron microscopy, which is synthesized by a hydrothermal method. The results illustrate that 4 mol/L NaOH leads to not only the epitaxial growth of the BNT film but also the mutual diffusion of elements. The uneven distribution of local elements in BNT films is observed and confirmed to impact the cation displacements of B-site and lattice distortion. However, the overall trend of B-site cation displacement at the BNT/STO heterointerfaces is dominated by the interfacial strain. Additionally, the oxygen octahedral tilt exhibits continuous tilt patterns of a0a0a0 - a−b0c− - a0a0c− - a0b−c− - a−b−c− from the substrate to BNT film due to the constraint of the substrate and presents a strong correlation with cation displacement. These results are helpful to understand the underlying atomic structures and physical properties of BNT epitaxial thin films.

Spin canting and lattice symmetry in La2CuO4

While the dominant magnetic interaction in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ is superexchange between nearest-neighbor Cu moments, the pinning of the spin direction depends on weak anisotropic effects associated with spin-orbit coupling. The symmetry of the octahedral tilt pattern allows an out-of-plane canting of the Cu spins, which is compensated by an opposite canting in nearest-neighbor layers. A strong magnetic field applied perpendicular to the planes can alter the spin canting pattern to induce a weak ferromagnetic phase. In light of recent evidence that the lattice symmetry is lower than originally assumed, we take another look at the nature of the field-induced spin-rotation transition. Comparing low-temperature neutron diffraction intensities for several magnetic Bragg peaks measured in fields of 0 and 14 T, we find that a better fit is provided by a model in which spins rotate within both neighboring planes but by different amounts, resulting in a noncollinear configuration. This model allows a more consistent relationship between lattice symmetry and spin orientation at all Cu sites.

Emergence of Rashba-/Dresselhaus effects in Ruddlesden–Popper halide perovskites with octahedral rotations

Ruddlesden–Popper halide perovskites are highly versatile quasi-two-dimensional energy materials with a wide range of tunable optoelectronic properties. Here we use the all-inorganic CsPb n X Ruddlesden–Popper perovskites with X = I, Br, and Cl to systematically model the effect of octahedral tilting distortions on the energy landscape, band gaps, macroscopic polarization, and the emergence of Rashba-/Dresselhaus splitting in these materials. We construct all unique n = 1 and n = 2 structures following from octahedral tilts and use first-principles density functional theory to calculate total energies, polarizations and band structures, backed up by band gap calculations using the GW approach. Our results provide design rules for tailoring structural distortions and band-structure properties in all-inorganic Ruddlesden–Popper perovskites through the interplay of the amplitude, direction, and chemical character of the antiferrodistortive distortion modes contributing to each octahedral tilt pattern. Our work emphasizes that, in contrast to three-dimensional perovskites, polar structures may arise from a combination of octahedral tilts, and Rashba-/Dresselhaus splitting in this class of materials is determined by the direction and Pb-I orbital contribution of the polar distortion mode.

Atomically engineered cobaltite layers for robust ferromagnetism.

Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit-thick syntactic layers of cobaltites within a strongly tilted octahedral matrix via atomically precise synthesis. The octahedral tilt patterns of adjacent layers propagate into cobaltites, leading to a continuation of octahedral tilting while maintaining substantial misfit tensile strain. These effects induce severe rumpling within an atomic plane of neighboring layers, further triggering the electronic reconstruction between the splitting orbitals. First-principles calculations reveal that the cobalt ions transit to a higher spin state level upon octahedral tilting, resulting in robust ferromagnetism in ultrathin cobaltites. This work demonstrates a design methodology for fine-tuning the lattice and spin degrees of freedom in correlated quantum heterostructures by exploiting epitaxial geometric engineering.

Sex differences in spatial and mechanical tilt: Support for investment theories

Tilt refers to a pattern of specific abilities and is based on within subject differences in two abilities (spatial and academic), yielding relative strength in one ability (spatial) and weakness in another ability (academic). The current study examined sex differences in spatial and mechanical tilt. Tilt was measured using the Armed Services Vocational Aptitude Battery in the National Longitudinal Survey of Youth. Tilt was based on spatial and mechanical abilities contrasted with academic abilities (math or verbal), producing spatial and mechanical tilt (spatial/mechanical > academic) and academic tilt (academic > spatial/mechanical). For males and females, spatial and mechanical tilt correlated negatively with verbal and math scores on college tests (SAT, ACT, PSAT), indicating that spatial and mechanical tilt predicted low scores on the college tests. In addition, for males and females, spatial and mechanical tilt predicted jobs and college majors in science, technology, engineering, and math (STEM), with stronger effects for spatial and mechanical tilt contrasted with verbal ability. Although levels of mechanical tilt were higher for males, levels of spatial tilt showed no sex differences, a pattern that could be attributed to the spatial ability measured (visualization). In addition, no consistent sex differences in tilt relations with diverse criteria (tests, jobs, majors) were found. The results support investment theories and research on sex differences in vocational preferences for people versus things. Such theories assume that males prefer working with things and machines, boosting mechanical tilt, whereas females prefer working with people and in (non-technical) humanities fields, boosting academic tilt. Future research should consider tilt patterns in other developmental periods and examine whether tilt varies with ability level, as predicted by differentiation theories.

International differences in math and science tilts: The stability, geography, and predictive power of tilt for economic criteria

Tilt represents an ability pattern and is based on differences in two dimensions (e.g., math and reading), yielding strength in one dimension (math) and weakness in the other (reading). Whereas prior research examined tilt relations with economic productivity at the individual or country levels, the current study is the first to examine the stability, geography, and predictive power of math, verbal, and science tilts for economic growth at the country level. Tilt was based on math, science, and reading results in seven PISA (Programme for International Student Assessment) waves from 2000 to 2018 (Nmax = 86 countries). Tilt was computed by contrasting math or science scores with reading scores, yielding math tilt (math > reading) and science tilt (science > reading). Tilt patterns were stable across the seven PISA waves (mean stabilities math-reading r = .74 and science-reading r = .53). Tilt showed a geographic gradient, with math/science tilt in East Asia and verbal tilt in Europe and the Americas (r = .38 to .42, for tilt and geographic coordinates). Tilt did not mediate relations between country level ability and economic growth. However, math tilt (math > reading) directly and positively predicted higher economic growth (β = .25 to .45), whereas science tilt (science > reading) was negligibly related to economic growth (β = .08 to .18). The results were insensitive to the year of data collection and the parametric procedure used. Future research should consider factors that influence tilt such as differential investment of educational resources.

Wideband Low-Sidelobe Slot Array Antenna With Compact Tapering Feeding Network for E-Band Wireless Communications

In this work, a wideband low-sidelobe slot array antenna for wireless communication system at E-band is presented. The slots are excited by a parallel feeding network where the Taylor distribution is applied. Two types of wideband unequal power divider with compact structures are designed for various output ratios to fit the limited feeding network space. One is integrated vertically inside a 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 1 elements subarray using a set of pins with asymmetrical offset. The other type is designed inside an E-plane T-junction with unbalance impedance at two output ports. To avoid main lobe tilt and asymmetrical radiation patterns, the feeding network is designed to be biaxially symmetrical, and the feed port is placed in the center of the back. A prototype is fabricated with brass and measured in an anechoic chamber. It achieves a peak gain of 25.8 dBi and a fractional impedance bandwidth of 19.2% with the reflection coefficient below −10 dB. The measured radiation patterns agree with the simulated patterns very well, and the sidelobe level (SLL) is lower than −21 dB.

Identification of the failure mode of corroding steel rebars in a viaduct in service through hardness measurements

In the present work, rebars extracted from a reinforced concrete viaduct that had to be demolished has been studied because they broke in service with an unusual tilted pattern for the rebars without reduction in diameter. The structure was a bridge suffering alkali-silica reaction and reinforcement corrosion. The determination of rebar's in service failure mode is very complex due, among other causes, to their high degree of degradation by corrosion. In this work it has been shown that by performing hardness measurements along a diameter in the cross section of the bars it is possible to determine their failure mode, tensile or fatigue. The results show a similar probability of in service fatigue or tensile plastic collapse failure. In both cases, a brittle fracture has occurred. The corrugations of the rebars generate a stress concentration that explains the observed tilted appearance of the fracture surface. The influence of different corrugation patterns, as for example B 500SD, in service fracture appearance should be studied to extend the conclusions reached in the present work. (172).

Orbital-order phase transition in Pr1−xCaxMnO3 probed by photovoltaics

The phase diagram of $Pr_{1-x}Ca_xMnO_3$ is modified x $\le$ 0.3, which suggests a reevaluation of the phase diagram of other manganites in that doping region. Rather than an orbital ordered phase reaching up to high temperatures of approximately 800-1100 K, we propose a loss of spontaneous orbital order already near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. The reevaluation of the orbital order transition is based on observed anomalies of various physical properties at a temperatures of 220-260 K in epitaxial thin films of $Pr_{1-x}Ca_xMnO_3$ x=0.1, i.e.in the photovoltaic effect, electric transport, magnetization, optical and ultrafast transient pump probe studies. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital order phase transition at $T_{OO} \approx$ 300 K for x=0.1. This is consistent with the experimental observation of a temperature dependent change in lattice parameter for bulk samples of the same doping at 300 K for x=0.1 and 350 K for x=0, typical for a second order phase transition. Since our reassignment of the orbital order phase transition towards lower temperatures challenges a well-established and long-accepted picture, we provide results of multiple complementary measurements as well as a detailed discussion.

Structural and magnetic phase diagram of epitaxial La0.7Sr0.3MnO3 from first principles

ABO3 perovskites host a huge range of symmetry lowering structural distortions, each of which can tune, or even switch on or off, different functional properties due to the strong coupling between the lattice, spin and charge degrees of freedom in these materials. The sheer number of different meta-stable structures present in perovskites creates a challenge for materials design via theory and simulation. Here, we tackle this issue using a first principles structure searching method on a prototypical half-metallic perovskite, La0.7Sr0.3MnO3, to predict how epitaxial strain can engineer structural and magnetic properties. We reveal a rich structural phase diagram through strain engineering in which the octahedral tilt pattern, and hence the crystal symmetry, is altered from the bulk. We show how the low-symmetry of the various phases in turn induces new structural modes, an increase in the magnetic anisotropy energy, and weak antiferromagnetic spin-canting.

Phase transition pathway of hybrid halide perovskites under compression: Insights from first-principles calculations

Lead halide perovskites undergo pressure-induced phase transition, facilitated by bond compressions and different tilt patterns of the ${\mathrm{PbI}}_{6}$ octahedra. Utilizing first-principles calculations we investigated 14 possible tilt systems of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ under compression, derived from the high-symmetry $Pm\overline{3}m$ structure. Fr substitution is adopted to mimic the rotational disorder effect of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ cation with the similar size (at room temperature or higher). Analyses of these phases reveal an interplay between tilting and distortion of the ${\mathrm{PbI}}_{6}$ octahedra. Dynamical fluctuations at finite temperature provide additional insight into the phases' stability. Drawing from the trends observed in $\mathrm{Fr}\mathrm{Pb}{\mathrm{I}}_{3}$ and additional calculations for perovskites involving differently ordered organic cations, we propose that phase transition in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ occurs from tetragonal at ambient pressure to orthorhombic under high pressure via the pathway I4/mcm$\ensuremath{\rightarrow}$P4/mbm$\ensuremath{\rightarrow}$Imm2. The distinct discontinuity in the calculated volume-pressure curve of I4/mcm and band-gap evolution of the proposed phases are consistent with photoluminescence shifts observed in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ under pressure.

Dielectric Gratings Enhancing the Field of View in Low Dielectric Permittivity Elliptical Lenses

Low relative permittivity plastic elliptical lenses in combination with integrated focal plane arrays are a promising solution to be used in the future mm- and sub-mm-wave systems. Their appeal lies in the availability of materials with moderate loss and lightweight, and the possibility to use cost-effective manufacturing techniques. However, the achievable scanning angular range is relatively small with low permittivity lenses. In this article, we explore the use of dielectric gratings with modulated height integrated in the lens material, with the aim of enlarging the steering angle. The dielectric gratings synthesize a tilted feed pattern, reducing the reflection loss and spillover when illuminating the lens off-focus. A quasi-analytic approach based on the Floquet mode analysis of the gratings is used to synthesize the grating profile. This method is combined with an analysis in reception of the lens antenna. A wideband prototype in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> -band (140–220 GHz) has been fabricated, achieving a field of view of ±25° with gain >30 dBi.

Machine learning classifies predictive kinematic features in a mouse model of neurodegeneration

Motor deficits are observed in Alzheimer's disease (AD) prior to the appearance of cognitive symptoms. To investigate the role of amyloid proteins in gait disturbances, we characterized locomotion in APP-overexpressing transgenic J20 mice. We used three-dimensional motion capture to characterize quadrupedal locomotion on a treadmill in J20 and wild-type mice. Sixteen J20 mice and fifteen wild-type mice were studied at two ages (4- and 13-month). A random forest (RF) classification algorithm discriminated between the genotypes within each age group using a leave-one-out cross-validation. The balanced accuracy of the RF classification was 92.3 ± 5.2 % and 93.3 ± 4.5 % as well as False Negative Rate (FNR) of 0.0 ± 0.0% and 0.0 ± 0.0% for the 4-month and 13-month groups, respectively. Feature ranking algorithms identified kinematic features, which when considered simultaneously, achieved high genotype classification accuracy. The identified features demonstrated an age-specific kinematic profile of the impact of APP-overexpression. Trunk tilt and unstable hip movement patterns were important in classifying the 4-month J20 mice, whereas patterns of the shoulder and iliac crest movement were critical for classifying 13-month J20 mice. Examining multiple kinematic features of gait simultaneously could also be developed to classify motor disorders in humans.