Tilt Distortion Research Articles

Nature of the structural symmetries associated with hybrid improper ferroelectricity inCa3X2O7(X=Mnand Ti)

In hybrid improper ferroelectric systems, polarization arises from the onset of successive nonpolar lattice modes. In this work, measurements and modeling were performed to determine the spatial symmetries of the phases involved in the transitions to these modes. Structural and optical measurements reveal that the tilt and rotation distortions of the MnO6 or TiO6 polyhedra relative to the high symmetry phases driving ferroelectricity in the hybrid improper Ca3X2O7 system (X=Mn and Ti) condense at different temperatures. The tilt angle vanishes abruptly at T$_T$ ~ 400 K for Ca3Mn2O7 (and continuously for X=Ti) and the rotation mode amplitude is suppressed at much higher temperatures T$_R$ ~1060 K. Moreover, Raman measurements in Ca3Mn2O7 under isotropic pressure reveal that the polyhedral tilts can be suppressed by very low pressures (between 1.4 and2.3 GPa) indicating their softness. These results indicate that the Ca3Mn2O7 system provides a new platform for strain engineering of ferroelectric properties in film based systems with substrate induced strain.

Stabilization of anti-ferroelectric Pbcm phase over ferroelectric P21ma phase in intermittent ferroelectric NaNbO3 by incorporating CaTiO3

This study focuses on the structural aspect of stabilization of anti-ferroelectric (AFE) Pbcm phase over ferroelectric P21ma phase by introducing CaTiO3 in NaNbO3 perovskite. For this work, Na1−xCaxNb1−xTixO3 (x = 0.00, 0.03, 0.04, 0.05) ceramic compositions are chosen for investigation. Structural and dielectric studies are performed on these compositions to see the effect of dopants on phase transitions involved. X-ray diffraction and their respective refinement results confirmed orthorhombic structure with Pbcm space group for all the compositions. Structures are generated using refined x-ray diffraction data in order to understand the effect of doping on octahedral tilting, which plays a huge role in increasing the energy gap required for Pbcm to P21ma conversion and is responsible for establishing stable antiferroelectric phase. To get more insights into the AFE-FE phase transition and stabilization of AFE phase with respect to composition, dielectric permittivity versus temperature and polarization versus electric field studies were performed for all the above-mentioned compositions. Based on PE and structural analysis, high electric field stable AFE phase can be established for x = 0.04 composition. Reasons for high electric field stable antiferroelectric phase at x = 0.04 are presented where it is argued that the octahedral tilt distortion of perovskite structure is responsible for the increase in energy barrier required for AFE ↔ FE transformation, thus stabilizing the antiferroelectric phase.

Phase compensation in digital holographic microscopy using a quantitative evaluation metric

Abstract This study presents a simple metric to quantitatively evaluate the compensation of phase distortions in digital holographic microscopy. The metric calculates the difference between the minimum and maximum values in an unwrapped phase image. A phase compensation method based on the metric is proposed. Both the tilt and curvature distortions can be automatically compensated using this method, and specimen-free zones are unnecessary. It is also suitable for thick objects located in wide areas. The method using this evaluation metric has been tested and validated on several samples. With strong robustness and low computational complexity, this approach can result in accurate and effective quantitative phase imaging in holographic microscopy.

Numerical reference plane algorithm for effectively solving tilt distortion of a phase image in digital off-axis holography

It is a simple and commonly-used approach to use an inclined plane reference wave to remove zero-order diffraction and conjugated image in digital off-axis holography. However, this method is encountering a difficulty, since an additional carrier frequency is incorporated into the inclined reference wave and it is difficult to accurately obtain this additional carrier frequency via experimental measurement, a certain tilt distortion of the phase image will occur in the hologram reconstruction. In this paper, a numerical reference plane algorithm is proposed to solve this problem. This method innovatively constructs a numerical reference plane which is able to exactly characterize the tilt of the phase image by choosing three different points from a local flat of the reconstructed image, and establishes a mathematical relation between the plane parameters and the carrier frequency of the reference wave, which is used as a criterion of correcting the tilt distortion of the phase image in the subsequent iterative computation. The procedures of the algorithm are as follows. 1) Input the nominal carrier frequencies, (fx', fy') of the plane reference wave and reconstruct the hologram. 2) Unwrap the phase with PUMA algorithm and suppress the noise using bilateral filtering and short time Fourier transform with wavelet shrinkage. 3) Construct the numerical reference plane reflecting the image inclination and establish the mathematical relation between the plane parameters and the carrier frequencies of the reference wave. 4) Perform the iterative computation to correct the nominal carrier frequencies, (fx', fy') by using the differential coefficients, (a, b) of the reference plane equation as the criterion. 5) Output the computation result and the corrected phase image. The algorithm is simple and effective. It is able not only to achieve accurate correction to the tilt phase distortion, but also to exactly obtain the additional carrier frequency of the inclined plane reference wave. Since in the phase unwrapping reconstruction, the proposed approach combines with bi-lateral filtering processing, wavelet shrinking and short time Fourier transform to remove the noise influence while the image details are preserved, the method would still be valid under the influences of environmental and system noise. The experimental result supports the theoretical prediction very well.

Tuning the metal-insulator transition in d1 and d2 perovskites by epitaxial strain: A first-principles-based study

We investigate the effect of epitaxial strain on the Mott metal-insulator transition (MIT) in perovskite systems with d^1 and d^2 electron configuration of the transition metal (TM) cation. We first discuss the general trends expected from the changes in the crystal-field splitting and in the hopping parameters that are induced by epitaxial strain. We argue that the strain-induced crystal-field splitting generally favors the Mott-insulating state, whereas the strain-induced changes in the hopping parameters favor the metallic state under compressive strain and the insulating state under tensile strain. Thus, the two effects can effectively cancel each other under compressive strain, while they usually cooperate under tensile strain, in this case favoring the insulating state. We then validate these general considerations by performing electronic structure calculations for several d^1 and d^2 perovskites, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We isolate the individual effects of strain-induced changes in either hopping or crystal-field by performing DMFT calculations where we fix one type of parameter to the corresponding unstrained DFT values. These calculations confirm our general considerations for SrVO_3 (d^1) and LaVO_3 (d^2), whereas the case of LaTiO_3 (d^1) is distinctly different, due to the strong effect of the octahedral tilt distortion in the underlying perovskite crystal structure. Our results demonstrate the possibility to tune the electronic properties of correlated TM oxides by using epitaxial strain, which allows to control the strength of electronic correlations and the vicinity to the Mott MIT.

Hidden phases revealed at the surface of double-layeredSr3(Ru1−xMnx)2O7

Double-layered Sr3Ru2O7 has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of Sr3(Ru1-xMnx)2O7. Driven by the tilt distortion of RuO6 octahedra, the surface of Sr3Ru2O7 is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn-doping, the surface of Sr3(Ru0.84Mn0.16)2O7 is metallic while the bulk is insulating. Our result demonstrates that the electronic and structural properties at the surface are intimately coupled and consistent with quasi two-dimensional character.

Probing structural distortions in rare earth chromites using Indian synchrotron radiation source

Effect of R site ionic radius on the structural distortions in RCrO3 (R: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y) rare earth perovskite chromites is investigated through synchrotron based X-ray diffraction technique. The said structural distortions are measured in the terms of CrO bond lengths, CrO6 octahedral tilt, tolerance factor (t) and RO12 polyhedral distortion. It is observed that the lattice parameters (b and c), RO bond length, tolerance factor and Cr–O–Cr bond angle increases with increase in the radius of R site ion; whereas octahedral tilts (along [010] and [101]) and RO12 polyhedral distortion decreases with increase in the radius of R site ion. Further it is observed that the radius of R site cation does not have significant effect on lattice parameter ‘a’ and CrO bond length (hence on the distortion of CrO6 octahedra). A comparison of the crystal structure parameters obtained using refinement of synchrotron powder X-ray data and that estimated theoretically using modern bond valence parameters (SPuDS) has been carried out. A one to one correlation between the experimental and theoretical results has been observed.

Thermal Expansion and Second Harmonic Generation Response of the Tungsten Bronze Pb2AgNb5O15.

The incorporation of transition metal element Ag was performed to explore negative thermal expansion (NTE) materials with tetragonal tungsten bronze (TTB) structures. In this study, the structure and thermal expansion behaviors of a polar TTB oxide, Pb2AgNb5O15 (PAN), were systematically investigated by high-resolution synchrotron powder diffraction, high-resolution neutron powder diffraction, transmission electron microscope (TEM), and high-temperature X-ray diffractions. The TEM and Rietveld refinements revealed that the compound PAN displays (√2a(TTB), √2b(TTB), 2c(TTB))-type superstructure. This superstructure within the a-b plane is caused by the ordering of A-site cations, while the doubling of the c axis is mainly induced by a slight tilt distortion of the NbO6 octahedra. The transition metal Ag has larger spontaneous polarization displacements than Pb, but the Pb-O covalence seems to be weakened compared to the potassium counterpart Pb2KNb5O15 (PKN), which may account for the similar Curie temperature and uniaxial NTE behavior for PAN and PKN. Powder second harmonic generation (SHG) measurement indicates that PAN displays a moderate SHG response of ∼0.2 × LiNbO3 (or ∼100 × α-SiO2) under 1064 nm laser radiation. The magnitudes of the local dipole moments in NbO6 and PbOx polyhedra were quantified using bond-valence approach. We show that the SHG response stems from the superposition of dipole moments of both the PbO(x) and NbO6 polyhedra.

A Novel Error Model of Optical Systems and an On-Orbit Calibration Method for Star Sensors

In order to improve the on-orbit measurement accuracy of star sensors, the effects of image-plane rotary error, image-plane tilt error and distortions of optical systems resulting from the on-orbit thermal environment were studied in this paper. Since these issues will affect the precision of star image point positions, in this paper, a novel measurement error model based on the traditional error model is explored. Due to the orthonormal characteristics of image-plane rotary-tilt errors and the strong nonlinearity among these error parameters, it is difficult to calibrate all the parameters simultaneously. To solve this difficulty, for the new error model, a modified two-step calibration method based on the Extended Kalman Filter (EKF) and Least Square Methods (LSM) is presented. The former one is used to calibrate the main point drift, focal length error and distortions of optical systems while the latter estimates the image-plane rotary-tilt errors. With this calibration method, the precision of star image point position influenced by the above errors is greatly improved from 15.42% to 1.389%. Finally, the simulation results demonstrate that the presented measurement error model for star sensors has higher precision. Moreover, the proposed two-step method can effectively calibrate model error parameters, and the calibration precision of on-orbit star sensors is also improved obviously.

Room-temperature polarisation and magnetisation in a bulk layered perovskite by control of octahedral tilt distortions

Room-temperature polarisation and magnetisation in a bulk layered perovskite by control of octahedral tilt distortions

Electronic correlations, magnetism, and Hund's rule coupling in the ruthenium perovskitesSrRuO3andCaRuO3

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials CaRuO$_3$ and SrRuO$_3$ is presented. Phase diagrams are determined for both materials as a function of Hubbard repulsion $U$ and Hund's rule coupling $J$. Metallic and insulating phases are found, as are ferromagnetic and paramagnetic states. The locations of the relevant phase boundaries are determined. Based on the computed phase diagrams, Mott-dominated and Hund's dominated regimes of strong correlation are distinguished. Comparison of calculated properties to experiments indicates that the actual materials are in the Hund's coupling dominated region of the phase diagram so can be characterized as Hund's metals, in common with other members of the ruthenate family. Comparison of the phase diagrams for the two materials reveals the role played by rotational and tilt (GdFeO$_3$-type) distortions of the ideal perovskite structure. The presence of magnetism in SrRuO$_3$ and its absence in CaRuO$_3$ despite the larger mass and larger tilt/rotational distortion amplitude of CaRuO$_3$ can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-$T$ properties of CaRuO$_3$ to those of SrRuO$_3$ provides insight into the effects of magnetic order on the properties of a Hund's metal. The study provides a simultaneous description of magnetism and correlations and explicates the roles played by band theory and Hubbard and Hund's interactions.

On the calibration of high-energy X-ray diffraction setups. II. Assessing the rotation axis and residual strains

The calibration of high-energy X-ray diffraction setups using an area detector and a rotation axis is discussed. The characterization of the tilt and spatial distortions of an area detector was discussed in part one of this series [Borbély, Renversade, Kenesei & Wright (2014).J. Appl. Cryst.47, 1042–1053]. Part II links the detector frame to the laboratory frame comprising an additional rotation axis and introduces a general diffractometer equation accounting for all sources of misalignment. Additionally, an independent high-accuracy method for the evaluation of the crystallographic orientation and cell parameters of the undeformed reference crystal is presented. Setup misalignments are mainly described in terms of a residual strain tensor, considered as a quality label of the diffractometer. The method is exemplified using data sets acquired at beamlines ID11 (European Synchrotron Radiation Facility) and 1-ID (Advanced Photon Source) on Al and W single crystals, respectively. The results show that the residual strain tensor is mainly determined by the detector spatial distortion, and values as small as 1–2 × 10−4can be practically achieved.

On the calibration of high-energy X-ray diffraction setups. I. Assessing tilt and spatial distortion of the area detector

The geometry of high-energy X-ray diffraction setups using an area detector and a rotation axis is analysed. The present paper (part 1) describes the methodology for determining continuously varying spatial distortions and tilt of the area detector based on the reference diffraction rings of a certified powder. Analytical expressions describing the degeneration of Debye rings into ellipses are presented and a robust calibration procedure is introduced. It is emphasized that accurate detector calibration requires the introduction of spatial distortion into the equation describing the tilt. The method is applied to data sets measured at the Advanced Photon Source and at the European Synchrotron Radiation Facility using detectors with different physical characteristics, the GE 41RT flat-panel and the FReLoN4M detector, respectively. The spatial distortion of the detectors is compared with regard to their use in structural and strain tensor analysis, a subject treated in part 2 of the calibration work [Borbély, Renversade & Kenesei (2014).J. Appl. Cryst.Submitted].

Atomic-scale fingerprint of Mn dopant at the surface of Sr₃(Ru₁-xMnx)₂O₇.

Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1−xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the “chirality” of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale.

Electrically induced decrease of magnetization in Ca3Mn2O7

The magnetoelectric effect of Ca3Mn2O7 is verified by measuring the electric field dependence of magnetization. The magnetization is reduced by the electric field, as much as nearly 6% at 4 K under 100 Oe magnetic field and 40 kV/m electric field. There are two theoretical models in previous researches. Harris's model [A. B. Harris, Phys. Rev. B 84, 064116 (2011)], based on the rotating effect of electric field predicts electric-field-direction dependence of the magnetization decrease. Benedek and Fennie's model [N. A. Benedek and C. J. Fennie, Phys. Rev. Lett. 106, 107204 (2011)] emphasizes the stretching effect of electric field and predicts direction independence. The experimental results support Benedek and Fennie's framework and conflict with Harris's prediction. We argue that the large anisotropy and the antiferromagnetic nature impede the rotation of domains and suppress the rotating effect. In the coupling of magnetic ordering and ferroelectric ordering, the oxygen octahedron tilt distortion (X3−) plays an essential role.

Manifestations of Broken Symmetry: The Surface Phases ofCa2−xSrxRuO4

The surface structural phases of Ca(2-x)SrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the RuO6 rotational distortion while diminishing the instability against the RuO6 tilt distortion occurring within the bulk crystal. As a result, suppressed structural and electronic surface phase transition temperatures are observed, including the appearance of an inherent Mott metal-to-insulator transition for x=0.1 and possible modifications of the surface quantum critical point near x(c) approximately 0.5.

Thermodynamic properties of [formula omitted] in magnetic fields

We have studied the influence of a magnetic field on the thermodynamic properties of Ca 2 - x Sr x RuO 4 in the intermediate metallic region with tilt and rotational distortions ( 0.2 ⩽ x ⩽ 0.5 ) . We find strong and anisotropic thermal expansion anomalies at low temperatures, which are suppressed and even reversed by a magnetic field. The metamagnetic transition of Ca 1.8 Sr 0.2 RuO 4 is accompanied by a large magnetostriction. Furthermore, we observe a strong magnetic-field dependence of c p / T and that can be explained by magnetic fluctuations.

Objective Sizing of Upper Airway Stenosis: A Quantitative Endoscopic Approach

In patients with airway stenosis, anatomy of the lesion determines the magnitude of the biomechanical ventilatory disturbance and thus the nature and severity of symptoms. It also gives information about biology, likelihood of response to treatment, and prognosis of laryngotracheal lesions. Accurate airway sizing throughout treatment is therefore central to managing this condition. We developed a method for objective assessment of airway lesions during endoscopy. We used airway simulations to investigate the effects of endoscope tilt and lens distortions on measurement accuracy, devising and validating clinical rules for quantitative airway endoscopy. A calibrator was designed to assess lesion length, location, and cross-section during tracheoscopy. It proved possible to calculate the length and location of the stenosis using simple mathematics. Cross-section measurements were more than 95% accurate, independent of endoscope tilt and without making assumptions about endoscope optics and visuospatial distortion, for both pediatric and adult airway dimensions. The technique was used to characterize airway lesions in 10 adult patients with an average age of 48 years undergoing therapeutic laryngotracheoscopy. Lesions occurred on average 36 mm below the glottis (range, 21-54 mm) and were 9.3 mm long (5-17 mm). The average pretreatment airway cross-section was 48.3 mm, increasing to 141.1 mm after laser therapy. Two independent observers calculated airway cross-sections, achieving an interobserver concordance of 0.98. This method can be used to objectively and precisely determine the anatomy of airway lesions, allowing accurate documentation of lesion characteristics and surgical results, serial monitoring throughout treatment, and comparison of outcomes between different centers.

Neutron-diffraction study of the crystal structure of Ca 1.5 Sr 0.5 RuO 4

We have investigated the temperature dependence of the crystal structure of Ca1.5Sr0.5RuO4 by powder and single-crystal neutron diffraction in the temperature range from 1.5 to 300 K. Both an anomaly in the temperature dependence of the c lattice parameter and the observation of characteristic superstructure reflections indicate a continuous transition from the I41/acd structure, in which the corner-sharing RuO6 octahedra are rotated about the c axis, to a low-temperature phase with an additional rotation of the octahedra around an axis lying in the RuO2 planes. The occurrence of this additional tilt distortion coincides with the previously observed change of the magnetic properties, from the nearly ferromagnetic state to the antiferromagnetic correlated state.

Automated extraction of grain-size data from gravel surfaces using digital image processing

This paper describes and tests a method for the automated extraction of grain-size data from digital imagery. It combines two basic image processing methods for this purpose: grey-scale thresholding to create a binary image and watershed segmentation to grow edges on the binary image to allow the identification of individual grains. The method is subject to rigorous testing in terms of edge detection and automatic measurement of grain-size information from the edge images, and is also compared with the results obtained from simple direct clast sampling. The edge detection methods are tested with respect to manually-identified edges. This suggests that simple thresholding of raw imagery produces grain-size estimates that are: (i) in excellent agreement with manual estimates, above a critical particle size defined by the scale of the photography; (ii) downgraded with the inclusion of additional edge information from analysis of high resolution digital elevation models (DEMs); and (iii) not affected by the use of raw imagery as opposed to imagery that has been rectified to deal with geometric, tilt and relief distortion effects. The automated ellipse-based measurement method is shown to produce a good estimate of two-dimensional a- and b- axes as they appear as long and short axes on the edge images. Thus, the research shows that it can be used to map and quantify very rapidly spatial variations in grain-size characteristics, although it cannot deal with the longrecognised problem of the relationship between two-dimensional planform grain-size estimates and actual a- and b- axes obtained by direct grain sampling.