Pseudotetragonal Structure Research Articles

The impact of co-doping Ce, W, and Mn on the microstructure and piezoelectric performance of high-temperature piezoelectric ceramics based on CaBi2Nb2O9

CaBi2Nb2O9 (CBN) is a bismuth layered ferroelectric material with extremely high Curie temperature and low dielectric loss, which has great application potential in ultra-high temperature field. However, because of its low piezoelectric coefficient, its practical application still faces great challenges. This study successfully prepared Ca1-xCexBi2Nb1.98(W2/3Mn1/3)0.02O9 (x = 0–0.05) two-layer ferroelectric ceramics using direct-reaction sintering method. A pseudo tetragonal structure was successfully constructed near x = 0.02 using the substitution method of Ce4+ at site A, and W6+ and Mn4+ at site B, which altered the crystal structure of CBN based ceramics. The properties of CBN-based piezoelectric ceramics have been significantly improved by this modification. It has a higher piezoelectric constant (d33 = 20.6 pC/N) and Curie temperature (Tc = 950 °C), while having a lower dielectric loss (tanδ = 0.031 at 500 °C). In addition, CaBi2Nb2O9 based piezoelectric ceramics also show excellent thermal stability. After annealing at 900 °C for 2 h, the d33 value remained high at 19.2 pC/N, retaining about 93.2% of its original value. Therefore, these findings indicate that Ca1-xCexBi2Nb1.98(W2/3Mn1/3)0.02O9 is a high-temperature piezoelectric material with great potential.

Preparation (HgBa2Ca2Cu2.85La0.15O8+δ) compound and Study of the impact of partial substitution of mercury with (In2O3)on the structural characteristics

In the current work, we focused on the impact of Indium oxide partial substitution at the (Hg) site on the structural characteristics of the (HgBa2Ca2Cu2.85La0.15O8+δ) compound with (0,0.05,0.1,0.15,0.2 and 0.25). Samples were prepared by the (SSR) method. The combined powder was crushed with a pressure of 7 tons per square centimeter to create a disc with a thickness of (0.2 cm) and a diameter of (1.5 cm). The samples sintered for 24 hours at a rate that was changed from ambient temperature to 820 °C. For the samples which were doped with (In=0.05) in comparison to other samples, the highest value of the c-axis lattice parameter and a pseudo-tetragonal structure were discovered using (XRD) analysis. It was discovered that altering the In concentrations in all of the results of our sample changes the volume fraction VPh(1223), mass density dm, and c/a. It was also determined that the change in lattice parameters, degree of crystallinity, crystal size, and strain (in different ways) resulted from a change in the indium oxide concentrations for all samples.

Single-crystal thin film growth of the Mott insulator EuVO3 under biaxial substrate strain

We report the single-crystal thin film growth of EuVO3 of a Mott insulator with V3+ (d2,S=1). We used the pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) methods and obtained the well-oriented epitaxial thin films with a pseudo-tetragonal crystal structure of EuVO3 which is distinct from the orthorhombic structure in bulk. The high quality of EuVO3 films is verified by the valence state of europium and vanadium ions with X-ray absorption near-edge structure (XANES) measurements showing clear signs from Eu3+ and V3+. The electrical resistivity exhibits the insulating temperature dependence; however, the magnitude of the resistivity is reduced from that of bulk EuVO3, possibly due to biaxial strain from substrates.

Cu doping effect on the structure and charge transport behavior of Nd5/3Sr1/3NiO4

Nd5/3Sr1/3Ni1-yCuyO4 (0≤y≤0.6) polycrystals have successfully synthesized, and X-ray diffraction analysis and electrical resistivity have been measured. Rietveld analyses show a pseudo-tetragonal K2NiF4-type structure with the Ni(Cu)-O octahedron being elongated and tilted by Cu doping. Resistivity jump and irreversible behavior at T≤TCO=221 K was observed in the undoped Nd5/3Sr1/3NiO4, such a first-order transition was suppressed by Cu doping at Ni site. The Cu doping also improves the conductivity by reducing the activation energy, which may be related with the enhancement of grain growth. For the Cu-doped Nd5/3Sr1/3NiO4, the charge conduction still follows the thermally activated small polaron hopping conduction mode above TCO; and below, it follows variable range hopping mode.

Temperature dependent partitioning mechanisms and its associated microstructural evolution in a CMnSiAl quenching and partitioning (Q&P) steel

The effect of temperature (350 °C ¡ MS ¡ 450 °C) on the partitioning mechanisms and the final microstructure evolution in a CMnSiAl quenching and partitioning (Q&P) steel was investigated. The microstructure of both the Q&P specimens, comprised of distorted BCC or pseudo tetragonal martensite structure with two different characteristics namely (i) tempered or carbon depleted martensite that formed during initial quenching (Mf ¡ 240 °C ¡ MS) and partitioning step and (ii) carbon enriched fresh martensite that formed after partitioning step and final quenching (RT) together with blocky and inter-lath films of retained austenite. In addition, packets of M/A constituents were observed in Q&P-350-1min specimen and some traces of carbide and plate martensite were observed in Q&P-450-1min specimen. The increase in partitioning temperature led to nearly 2% increase in the amount of retained austenite (both blocky and inter-lath) with increased carbon content of 0.27 wt.%. Along with carbon partitioning, slight interface mobility/isothermal martensite formation was also observed in the case of specimen partitioned at 350 °C, whereas tempering effect was predominantly seen in the case of specimen partitioned at 450 °C. Irrespective of the partitioning temperature, the amount of carbon required to stabilize the retained austenite at RT was found to be about 1.15 wt.% and was confirmed through APT analysis.

Vibrational Properties of a Naturally Occurring Semiconducting van der Waals Heterostructure

We present vibrational properties of Franckeite, which is a naturally occurring van der Waals heterostructure consisting of two different semiconducting layers. Franckeite is a complex layered crystal composed of alternating SnS$_2$ like pseudohexagonal and PbS-like pseudotetragonal layers stacked on top of each other, providing a unique platform to study vibrational properties and thermal transport across layers with mass density and phonon mismatches. By using micro-Raman spectroscopy and first-principles Raman simulations, we found that the PbS-like pseudotetragonal structure is mostly composed of Pb$_3$SbS$_4$. We also discovered several low-frequency Raman modes that originate from the intralayer vibrations of the pseudotetragonal layer. Using density functional theory, we determined all vibrational patterns of Franckeite, whose signatures are observed in the Raman spectrum. By studying temperature dependent Raman spectroscopy (300 K - 500 K), we have found different temperature coefficients for both pseudotetragonal and pseudohexagonal layers. We believe that our study will help understand the vibration modes of its complex heterostructure and the thermal properties at the nanoscale.

Structural phase transitions of LaScO3 from first principles

We report a density functional theory study of LaScO3 perovskite using the SCAN meta-GGA potential. LaScO3 is of interest for applications such as hydrogen gas sensors, protonic ceramic fuel cells, and inorganic phosphors because of the useful electrical and optical properties it displays when doped, but little is known about its structural phase stability or phase transition behavior. We thus examine the relative stabilities of various candidate structures of LaScO3 and identify the likely transition sequence as a function of temperature. The low-temperature phase is confirmed to have an orthorhombic crystal structure with Pnma symmetry, with cooperative tilting of ScO6 octahedra about three pseudo-cubic axes, corresponding to Glazer tilt system a−b+a−. With increasing temperature, LaScO3 is predicted to undergo a second-order transition to a pseudo-tetragonal Imma structure (a−b0a−), followed by a first-order transition to a tetragonal I4/mcm structure (a0a0c−), and finally another second-order transition to the aristotype (cubic Pm3¯m; a0a0a0) structure below the melting point. The large difference in lattice enthalpy between the orthorhombic and cubic structures is consistent with the experimental difficulty in stabilizing the cubic structure at room temperature, although the results suggest other polymorphs may be accessible through synthesis at non-standard pressures or application of strain.

Enhanced piezoelectric properties and low electrical conductivity of Ce-doped Bi7Ti4.5W0.5O21 intergrowth piezoelectric ceramics

New types of Ce-doped CexBi7-xTi4.5W0.5O21 (BTW-BIT-xCe) Aurivillius intergrowth ceramics with high Curie temperatures were synthesized to improve the piezoelectric performances as well as the conduction behaviour, and these ceramics exhibit great potential for high-temperature lead-free piezoelectric applications. The crystal structure, electrical properties and conduction behaviour of BTW-BIT-xCe samples were analysed thoroughly. The XRD patterns combined with Rietveld refinements of the patterns showed that the crystal structure transformed from orthorhombic structure towards pseudo-tetragonal structure with increasing CeO2 dopant, indicating that a higher symmetry was obtained. The dielectric properties of Ce-doped samples were improved, accompanied by a significant drop in the dielectric loss and a slight decreased Curie temperature (705 °C–683 °C). An enhanced piezoelectric constant d33 of 25.3 pC/N was obtained in BTW-BIT-0.12Ce, which may be attributed to a common decrease in the electrical conductivity and coercive field. Besides, a low electrical conductivity of 2 × 10-6 S/cm at 540 °C was achieved in the same component owing to a decreased concentration of the oxygen vacancies, which was verified by analyses on XPS spectra. The above results indicate that Ce-doped BTW-BIT samples have great development potential for high temperature piezoelectric applications.

Structural and Magneto-Electrical Properties of Bi2-xSmxSr2CaCu2O8 + δ High Tc Superconductor Prepared by Pechini Method

Polycrystalline superconducting samples of Bi2-xSmxSr2CaCu2O8+δ were synthesized using Pechini method with different doping Sm concentrations (x = 0.0, 0.05, and 0.10) and investigated. FTIR spectroscopy showed that our final samples are totally free of organic elements. Le Bail refinement revealed that they were crystallized in the Bi-2212 phase. A small amount of the omnipresent Bi-2201 phase appeared as an impurity phase in all samples, whereas a small amount of Bi-2223 appeared only in the case of x = 0.5. In one hand, the Sm doping decreases the cell parameters, and in other hand, it leads to a transformation into a pseudo-tetragonal structure in the case of x = 0.10.The modulation was found to be incommensurate and increases with Sm content. The magneto-electrical resistivity curves show a transition to the superconductor state. The critical temperature TC decreases with the Sm doping, whereas the activation energy U0 increased from 53.4 meV to 68.9 meV.

Maximization of ferromagnetism in LaCoO3 films by competing symmetry

Spin state transition in perovskite cobaltite is delicately controlled by the competition between exchange interaction energy and crystal-field energy. The latter is mainly governed by the change of bond length and bonding angle. Previous work has revealed that the electronic configuration associated with spin state transition in $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO) thin films can be effectively modulated by epitaxial strain. However, a systematic study on the spin state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions in LCO films with different crystallographic symmetry is still missing. Here, keeping the in-plane strain unchanged, we report that the magnetization of LCO films can be manipulated with crystallographic symmetry. The ultrathin LCO layers, constrained by the cubic substrate, have pseudotetragonal structure and small magnetization. Upon increasing the layer thickness, the monoclinic structure dominates the LCO film and maximizes its ferromagnetism. For the LCO films with a thickness beyond 35 unit cells, the symmetry relaxes gradually towards its rhombohedral bulk form, and meanwhile the magnetization reduces. These results highlight the importance of spin-lattice entanglement in a ferroelastic material and provide a concise way to maximize its functionality using symmetry engineering.

Effect of Cesium Substitution on the Superconducting Properties of Bi-2212 Samples Prepared Via Solid-State Reaction and Laser Floating Zone Technique

In this work, the effect of cesium substitution on the structural and superconducting properties of Bi2Sr2Ca1-xCsxCu2O+y (where x = 0.0, 0.025, 0.050, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction route and by directional growth using the laser floating zone (LFZ) technique has been studied. X-ray diffraction has shown that both kinds of samples present the 2212 phase as the major one, with pseudo tetragonal structure, accompanied by minor amounts of (Sr,Ca)CuO2 and Bi2(Sr,Ca)4O6 + d secondary phases. SEM-EDX results indicate that Bi-2212 phase is the major one, with small amounts of secondary phases. Cs substitution diminishes the size and content of secondary phases. Magnetization measurements reflect different behavior for sintered and LFZ grown samples. Although the diamagnetic transition is very wide for the former samples, it is very sharp for the last ones. The critical onset temperatures, TC, are around 80 and 90 K, for sintered and LFZ grown samples, respectively. Moreover, M–H measurements indicate that LFZ grown samples exhibit larger areas than the sintered ones. The intragranular critical current, JC, calculated from the M–H loops and Bean’s formula, is maximum for x = 0.025Cs sintered and x = 0.05Cs LFZ grown samples. The maximum pinning forces, FPmax, determined in these samples are higher than those obtained in undoped ones, reaching 9 × 107 Oe*A/cm2 and 10 × 108 Oe*A/cm2, at 10 K, for sintered and LFZ grown samples, respectively.

Polarization switching, dielectric, structural and elastic properties of 2-Methylbenzimidazole crystals and films

Single crystals and films of 2-methylbenzimidazole were grown from ethanol solution by slow cooling and evaporation method. 3D XRD study confirms pseudo tetragonal structure of MBI crystals. MBI films have a texture formed by spherulite type blocks with in-plane orientation of pseudo-tetragonal [001] axis. MBI crystals and films surface were studied by atomic force microscopy to evaluate Young’s modulus and surface potential distributions related to small charge of domain walls. Dielectric hysteresis loops were simulated in frames of particular case of KAI model. Strong decrease of activation field with temperature increase indicates a potential ferroelectric transition above melting point.

Optical modulation and magnetic transition in PbTi1−xPdxO3−δ ferroelectric thin films

The PbTi1−xPdxO3−δ (xPTPO) thin films prepared by chemical solution deposition have been investigated by means of structural characterizations, optical and magnetic measurements. X-ray diffraction patterns show that all the films have a pseudotetragonal perovskite structure, but also exhibit a lattice dilatation behavior and increased internal strain as the x increases. A possible mechanism for strain-induced structural evolution is discussed. Raman scattering further corroborates this change in average structure, where the characteristic variation of phonon modes, indirectly reveal the incorporation of Pd2+ ions into host lattice. Transmittance spectra analysis indicates that Pd doping has a key effect on the energy band structure. The optical bandgap of xPTPO films decreases significantly with increasing Pd content, expressed by (3.5–9.0x) eV (0≤x≤0.09). Also, magnetic switching driven by doping has been confirmed in the films, which is attributed to the competition between ferromagnetic and paramagnetic/antiferromagnetic components.

Study of dielectric and ferroelectric properties of five-layer Aurivillius oxides: A2Bi4Ti5O18 (A = Ba, Pb and Sr) synthesized by solution combustion route

This paper presents the ferroelectric and dielectric properties of five-layer Aurivillius oxides (Ba2Bi4Ti5O18, Pb2Bi4Ti5O18 and Sr2Bi4Ti5O18) prepared by a solution combustion route with glycine as a fuel at low calcination temperature. The phase formation of these materials with pseudo-tetragonal structure was achieved after calcination at 750°C for 3 h; as confirmed by X-ray diffraction studies. Scanning electron microscopy of the sintered ceramics shows that the grains exhibit a plate-like morphology. The ferroelectric to paraelectric transition temperature (T c) for Ba-, Pb- and Sr-based bismuth titanate ceramics was found to be 350, 280 and 260°C, respectively. All three materials show multiple relaxation phenomena and their electrical conductivity was found to be temperature dependent. The Pb2Bi4Ti5O18 ceramic possessed the highest value of activation energy (0.68 eV) and hence shows better ferroelectric properties, as compared to barium and strontium bismuth titanates.

Magnetic Domains and Twin Microstructure of Single Crystal Ni–Mn–Ga Exhibiting Magnetic Shape Memory Effect

Magnetic Shape Memory Effect or more precisely magnetic-field-induced-structural reorientation (MIR) generate large strain (up to 12 %) and fast response (around 1 kHz) in a moderate magnetic field below 1 T. The strain is caused by twin microstructure reorientation in which the maximum generated strain is determined by difference of lattice constants of the pseudotetragonal structure a = b > c, i.e. e MAX = e 0 = 1−c/a. The reorientation is mediated by twin boundary motion. Very high mobility is, therefore, necessary condition for the existence of the effect. Recently considering more precise structural description of 10M martensite of Ni-Mn-Ga alloys as monoclinic, i.e. a > b > c and γ π 90, we showed that there are two different kinds of mobile a-c twin boundary, Type I and II with different microstructure [1]. The boundaries differ not only in magnitude of twinning stress needed to move twin boundary, i.e. in mobility, but also in temperature dependence of the mobility or twinning stress [2]. Despite of the simplicity of the moving interface the twinned structure is complex [3]. The model of the movable Type II twin boundary is shown in the figure. The twinned structure consists apart of the moving a-c twin boundary the modulation twinning bands and a-b twinning.

Strain-induced structural phase transition, ferromagnetic and optical properties of Bi1−xTbxFeO3 thin films

In this paper, we report on the structural phase transition, surface morphologies, ferromagnetic and optical properties that have been observed in Bi1−xTbxFeO3 (BTFO) thin films with (x = 0, 0.03, 0.06 and 0.09). X-ray diffraction (XRD) indicated that all the films have a single phase with rhombohedral structure and exhibit gradual transition behavior to pseudo-tetragonal structure with the increase of x, the strain values were estimated to be 0.1–0.8, depending on composition. A possible mechanism, strain-induced structural phase transition, is discussed. The micro-Raman spectroscopy analysis was in good agreement with structural phase transition verified by the XRD patterns. Moreover, with an increase in Tb3+ ions from x = 0–0.06, the saturation magnetization of the BTFO thin films shows a linear increase, which is mainly due to the Tb3+ ions. However, a sharp increase emerges with x from 0.06 to 0.09, the phase transition mainly dominates the higher ferromagnetism in BTFO films. Moreover, BTFO films with x > 0 exhibit a lower band-gap than that with x = 0, originating from local rhombohedral to pseudo-tetragonal structure transition. These results are helpful for a deeper understanding of structural transition, magnetic, and optical properties in perovskite oxides and show the potential roles, which such materials can play in multiferroic applications and solar energy devices.

Effect of Tb-doping on structural, magnetic and optical properties of BiFeO3 films prepared by chemical solution deposition

Single-phase multiferroic Bi1-xTbxFeO3 (xBTFO, x=0.00, 0.03, 0.06 and 0.09) films were deposited on quartz substrates by chemical solution deposition. X-ray diffraction patterns indicate that all films exhibit a distorted pseudo-tetragonal perovskite structure with a high degree of (110) preferred orientation. The Tb-doping at A-site of BiFeO3 (BFO) results in lattice contraction, improves surface morphology and reduces the average grain size of the films. One A mode and five E modes of films are observed in Raman scattering spectra, and the position of A mode shifts to higher wavenumber with the increasing of x due to the enhanced Bi–O covalent by Tb doping. With increasing x from 0.00 to 0.09, the saturation magnetization of xBTFO films increases and can be expressed by Ms=(34.6x+1.1) emu/cm−3, originating from different bond interactions between 4f, 5d orbitals of Tb–O and 6p orbitals of Bi–O. Compared with BFO film, the band gap of xBTFO films decreases, which is mainly due to the increased the tailing of the conduction band edge into the band-gap, which is leading to decreasing the band gap.

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. These findings may open up a route to explore promising perovskite oxides as candidate materials for use in multiferroics and solar-energy devices.

Sequence of structural transitions in BiFeO3–RMnO3 thin films (R=Rare earth)

The BiFeO3 (BFO)–RMnO3 (R=rare earth) system affords an interesting combination of good ferroelectric polarization and magnetization properties at room temperature that is prerequisite for intrinsic multiferroism. In the present work we report on the structural properties of the BFO–RMnO3 (R=Ho, Gd, La) solid solution in a range of concentrations. Using XRD and particularly Raman scattering we show the BFO–RMnO3 system undergoes a structural transition from monoclinic toward ferroelectric orthorhombic structure for RMnO3 concentrations of up to 10% and beyond it to paraelectric pseudo tetragonal structure for large ions or to pseudocubic for the small ones. The phase transitions are explained in terms of the change of the nature of octahedral distortion which is found to be sensitive to the concentration and size of the rare earth element, and the presence of manganese.

Raman spectroscopy insights into the size-induced structural transformation in SnSe nanolayers.

Raman spectroscopy is used to probe the structural changes in [SnSe]m[MoSe2]n ferecrystal thin films as a function of m, the number of bilayers of SnSe. In spite of the interleaved structure in the intergrowths, Raman spectra can be described as a superposition of spectra from the individual components, indicating that the interaction at the interface between the components is relatively weak. Analysis of room-temperature Raman spectra indicate that the MoSe2 layers separating the SnSe layers are nanocrystalline in all of the samples studied, with little change as the number of Se-Mo-Se trilayers (n) or SnSe bilayers (m) increases, reflecting the rotational disorder between adjacent trilayers. A thickness-dependent, continuous transition occurs in the SnSe layer as m is increased, from a pseudotetragonal structure when the layers are thin to a bulk-like orthorhombic SnSe structure when the SnSe layer thickness is increased. Polarization analysis of the Raman scattering from these materials allows the symmetry evolution of the SnSe layers through this transition to be determined.