Composition-dependent Phase Transition Research Articles

The role of Ca/Zr ratio on the local structure and phase transitions in lead-free (Ba,Ca)(Zr,Ti)O3

In this study, the structural phase transitions are investigated as a function of composition and temperature for polycrystalline x(Ba0.7Ca0.3)TiO3-(1 − x)Ba(Zr0.2Ti0.8)O3 (x = 0.40, 0.45, 0.50, 0.55, and 0.60) through a combination of Raman spectroscopy, synchrotron X-ray diffraction, and dielectric spectroscopy. The aim is to gain insight into the complex phase boundary region responsible for the excellent electromechanical properties. The results demonstrate the correlation between local site substitutions based on the stoichiometric variations to the microstructure and dielectric properties. The dielectric response has been correlated with the BCT/BZT content displaying a maximum depending on the phase content. Additionally, in situ temperature-dependent Raman, permittivity, and pair distribution function (PDF) studies were performed from −60 °C to 130 °C to highlight the structure and phase evolution. In particular, the in situ temperature-dependent Raman measurements reveal sudden discontinuities in the vibrational modes that correspond to the structural changes in the perovskite structure. Therefore, the results of the permittivity response, based on the average of a large volume, are consistent with the local structural changes obtained by other techniques. Indeed, correlations of Raman, X-ray diffraction, and PDFs obtained from synchrotron X-ray total scattering data, along with permittivity measurements allowed the identification of the discrete ferro- to paraelectric phase transitions and a more robust characterization. The rotation and distortion of the octahedral caused by oxygen displacement are driving forces behind symmetry changes and phase transitions, explaining the mechanism of polymorphic phase transition based on A- and B-site substitution. Therefore, this work provides a comprehensive understanding of temperature and composition-dependent phase transitions in BCZT.

Enhanced relaxor behavior and high energy storage efficiency in niobium substituted (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics

The microstructure, dielectric properties, relaxor behavior, and energy storage efficiency of un-substituted and niobium (Nb) substituted (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 (for x = 0, 0.02 and 0.05) samples prepared by the solid-state reaction method has been studied in detail. All the samples exhibited perovskite structure with no trace of impurity. Composition-dependent phase transition was also observed on the addition of Niobium. At room temperature, the co-existence of rhombohedral and tetragonal phases is observed in the unsubstituted samples. As the composition changes from x = 0.02 to x = 0.05, a structural change from tetragonal to cubic is observed. A remarkable reduction in grain size, from 90 μm (for x = 0) to 1.21 μm for (x = 0.05), is observed with the addition of Niobium. This result suggests that Niobium acts as a grain growth inhibiter in barium calcium zirconium titanate (BCZT) ceramics. The effect of Niobium on transition temperature is studied from the temperature-dependent dielectric permittivity graph. It was clear that the transition temperature shifted to a lower temperature region, and for x = 0.05, at a very low temperature (−23 °C/250 K) the tetragonal to cubic transition was observed. At x = 0.05, the temperature-dependent dielectric permittivity showed a broadened curve, indicating a diffuse phase transition. The diffuse phase transition in Nb substituted samples is explained by Uchino and Nomura modified Curie Weiss law.Moreover, the observations on temperature-dependent dielectric permittivity measurements at various frequencies suggest that the substitution of Nb5+ induces relaxor behavior. The energy storage efficiency of unsubstituted and Nb substituted samples was calculated from the polarization versus electric field graph. A high storage efficiency of 84% was obtained for the Nb substituted sample (x = 0.05) at 12 kV cm−1. Enhanced relaxor behavior and increased storage efficiency were observed in (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 at x = 0.05. Thus we suggest that these are promising materials for energy storage applications.

Composition dependence of the charge-driven phase transition in group-VI transition metal dichalcogenides

Materials exhibiting multiple stable phases can be used as functional components in electronic and optical applications if the phase transition is controllable. Group-VI transition metal dichalcogenides (TMDs, MX$_2$, where M=Mo, W and X = S, Se) are known to undergo charge induced transitions from semi-conducting H phases to metallic T phases. This occurs, for example, when bulk TMDs are exfoliated with the aid of alkali ion intercalants. However, it is difficult to experimentally decouple the effect of composition-dependent phase transition barriers from indirect effects related to the exfoliation process. Here, using first-principles calculations, we study the energetics of transition between the different structural polytypes of four group-VI TMDs upon lithium adsorption. We find that both the activation barrier from the H phase to the metallic phase in charged monolayers and the reverse barrier in neutral monolayers are required to explain experimental results. We show that the high proportion of metallic phase found in WS$_2$ monolayers after alkali treatment can be explained by high transition barriers to revert back to the H phase once in a neutral state. The calculated barriers however cannot explain the low proportion of metallic phase found in MoS$_2$ monolayers in some experiments and so non-electronic effects must also play a role.

Composition dependent phase transition and its induced hysteretic effect in the thermal conductivity of WxMo1−xTe2

Recently, transition metal dichalcogenide (TMD) materials have shown promise in electronics and optoelectronics applications. Most of their properties are closely related to their abundant structural phases and phase transitions. For more practical applications in the future, it is necessary to tune the phase transitions in this material system. Here, we demonstrate the modulation of phase transitions in miscible WxMo1−xTe2 samples by appropriate alloying. The temperature dependent thermal conductivity along the c-axis, which strongly relates to the phase structures and the defect level, has been measured using the time-domain thermoreflectance method. In addition, a tunable hysteretic effect, induced by phase transitions, is observed in both thermal and electrical transport properties and confirmed by the consistent hysteresis in the Raman spectroscopic study. This hysteretic effect can be applied to realize phase-change storage devices. Furthermore, we provide a phase diagram to illustrate the composition dependent phase transition in WxMo1−xTe2. This work demonstrates an approach to modulate phase transition and thermal hysteresis in such a TMD material system by alloying engineering.

Temperature and composition dependent phase transitions of lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 thin films.

Extensive studies on (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 - x%) (Bi0.5Na0.5)TiO3-x% BaTiO3 (x = 0-15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol-gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.

Linear composition-dependent phase transition behavior and energy storage performance of tetragonal PLZST antiferroelectric ceramics

The composition dependent dielectric, ferroelectric and energy storage properties of tetragonal lead lanthanum zirconate stannate titanate (PLZST) antiferroelectric ceramics were systematically studied in this paper. Two sets of simple linear scaling relations were established for the forward switching field EAFE-FE, the backward switching field EFE-AFE and recoverable energy density Wre. As Zr content increases (Ti fixed), EAFE-FE, EFE-AFE, Wre decreases linearly, while the stored energy density Wst declines first and then increases. In addition, decreasing Ti content (Zr fixed) leads to linear increments in all of EAFE-FE, EFE-AFE, Wre and Wst. These results not only reveal the superiority of PLZST with lower Zr and lower Ti contents for energy storage capacitors, but also provide a fast way to design PLZST ceramics with specific energy storage properties. Our work may also be very helpful for better understanding the mechanism of phase transition behaviors of antiferroelectric materials.

The effects of phase transitions and compositional layering in two-dimensional kinematic models of subduction

Subduction zones exhibit a wide range of behaviour, from slab stagnation at the base of the transition zone, to direct sinking into the lower mantle. Numerical simulations have utilised a range of approximations to improve computational efficiency, such as limiting the phase transitions or compositional effects included in a model: the net effect of these approximations on slab dynamics remains unclear. With the goal of developing more complete, self-consistent, and less idealised simulations, we analyse the importance of various modelling choices on slab evolution: the presence of shear, adiabatic and latent heating, compositional layering and composition-dependent phase transitions. We find that slabs in models with additional heating terms (extended Boussinesq approximation) are warmer, weaker and exhibit more folding and stretching than in models without these terms (Boussinesq approximation). The presence of compositional layers and the stress-dependence of rheology acts to strengthen the slab, decreasing slab dip and leading to less folding and lateral migration of the slab. We also find that the extent of slab folding and stagnation is overestimated by modelling only the olivine phase transitions at 410 and 660km. A more realistic subduction zone with compositional layers and all the pyrolite phase transitions exhibits almost no slab stagnation at 660km and is instead characterised by broad, periodic, slab folding. This slab behaviour is more similar to a model with no phase changes or compositional layers. Therefore, for studies in which a complete treatment of compositional layers and phase transitions is not possible, rather than including an incomplete approximation that over-predicts folding, the large-scale deformation of slabs is best approximated by a model with no phase transitions and no layers.

Investigation on structural, Mössbauer and ferroelectric properties of (1−x)PbFe0.5Nb0.5O3–(x)BiFeO3 solid solution

In this study, (1−x)PbFe0.5Nb0.5O3(PFN)–(x)BiFeO3(BFO) multiferroic solid solutions with x=0.0, 0.1, 0.2, 0.3 and 0.4 were synthesized through single step solid state reaction method and characterized thoroughly through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FTIR), Raman, Mössbauer spectroscopy and ferroelectric studies. The room temperature (RT) XRD studies confirmed the formation of single phase with negligible amount of secondary phases (x=0.2 and 0.4). The zoomed XRD patterns of (1−x)PFN–(x)BFO solid solutions showed the clear structural phase transition from monoclinic (Cm) to rhombohedral (R3c) at x=0.4. The Raman spectra of the (1−x)PFN–(x)BFO solid solutions showed the composition dependent phase transition from monoclinic (Cm) to rhombohedral (R3c). With increasing x in PFN, the modes related monoclinic symmetry changes to those of rhombohedral symmetry. The RT Mössbauer spectroscopy results evidenced the existence of composition dependent phase transition from paramagnetic to weak antiferromagnetic ordering and weak antiferromagnetic to antiferromagnetic ordering. The Mössbauer spectroscopy showed paramagnetic behavior with a doublet for x=0.0, 0.1 and 0.2 are shows the weak antiferromagnetic with paramagnetic ordering. For x=0.3 and 0.4 shows the sextet pattern and it is a clear evidence of antiferromagnetism. The ferroelectric (P–E) loops at RT indicate the presence of small polarization, as the x concentration increases in PFN, the remnant polarization and coercive field were decreased, which may due to the increase in the conductivity and leaky behavior of the samples.

Optimizing thermoelectric property of antimony telluride nanowires by tailoring composition and crystallinity

In this work, an effect of the composition and microstructure changes on the electrical and thermoelectric properties of antimony telluride (SbxTey) nanowires was investigated to determine the optimum conditions that give the highest Seebeck coefficient. For a systematic study, a facile template-directed electrodeposition technique was utilized to prepare amorphous SbxTey nanowires with a well-controlled dimension and composition. The composition-dependent phase transition behavior of the nanowires, corresponding to the amorphous-to-crystalline solid-state transition, was investigated by measuring their electrical resistivity changes, where the electrical resistivity variation ratio (Δρ/ρ0 = (ρ − ρ0)/ρ0) was highly dependent on the Sb content. Temperature-dependent electrical properties (electrical resistivity, thermal activation energy (Ea), and field effect transistor (FET) mobility) of the amorphous and crystalline nanowires were also characterized before/after the phase transition. Material properties of the nanowires, such as crystallinity, grain size, and phase segregation as a function of the Sb content, were tailored by a post-annealing process. The tailored material properties successfully altered the electrical and thermoelectric properties of the nanowires. The highest Seebeck coefficient of 318 μV K−1 at the carrier concentration of 1.1 × 1019 cm−3 was observed in the Te-segregated Sb2Te3 (i.e., Sb32Te68) nanowires.

Discovery of New Solid Solution Phosphors via Cation Substitution-Dependent Phase Transition in M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) Quasi-Binary Sets

The cation substitution-dependent phase transition was used as a strategy to discover new solid solution phosphors and to efficiently tune the luminescence property of divalent europium (Eu2+) in the M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors including the greenish-white SrCa2(PO4)2:Eu2+, the yellow Sr2Ca(PO4)2:Eu2+, and the cyan Ba2Ca(PO4)2:Eu2+ were reported, and the drastic red shift of the emission toward the phase transition point was discussed. Different behavior of luminescence evolution in response to structural variation was verified among the three M3(PO4)2:Eu2+ joins. Sr3(PO4)2 and Ba3(PO4)2 form a continuous isostructural solid solution set in which Eu2+ exhibits a similar symmetric narrow-band blue emission centered at 416 nm, whereas Sr2+ substituting Ca2+ in Ca3(PO4)2 induces a composition-dependent phase transition and the peaking emission gets red shifted to 527 nm approaching the phase transition point. In the Ca3–xBax(PO4)2:Eu2+ set, the validity of crystalloc...

Structural investigation of tungsten bronze-type relaxor ferroelectrics in the BaxSr3−xTiNb4O15system

Compounds in the BaxSr3−xTiNb4O15series withx = 0.0–3.0 were synthesised. Synchrotron X-ray diffraction data were collected for each member synthesised. Rietveld refinements were performed for all compositions. A composition-dependent phase transition between orthorhombic and tetragonal symmetries was found. Cation ordering was observed between barium and strontium atoms in the two distinctAsite cavities

Composition–phase structure relationship and thickness-dependent ferroelectricity of rhombohedral phase in [1 1 1]-textured Nb-doped Pb(Zr,Ti)O3 thin films

2% Nb-doped [111]-oriented lead zirconate titanate films (PNZT) of different Zr/Ti ratios ranging from 30/70 to 70/30 with a fixed thickness were deposited on the Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method. The orientation degree, film morphology and phase transition of the PNZT films were investigated as a function of Zr/Ti ratios. The experimental results verified the composition-dependent phase transition from tetragonal to rhombohedral of the PNZT films with a visible [111] preferred orientation. To obtain high performance by making use of rhombohedrel [111]-oriented thin films, Pb(Zr0.6Ti0.4)Nb0.02O3 films with various thicknesses varying from 70nm to 300nm were studied to reveal the relationship among the film thickness, stress and ferroelectric response. It was found that ferroelectric property reached a high remnant polarization (Pr) of 61.7μC/cm2 and a minimum coercive field (Ec) of 70.7kV/cm at 300nm due to the release of residual tensile stress.