Structural Phase Transition Temperature Research Articles

On the Theory of Initiating Effect of Continuous Heating on Structural Transformations and Phase-Transition Temperature in Crystals

A model and kinetic theory of structural changes in crystals upon continuous heating are proposed; this theory takes into account nonadiabatic atomic transitions between different states of the system of nuclei and electrons. The nonadiabatic transitions initiate structural transformations determined by a change in the short-range order during the electron-density redistribution. It is shown that continuous heating stimulates structural changes in a crystal and reduces the temperature of the known phase transition.

Coexistence of odd-parity and even-parity order parameters in the multipole order phase of the spin-orbit coupled metal Cd2Re2O7

Magnetic torque measurements have been performed to investigate the phase transitions of the metallic pyrochlore compound $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$, in which a spin-orbit (SO) interaction leads to unique multipole orders associated with an inversion symmetry breaking. We find that the magnetic torque signals with a fourfold symmetry (${\ensuremath{\tau}}_{4}\ensuremath{\propto}{H}^{4}$) as well as a twofold symmetry (${\ensuremath{\tau}}_{2}\ensuremath{\propto}{H}^{2}$) are significantly enhanced at low temperatures below a structural phase transition temperature (\ensuremath{\sim}200 K). The analyses of the torque curve symmetries show that the ${\ensuremath{\tau}}_{4}$ term arises from an even-parity order parameter (OP) with the irreducible representation ${E}_{\mathrm{g}}$, whereas the ${\ensuremath{\tau}}_{2}$ term from an odd-parity OP with ${E}_{\mathrm{u}}$, ${T}_{1\mathrm{u}}$ or ${T}_{2\mathrm{u}}$. The parity mixing of the primary OPs shows a peculiar phase transition in the SO coupled $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$. The coexistence of the two OPs provides important insights into the origin of the multipole orders induced by the SO interaction.

Ab Initio Studies of Phase Transformations in Fe100 – xSix

A concentration phase diagram for Fe100 – xSix (9.375 ≤ x ≤ 25.0 at %) alloys has been built on the base of the structural and magnetic phase transition temperatures estimated theoretically from the first principles. The structural phase transition temperatures for the experimentally observed crystal structures are obtained from the geometric optimization of the crystal structure. The Curie temperatures are estimated in a molecular field approximation using the constants of magnetic exchange interaction calculated ab initio. As temperature increases, the structural transitions from the ordered cubic phase to a completely disordered structure occur via a partially disordered structure over the entire concentration range. As for the magnetic transformations, the ferromagnet–paramagnet transition is observed for all the compositions, but in various crystal phases.

Correlations between electronic order and structural distortions and their ultrafast dynamics in the single-layer manganite Pr0.5Ca1.5MnO4

Time-resolved x-ray diffraction experiments on the half-doped single-layered manganite $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ are used to monitor the ultrafast photoinduced dynamics of the structural distortion associated with the charge and orbital ordering (CO/OO). As in the nonlayered three-dimensional counterpart, the ordered phase melts in less than 100 fs after 800-nm photoexcitation and subsequently partially recovers due to thermal equilibration of electronic and vibrational systems. Photoexciting $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ below the transition temperature of a second structural phase transition that occurs around 146 K (deep inside the CO/OO phase) releases this structural transition, but progresses on a much slower timescale. This additional reduction of crystal symmetry, which we ascribe to a further tilt of the oxygen octahedra, can thus be considered to be only weakly coupled to CO/OO. Furthermore, static hard-x-ray and resonant soft-x-ray diffraction at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edges experiments identify correlations between structural distortions and electronic order in thermal equilibrium.

Electric-field-induced phase transition in 2D layered perovskite (BA)2PbI4 microplate crystals

Two-dimensional (2D) lead halide perovskite materials are solution-processable semiconductor materials, which would find promising applications in optoelectronic devices. The fundamental understanding of the structural phase transition in two-dimensional perovskites is of great importance for fully exploiting their potential applications in electronic and optoelectronic devices. Here, we report on how the external electric field affects the structural phase transition in 2D perovskite (BA)2PbI4 microplates via temperature-dependent photoluminescence spectroscopy. A high-temperature phase and a low-temperature phase can coexist in a wider range of temperatures. The external electric field would enhance the phase transition to the dominant phase depending on the surrounding temperature. This field-induced phase transition might be ascribed to the release of strain in the perovskite microplate induced by the applied electric field, leading to the change in the structural phase transition temperature. Our findings are of great significance not only to the fundamental understanding of phase transition but also to the design and optimization of two-dimensional perovskite based electronic and optoelectronic devices.

Correlation between structural phase transition and surface chemical properties of thin film SrRuO3/SrTiO3 (001).

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO3 (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment. The directions and behavior of the core level shift of Ru and Sr are attributed to the formation of oxygen vacancy across the SPT temperature of SRO. The analysis of in situ XRD and AP-XPS results provides an evidence for the formation of metastable surface oxide possibly due to the migration of internal oxygen atoms across the SPT temperature, indicating the close relationship between oxygen vacancy and SPT in SRO thin films.

Фазовые превращения в сплавах Fe-=SUB=-100-x-=/SUB=-Si-=SUB=-x-=/SUB=-: исследования ab initio

In present work, on the basis of structural and magnetic phase transition temperatures estimated theoretically from the first principles, the concentration phase diagram for Fe100-xSix (9.375 ≤ x ≤ 25.0 at.%) was plotted. Structural phase transition temperatures for the experimentally observed crystal structures were obtained from the structural optimization. The Curie temperatures were estimated from mean field approximation using ab initio calculated exchange coupling constants. Both structural and magnetic phase transitions temperatures are found in qualitative agreement with the experiment. For the whole considered concentration range, structural transitions from the ordered cubic to fully disordered through partially disordered one are appeared with temperature increasing. As for magnetic transformations, ferromagnetic-paramagnetic transition is observed for all compositions, however, it happens in different crystal phases.

Temperature Dependence of the Vibrational Mode of Pb1 –xSnxTe Films Grown by MBE on the GaAs/CdTe Hybrid Substrate

Reflectance spectra of thin Pb1 –xSnxTe films (~60 nm) with x = 0.25, 0.53, and 0.59, grown by MBE on hybrid GaAs/CdTe substrates, are measured in the frequency range of 20–5500 cm–1 and the temperature range of 5–300 K. The temperature dependence of transverse optical phonon frequencies and structural phase transition temperatures are determined based on a dispersion analysis. An increase in the plasma frequency with decreasing temperature in the range 300‒77 K in all films is detected.

Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr2-xNdxNiO4+δ under controlled oxygen partial pressures.

The behavior of the structural orthorhombic-tetragonal phase transition of Pr2-xNdxNiO4+δ, a candidate material for solid oxide fuel cells and oxygen permeation membranes, was investigated by differential scanning calorimetry and thermogravimetry (TG), under controlled oxygen partial pressures, P(O2). The structural phase transition temperature, TP, of Pr2-xNdxNiO4+δ increased with increasing Nd content, x, or increasing P(O2). The phase transitions of all compositions involved discrete variations in the oxygen content, Δδ, which were observed in the TG curves under various P(O2) values. Δδ of Pr2-xNdxNiO4+δ with 0.5 ≤x≤ 1.5 were between those of Nd2NiO4+δ and Pr2NiO4+δ, regardless of P(O2), and were slightly increased with decreasing P(O2). It was proposed that the effect of the valence change of the Pr ion on Δδ was decreased with increasing Nd content. The standard enthalpy change, ΔH°, and entropy change, ΔS°, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship between P(O2) and TP using an ideal solution model. ΔS° was decreased with increasing Nd content for the specimens with 0.0 ≤x≤ 1.5. The ΔH° of Pr2-xNdxNiO4+δ with 0.0 ≤x≤ 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr2-xNdxNiO4+δ with increasing x from 0.0 to 1.5 was successfully explained using the calculated values of ΔH° and ΔS°.

Phase transitions in Fe3Al-based alloys: ab initio study

ABSTRACTIn this work, based on structural and magnetic phase transition temperatures estimated theoretically from the first principles, the concentration phase diagram for FeAl ( at.%) was plotted. Structural phase transition temperatures for the experimentally observed crystal structures were obtained from the structural optimization. It is shown that for x = 28.125 at.% L1-like phase with slight distortions is energetically favorable. The ground state energy and magnetic moments of FeAl with different structures were calculated within GGA and GGA+U approaches. It is shown that in the case of GGA+U approximation, L1 phase becomes more stable compared with D0. The Curie temperatures were estimated from mean field approximation and Monte Carlo simulations using ab initio calculated exchange coupling constants. Both structural and magnetic phase transition temperatures are found in qualitative agreement with the experiment. Depending on Al concentration, five different sequences of phase transitions are observed in the phase diagram.

Annealing effect on phase transition and thermochromic properties of VO2 thin films

Abstract Vanadium dioxide (VO2) is one kind of desired thermochromic material for many smart devices because of its notable temperature-responsive infrared modulation and metal−insulator transition with underlying structural phase transition (SPT). Understanding on the tuning of these properties in an anticipated manner is essential to accomplish device realization. Here, we report the annealing time induced modifications in SPT and the thermochromic properties of VO2 thin films. Using RF sputtering deposition, VO2 thin films were grown at room temperature and their ex-situ annealing was carried out at 600 °C for different time from 2 min to 60 min. Structural, electronic and thermochromic properties of these films were investigated. VO2 thin films samples annealed for longer time exhibit larger crystallite size, higher surface roughness and SPT temperature, and the reduced hysteresis width of SPT during heating and cooling cycle. X-ray absorption spectroscopy results indicate that the variation in the annealing time do not alter the electronic structure significantly. Nevertheless, VO2 thin film samples with prolonged annealing display higher IR transmittance modulation across the SPT.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical, electronic and optoelectronic properties of the materials, especially low-dimensional materials. Two-dimensional (2D) organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field; nevertheless, studies on this remain elusive. Here we report on how the surface depletion field affects the structural phase transition, quantum confinement and Stark effect in 2D (BA)2PbI4 perovskite microplates by the thickness-, temperature- and power-dependent photoluminescence (PL) spectroscopy. Power dependent PL studies suggest that high-temperature phase (HTP) and low-temperature phase (LTP) can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates. With the decrease of the microplate thickness, the structural phase transition temperature first gradually decreases and then increases below 25 nm, in striking contrast to the conventional size dependent structural phase transition. Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase, the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs. This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes. Along with the thickness dependent phase transition, the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

Effect of Ti:ZnO Layer on the Phase Transition and the Optical Properties of VO2 Film

VO2 single-layer and VO2/Ti:ZnO bilayer thin film samples were stabilized on quartz substrates by using sputtering technique, and their structural, morphological and optical properties were investigated. The X-ray diffraction (XRD) results confirmed that VO2 stabilized in the mono-clinic structure in both the samples. Temperature-dependent XRD results demonstrated that the monoclinic-to-rutile structural phase transition temperature had shifted in bilayer sample as compared to VO2 single-layer sample. A uniform surface morphology was realized in both samples, as shown by atomic force microscopy. UV-vis spectroscopy results revealed a significant improvement in the optical transmission in the VO2/Ti:ZnO bilayer thin-film sample. Improved visible transmittance of bilayer sample identifies it as a good candidate for smart window applications.

Manipulation of dielectric, ferroelectric and magnetic anomalies in multiferroic, morphotropic phase boundary quenched BiFeO3-0.35PbTiO3 solid solutions

The effect of thermal quenching on physical properties of morphotropic phase boundary BiFeO3-0.35PbTiO3 composition, composed of Pnma, R3c and P4mm phases, has been investigated in detail. We detected and quantified role of quenching through investigation of magnetic, dielectric and ferroelectric anomalies. Quenching significantly i) enhances the tetragonal phase percentage (hence affects domain structure), ii) reduces domain wall clamping with a large increase in electrical polarization, iii) increases magnetization at the structural phase transition temperature (tunable magneto-electric coupling), iv) magnifies intrinsic property i.e. intra grain relaxation dynamics (which are otherwise suppressed due to the pinning of the defect dipoles), etc. All these findings clearly verify the role of quenching which noticeably enhances multiferroic properties at the well-known morphotropic phase boundary.

Influence of europium doping on the structural phase-transition temperature of β− and α−CoMoO4 polymorphs

This work presents a new possibility to increase the phase transition temperature of CoMoO4 polymorphs (β and α) by europium doping. The material Co0.98Eu0.02MoO4 was produced by the co-precipitation method and its thermochromic property was investigated. An accurate characterization of the material was performed using X-ray diffraction, Raman spectroscopy, diffuse reflectance spectroscopy, thermal analysis, and morphological analysis. The results showed that an irreversible first order phase transition between the two allotropic forms occurred at around 950 °C, and it is associated with a drastic color change from purple (beta) to green (alpha). This color change confirmed the promising potential of the material for use as a temperature indicator that could be observed by the naked eye. Photoluminescence spectroscopy helped to elucidate the effect of the presence of europium added to the cobalt molybdate, which also enabled the material to be used as a light emitter under UV excitation.

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

Abstract The (K0.90Li0.10) (Nb0.80Ta0.20)0.99Mn0.01O3 (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.

Thickness dependent uniaxial magnetic anisotropy due to step-edges in (1 1 1)-oriented La0.7Sr0.3MnO3 thin films

Abstract The magnetic anisotropy of films of La0.7Sr0.3MnO3 grown on vicinal (1 1 1)-oriented SrTiO3 substrates are investigated. For temperatures above the tetragonal – cubic structural phase transition temperature of the substrate, a step edge induced uniaxial magnetic anisotropy is found at remanence, with a thickness-driven change in easy axis direction, from perpendicular to the step edges to parallel to the step edges with increasing thickness. The anisotropy constant for the investigated (1 1 1)-oriented samples is of the same magnitude as for previously reported (0 0 1)-oriented samples. The data is discussed in the framework of in-plane rotations of the oxygen octahedra resulting in a uniaxial anisotropy. Furthermore, the magnetic anisotropy is sensitive to the structural phase transition at 105 K of the substrate, and the anisotropy constant increases drastically as the temperature is lowered below 105 K.

Contrasting the magnetism in La2−xSrxFeCoO6(x=0,1,2) double perovskites: The role of electronic and cationic disorder

The magnetism of the double perovskite compounds \SLFCOx\ ($x$ = 0, 1, 2) are contrasted using magnetization, neutron diffraction and electron paramagnetic resonance with the support from density functional theory calculations. \LFCO\ is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at $T_N$ = 270~K, accompanied by a low temperature structural phase transition at $T_S$ = 200~K. The structural phase transformation at $T_S$ occurs from $R\overline{3}c$ at 300~K to $Pnma$ at 200~K. The density functional theory calculations support an insulating non-compensated AFM structure. The long-range ordered magnetism of \LFCO\ transforms to short-range glassy magnetism as La is replaced with Sr in the other two compounds. The magnetism of \LFCO\ is differentiated from the non-equilibrium glassy features of \SFCO\ and \SLFCO\ using the {\em cooling-and-heating-in-unequal-fields} (CHUF) magnetization protocols. This contransting magnetism in the \SLFCOx\ series is evidenced in electron paramegnetic resonance studies. The electronic density-of-states estimated using the density functional theory calculations contrast the insulating feature of \LFCO\ from the metallic nature of \SFCO. From the present suite of experimental and computational results on \SLFCOx, it emerges that the electronic degrees of freedom, along with antisite disorder, play an important role in controlling the magnetism observed in double perovskites.

Room temperature crystal structure and high temperature structural and magnetic phase transitions in Sr(Fe0.5Nb0.5)O3 ceramic

The room temperature structure of the perovskite Sr(Fe0.5Nb0.5)O3 ceramic has been investigated by Rietveld structure refinement using high resolution X-ray diffraction (XRD) data. The correct structure is determined to be tetragonal in the space group I4/mcm ruling out orthorhombic (Pbnm), tetragonal (P4 mm), and monoclinic structures previously reported by authors. The high temperature structural phase transition has been studied by Rietveld analysis using the high resolution XRD data in the temperature range of 300–850 K. A high temperature structural phase transition is observed from tetragonal (I4/mcm) to cubic (Pm3̅m) phase around ∼630 K. This phase transition was also confirmed by the heat flow experiment using differential scanning calorimetry. To confirm the nature of phase transition, we have studied the variation of integrated intensity (ISL) of superlattice peak and the oxygen octahedral tilt angle (φ) in a tetragonal structure with temperature. The temperature variations of these two parameters provide evidence for the tricritical nature of the phase transition. A magnetic phase transition is discovered in temperature dependence of magnetization M(T) around ∼708 K, which coincides with the dielectric anomaly temperature in loss tangent.

Magnetostructural transition in Co-Mn-Ge systems tuned by valence electron concentration

The effects of annealing process and chemical pressure on the structural and magnetocaloric properties of CoMn1−xVxGe (x = 0.05 and 0.10) alloys have been studied. Rietveld refinement of XRD patterns revealed that all samples are single phase and they crystallize in hexagonal structure with a space group (P63/mmc) at room temperature. The structural phase transition temperature Tstruct decreases with increasing vanadium (V) content drastically in compare to CoMnGe alloy which exhibit a first order structural transition at around 450 K. The decrease in Tstruct. by substituting fourth element in CoMnGe-based alloys are discussed in frame of changing its valence electron concentration (e/a) values. We found that, alloys exhibiting a first order magneto-structural transition (FOMT) around room temperature, have an e/a value of 6.62–6.67. The e/a dependence of the magnetostructural transition temperature can be used as a guide to obtain an alloy with FOMT around room temperature.