High-temperature Tetragonal Phase Research Articles

Disclosing the magnetic ground state of PrBaMn2O6

In this work, we report a neutron diffraction study on a single crystal of the layered PrBaMn2O6 perovskite. This compound undergoes two consecutive magnetic transitions with decreasing temperature. At TC ≈ 309 K, a long-range ferromagnetic ordering is established with the Mn magnetic moments oriented along the c axis. Below TN ≈ 240 K, a spin reorientation gives rise to a sudden decrease of the magnetization and the formation of an antiferromagnetic order. This magnetic transition is also accompanied by an electronic localization caused by a structural phase transition from a high-temperature tetragonal phase (P4/mmm) into a low-temperature orthorhombic one (P21am). Our neutron diffraction study at 12 K has unambiguously identified that the ground magnetic structure for this compound is a layered antiferromagnetic structure along the c axis with the Mn moments oriented in the ab plane along one diagonal of the primitive tetragonal cell. No sign of magnetic scattering associated to a CE-type magnetic structure was found in our study, discarding its occurrence in this compound. This result excludes the occurrence of a charge order transition to account for the electronic localization and it is fully consistent with previous structural studies showing a unique crystallographic site for Mn atom in the low-temperature phase. A symmetry analysis was carried out to identify the magnetic space group of the ferromagnetic order above TN and the two possible groups for the antiferromagnetic ordering below TN.

Ferroelectric relaxor behaviour in Bi and K/Na-substituted solid solutions of tetragonal tungsten bronze PbNb2O6

Lead metaniobate (PbNb2O6 or PNO) belongs to the tetragonal tungsten bronze (TTB) family. There are three polymorphic forms of PNO: tetragonal (stable at high temperature), rhombohedral (stable at room temperature), and the metastable orthorhombic phase, which has ferroelectric properties. The metastable orthorhombic phase is formed by quenching from the high temperature tetragonal phase due to the reconstructive/sluggish tetragonal to rhombohedral phase transition. A suitable ionic substitution can also lead to the stable ferroelectric orthorhombic phase. Stabilization of the ferroelectric orthorhombic phase by ionic substitution, and the formation of ferroelectric relaxors with increased amounts of dopants is hereby reported for the compositions Pb(1-x)Bi(x/2K(x/2)Nb2O6 and Pb(1-x)Bi(x/2)Na(x/2)Nb2O6. The solid solutions of PNO showed evidence of linked dielectric relaxation and ferroelectricity, validating the distinctive ferroelectric relaxor behaviour for the compositions Pb0.6Bi0.2K0.2Nb2O6 and Pb0.6Bi0.2Na0.2Nb2O6. As the concentrations of bismuth (Bi) and potassium/sodium (K/Na) dopants increased, the Curie temperature (Tc), or the temperature corresponding to the maximum dielectric permittivity (Tm), dropped. By using the Uchino-Nomura relation as a modification of the Curie-Weiss law, it was established that the Pb0.6Bi0.2K0.2Nb2O6 solid solution exhibit higher degree of diffuseness than Pb0.6Bi0.2Na0.2Nb2O6 at the transitions. The width of the ferroelectric hysteresis loop increased as the percentage of dopants increased, from x = 0.1 to x = 0.3. At x = 0.4, a narrow ferroelectric hysteresis loop and a frequency-dependent Tm abruptly formed, which is indicative of a ferroelectric relaxor.

Origin of morphotropic phase boundary in thin-film Hf0.5Zr0.5O2 on the TiN electrode

Our study aims to clarify the morphotropic phase boundary observed in Zr-doped hafnia systems. We utilize density-functional-theory calculations to examine various structural phases of (Hf,Zr)O2 thin films on TiN electrodes. We account for Zr composition, film thickness, and temperature to model the free energy of (Hf,Zr)O2 on TiN electrodes. Our assessment of the thermodynamic stability of each structural phase in terms of surface and interface energies under the substrate strain allows us to determine that the substrate strain and temperature significantly reduce the energy differences between different phases. Our findings lead to the energy reversal between tetragonal and orthorhombic phases when the film thickness increases. Based on our results, we propose that the formation of a high-temperature tetragonal phase, arising from the rapid thermal or annealing processes, is crucial to the appearance of the morphotropic phase boundary in Hf0.5Zr0.5O2. Understanding the origin of the morphotropic phase boundary can have significant implications for device applications.

Stress dynamics during O-T phase transitions in lead-free KNN-based piezoelectric ceramics

In this study, we have investigated the stress dynamics arising from the Orthorhombic-Tetragonal Polymorphic Phase Boundary (PPB) in lead-free potassium sodium niobate (KNN)-based ceramics and its temperature dependence, as a means to elucidate the characteristics of PPBs in lead-free piezoelectric oxides. The dynamics are dictated by the phase transitions that occur upon cooling from the cubic phase and the coexistence of different crystal structures. Our findings reveal that the growth of Orthorhombic phases is constrained by the high-temperature tetragonal phase distributions. In particular, it is evidenced that two main mechanisms regulate the decrease of stress processes in which structural and microstructural effects are correlated; the first one is associated to a purely microstructural effect in which bimodal grain distribution hinders the formation of non-180° domains. By contrast, the second is mainly governed by ferroelectric domain distribution and the occurrence of pseudo-cubic regions around room temperature associated with local structural heterogeneity (that is, polar nano regions, PNRs). Specifically, these mechanisms generate regions with different stress state and explain the widening of the phase transition due the phase coexistence.

Evidence of a stable tetragonal (P4bm) phase in rhombohedral (R3c) complex perovskite (Na[formula omitted]Bi[formula omitted])[formula omitted]Ca[formula omitted]TiO[formula omitted] lead-free piezoelectric materials

The room temperature crystal structure and phase evolution of Ca-substituted Na1/2Bi1/2TiO3(NBCT) at A-site were investigated systematically using X-ray diffraction (XRD) combined with Raman spectroscopy. While XRD patterns of calcined powders exhibited the expected rhombohedral (R3c) phase at room temperature due to low Ca concentration, the corresponding sintered samples displayed peak splitting for all Bragg peaks, appearing as shoulders for all studied compositions. This departure from the average rhombohedral (R3c) to a non-rhombohedral structure is an indication of a new phase evolution of this system. The optimal Rietveld refinement corresponds to the coexistence of two phases, rhombohedral and tetragonal (R3c+P4bm) in the equilibrium state at room temperature. The proposed model describes the growth of a stable high-temperature tetragonal (P4bm) phase within the rhombohedral (R3c) matrix at room temperature. This work is the first XRD macroscale detection of a stable tetragonal (P4bm) phase in rhombohedral complex perovskite at room temperature.

Nanoscale structural analysis of Bi0.5Na0.5TiO3 in high-temperature phases

We performed a local structure analysis of the ferroelectric perovskite Bi0.5Na0.5TiO3 (BNT) in high-temperature tetragonal and cubic phases by combining high-energy synchrotron X-ray diffraction and atomic pair-distribution function. BNT is a promising lead-free piezoelectric material, although it has the problem of narrow working temperature range due to its low depolarization temperature. Since the depolarization temperature of BNT is caused by a diffuse order-disorder phase transition similar to a relaxer ferroelectrics, local structural analysis over a wide temperature range is required to elucidate the depolarization phenomenon. Using this approach, we found that the local structure is largely contributed by the diffuse component, and the deviation from the average structure is caused by the shift of Bi atoms. The amount of shift increased with increasing temperature. Changes in nanoscale structure with distance are closely related to BNT depolarization phenomena. The results show the model needed to develop highly functional piezoelectric materials.

Doubling of the Phase Transition Temperature of VO2 by Fe Doping.

Vanadium dioxide (VO2) undergoes a fully reversible first-order metal-insulator transition from the M1 monoclinic phase (P21/c) to a high-temperature tetragonal phase (P42/mnm) at around 68 °C. Modulation of the phase transition of VO2 by chemical doping is of fundamental and technological interest. Here, we report the synthesis of highly crystallized Fe-doped VO2 powders by a carbo-thermal reduction process. The impact of Fe doping on the structural and phase transition of VO2 is studied. The as-prepared Fe-doped VO2 samples crystallize in the M2 monoclinic form (C2/m), which is linked to segregation of the doping ions in the V2 zigzag chains. A large increase in the transition temperature to 134 °C is observed, which does correspond to a breakthrough in VO2-type thermochromic materials.

Compression effect on structure of the Li-stabilized high-temperature phase of Mn3(VO4)2 with composition Li0.2Mn2.9(VO4)2 - Raman spectroscopic and X-ray diffraction investigations

Tetragonal Mn3(VO4)2 is a metastable high temperature phase of Mn3(VO4)2 showing unusual coordination of Mn atoms. A nominal Li-substitution helps to stabilize the tetragonal modification. We report room temperature compression of Li-stabilized high-temperature tetragonal phase of Mn3(VO4)2 with composition Li0.2Mn2.9(VO4)2 studied by in-situ Raman spectroscopy and synchrotron X-ray diffraction up to pressures of 20 and 26.5 GPa respectively. Raman spectroscopy suggests onset of a structural phase transition at around 10 GPa which completes above 13 GPa. X-ray diffraction also suggested a first order structural transition above 10 GPa, with a coexistence range of two phases up to 13 GPa. High pressure phase is found to be different from the known thermodynamically stable orthorhombic structure of Mn3(VO4)2. The equation of state data of the ambient tetragonal phase has been obtained. Anisotropic compression is observed with the c-axis being more compressible than a-axis. Using experimental bulk modulus, the mode Grüneisen parameters for the observed Raman frequencies for the ambient phase have been obtained. On pressure release, the high pressure phase could be retrieved.

Improved thermal stability of zirconia macroporous structures via homogeneous aluminum oxide doping and nanostructuring using atomic layer deposition

Dopants are regularly used in sol-gel and powder metallurgy routes, however, the controlled insertion of such is quite challenging, especially in the case of nanostructures. Here we investigate the use of atomic layer deposition (ALD) as a potential technique to precisely introduce aluminum oxide as dopant or second phase into zirconia 3D macroporous nanostructures. The results show that the introduction of high Al2O3 contents into the zirconia nanostructures successfully inhibited sintering when in comparison to undoped zirconia. Moreover, for the multi-nanolaminated and full-mix structures, the tetragonal phase was stabilized up to 1200 °C. Furthermore, the structures presented a photonic band gap even after heat treatment at 1200 °C for 2 h, enabling its application as inverse opal photonic crystals in high-temperature environments. The enhancement of thermal stability and high-temperature tetragonal phase stabilization is enabled jointly by the nanostructuring and homogeneous distribution of aluminum oxide provided by ALD super-cycles.

Contrasting Behaviors of FA and MA Cations in APbBr3.

It is known that the organic units in hybrid halide perovskites are free to rotate, but it is not clear if this freedom is of any relevance to the structure-property relationship of these compounds. We have employed quasi-elastic neutron scattering using two different spectrometers, thus providing a wide dynamic range to investigate the cation dynamics in methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) over a large temperature range covering all known crystallographic phases of these two compounds. Our results establish a plastic crystal-like phase forming above 30 K within the orthorhombic phase of MAPbBr3 related to 3-fold rotations of MA units around the C-N axis with an activation energy, Ea, of ∼27 meV, which has no counterpart in the FA compound. MA exhibits an additional 4-fold orientational motion of the whole molecule via rotation of the C-N axis itself with an Ea of ∼68 meV common for the high-temperature tetragonal and cubic phases. In contrast, the FA compound exhibits only an isotropic orientational motion of the whole FA unit with Ea ≈ 106 meV within the orthorhombic phase and a substantially reduced common Ea of ∼62 meV for the high-temperature tetragonal and cubic phases. Our results suggest that the rotational dynamics of the organic units, crystallographic phases, and physical properties of these compounds are intimately connected.

Role of the ferroelastic strain in the optical absorption of BiVO4

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical bandgap is found to be very sensitive to the temperature, and to the tetragonal-to-monoclinic ferroelastic transition at 523 K. The anisotropy, as measured by the difference in the absorption edge for the light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth vanadate-based light absorbers in photocatalytic devices.

WO3 thermodynamic properties at 80–1256 K revisited

Thermodynamic properties (heat capacity Cp, entropy S°, enthalpy change ΔH° and reduced Gibbs energy ΔΦ°) of crystalline WO3 in a temperature range of 80–1256 K were refined using two complementary calorimetric methods, namely differential scanning calorimetry and adiabatic vacuum calorimetry. Enthalpy changes during a number of successive phase transitions in WO3 were calculated from differential scanning calorimetry data in both heating and cooling regimes and compared to previously reported values. For the first time, enthalpy changes for the orthorhombic to monoclinic and monoclinic to tetragonal high-temperature phase transitions in WO3 were estimated.

Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr0.67Na0.33Fe2As2

We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the SC transition at $T_c\simeq$10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the dramatic decrease in the low-frequency optical conductivity signals the formation of a SC energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron- and hole-like bands at the corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below $T_r$ the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ~20 meV. The magnetic states, with resulting SDW and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the SC in this material.

Phase transitions in Bi4Ti3O12.

Bi4Ti3O12 is a representative of the Aurivillius family of layered perovskites. These are high-temperature ferroelectric materials with prospects for applications in random-access memory and are characterized by an extremely confused interaction of their structural degrees of freedom. Using group-theoretical methods, structural distortions in the Bi4Ti3O12 high-symmetry phase, caused by rotations of rigid octahedra and their displacements as a single unit, have been investigated, taking into account the connections between them. Within the Landau theory, a stable thermodynamic model of phase transitions with three order parameters has been constructed. It is shown that, according to the phenomenological phase diagram, the transition between the high-temperature tetragonal phase and the low-temperature ferroelectric can occur both directly and through intermediate states, including those observed experimentally. The role of improper ordered parameters and possible domain configurations in the structure formation of the low-temperature ferroelectric phase are discussed.

Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1−xCox)2As2

We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ with $x=0.048$ and 0.054. Both samples exhibit a tetragonal-to-orthorhombic structural transition at ${T}_{s}$, a collinear static antiferromagnetic order at wave vector ${\mathbf{Q}}_{1}={\mathbf{Q}}_{\mathrm{AF}}=(1,0)$ below the $\mathrm{N}\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}\mathrm{el}$ temperature ${T}_{N}$, and superconductivity below ${T}_{c}$ (${T}_{s}>{T}_{N}>{T}_{c}$). In the high-temperature paramagnetic tetragonal phase ($T\ensuremath{\gg}{T}_{s}$), spin excitations centered at ${\mathbf{Q}}_{1}$ and ${\mathbf{Q}}_{2}=(0,1)$ are gapless and have fourfold (${C}_{4}$) rotational symmetry. On cooling to below ${T}_{N}$ but above ${T}_{c}$, spin excitations become highly anisotropic, developing a gap at ${\mathbf{Q}}_{2}$ but still are gapless at ${\mathbf{Q}}_{1}$. Upon entering into the superconducting state, a neutron spin resonance appears at ${\mathbf{Q}}_{1}$ with no magnetic scattering at ${\mathbf{Q}}_{2}$. By comparing these results with those from angle-resolved photoemission spectroscopy experiments, we conclude that the anisotropic shift of the ${d}_{yz}$ and ${d}_{xz}$ bands in detwinned $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ below ${T}_{s}$ is associated with the spin excitation anisotropy, and the superconductivity-induced resonance arises from the electron-hole Fermi surface nesting of quasiparticles with the ${d}_{yz}$ orbital characters.

Martensitic transformations in VO2 films

Martensitic transformation is considered to be non-diffusive since the atoms in the transition are moved to a distance of the order of interatomic, and neighboring atoms in the initial phase remain adjacent and in the new one, only their position changes. In this paper, for the first time, an experimental study of the phase transition metal-dielectric (FPMD) in thin films of vanadium dioxide is accompanied by a structural transition of martensitic type, so this material is a convenient model object that allows you to clearly identify both the processes of defect formation and their influence on its physical properties in the vicinity of the phase transition point. The obtained results are naturally associated with the effect of adhesion and epitaxy on the phase transition, which can be reduced to the contribution to the energy balance of the component σ/d, where σ is the density of the surface energy; d is the film thickness. Physically, this may mean that since the growth of films occurs at a temperature above the FPMD temperature, the orientation of the crystal axes of the high-temperature tetragonal phase is stabilized by epitaxial forces from the substrate side, and this makes it difficult to reconstruct the crystal structure during the phase transition. Phase stabilization in thin layers due to surface effects is a common phenomenon in the physics of thin films.

Quantitative characterization of short-range orthorhombic fluctuations in FeSe through pair distribution function analysis

Neutron and x-ray total scattering measurements have been performed on powder samples of the iron chalcogenide superconductor FeSe. Using pair distribution function (PDF) analysis of the total scattering data to investigate short-range atomic correlations, we establish the existence of an instantaneous, local orthorhombic structural distortion attributable to nematic fluctuations that persists well into the high-temperature tetragonal phase, at least up to 300 K and likely to significantly higher temperatures. This short-range orthorhombic distortion is correlated over a length scale of about 1 nm at 300 K and grows to several nm as the temperature is lowered toward the long-range structural transition temperature. In the low-temperature nematic state, the local instantaneous structure exhibits an enhanced orthorhombic distortion relative to the average structure with a typical relaxation length of 3 nm. The quantitative characterization of these orthorhombic fluctuations sheds light on nematicity in this canonical iron-based superconductor.

Reaping the remarkable benefits of a ‘burst nucleation’ approach for a ceria doped zirconia system

Through this work, we show the efficacy of a novel processing step (during co-precipitation synthesis) that ensured a ‘burst nucleation’ scenario and yielded crucial benefits for a stabilized zirconia (Ce0.2Zr0.8O2) system. In addition to the high surface area and near-spherical morphology of the Ce0.2Zr0.8O2 nanoparticles, high temperature (≥1350 °C) tetragonal phase stability was noted for both Ce0.2Zr0.8O2 powders and ceramics. Ce0.2Zr0.8O2 powders produced near-theoretical density (∼97%) ceramic with equiaxed grain size (0.17 ± 0.05 μm) via conventional sintering at 1250 °C. Significant improvement was noted for the hardness values (∼14.5 GPa) for the Ce0.2Zr0.8O2 ceramic compared to all previously published literature. Tetragonal→monoclinic transformation (for synthesis plan without burst nucleation approach) was correlated to anisotropic and irregular particle shapes, wide dispersion in particle sizes that induced internal stress in ZrO2 matrix.

Low-cost VO2(M1) thin films synthesized by ultrasonic nebulized spray pyrolysis of an aqueous combustion mixture for IR photodetection.

We report detailed structural, electrical transport and IR photoresponse properties of large area VO2(M1) thin films deposited by a simple cost-effective two-step technique. Phase purity was confirmed by XRD and Raman spectroscopy studies. The high quality of the films was further established by a phase change from low temperature monoclinic phase to high temperature tetragonal rutile phase at 68 °C from temperature dependent Raman studies. An optical band gap of 0.75 eV was estimated from UV-visible spectroscopy. FTIR studies showed 60% reflectance change at λ = 7.7 μm from low reflectivity at low temperature to high reflectivity at high temperature in a transition temperature of 68 °C. Electrical characterization showed a first order transition of the films with a resistance change of four orders of magnitude and TCR of −3.3% K−1 at 30 °C. Hall-effect measurements revealed the n-type nature of VO2 thin films with room temperature Hall mobility, μe of 0.097 cm2 V−1 s−1, conductivity, σ of 0.102 Ω−1 cm−1 and carrier concentration, ne = 5.36 × 1017 cm−3. In addition, we fabricated a high photoresponsive IR photodetector based on VO2(M1) thin films with excellent stability and reproducibility in ambient conditions using a low-cost method. The VO2(M1) photodetector exhibited high sensitivity, responsivity, quantum efficiency, detectivity and photoconductive gain of 5.18%, 1.54 mA W−1, 0.18%, 3.53 × 1010 jones and 9.99 × 103 respectively upon illumination with a 1064 nm laser at a power density of 200 mW cm−2 and 10 V bias voltage at room temperature.

Synthesis, microstructure and electrical properties of nanocrystalline calcium doped lanthanum orthoniobate

The single phase lanthanum orthoniobate with tetragonal structure has been synthesized by the means of mechanosynthesis method. The studies have shown the crystal structure of La0.98Ca0.02NbO4 depends on the synthesis stage. The samples were predominantly in the tetragonal phase with a trace amount of the monoclinic phase. The SEM studies of morphology and microstructure have shown nanocrystallinity of the materials. The Raman studies showed the significant difference between the Raman spectra of nano- and microcrystalline La0.98Ca0.02NbO4. A decrease of Debye temperature, shown a change of thermal properties due to the presence of nano-sized particles in La0.98Ca0.02NbO4. Electrical characterization results suggest the protons are predominant charge carries in the material for the whole measured range of temperature and shows the total conductivity value of 4.0 × 10−5 S cm−1 at 600 °C in wet air. The calculated activation energies were equal to (0.81 ± 0.04) eV under dry and (0.73 ± 0.06) eV under humidified conditions. That indicates that nanostructuring was successfully stabilized the high-temperature tetragonal phase.