Monoclinic-tetragonal Phase Transition Research Articles

Controlled growth of single-walled carbon nanotubes with narrow diameter distribution and high areal density on Mn-doped ZrO2-supported Fe catalyst

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution and high areal density hold great promise for a variety of applications, particularly in advanced electronics and biomedical devices. This study utilizes Mn ion doping as a strategic approach both to stabilize the ZrO2-supported Fe catalyst and regulate lattice oxygen release, thereby facilitating the growth of SWCNTs with high areal density and a narrow diameter distribution. The ZrO2-supported Fe catalyst was meticulously dispersed on Si/SiO2 (300 nm) substrates through the optimization of spin-coating speed and solvent viscosity, aiming to enhance the dispersion and uniformity. Investigation into the precise control over the Mn ion content was conducted to regulate the structure and stability of the Mn-doped ZrO2-supported Fe catalyst. Mn ions perform a dual role: they maintain a higher proportion of ZrO2 in the monoclinic phase, which in turn alleviates the monoclinic-tetragonal phase transition during chemical vapor deposition (CVD) growth. Concurrently, the conversion of Mn3+ to Mn4+ ions captures the lattice oxygen released during the formation of zirconium transition oxides. Additionally, first-principles simulations have confirmed the stabilizing impact of Mn ions on the catalyst, which reduces aggregation and maturation, ultimately resulting in a more concentrated diameter distribution of as-grown SWCNTs. This approach has successfully achieved the controlled growth of high areal density SWCNTs with a diameter distribution of 1.73 ± 0.08 nm.

Regulation of crystal and microstructures of RETaO4 (RE = Nd, Sm, Gd, Ho, Er) powders synthesized via co-precipitation

Ferroelastic rare earth tantalates (RETaO4) are widely researched as the next-generation thermal barrier coatings (TBCs), and RETaO4 powders are hugely significant for synthesizing their coatings. The current research used chemical co-precipitation within an automated experimental device to synthesize RETaO4 (RE = Nd, Sm, Gd, Ho, Er) powders. The device automatically monitored and controlled the solutions’ pH, improving the chemical co-precipitation efficiency. The crystal structure and microstructure of the RETaO4 powders can be controlled by changing the annealing temperature, and the materials undergo an m'-m phase transition. The m'-RETaO4 powders exhibit nano-size grains, while m-RETaO4 powders evince micron-size grains, altered by the annealing temperatures. A simultaneous thermal analysis estimates the reversive ferroelastic tetragonal-monoclinic phase transition temperatures. Overall, this research focuses on the synthesis, crystal structures, microstructures, and phase transition of the fabricated RETaO4 powders.

Characteristics of ferroelastic domains and thermal transport limits in HfO2 alloying YTaO4 ceramics

Low thermal conductivity and a ferroelastic tetragonal-monoclinic phase transition are essential for the use of YTaO4 in thermal barrier coatings. The phonon scattering mechanism of HfO2 alloying YTaO4 is elucidated via the analysis of microstructural characteristics, and a limit thermal conductivity (1.3 W·m−1·K−1) is achieved. Furthermore, a revised model is developed to successfully derive high-temperature phonon thermal conductivity, showing a decrease in the phonon scattering coefficient as strain field fluctuations decrease. This proves that the phonon scattering coefficient is temperature dependent. Atomic weight disorder plays a significant role in thermal conductivity reduction, whereas HfO2 alloying enhances lattice symmetry and weakens the phonon scattering, and thus mitigates thermal conductivity reduction. Additionally, domain boundary width and spacing mismatch between neighboring domains cause the scattering of phonons and further reduce the thermal conductivity. Finally, evidence of a diffusive domain boundary validates that tetragonal-monoclinic transition is a second-order process.

Luminescence properties of PrNbO4 and EuNbO4 orthoniobates and investigation of their structural phase transition by high-temperature Raman spectroscopy

This work reports the photoluminescence (PL) properties at room temperature for PrNbO4 and EuNbO4 orthoniobates synthesized by solid-state reaction. An accurate colorimetric study was carried out for these materials, with determination of the most important parameters for the development of optical devices: emission coordinates, color purity, and emitted color temperature. The emission spectra showed the characteristic lines for the rare earths Pr3+ and Eu3+, with the 3P0 → 3F2 and 5D0 → 7F2 emissions being the most intense for PrNbO4 and EuNbO4, respectively. The samples emitted in the orange (Pr3+) and reddish-orange (Eu3+) regions, having the emission coordinates (0.5723, 0.4053) for PrNbO4 and (0.5868, 0.3938) for EuNbO4. The samples exhibited color purities higher than 90%, so they could be considered promising red phosphors for application in LEDs. This work also demonstrates the suitability of high-temperature Raman spectroscopy (HT-Raman) as a good alternative method for analysis of the monoclinic-tetragonal phase transition in orthoniobates. In situ HT-Raman analyses in the temperature range 25–700 °C showed that the PrNbO4 material changed from the C2/c (#15) space group to I41/a (#88), at ~600 °C. On the other hand, the EuNbO4 ceramic showed no alteration of the crystal structure, in the temperature range studied. The observed structural phase transition from fergusonite-type to scheelite-type was reversible and could be confirmed by Raman measurements during the cooling process. The main contribution of this work is the approach used to determine the PL properties of the PrNbO4 sample, for which there have been no previous reports in the literature, to the best of our knowledge. Furthermore, the RT-Raman technique was successfully used, for the first time, to investigate the structural phase transitions of rare earth orthoniobates.

On the question of the critical end point in the (1 − x)PbMg1/3Nb2/3O3-xPbTiO3 relaxor ferroelectric single crystals based on acoustic emission data studies

()PbMgNbOPbTiO3, [100]-oriented relaxor ferroelectric crystals were studied by means of dielectric and acoustic emission methods in a temperature range up to 200 °C and under a dc bias electric field up to 1.2 kV/cm. The monoclinic-tetragonal and tetragonal-cubic ferroelectric phase transitions were successfully detected by both experimental methods. The bias electric field dependence of the temperature of a dielectric constant maximum, related to tetragonal-cubic phase transitions, exhibits a V-shape effect with a threshold field of 0.5 kV/cm. In contrast, the bias electric field dependence of the temperature of a monoclinic-tetragonal phase transition exhibits no V-shape effect but points out a tricritical point at 0.7 kV/cm. These results have not confirmed the existence of the critical end point in the crystals under studies.

Structure, phase composition and properties of vacuum pressureless sintered ZrO2-based composites reinforced with carbon nanotubes

In this work, composites based on 3 mol. % yttria-stabilized zirconia (3YSZ) with additives of 1, 5 and 10 wt. % multi-walled carbon nanotubes (MWCNT) were investigated. Samples were obtained by cold pressing followed by vacuum pressureless sintering at a temperature of 1500 °C. It was found that MWCNTs retain their structure after high-temperature sintering, they are located along the zirconia grain boundaries and they lead a grain refining effect for zirconia matrix. Nanotubes can significantly affect the phase composition of composites. Additives 5 wt. % and 10 wt. % MWCNT partially limits the monoclinic-tetragonal phase transition of ZrO2 and leads to the formation of a new phase of zirconium carbide. An increase of MWCNTs content in composites leads to a smooth decrease of the relative density from 99.2 % to 81.2 %. To obtain fully dense 3YSZ/MWCNT composites, longer sintering at higher temperatures is required compared to the conditions for sintering 3YSZ ceramics without additives.

Undoped tetragonal ZrO2 obtained by the Pechini method: thermal evaluation of tetragonal–monoclinic phase transition and application as catalyst for biodiesel synthesis

Zirconia (ZrO2) is a well-known polymorphic material, and the monoclinic phase of this compound is the most stable phase at room temperature. Nevertheless, tetragonal zirconia has been obtained at room temperature without doping using chemical synthesis methods. In the present work, tetragonal undoped ZrO2 was obtained by the Pechini method, and the tetragonal–monoclinic phase transition was evaluated by thermogravimetry, differential thermal analysis (DTA) and in situ high-temperature X-ray diffraction (HT-XRD). The tetragonal phase was obtained as the major phase after calcination at and below 800 °C, while most of the transition to the monoclinic phase took place during cooling at temperatures below 500 °C, as indicated by DTA and HT-XRD. The pure monoclinic phase was obtained after calcination at 1000 °C. Samples calcined at 600 and 1000 °C were sulphated and applied in the ethylic transesterification of soybean oil. No meaningful difference was observed in the activity, and high efficiency was attained for both catalysts for the reaction performed at 100 °C for 2 h.

Структура, фазовый состав и физико-механические свойства композитов на основе ZrO2 и многостенных углеродных нанотрубок

In this work, composites based on yttria-stabilized zirconia (3Y-TZP), with additives of 1, 5 and 10 wt. % multi-walled carbon nanotubes (MWCNTs) were investigated. Samples were obtained by spark plasma sintering at a temperature of 1500 °C. It was found that MWCNTs retain their structure after high-temperature sintering, they are located along the grain boundaries of ZrO2, forming a network structure. Found that the addition of 1 wt. % MWCNTs increase the relative density of the composite from 98.3 % to 99.0 %. It is noted that nanotubes can significantly affect the phase composition of composites. Additive 5 wt. % MWCNT partially limits the monoclinic-tetragonal phase transition of ZrO2, and the addition of 10 wt. % MWCNTs leads to the formation of a cubic phase of zirconium carbide. It was found that the fracture toughness of the composite with 10 wt. % MWCNTs increases from 4.0 to 5.7 MPa·m1/2.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties.

We have investigated the high-pressure behavior of PbCrO4. In particular, we have probed the existence of structural transitions under high pressure (at 4.5 GPa) by single-crystal X-ray diffraction and density functional theory calculations. The structural sequence of PbCrO4 is different than previously determined. Specifically, we have established that PbCrO4, under pressure, displays a monoclinic-tetragonal phase transition, with no intermediate phases between the low-pressure monoclinic monazite structure (space group P21/ n) and the high-pressure tetragonal structure. The crystal structure of the high-pressure polymorph is, for the first time, undoubtedly determined to a tetragonal scheelite-type structure (space group I41/ a) with unit-cell parameters a = 5.1102(3) Å and c = 12.213(3) Å. These findings have been used for a reinterpretation of previously published Raman and optical-absorption results. Information of calculated infrared-active phonons will be also provided. In addition, the pressure dependence of the unit-cell parameters, atomic positions, bond distances, and polyhedral coordination are discussed. The softest and stiffest direction of compression for monazite-type PbCrO4 are also reported. Finally, the theoretical pressure dependence of infrared-active modes is given, for the first time, for both polymorphs.

57Fe nuclear resonant inelastic scattering of Fe1.1Te

This study reports results of 57Fe Mossbauer spectroscopy and nuclear resonant inelastic scattering (NRIS) in Fe1.1Te. The Fe-specific partial phonon densities of states (PDOSs) were extracted from the observed spectra. No detectable change at the tetragonal-monoclinic phase transition around 65 K was observed. Therefore the variations in lattice constants and/or distortion in FeTe4 tetrahedron through the structural transition was minute. From the observed Fe-PDOS, the Lamb-Mossbauer factor was 0.90(3) at 10 K and the Debye temperature was evaluated as 300(10) K. In 57Fe Mossbauer spectroscopy, the clear magnetic spittings of spectra were measured below Neel temperature. The obtained values of isomer shifts showed characteristics of Fe2+ species and clear difference with the phase transition was observed in temperature dependence of quadrupole splitting (QS). The change of QS must be caused by substantial change of the charge density distribution around the Fe site. This suggests that the band structure or the nature of the Fe-Te bonds becomes highly anisotropic. These results might provide additional information about the respective mechanisms for FeTe1−xSex superconductivity.

Spectroscopic studies of a tetragonal–monoclinic phase transition in ZrO2–Y2O3–CeO2–Nd2O3 crystals

The character of the distribution of cerium and neodymium dopants in 95.14 wt % ZrO2–3.67 wt % Y2O3–0.89 wt % CeO2–0.30 wt % Nd2O3 crystals has been studied. It has been shown that the crystal has defect regions due to the violation of normal growth of the crystal, which are depleted in the stabilizing Y2O3 oxide and enriched in Nd2O3 and CeO2 dopants. The existence of inclusions of a monoclinic phase in defect regions has been revealed using Raman and optical spectroscopy. A tetragonal–monoclinic phase transition initiated in the crystal by annealing in air has been investigated.

Influence of antimony substitution on spontaneous strain and thermodynamic stability of lanthanum orthoniobate

Analysis of the influence of antimony substitution on the temperature dependence of unit cell distortion in lanthanum orthoniobate has been performed. The values of spontaneous strain and Landau order parameter for three different antimony contents have been calculated. The monoclinic–tetragonal phase transition occurring for antimony substituted lanthanum orthoniobate was found to be of the second order. High temperature oxide melt solution calorimetry has shown that antimony substitution has little influence on the stability of the monoclinic phase. The average value of enthalpy of formation of antimony substituted lanthanum orthoniobate with fergusonite structure is −132.0±0.8kJ/mol, and that of scheelite structure is −126.4±1.5kJ/mol, implying greater stability for the former polymorph.

Refined crystal structures and phase transitions in 0.24Pb(In1/2Nb1/2)O3–0.43Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystal

Local lattice structures of 0.24Pb(In1/2Nb1/2)O3–0.43Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (0.24PIN–0.43PMN–0.33PT) single crystal have been investigated by Micro-Raman spectroscopy. A total of 12 Raman active modes were ascertained using the Gauss–Lorentzian line shape approximation, which confirmed that the 0.43PIN–0.26PMN–0.33PT single crystal has monoclinic symmetry at room temperature. The phase transitions were investigated by analyzing the temperature-dependent relative dielectric permittivity of both poled and unpoled 0.43PIN–0.26PMN–0.33PT single crystals measured upon heating. The monoclinic–tetragonal phase transition occurs at TM–T = 82 °C, and the Curie temperature is 199 °C. The splitting of (200) and (002) peaks in detailed X-ray diffraction spectra in the 2θ range of 44.6–45.2° during heating from room temperature to 200 °C was also studied. For a sample being poled along [111]c pseudo-cubic direction, several sharp peaks arise in the thermally stimulated depolarization current from 67 to 82 °C with the main peak occurs at 67 °C, which indicates that the depolarization temperature is 15 °C lower than the monoclinic to tetragonal phase transition and there are coexistence of monoclinic and rhombohedral phases in the [111]c poled single crystal.

Improving the piezoelectric thermal stability by tailoring phase transition behavior in the new (1 − x)[0.65PbMg 1/3Nb 2/3O 3–0.35PbTiO 3]– xBiZn 1/2Ti 1/2O 3 perovskite solid solutions

The phase transition behavior and its effect on thermal stability of the piezoelectric properties of the (1 − x)[0.65PbMg 1/3Nb 2/3O 3–0.35PbTiO 3]– xBiZn 1/2Ti 1/2O 3 ceramics with 0 ≤ x ≤ 0.06 were investigated. The phase transition from the monoclinic to tetragonal phase was determined by the dielectric constant and elastic constant measurements. The temperature independent piezoelectric response with − d 31 = 188 pC/N was obtained from 175 to 337 K for the composition with x = 0.02. The enhanced thermal stability of piezoelectric response was achieved by shifting the monoclinic–tetragonal phase transition to the lower temperature.

Stress analysis and ferroelectric properties of Pb(Zr0.52Ti0.48)0.96Nb0.04O3 thin film grown on different thickness of BaPbO3 electrodes

Pb ( Zr 0.52 Ti 0.48 ) 0.96 Nb 0.04 O 3 (PZTN) thin films were deposited on BaPbO3 (BPO) electrodes by rf-magnetron sputtering. 34, 68, 135, and 270 nm thick BPOs were adopted in this study. The preferred orientation changes from slightly (100)/(001) to slightly (101)/(110) as the BPO thickness increased. The mean grain sizes obtained by Williamson–Hall plots are 81 nm, 120 nm, 146 nm, and 90 nm, respectively. The same tendency was observed by atomic force microscopy method. In residual stress analysis, tensile stress was observed in every film. The stress magnitude is the maximum in the film with 135 nm thick BPO. Through a simple calculation, we suggest that the tensile stress in our films mainly originates from the phase transformation stresses. We also note that the ferroelectric and dielectrics properties are improved with the raise of tensile stresses. A possible reason is that the tensile stress benefits the tetragonal–monoclinic phase transition in the PZTN films with composition near morphotropic phase boundary. The other possible reason is that the raise of the tensile stress is consistent with the increasing of grain size, which decreases the grain boundary density and facilitate domain mobility.

Characterization of structural dynamics ofVO2thin film onc-Al2O3using in-air time-resolved x-ray diffraction

The lattice motion and displacement of atoms in the unit cell in vanadium dioxide $({\text{VO}}_{2})$ grown on $c{\text{-Al}}_{2}{\text{O}}_{3}$ were characterized by static and time-resolved x-ray diffraction (XRD) measurements. The monoclinic-tetragonal phase transition of the ${\text{VO}}_{2}$ unit cell and the twist motion of vanadium atoms in the unit cell were observed. The time-resolved XRD measurements were performed in air using a tabletop high-repetition femtosecond laser. The results obtained from the time-resolved XRD measurements suggested that the unit cell of the low-temperature monoclinic ${\text{VO}}_{2}$ transformed into the high-temperature tetragonal phase extremely rapidly (within 25 ps); however, the atoms in the unit cell fluctuated or vibrated about the center of the tetragonal coordinates, which abated within $\ensuremath{\sim}100\text{ }\text{ps}$. Thus, the time-resolved XRD measurements of the Bragg angle, intensity, and width of the diffraction lines simultaneously revealed the phase transition of ${\text{VO}}_{2}$ and the atomic motion in the unit cell.

Polarization System and Phase Transition on (Li,Na,K)NbO3 Ceramics

The polarization system and phase transition on Li0.055(Na0.50K0.50)0.945NbO3 (LNKN) ceramics were investigated to determine the piezoelectric properties, relative permittivities and X-ray diffraction (XRD). The piezoelectric properties and relative permittivities were strongly dependent on the poled state, which poling temperatures were 25 or 150 °C. The XRD did not exhibit orthorhombic–tetragonal phase transition around the phase transition temperature because no orthorhombic phase was observed below the phase transition temperature. The LNKN ceramics had multiple poled states, the origin of the multiple poled states was considered to be attributable to the multiple spontaneous polarization (Ps) direction on perovskite structure. From these results, we proposed that the phase transition on LNKN was not an orthorhombic–tetragonal phase transition but a monoclinic-tetragonal phase transition. The monoclinic perovskite structure [Z=1, Pm, a=3.9825(5) Å, b=3.9438(4) Å, c=4.0217(5) Å, β=90.292(3)°] can provide multiple Ps directions, such as [101]pc and [001]pc.

In situ X-ray diffraction analysis of Pb(Zr0.52Ti0.48)O3 phase transition in CoFe2O4/Pb(Zr0.52Ti0.48)O3 2-2-type bilayer films

A 2-2-type nanostructure bilayer film of CoFe2O4/Pb(Zr0.52Ti0.48)O3 was successfully prepared on the (111)Pt/Ti/SiO2/Si substrate. The Pb(Zr0.52Ti0.48)O3 layer in the bilayer film is (111) oriented and is a mixture of tetragonal and monoclinic phases. The results from an in situ X-ray diffraction analysis of the multiferroic bilayer film under statistic magnetic field indicate that the monoclinic-tetragonal phase transition was induced by magnetostriction of the CoFe2O4 layer. A large magnetoelectric effect was obtained probably because of the different polarization directions of the tetragonal and monoclinic phases.

Synthesis of Yttria‐Stabilized Zirconia Nanopowders by a Thermal Plasma Process

A thermal plasma process was used to synthesize nanosized zirconia and yttria‐stabilized zirconia powders. This paper presents a new method on the synthesis of nanosized zirconia and yttria‐stabilized zirconia powders from the carbonates (zirconium carbonate and/or yttrium carbonate). The products from this process using zirconium carbonate were mixtures of monoclinic and tetragonal phases of zirconia. The amount of the tetragonal phase in the mixture increased as the plasma torch power or plasma gas flow rate increased. This is attributed to the formation of metastable tetragonal phase by rapid quenching. With the addition of yttria precursor, two combinations of zirconia phases were formed depending on the yttria contents. Pure tetragonal phase was obtained at 3.8 mol% yttria, while a mixture of tetragonal and cubic phases was produced at 7.7 mol% yttria. The yttria‐doped tetragonal phase was stable at high temperatures without the tetragonal–monoclinic phase transition. The grain size of each phase, the particle size, and the BET are also discussed in this paper.

Influence of phase transition induced by residual stress on ferroelectric properties of highly (1 0 0)-oriented Pb(Zr 0.52Ti 0.48)O 3 thin films

Highly (1 0 0)-oriented polycrystalline Pb(Zr 0.52Ti 0.48)O 3 films with different thicknesses were fabricated on Pt/Ti/SiO 2/Si substrate using typical sol–gel method. The study results indicate that, for the PZT film with composition near morphotropic phase boundary, tensile stress benefits the tetragonal-monoclinic phase transition and the phase transition is very sensitive to stress. The difference in the polarization behavior of the films with different thickness is considered as the result of the preferred orientation and the different monoclinic phase contents in the films.