Phase Transition Temperature Tc Research Articles

Impact of surface phase coexistence on the development of step-free areas on Si(111)

The step-flow growth condition of Si on Si(111) near the (7×7)-“1×1” surface phase transition temperature T C are analyzed within the framework of Burton-Cabrera-Frank theory. In particular, coexistence of both surface phases well below T C and their specific influence on the step-flow growth behavior are considered. We presume that under dynamical condition of growth, the surface initially covered by only the (7×7) phase separates into domains surrounded by “1×1” areas. On such a surface, the overall supersaturation should be reduced drastically compared to a surface with only (7×7), resulting in much larger critical terrace width for nucleation.

On-line ultrasonic characterisation of barium doped lanthanum perovskites

Perovskite manganite samples La1−xBaxMnO3 with the composition of x=0.30, 0.33 and 0.36 were prepared by employing solid state reaction technique. The X-ray diffraction (XRD) patterns confirmed the rhombhedral structure with R3c space group of the samples. The obtained energy dispersive analysis X-rays (EDX) spectra of the samples have confirmed the elemental composition of the samples. The scanning electron microscope (SEM) images of the samples were used to find out the size of the particles. In-situ ultrasonic measurements were carried out on the samples by through transmission method. The temperature dependence of the ultrasonic parameters revealed interesting features of the samples. The observed ultrasonic velocities and attenuation both in longitudinal and shear mode are related to the paramagnetic (PM) to ferromagnetic (FM) phase transition in the prepared samples. The results confirmed that an increase in the barium content in the sample leads to an increase in the phase transition temperature TC.

Influence of (7×7)–“1×1” phase transition on step-free area formation in molecular beam epitaxial growth of Si on Si (111)

The step-flow growth condition of Si on Si (111) near the (7×7)–“1×1” surface phase transition temperature TC is analysed within the framework of Burton–Cabrera–Frank theory. In particular, coexistence of both surface phases well below TC and their specific influence on the step-flow growth behaviour is considered. We presume that under dynamical condition of growth, the surface initially covered by only the (7×7) phase separates into domains surrounded by “1×1” areas. On such a surface, the overall supersaturation should be reduced drastically compared to a surface with only (7×7), resulting in much larger critical terrace width for nucleation.

Mechanism for microwave heating of 1-(4′-cyanophenyl)-4-propylcyclohexane characterized by in situ microwave irradiation NMR spectroscopy

Microwave heating is widely used to accelerate organic reactions and enhance the activity of enzymes. However, the detailed molecular mechanism for the effect of microwave on chemical reactions is not yet fully understood. To investigate the effects of microwave heating on organic compounds, we have developed an in situ microwave irradiation NMR spectroscopy. 1H NMR spectra of 1-(4′-cyanophenyl)-4-propylcyclohexane (PCH3) in the liquid crystalline and isotropic phases were observed under microwave irradiation. When the temperature was regulated at slightly higher than the phase transition temperature (Tc=45°C) under a gas flow temperature control system, liquid crystalline phase mostly changed to the isotropic phase. Under microwave irradiation and with the gas flow temperature maintained at 20°C, which is 25°C below the Tc, the isotropic phase appeared stationary as an approximately 2% fraction in the liquid crystalline phase. The temperature of the liquid crystalline state was estimated to be 38°C according to the line width, which is at least 7°C lower than the Tc. The temperature of this isotropic phase should be higher than 45°C, which is considered to be a non-equilibrium local heating state induced by microwave irradiation. Microwaves at a power of 195W were irradiated to the isotropic phase of PCH3 at 50°C and after 2min, the temperature reached 220°C. The temperature of PCH3 under microwave irradiation was estimated by measurement of the chemical shift changes of individual protons in the molecule. These results demonstrate that microwave heating generates very high temperature within a short time using an in situ microwave irradiation NMR spectrometer.

Hydrothermal treatment with VO2(B) nanobelts for synthesis of VO2(A) and W doped VO2(M) nanobelts

In this paper, the conversion of VO2(B) nanobelts under the hydrothermal condition in the absence/presence of dopant was studied. The synthetic conditions, such as the reaction temperature, reaction time, quantity of the dopants and kinds of dopants, were briefly discussed to prepare VO2(A) and W doped VO2(M) solid solution. It was found that the dopant played a crucial role in controlling VO2 polymorph, and the results showed that VO2(A) was obtained in the absence of dopant, whereas W doped VO2(M) was obtained in the presence of tungstenic acid. The transformation mechanism from VO2(B) to VO2(A) and W doped VO2(M) was discussed in detail. The phase transition temperature (Tc) of VO2(A) and W doped VO2(M) was evaluated by differential scanning calorimetry. It is found that doped atoms can be successfully doped into the crystal lattice of VO2(M) matrix, and the Tc of W doped VO2(M) can be reduced by W doping. The optical switching properties were studied by the variable temperature infrared spectra, and it was found that the as obtained VO2(A) and W doped VO2(M) had outstanding thermochromic characters and optical switching properties.

Effect of Polarization on Mechanical Properties of Lead Zirconate Titanate Ceramics

This study concerns two following commercial lead zirconate titanate (PZT) materials, PZT-5H and PZT-8 piezoelectric ceramics. Young's modulus and internal friction versus temperature at different vibration frequencies were investigated by means of Dynamic Mechanical Analysis (DMA) before and after polarization. The phase transition temperatures (Tc) of PZT-5H and PZT-8 ceramics were 438 K and 550 K. PZT-8 ceramic had higher Young's modulus than PZT-5H ceramic at ferroelectric phase, which resulted from the increase of internal local stress in PZT-8 ceramic. After polarization, the Young's modulus of both PZT ceramics increased, the influence of the piezo- electric effect was investigated. A relaxation peak emerges in internal friction curve of each PZT sample. Considering the hard ceramic, this peak is a pure relaxation mechanism controlled by oxygen vacancies diffusion. Considering the soft ceramic, the relaxation peak can be related to viscous motion of domain walls and the interaction between domain walls and point defects. The activation energy of the relaxation peak increased after polarization, which resulted from the increased local internal stress and directional arranged space charges caused by polarization.

Transport Characteristics of Charge Carriers in Normal State Superconductor YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub>

The general expressions based on the Fermi distribution of the free charge carriers are applied for estimation of the transport characteristics in superconductors at the temperature well above the superconducting phase transition temperature TC. The Hall-effect experimental results in the normal state of the superconductor YBa2Cu3O7-δ are not finally explained. On the ground of the randomly moving charge carriers, the transport characteristics of the randomly moving charge carriers for both single type and two types of the charge carriers are presented. The particular attention has been pointed to the Hall-effect measurement results of the high-TC superconductor YBa2Cu3O7-δ. It is at the first time derived the Hall coefficient expression for two type of highly degenerate charge carriers (electrons and holes) on the ground of the randomly moving charge carriers at the Fermi surface. It is shown that the Hall coefficient and other transport characteristics are determined by the ratio between the electron-like and hole-like densities of states at the Fermi surface.

Phase transitions and thermal-stress-induced structural changes in a ferroelectric Pb(Zr0.80Ti0.20)O3 single crystal

A single crystal of lead-zirconate-titanate, composition Pb(Zr0.80Ti0.20)O3, was studied by polarized-Raman scattering as a function of temperature. Raman spectra reveal that the local structure deviates from the average structure in both ferroelectric and paraelectric phases. We show that the crystal possesses several, inequivalent complex domain boundaries which show no sign of instability even 200 K above the ferroelectric-to-paraelectric phase transition temperature TC. Two types of boundaries are addressed. The first boundary was formed between ferroelectric domains below TC. This boundary remained stable up to the highest measurement temperatures, and stabilized the domains so that they had the same orientation after repeated heating and cooling cycles. These domains transformed normally to the cubic paraelectric phase. Another type of boundary was formed at 673 K and exhibited no signs of instability up to 923 K. The boundary formation was reversible: it formed and vanished between 573 and 673 K during heating and cooling, respectively. A model in which the crystal is divided into thin slices with different Zr/Ti ratios is proposed. The physical mechanism behind the thermal-stress-induced structural changes is related to the different thermal expansion of the slices, which forces the domain to grow similarly after each heating and cooling cycle. The results are interesting for non-volatile memory development, as it implies that the original ferroelectric state can be restored after the material has been transformed to the paraelectric phase. It also suggests that a low-symmetry structure, stable up to high temperatures, can be prepared through controlled deposition of layers with desired compositions.

Spin-gap phenomenon in a strongly interacting ultracold Fermi gas

We investigate magnetic properties of a strongly interacting ultracold Fermi gas. Within the framework of an extended T-matrix approximation, we calculate the spin susceptibility χ in the unitarity limit. We show that effects of pairing fluctuations on this magnetic quantity are quite different in between the normal state and the superfluid phase. In the normal state, pairing fluctuations cause spin-gap phenomenon near the superfluid phase transition temperature Tc, where χ is anomalously suppressed. In the superfluid phase, on the other hand, the ordinary suppression of χ by the BCS energy gap is weakened by pairing fluctuations, because they induce finite density of states inside the gap. Our results indicate that the spin susceptibility is a useful quantity for the study of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Fermi gas.

Experimental Electron Density of Ammonium Dihydrogen Phosphate in the Paraelectric as well as Antiferroelectric Phases by the Maximum Entropy Method

The experimental electron density of ammonium dihydrogen phosphate (ADP) crystal in the paraelectric phase (155 K) as well as antiferroelectric phase (100 K) is obtained from its high resolution X-ray diffraction data using the maximum entropy method. Marked redistribution of electron density has been observed in ADP crystals as the crystal temperature is lowered below the phase transition temperature Tc = 148 K. The nature of very strong O–H–O hydrogen bonds between phosphate anions changes from an ideal covalent interaction to a polar covalent interaction as the temperature is altered from 155 to 100 K. The influence of intermolecular interaction like the dipolar interaction on the electron density particularly in the intermolecular region is clearly visible in the electron density maps. One of the most striking features of the electron density of ADP is the presence of non nuclear maxima (NNM) within the “ab” planes. It is argued that the appearance of these NNMs is a normal consequence of the chemical bonding between homonuclear groups in ADP. The manuscript describes the experimental electron density of ammonium dihydrogen phosphate (ADP) obtained from its high resolution X-ray diffraction data recorded at two crystal temperatures namely 100 and 155 K.

Structure, phase transitions, 55Mn NMR and magnetoresistive properties of Pr0.6−xNdxSr0.3Mn1.1O3−δ (x = 0−0.6)

Ceramic samples of Pr0.6−xNdxSr0.3Mn1.1O3−δ (x = 0−0.6) were studied by x-ray diffraction, resistive, magnetic (χac and 55Mn NMR), magnetoresistive and electron microscopy methods. It was shown that with increasing the concentration x, the type of unit cell distortion changes from orthorhombic (x = 0–0.2) to pseudo-cubic (x = 0.4–0.6), and the imperfection of the structure, which contains anion and cation vacancies, is increased. A decrease in the temperatures of metal–semiconductor (Tms) and ferromagnetic–paramagnetic (TC) phase transitions and an increase in the resistivity and activation energy with increasing x was explained by an increase in the concentration of vacancies, which weakens high-frequency electronic double-exchange Mn3+ ↔ Mn4+. It was found that the compositions with a higher content of neodymium exhibit a transition to the antiferromagnetic state at temperatures below 130 K. Two types of magnetoresistive effects were observed. The magnitude of the first effect, which occurs near the phase transition temperatures Tms and TC, increases with concentration x. The magnitude of the second effect, which was observed at low temperatures, exceeds that of the first one. The magnetic phase diagrams which describe strong correlations between the composition, structure defects, phase transitions, and functional characteristics, including magnetoresistive effect, were constructed.

Temperature dependence of dielectric and electromechanical properties of (K,Na)(Nb,Ta)O3 single crystal and corresponding domain structure evolution.

Domain structures and their evolution with temperature in the [001] C oriented (K,Na)(Nb,Ta)O3 (KNNT) single crystal have been studied before and after poling by polarizing light microscopy. The results indicate that the KNNT crystal is difficult to be completely poled by the room temperature poling process. The domain structure is rather stable in the orthorhombic phase, but exhibits substantial changes near the phase transition temperatures TO-T and TC. Narrower stripe domains are formed during both the orthorhombic-tetragonal and tetragonal-cubic phase transition processes, no intermediate phases were found during the phase transitions. The temperature dependence of the dielectric and piezoelectric properties were measured, and the influence of domain structures on the dielectric and electromechanical properties were quantified.

Evolution from Diffuse Ferroelectric to Relaxor Ferroelectric in Pb1−xBax(Fe1/2Nb1/2)O3 Solid Solutions

Dielectric and ferroelectric characteristics for Pb1−xBax(Fe1/2Nb1/2)O3 (x = 0, 0.05, 0.1, 0.15, and 0.2) ceramics are determined together with their structures. X-ray diffraction (XRD) analysis confirms the solid solutions with the cubic structure. The dielectric nature changes from diffuse ferroelectric to relaxor ferroelectric with increasing x, while the phase transition temperature TC (or Tm) decreases monotonously. The diffuse ferroelectric phase transition is observed in the solid solutions with 0 ≤ x ≤ 0.05. For Pb1—xBax(Fe1/2Nb1/2)O3 with 0.1 ≤ x ≤ 0.2, relaxor ferroelectric behavior is determined, and the Vogel-Fulcher equation is used to describe the relaxor behavior. The 1/ε versus T plots reveal the diffusion dielectric characteristics in both diffuse and relaxor ferroelectrics.

Direct evidence of Ni magnetic moment in TbNi2Mn—X-ray magnetic circular dichroism

We have investigated the individual magnetic moments of Ni, Mn and Tb atoms in the intermetallic compound TbNi2Mn in the Laves phase (magnetic phase transition temperature TC ~131K) by X-ray magnetic circular dichroism (XMCD) studies at 300K, 80K and 20K. Analyses of the experimental results reveal that Ni atoms at 20K in an applied magnetic field of 1T carry an intrinsic magnetic moment of spin and orbital magnetic moment contributions 0.53±0.01μB and 0.05±0.01μB, respectively. These moment values are similar to those of the maximum saturated moment of Ni element. A very small magnetic moment of order <0.1μB has been measured for Mn. This suggests that Mn is antiferromagnetically ordered across the two nearly equally occupied sites of 16d and 8a. A magnetic moment of up to ~0.3μB has been observed for the Tb atoms. Identification of a magnetic moment on the Ni atoms has provided further evidence for the mechanism of enhancement of the magnetic phase transition temperature in TbNi2Mn compared with TbNi2 (TC~37.5K) and TbMn2 (TC~54K) due to rare earth–transition metal (R–T) and transition metal–transition metal (T–T) interactions. The behaviour of the X-ray magnetic circular dichroism spectra of TbNi2Mn at 300K, 80K and 20K – above and below the magnetic ordering temperature TC ~131K – is discussed.

Effect of Zr/Ti ratio on the dielectric and piezoelectric properties of Mn-doped Ba(Zr, Ti)O3 ceramics

Lead-free piezoelectric Ba(ZryTi1−y)O3:0.5 mol%Mn (abbreviated as BZTM-100y, y = 0.04, 0.045, 0.05, and 0.055) ceramics were prepared via the conventional solid-state reaction method. The cubic to tetragonal phase transition temperature (Tc) shifts to lower temperature with increasing Zr concentration, and the transition temperature from tetragonal to orthorhombic (To−t) which is a shift to higher temperature from 26 to 47 °C with increasing y value from 0.04 to 0.055. And a small amount of Mn can improve effectively the electrical properties, especially the piezoelectric properties of the BZT ceramics. Compared to the pure BZT ceramics, the Mn-doped BZTM ceramic shows an excellent piezoelectric coefficient d33 as 213 pC/N.

Real-time crystallization in fluorinated parylene probed by conductivity spectra

Dielectric relaxation spectroscopy experiments were performed at high temperature on fluorinated parylene films during the occurrence of the isothermal crystalline phase transition. For this polymer, since the difference between the glass transition temperature (Tg) and the phase transition temperature (Tc) is very strong (Tc ≥ 4Tg), segmental and dipolar relaxation usually used to probe the crystallization are not shown in the experiment frequency window (10−1 to 106 Hz) during the crystallization. The charge diffusion becomes the only electrical marker that allows probing the phase transition. During the transition phase, a continuous decrease of about two orders of magnitude is observed in the conductivity values below an offset frequency (fc) with a tendency to stabilization after 600 min. Below the offset frequency, the decrease of the normalized conductivity to the initial value as function of time is frequency independent. The same behavior is also observed for the fc values that decrease from 160 Hz to about 20 Hz. Above the offset frequency, the electronic hopping mechanism is also affected by the phase transition and the power law exponent (n) of the AC conductivity shows a variation from 0.7 to 0.95 during the first 600 min that tend to stabilize thereafter. Accordingly, three parameters (n, fc, and AC conductivity values for frequencies below fc) extracted from the AC conductivity spectra in different frequency windows seem suitable to probe the crystalline phase transition.

Thermally Stable BaTiO3-Bi(Mg0.75W0.25)O3 Solid Solutions: Sintering Characteristics, Phase Evolution, Raman Spectra, and Dielectric Properties

(1 − x)BaTiO3-xBi(Mg0.75W0.25)O3 [(1 − x)BT-xBMW, 0.02 ≤ x ≤ 0.24] ceramics were synthesized by a two-step solid-state reaction technique. X-ray diffraction (XRD) patterns show that a systematic structure evolution from tetragonal to pseudocubic phase was observed at x = 0.07. Raman spectra analysis illustrates that a change in average structure was observed with increasing x, and the local crystal symmetry which deviated from the idealized cubic perovskite structure appeared as x ≥ 0.07. Temperature dependence of dielectric properties indicates that the phase transition temperature (Tc) decreased with increasing x. Moreover, (1 − x)BT-xBMW (0.07 ≤ x ≤ 0.24) ceramics show good dielectric thermal stability over a wide temperature range, which indicates that these ceramics are candidates for thermal stability devices.

A theoretical study of soft mode behavior and ferroelectric phase transition in 18O-isotope exchanged SrTiO3: evidence of phase coexistence at the quantum critical point

Motivated by recent experiments, the dynamics of the ferroelectric soft mode and the ferroelectric phase transition mechanism in 18O isotope exchanged systems SrTi(16O1−x18Ox)3 (abbreviated as STO18-x) are reinvestigated as a function of the 18O isotope exchange rate x, within a quasiharmonic model (QHM) for quantum ferroelectric modes in double-Morse local potential with mean-field approximation interactions between modes. The approach was realized within the framework of the variational principle method at finite temperature through the quantum mean-field approximation and by taking into account the effect of isotope replacement through the predominant mass effect, the cell volume effect, homogeneity of the composition throughout the material and the concentration-dependent ferroelectric mode distortion effect. The dynamics of the lowest-frequency soft phonon mode clearly presents an increased softening phenomenon with increasing x and a complete one at the corresponding phase transition temperature Tc, demonstrating the perfect soft-mode-type quantum ferroelectric phase transition for x ⩾ xc. Also, a ferroelectric–paraelectric phase coexistence state has been found near the quantum critical point xc and its origin is discussed. The ferroelectric phase transition mechanism is analyzed and its nature discussed, where a second-order phase transition close to the tricritical point is predicted. In addition, the effect of quantum fluctuations on the soft mode dynamics is discussed which reveals its reduction with increasing x and the crossover of the soft mode dynamics from the quantum to the classic one at the full 18O exchange limit x = 1, for which the origin seems to lie in the new homogeneity associated with the direct reduction of quantum fluctuations effects on the soft mode behavior. Within the QHM, consistent agreement with some of the previous experimental results and theoretical predictions of quantum ferroelectricity throughout the full range of x are achieved.

An effective growth method to improve the homogeneity of relaxor ferroelectric single crystal Pb(In1/2Nb1/2)O3‐ Pb(Mg1/3Nb2/3)O3‐PbTiO3

Compositional segregation usually has negative effects on the growth of solid solution ferroelectric single crystals of Pb(In1/2Nb1/2)O3‐Pb(Mg1/3Nb2/3)O3‐PbTiO3 (abbr. PIN‐PMN‐PT or PIMNT). A modified Bridgman method was adopted in this work to control the segregation and improve the compositional homogeneity significantly. The characteristic of this work is to use multiround growths and gradient composition raw materials in order to keep the PbTiO3 concentration constant during the crystal growth. As an example, the two‐round growth of ternary PIN‐PMN‐PT single crystal is conducted in the same Pt crucible with gradient raw materials, where the first‐round boule was used as the seed crystal for the second‐round growth. Our results show that the as‐grown (Ф80 mm × 270 mm) PIN‐PMN‐PT crystals exhibit higher phase transition temperatures (Tc∼180 °C, Tr/t∼110 °C) and larger coercive field (Ec∼5–5.5 kV/cm), which are much better than the performances of Pb(Mg1/3Nb2/3)O3‐PbTiO3 crystals, and similar dielectric and piezoelectric performances (ε∼5000, tanδ∼1.25%, d33∼1500 pC/N, kt∼60%). And about 85 percent of the crystal boule grown by the two‐round growth technique could maintain its compositions around the morphotropic phase boundary.

The synthesis and performance of Zr-doped and W–Zr-codoped VO2 nanoparticles and derived flexible foils

The effect of Zr doping on the Tc, Tlum and ΔTsol values of VO2.