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1.
In this study using a mean field model we calculate the phase line equations for the β-δ, β-γ and γ-δ phase transitions in the NH4BrxCl1 x system. We then fit our phase line equations to the experimentally observed T-X Br phase diagram for this system. Our calculated phase diagram agrees well with the observed one for the NH4BrxCl1 x system.  相似文献   

2.
We have studied the influence of Ar atoms on the high-pressure phase behaviour of N2 by a Monte Carlo study of Ar x (N2)1-x mixed solids. The results were obtained on the N2 rich side (0 ≤ x ≤ 0.25), in the region where the ? and δ phases exist in pure N2. The ??—δ? transition shifts rapidly to lower temperatures with x: it has vanished for x ? 0.05. For x ? 0.12, a cubic orientationally ordered phase appears. The transition from this phase to the δ? phases shifts to higher temperatures with increasing x. The temperature of the onset as well as the completion of the cascade process, responsible for the δ?loc—δ?rot transition, is independent of composition. This transition disappears in the x-region where the ordered phase is present for all temperatures below the transition to the δ?rot phase. Therefore, the influence of the isotropic diluents on the orientational behaviour of the N2 molecules is threefold. In the δ?loc phase, the vibron frequencies show a splitting that shifts as a function of x, which might be related to the occurrence of a tetragonal modification of this phase.  相似文献   

3.
High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al0.5Ga0.5PO4. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C2221 phase transforms to orthorhombic Cmcm phase at 4.9?GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19?GPa. The values of bulk modulus for C2221 and Cmcm phases are 19(2) and 126(4)?GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C2221 phase to Cmcm phase has not observed up to 17.4?GPa. Instead, a new monoclinic phase P21 is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.  相似文献   

4.

In this work, we studied the behavior of the Nd–Dy–Fe–Co–Cu–B alloy for permanent magnets under high pressure torsion (HPT). In the initial state of the studied alloy, it mainly contained the crystalline phase τ1 (Nd, Dy)2(Fe, Co, Cu) 14B. After HPT at room temperature (THPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ1 phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ1 phase with the melt at the temperature Teff= ∼1100°C. The thus determined Teff value is called the effective temperature. When the THPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe2B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ1+ Fe2B + γ-Fe, which is observed in the temperature range from ∼950 to ∼1050°C. We explain this phenomenon by the fact that with an increase in the HPT temperature THPT, the rate of formation of defects during deformation remains constant, but the rate of their thermal relaxation (annihilation) increases. This is equivalent to decrease in the effective temperature Teff in the equilibrium phase diagram. The previously predicted decrease in Teff with an increase in THPT is observed for the first time.

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5.
A phase diagram identical to the x-T diagram for lead zirconate-titanate (PZT) solid solutions (PbZr1 − x Ti x O3) was obtained on the basis of a statistical 12-minima model in the vicinity of the cubic phase boundary. The applicability of this model to the phase transitions in PZT is discussed.  相似文献   

6.
The structural phase transition and electronic properties at ambient (B 1-phase) and high pressure (B 2-phase) of heavy rare earth monoantimonides (RESb; RE?=?Ho, Er, and Tm) have been studied theoretically using the self-consistent tight binding linear muffin tin orbital method. These compounds show metallic behavior under ambient condition and undergo a structural phase transition to the B 2 phase at high pressure. We predict a structural phase transition from the B 1 to B 2 phase in the pressure range 30.0–35.0?GPa. Apart from this, the ground state properties, such as lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.  相似文献   

7.
The temperature dependence of the crystalline structure and the lattice parameters of Pb1? x La x Zr0.40Ti0.60O3 ferroelectric ceramic system with 0.00 ≤ x ≤ 0.21 was determined. The samples with x ≤ 0.11 show a cubic-to-tetragonal phase transition at the maximum dielectric permittivity, T max. Above this amount and especially for the x = 0.12 sample, a spontaneous phase transition from a relaxor ferroelectric state (cubic phase) to a ferroelectric state (tetragonal phase) is observed upon cooling below the T max. Unlike what has been reported in other studies, the x = 0.13, 0.14, and 0.15 samples, which present a more pronounced relaxor behavior, also presents a spontaneous normal-to-relaxor transition, indicated by a cubic to tetragonal symmetry below the T max. The origin of this anomaly has been associated with an increase in the degree of tetragonality, confirmed by the measurements of the X-ray diffraction patterns. The differential thermal analysis (DSC) measurements also confirm the existence of these phase transitions.  相似文献   

8.
The influence of different regimes of application of a dc electric field (0 < E < 4 kV/cm) on structural phase transformations and behavior of the phase boundary in [001]-oriented single crystals PMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) with compositions near the morphotropic phase boundary (x = 28 and 32%) and far from it (x = 13%) has been investigated. The studies have been performed using optical methods, namely, optical transmittance and small-angle light scattering. It has been revealed that the number, symmetry, and stability of field-induced phases depend on the regime of electric field application. During cooling of the PMN-28PT crystals from the cubic phase in the field (FC regime), the tetragonal phase is induced already in weak fields of ∼0.5 kV/cm, whereas even a field of 3 kV/cm is insufficient for inducing this phase by applying the field in the ferroelectric phase. In the PMN-32PT crystal (which has a composition closer to the morphotropic phase boundary as compared to that of the PMN-28PT crystal) in the FC regime and when the field is applied in the ferroelectric phase, the tetragonal phase is induced already in weak electric fields, even though this phase in the latter case appears to be unstable. The E-T phase diagrams have been constructed for different regimes of field application. The possible factors responsible for the observed differences have been discussed.  相似文献   

9.
The CsCl-type compounds CeAg x In1–x have a martensitic phase transformation with properties similar to that in LaAg x In1–x , but with higher transition temperature. For other trivalent lanthanides, the temperature for the transition into the tetragonal low temperature phase decreases with increasing atomic number. In the Jahn-Teller like model for the phase transformation, this can be related to the increasing distance of the 5d-levels from the Fermi energy.  相似文献   

10.
The pressure- and photo-induced phase transition in mixed-valence gold complexes of Cs2Au2X6 (X = Cl, Br, and I) has been investigated by means of the Raman scattering. The Raman-active Au-X stretching modes were deactivated by the pressure, which indicates a pressure-induced phase transition from the mixed-valence (MV) state to the single-valence (SV) state. The electronic phase diagrams of Cs2Au2X6 (X = Cl and Br) as a function of pressure and temperature have been derived. A photoinduced phase transition from the MV state to the SV state has been found for Cs2Au2Br6. The observed time behavior accompanying this phase transition is successfully interpreted by the Avrami model, indicating the three-dimensional character of the MV cluster growth.  相似文献   

11.
Containerless solidification processes in undercooled Lu2O3 and Y2O3 melts were investigated using an aerodynamic levitation furnace and a high-speed video. Double recalescence, indicative of two successive phase transitions, was observed for both oxides. The melting point following the first recalescence (T M1) was higher than that of the second (T M2) for Y2O3, which suggests that Y2O3 has a phase transition from a melt to a high temperature phase at T M1, and then to a room temperature phase at T M2. Two solid phases of Lu2O3 were also identified at T M1 and T M2. However, in contrast to Y2O3, T M1 was lower than T M2 for Lu2O3. This implies that the first crystallized phase from a melt is a metastable phase. The life duration of this metastable Lu2O3 was in the order of tens of milliseconds.  相似文献   

12.
Low field (2 MHz) Nuclear Magnetic Resonance (NMR) proton spin–spin relaxation time (T2) distribution measurements were employed to investigate tetrahydrofuran (THF)—deuterium oxide (D2O) clathrate hydrate formation and dissociation processes. In particular, T2 distributions were obtained at the point of hydrate phase transition as a function of the co-existing solid/liquid ratios. Because T2 of the target molecules reflect the structural arrangements of the molecules surrounding them, T2 changes of THF in D2O during hydrate formation and dissociation should yield insights into the hydrate mechanisms on a molecular level. This work demonstrated that such T2 measurements could easily distinguish THF in the solid hydrate phase from THF in the coexisting liquid phase. To our knowledge, this is the first time that T2 of guest molecules in hydrate cages has been measured using this low frequency NMR T2 distribution technique. At this low frequency, results also proved that the technique can accurately capture the percentages of THF molecules residing in the solid and liquid phases and quantify the hydrate conversion progress. Therefore, an extension of this technique can be applied to measure hydrate kinetics. It was found that T2 of THF in the liquid phase changed as hydrate formation/dissociation progressed, implying that the presence of solid hydrate influenced the coexisting fluid structure. The rotational activation measured from the proton response of THF in the hydrate phase was 31 KJ/mole, which agreed with values reported in the literature.  相似文献   

13.
The phase diagram of the model of spinless fermions with repulsive nearest neighbour interaction is calculated analytically on a hypercubic lattice in infinite dimensions (d → ∞). In spite of its simplicity the model displays a rich phase diagram depending on the doping δ, the interaction U and the temperature T. The system can be in the homogeneous phase (HOM), the nonsegregated AB charge density wave (AB-CDW), the AB phase separation region (PS-AB/HOM; coexistence of AB-CDW and HOM), the incommensurate phase (IP) or the IP phase separation region (PS-AB/IP; coexistence of AB-CDW and IP). We identify three important values of the interaction UIPL = 0.572 < UIPH = 1.914 < UIP/PS = 4.212 which distinguish four intervals of U. These imply four different types of phase diagrams. In all the three phase diagrams with U below UIP/PS the IP appears. We propose a new general ansatz for the order parameter of this phase. A competition between the IP, the PS-AB/IP and the PS-AB/HOM is found. The relevance of our findings for the phase scenario of the Hubbard model is shown.  相似文献   

14.
The lattice QCD studies indicate that the critical temperature T c ≃ 260-280 MeV of the deconfinement phase transition in quenched QCD is considerably smaller than the lowest-lying glueball mass m G ≃ 1500-1700 MeV, i.e., T cm G. As a consequence of this large difference, the thermal excitation of the glueball in the confinement phase is strongly suppressed by the statistical factor e -mG/Tc ≃ 0.00207 even near TT c. We consider its physical implication, and argue the abnormal feature of the deconfinement phase transition in quenched QCD from the statistical viewpoint. To appreciate this, we demonstrate a statistical argument of the QCD phase transition using the recent lattice QCD data. From the phenomenological relation between T c and the glueball mass, the deconfinement transition is found to take place in quenched QCD before a reasonable amount of glueballs is thermally excited. In this way, quenched QCD reveals a question “what is the trigger of the deconfinement phase transition ?” Received: 18 November 2002 / Accepted: 4 February 2003 / Published online: 29 April 2003  相似文献   

15.
Abstract

Contrary to monocrystals powdered KCN displays, after grinding and without annealing, an intermediate monoclinic phase between the high-temperature pseudocubic and the low-temperature orthorhombic phases. The consequence of annealing is the total disappearance of this intermediate phase. A further effect of the annealing process is the reduction of the cubic distortion at the transition. In this paper, we analyse this effect on the intermediate rhombohedral phase observed in the (KCN) x (KBr)1?x mixed system.  相似文献   

16.
We investigate the physical properties of two coupled chains of electrons, with a nearly half-filled band, as a function of the interchain hopping t and the doping. We show that upon doping, the system undergoes a metal-insulator transition well described by a commensurate-incommensurate transition. By using bosonization and renormalization we determine the full phase diagram of the system, and the physical quantities such as the charge gap. In the commensurate phase two different regions, for which the interchain hopping is relevant and irrelevant exist, leading to a confinement-deconfinement crossover in this phase. A minimum of the charge gap is observed for values of t close to this crossover. At large t the region of the commensurate phase is enhanced, compared to a single chain. At the metal-insulator transition the Luttinger parameter takes the universal value K ρ * = 1, in agreement with previous results on special limits of this model. Received 31 July 2000  相似文献   

17.
This paper is devoted to the study of the influence of metal ion isomorphous substitution on the ferroelastic-ferroelectric phase transition and dispersion caused by the motion of domain walls in dimethylammonium metal sulfate hexahydrate DMAAl1?xCrxS ferroelectric crystals (x = 0, 0.065, 0.2). It is shown that such a substitution significantly changes the phase transition temperature and parameters of the dielectric dispersion. These changes are explained in terms of interaction between the metal-hydrate complexes and DMA groups that carry the dipole moment and due to this they are responsible for the phase transitions and motion of the domain walls.  相似文献   

18.
An X-ray diffraction and 57Fe Mössbauer spectroscopy investigation of Si100– x Fe x (0?<?x?<?80) thin films prepared by combinatorial sputtering methods is reported. Resulting Mössbauer spectra were fit to Voigt-based distributions of quadrupole doublets for paramagnetic spectral components and Zeeman split sextets for ferromagnetic spectral components. In conjunction with the X-ray measurements, these results show that the Si-rich films are a mixture of dilute Fe in amorphous Si and an approximately equiatomic amorphous SiFe phase. Fe-rich films show the presence of a ferromagnetically ordered phase. For x?<?73, this ferromagnetic phase is amorphous or nanostructured and for x?≥?73, the phase is shown to be a crystalline bcc phase. Results are discussed in terms of short-range structural ordering in these alloys.  相似文献   

19.
In order to clarify the mechanism of superprotonic conductivity (SPC) for the hydrogen-bonded compound CsHXO4 (X=S, Se), we have investigated the elastic properties of CsHXO4 with various transition temperatures (Tc), at which CsHXO4 exhibits a structural phase transition from a low-temperature monoclinic phase with low protonic conductivity (phase II) to a high-temperature SPC phase (phase I). It was found that, in CsHSxSe1 − xO4 (x=0–1), the transition temperature Tc decreases in proportion to the square of spontaneous strain b in the aIIbII plane of phase II. This result indicates that the spontaneous strain in the aIIbII plane plays an important role in the appearance of SPC in CsHXO4. Moreover, thermodynamic analyses suggest that proton migration in phase I is induced by the release of the strain energy. From these results, it is deduced that the phase transition temperature in CsHXO4 is determined by the competition between the kinetic energy of proton and the released-strain energy.  相似文献   

20.
The ferroelectric phase transition characteristics of the 0.32Pb(In1/2Nb1/2)O3-0.345Pb(Mg1/3Nb2/3)O3-0.335PbTiO3 (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FEM-FET) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FET-PC), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around TM-T and TC/Tm temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FEM-FET phase transition and the first-order transition feature of the FET-PC phase transition. The FEM-FET phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the Pr and Ec values, and extremums of the pyroelectric performance.  相似文献   

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