The crystal structure of antiferroelectric Pb2MgWO6 has been studied using neutron diffraction at high pressures to 5.4 GPa at room temperature and energy-dispersive X-ray diffraction at high pressures to 4 GPa in the temperature range 300–400 K. At normal conditions, in Pb2MgWO6, there is an antiferroelectric phase with the crystal structure described by the orthorhombic symmetry with space group Pnma. At temperature T = 313 K and normal pressure or at room temperature and pressure P ~ 0.9 GPa, the crystal under-goes a structural phase transition to the cubic phase with space group $Fm\bar 3m$ (paraelectric phase). The temperature and pressure dependences of the lattice parameters, unit cell volume, and interatomic bond lengths have been obtained, and the thermal expansion coefficients and the bulk moduli have been calculated for the antiferroelectric and paraelectric phases of Pb2MgWO6. 相似文献
The dynamics of precursor polar clusters in rhodium(Rh)-doped barium titanate single crystals were examined in the paraelectric phase by Brillouin light scattering and birefringence measurements in a wide temperature range. The longitudinal acoustic (LA) mode and central peaks in the inelastic light scattering spectrum were investigated by Brillouin scattering, while the temperature dependence of the birefringence was accurately determined by means of a birefringence imaging system. In a specific temperature range above the ferroelectric phase transition temperature, the LA mode frequency exhibited a significant softening concomitant with the substantial increase in the hypersonic damping, which was attributed to the formation of polar clusters and their interactions with acoustic waves. In this temperature range where acoustic anomalies were brought about, the birefringence showed non-zero values, indicating the existence of local non-centrosymmetric regions. All these results clearly indicated that the off-centered motions of Ti ions in the oxygen octahedra in Rh-doped BaTiO3 are correlated in the paraelectric phase inducing slowing down of the precursor dynamics and anomalous birefringence, similar to pure BaTiO3 (Ko et al., Phys. Rev. B 84, 094123 (2011)). The effect of rhodium doping in BaTiO3 enhanced the first-order character of the ferroelectric phase transition. 相似文献
Abstract Proton glasses are crystals of composition M1?x(NW4)xW2A04, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters. 相似文献
The acoustic behaviors of oxygen-reduced barium titanate (BaTiO3-δ) single crystals with δ∼0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO3 (BTO) showed two pronounced anomalies at approximately 112 °C and −11 °C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 °C compared to those of the pure BaTiO3 suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO3 were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point. 相似文献
The EPR T2+ center in NH4H2PO4 was studied in the range of temperature 85K – 175K which includes the antiferroelectric transition. The center has the symmetry of NH+4 site, where it goes substitutionally, in both the paraelectric and antiferroelectric phases. In the antiferrelectric phase there is a charge compensations mechanism involving the repulsion of one of the neighbor protons in the hydrogen bond. A measurement of the splitting of the resonance lines for the field along the cristallographic direction is presented as a function of temperature. This splitting is proportional to the order parameter. 相似文献
The polarized Raman spectra of (CH3NHCH2COOH)3CaCl2 (TSCC) have been obtained applying hydrostatic pressure in the paraelectric, ferroelectric and in the pressure-induced antiferroelectric phase. The phase transition between the paraelectric or the ferroelectric and the antiferroelectric phase appears to be of first order. No cell doubling could be observed in the antiferroelectric phase. The space group for TSCC in this phase is compatible with our experimental results. The pressure-dependence of the Raman-active soft mode is discussed qualitatively. 相似文献
Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP2Se6 crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at Tc?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics. 相似文献
Mixture of two liquid crystalline components exhibits the antiferroelectric phase in a broad temperature range at room temperatures,
though the two components separately do not show an antiferroelectric phase in a temperature range applied. The dielectric
spectroscopy technique combined with measurements of the selective light reflection was used for identification and characterization
of the phases and subphases existing in the mixture. In the SmC*A phase, the low frequency mode characteristic of antiferroelectric phase has been detected. In the broad temperature range
between SmC*A and SmA* phases, no relaxation mode has been detected. The soft mode registered near the phase transition to SmA* phase follows
the Curie-Weiss law. 相似文献
The combination of antiferroelectric PbZrO3 (PZ) and relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 was prepared via the columbite precursor method. The basic characterizations were performed using X-ray diffraction (XRD),
scanning electron microscopy (SEM), linear thermal expansion, differential scanning calorimetry (DSC) techniques, dielectric
spectroscopy, and hysteresis measurement. The XRD result indicated that the solid solubility limit of the (1−x)PZ–xPZN system was about x=0.40. The crystal structure of (1−x)PZ–xPZN transformed from orthorhombic to rhombohedral symmetry when the concentration of PZN was increased. A ferroelectric intermediate
phase began to appear between the paraelectric and antiferroelectric phases of pure PZ, with increasing PZN content. In addition,
the temperature range of the ferroelectric phase increased with increasing PZN concentration. The morphotropic phase boundary
(MPB) in this system was located close to the composition, x=0.20. 相似文献
For PbZrO3 and PbZr0.99Ti0.01O3 single crystals the first order Raman light scattering spectra in the paraelectric phase have been analysed. In lead zirconate two kinds of crystals, with and without a transient phase, were investigated. It was found that temperature dependence of the defect-induced soft mode frequency in the paraelectric phase has a negligible contribution to the low-frequency dielectric response. From the point of view of strong dielectric relaxation in the range of 106–109 Hz, which originates in a disorder of the lead sites, crossover of the order-disorder and displacive type of antiferroelectric phase transition in lead zirconate is considered: the phase transition mechanism is neither purely displacive nor order-disorder. Contribution of physical phenomena responsible for the ε(ω,T) dielectric response at low frequencies of an external electric field are described. 相似文献
The crystal structure of sodium niobate (NaNbO3) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO3 has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature. 相似文献
A first-principle method is used to calculate phonon density of states, Helmholtz free energy, internal energy, and entropy for ferroelectric and paraelectric SbSI. Theoretical phase transition temperature was obtained using the difference of the Helmholtz free energy, internal energy, and entropy term between ferroelectric and paraelectric phases on temperature. The obtained value is in reasonable agreement with the experimental second-order phase transition temperature Tc2 = 233 K. 相似文献
The integral equations for calculating ferroelectric and antiferroelectric phase transition temperatures, order parameters, and critical concentrations of solid solution components are derived. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is constructed taking into account the contribution of nonlinear effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature on the composition of a binary solid solution in the ferroelectric-antiferroelectric and ferroelectric-paraelectric systems is calculated. Numerical calculations are carried out for the PbTixZr1?xO3 and BaZrxTi1?xO3 solid solutions. The results obtained are in good agreement with the experimental phase diagrams of these systems. Analysis of the results indicates that any solid solution containing ferroelectric (antiferroelectric) and paraelectric components transforms into a relaxor state at sufficiently high concentrations of the paraelectric component. 相似文献
Ammonium dihydrogen phosphate NH4H2PO4 (ADP) is an antiferroelectric (AFE) compound belonging to the KDP-type family of hydrogen-bonded ferroelectrics. Recent ab initio results have shown that the optimization of the N–H–O bridges in ADP leads to the stabilization of the AFE state over a FE one. However, electron spin probe measurements have suggested that microregions of both phases may coexist above the critical antiferroelectric–paraelectric transition temperature. We have performed first principles studies of the energetics and relative stability of different AFE and FE defects embedded in a paraelectric (PE) matrix of ADP. Our analysis indicates that FE and AFE clusters are stable and may coexist in the PE phase, thus confirming the above suggestion. 相似文献
We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb0.50Sr0.50TiO3 thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb0.50Sr0.50TiO3 thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO3 perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f 相似文献
A compressible pseudo-spin Ising model hamiltonian is used to calculate the pressure-temperature phase diagram of quasi-one-dimensional hydrogen-bonded ferroelectric crystals such as CsD2PO4. We assume strong effective interactions, which are treated exactly, along chains, and weak volume dependent interactions, which are treated in the mean field approximation, between chains. In agreement with the experimental findings, the phase diagram exhibits a triple point and transition lines between ferroelectric, antiferroelectric, and paraelectric phases. However, the results suggest that the Ising model may be too simple to account for the detailed form of the phase diagram of CsD2PO4. 相似文献
Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmCA*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmCA* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmCA* phase. 相似文献
A new and unsual transition is reported in Ba2NaNb5O15: the crystal, which was previously known to undergo at 573 K a ferroelastic transition 4mm → mm2 (C4v → C2v), reverts back at 110 ± 5 K to a low temperature phase of 4mm (C4v) symmetry. Thus this crystal presents a unique example of a ferroelastic (mm2) phase stable in a wide temperature range (470 K) and sandwiched between two phases possessing the same higher point symmetry. 相似文献