A comparative study of cationic distribution in the systems Li1+5XM1−XO3 and Li1+XM1−XM′ XO3 (M=Nb,M′=Ti; M=Ta,M′=Ti,Zr, Sn)

Abstract

A comparative ferroelectric and NMR studies of the systems Li_1+5X M_1?XO₃ and Li_1+XM_1?XM^′ _XO₃ (M=Nb, M′=Ti M = Ta; M′=Ti, Zr, Sn) have indicated that plot of T_C versus composition can be qualitatively explained on the basis of cationic distribution. The strong drop of T_C has been found in systems with all cations in octahedral sites, whereas little change of T_C was observed along the solid solutions with Li-ions in interstitial tetrahedral sites.     

Investigation of ferroelectric transitions in the binary system LiTaO3 [sbnd] Fe2O3

Abstract

A continuous solid solution with the formula Li_1?XTa_1?XFe_2XO₃ O≤x≤. 125 has been identified. Hexagonal a_H axis decreases very slightly with increasing Fe₂O₃ content whereas simultaneously c_H passes through a maximum at x =.06. This behaviour has been tentatively explained on the basis of repulsive forces. The ferroelectric Curie temperature decreases as the composition deviates from LiTaO₃.     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

Inelastic neutron scattering study of the relaxor ferroelectric PbMg1/3Nb2/3O3 at high temperatures

Results of inelastic neutron scattering experiments between 300 K and 900 K on the relaxor ferroelectric PbMg _1/3 Nb _2/3 O₃ are presented. Within a mode-coupling analysis the data are consistent with the observation of a strongly damped quasi-optic excitation. It is demonstrated that below K a dynamical crossover takes place manifesting itself by a narrow central peak. This crossover is accompanied by the appearance of strong damping of the transverse acoustic phonons, with the damping constant proportional to q⁴. Different physical models of the crossover are discussed. Received 8 February 1999     

Lattice mediated interactions and ferroelectric anomalies in the crystal (CH3NH3)5Bi2Cl11 (PMACB)

Abstract

A two-sublattice compressible pseudospin model has been constructed to describe the behaviour of the spontaneous polarisation of PMACB in the temperature region comprising the ferroelectric phase transition (T=307K) and the isomorphous anomaly (T= 180 K). A strong coupling of pseudospins with a “pancake” strain ?(2ε₃₃ ? ε₁₁ - ε₂₂) /3 resulting from the comparison of the model with the experimental spontaneous polarisation agrees with the existing data on the dielectric susceptibility, and on the specific heat. The ferroelectric phase transition then turns out to be close to a tricritical point. The numerical values of the parameters of the model show how the ferroelastic phase transition (T=260K) can occur in this crystal without any discernible effect on the specific heat and on the dielectric susceptibility.     

Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : I. Susceptibility of clusters and domains

The complex dielectric susceptibility of Sr _0.61 Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ (SBN61:Ce) has been measured at frequencies and temperatures before and after poling. The relaxor behaviour with large polydispersivity observed above the ferroelectric phase transition temperature, T _c = 360 and 340 K for x (Ce) = 0 and 0.0066, respectively, is perfectly modeled within the framework of Chamberlin's dynamically correlated domain approach. Below T _c the dynamic nanodomain state crosses over into a ferroelectric state with polydispersive domain wall dynamics at very low frequencies. Presumably SBN61:Ce belongs to the three-dimensional random field Ising rather than to the dipole glass universality class. Received 1 October 1999     

Broad-band dielectric response of 0.5Ba(Ti0.8Zr0.2)O3–0.5(Ba0.7Ca0.3)TiO3 piezoceramics: soft and central mode behaviour

ABSTRACT

Dielectric properties of 0.5Ba(Ti_0.8Zr_0.2)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ ceramics were probed in the frequency range from 10 Hz to 100 THz in a broad temperature range (10–900 K). Polar soft phonon observed in infrared spectra softens with cooling; however, below 500 K, its frequency becomes temperature independent. Simultaneously, a central mode activates in terahertz and microwave spectra; and it actually drives the ferroelectric phase transitions. Consequently, the phase transitions strongly resemble a crossover between the displacive and order–disorder type. The central mode vanishes below 200 K. The dielectric relaxation in the radiofrequency and microwave range anomalously broadens on cooling below T_C1, resulting in the nearly frequency independent dielectric loss below 200 K. This broadening comes from a broad frequency distribution of ferroelectric domain wall vibrations. Raman spectra reveal new phonons below 400 K, i.e. already 15 K above T_C1. Several weak modes are detected in the paraelectric phase up to 500 K in Raman spectra. Activation of these modes is ascribed to the presence of polar nanoclusters in the material.     

Divide line between relaxor,diffused ferroelectric,ferroelectric and dielectric

The dielectric spectroscopic studies of relaxor ferroelectric Ba(Ti_0.70Sn_0.30)O₃ (BTS30), diffused ferroelectric Ba(Ti_0.70Sn_0.20)O₃ (BTS20), classical ferroelectric BaTiO₃ (BT0), and normal dielectric Ba(Fe_0.50Ta_0.50O₃) (BFT) were carried out in the temperatures ～100 K–650 K, and frequencies ～100 Hz–1 MHz, which showed relaxor behavior at 130 K with typical 30 K frequency dispersion, perfect diffused ferroelectric character at 190 K, classical ferroelectric, and dielectric relaxation at 450 K–550 K respectively. A divide line was sketched between the relaxors, ferroelectrics and normal dielectrics based on the Maxwell–Wagner space charge model. The impedance spectroscopy and ac conductivity spectroscopy were carried out to explain the relaxor and pseudo relaxor behavior.     

Broad-band dielectric spectroscopy of Ba 2NaNb 5O 15 single crystal

Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 10²-10¹⁴ Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T _c = 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase. Below T _i the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B₂-symmetry soft mode. Received 24 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: buixader@fzu.cz     

Critical anomaly in the electron spin-lattice relaxation at the ferroelectric phase transition of X-irradiated KDA

The spin-lattice relaxation of X-irradiated ferroelectric KDA has been investigated by means of the electron spin-echo method in the range between 2 and 200 K. In the vicinity of the phase transition point an anomalous increase of T₁ has been observed. This effect could not be detected for KDA-KDP mixed crystals with a high concentration of KDP. The anomaly of the spin-lattice relaxation at the phase transition is explained by the increased damping of the “hard” optical mode which governs the relaxation behaviour at this temperature region.     

The structure of ‘nearly’ ferroelectric RADP

X-ray scattering techniques have been used to study the structures of two crystals of Rb_1–x (NH₄) _x H₂PO₄ withx0.2, near to the boundary between ferroelectric and structural-glass behavior at low temperature. Below about 83K, both crystals develop shortrange incommensurate correlations with a wavevectorq0.13 a ^* which are presumably related to the glass properties. On cooling below 70 K, the crystal with the slightly larger NH ₄ ⁺ concentrationx, develops the ferroelectric structure in a small fraction of the crystal, while the bulk of the crystal retains the tetragonal structure. The other crystal transforms almost wholly to the ferroelectric phase. The transition to the ferroelectric structure shows considerable hysteresis on heating and cooling, and is spread over about 20 K. The transition is certainly of first order, and the spread in temperature may arise from concentration fluctuations. These results and the structure of the incommensurate modulations are compared with the predictions of a theoretical model for this system.     

Anomalous behavior of the elastic and inelastic properties in the ferroelectric phase of single-crystal (NH4)2SO4

The anomalous changes of the low-frequency elastic and inelastic properties of single-crystal (NH₄)₂SO₄ accompanying the phase transition from the paraelectric to the ferroelectric phase have been studied by the reverse torsion-pendulum method at 223 K and in the temperature region where the spontaneous polarization changes sign. Fiz. Tverd. Tela (St. Petersburg) 40, 2202–2205 (December 1998)     

Electrical and thermal properties of lead titanate glass ceramics

Glass samples with composition of (50−X)PbO-(25+X)TiO₂-25B₂O₃ (where X=0, 5, 10 and 12.5 mol%) were prepared using conventional quenching technique. The glass transition temperature, T_g and crystallization temperature T_c were determined from the DTA. These glass samples were converted to glass ceramics by following two stage heat treatment schedule. The glass ceramic samples were characterized by XRD, SEM and dielectric constant measurements. The XRD results revealed the formation of ferroelectric lead titanate (PT) as a major crystalline phase in the glass ceramics. The density increases and the CTE decreases for all glass ceramics with increase in X (mol%). This may be attributed to increase in PT phase. The SEM results which show rounded crystallites of lead titanate, also supports other results. Hysteresis loops observed at room temperature confirms the ferroelectric nature of glass ceramics. The optimized glass ceramic sample exhibits high dielectric constant which is of technical importance.     

Coexistence of glass and ferroelectric order in deuterated betaine phosphate0.05 betaine phosphite0.95 crystals

The dielectric spectra of ferroelectric hydrogen bonded betaine phosphate_0.05 betaine phosphite_0.95 (DBP_0.05DBPI_0.95) was investigated in the very wide temperature (300–20 K) and frequency (20–35 GHz) regions. The dielectric dispersion was analyzed in terms of distribution of relaxation times, using Tichonov regularization method. Strongly asymmetric and broad distribution of relaxation times below ferroelectric phase transition temperature T _c?≈?253 K clearly differs from the one that is usually observed in ferroelectrics. The observed disorder in deuterons system close to ferroelectric phase transition temperature is an embryo of coexistence ferroelectric order and dipolar glass disorder observed at low temperatures.     

Low temperature,high pressure Raman study of SbSBr

Abstract

We have measured the Raman spectra of the quasi-one-dimensional crystal SbSBr as a function of pressure at 295, 70, 37 and 25 K.

The pressure coefficients of the observed Raman modes have been determined and used to distinguish inter-from intrachain modes. Spectral features characteristic of the ferroelectric phase have been attributed to impurities or lattice imperfections and not to the presence of the ferroelectric phase, providing indication that the para-to ferroelectric phase transition occurs below 25 K.     

Ferroelectric properties of SrTiO3-PbTiO3 solid solutions

Dielectric properties of ceramic samples of Sr_1−x Pb_xTiO₃ solid solutions for x varying from 0 to 0.3 have been studied. The ferroelectric phase transition in this system has been established to persist down to x=0.005. Within the x values of 0.002 to 0.05, the ferroelectric transition temperature is shown to follow the relation T _c=A(x−x _c)^1/2, with A=440 K and x _c=0.002. Fiz. Tverd. Tela (St. Petersburg) 39, 714–717 (April 1997)     

Phase transitions in h-thiourea under electric‐field

Abstract

Dielectric constant, spontaneous polarization and pyroelectric response measurements were performed on thin samples (e < 100 μm) of H-thiourea as a function of temperature and applied electric field. Along with previous data obtained by optical birefringence, X-ray and neutron diffraction techniques, the results put in evidence three features in the modulated region between the ferroelectric state and the paraelectric one. The tricritical point is determined for E_tr ? 2075 V/mm and T_tr ? 192 K.     

Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.