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.     

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.     

Effect of γ-irradiation on the heat capacity of an [NH2(CH3)2]2 · CuCl4 crystal in the temperature range 80–300 K

The heat capacity of [NH₂(CH₃)₂]₂ · CuCl₄ crystals prior to and after γ-irradiation with doses of 1, 5, 10, and 50 MR is measured by the calorimetric method in the temperature range 80–300 K. It is found that, as the temperature decreases, the temperature dependence C _p (T) exhibits two anomalies which correspond to phase transitions from the incommensurate to the ferroelectric phase at T _c =281 K and from the ferroelectric to the ferroelastic phase at T ₁=255 K. The nature of the anomalies is typical of a first-order phase transition. In addition, a smeared anomaly in the form of a small increase in the heat capacity of the ferroelectric phase is observed at T≈275 K. It is demonstrated that when the dose of γ-irradiation increases, the anomalies decrease in magnitude and the phase transition temperatures are displaced: T _c increases and T ₁ decreases.     

Anomalous states of the structure of (NH4)2SO4 crystals in the temperature range 4.2–300 K

Crystals of the (NH₄)₂SO₄ ammonium sulfate are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameters exhibits an anomalous behavior, namely, a global hysteresis, and an anomalous increase in the lattice parameter a and the unit cell volume at temperatures below the ferroelectric phase transition point (T _c=223 K). The series of superstructure reflections observed corresponds to an incommensurate composite structure. Analysis of the temperature behavior of the mismatch parameters for the matrix (host) and superstructure (guest) lattices demonstrates that the (NH₄)₂SO₄ compound undergoes a number of phase transitions, including a transition to a three-dimensionally incommensurate composite phase and transitions to commensurate (along one of the crystallographic directions) composite phases.     

Phenomenological analysis of the phase transitions sequence in the ferroelectric crystal (CH3NH3)5Bi2Cl11 (PMACB)

A phenomenological model is proposed to describe the sequence of phase transitions in Pentakis (methylammonium) Undecachlorodibismuthate (111), (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB) crystal on the basis of the recent structural data. The ferroelectric phase transition at 307 K is attributed to the ordering in a sublattice of the methylammonium cations CH₃NH ₃ ⁺ } placed in centrosymmetric sites in the paraelectric phase, whereas the isomorphous anomaly at about 180 K is related to a variation of the order in the sublattice of the remaining CH₃NH ₃ ⁺ } cations. The phenomenological thermodynamic potential is constructed for this system of two nonequivalent sublattices and the numerical values of its coefficients are then estimated from the dielectric, pyroelectric and calorimetric data. The sublattices are found to be weakly coupled near the ferroelectric phase transition. The anomaly at 180 K is interpreted as a continuous trace of a first order phase transition in a field created by the cations already ordered in the ferroelectric phase transition. This is analogous to a cusp A₊₃ in the catastrophes theory. The comparison of the Curie constants with the saturation values of the spontaneous polarization suggests that the sublattices cannot be treated as consisting of simple two-states pseudospins.     

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     

Second harmonic generation in layered TunS2 in the vicinity of the low-temperature phase transitions

Second-harmonic generation in the ternary layered semiconductor TlInS₂ excited with the wavelength λ = 1.06 μm of YAG: Nd^{3 +} laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients d_eff, d₂₁, d₂₂ reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to T_i2 = 206K) is explained assuming that an incomplete lock-in transition in TlInS₂ takes place at T_c1 =204K within the temperature range between a ferroelectric (T_c2 = 201K) and an incommensurate (T_i2 = 206 K) phases. It is shown that the monoclinic point group symmetry C₂ is preserved also at temperatures lower than the phase transition temperature Tc4 = 79K to a weak ferroelectric state.     

Dielectric behavior and low temperature phase transition in NH4IO3

The electrical properties namely ac conductivity σ(ω,?T) and the complex dielectric permittivity (ε*) are measured at selected frequencies (5–100?kHz) as function of temperature (95?K?T?4IO₃. The ferroelectric hysteresis loops and the X-ray diffraction pattern are also measured. The analysis of the data indicates that the compound undergoes a structural phase transition at ～103?K and the behavior of σ(ω,?T) obeys the power law. The trend of the temperature dependence of the angular frequency exponent s (0?s?4IO₃; (2) the data indicate that the compound undergoes a structural phase transition at 103?K; (3) the originality of this transition has been confirmed by X-ray diffraction; (4) no evidence for the existence of a ferroelectric transition at 103?K as mentioned earlier; and (5) the quantum mechanical tunneling is proposed as the main mechanism of the electric conduction.     

On the nature of the phase transitions in cadmium pyroniobate

The existing, doubtful interpretations of the numerous phase transitions in cadmium pyroniobate Cd₂Nb₂O₇, which is a ferroelectric with a pyrochlore-type structure and a Curie temperature lying near 200 K, are critically analyzed. The manifestations of all phase transitions in cadmium pyroniobate, the mechanisms proposed for these transitions, and doubts about their validity are discussed. Various dielectric anomalies observed near 200 K are related to three phase transitions in this temperature range. The causes of the large number of phase transitions observed in this material, the dislocation structure of the crystal, the diffuseness of the ferroelectric phase transition, and the relaxor properties of the crystal are also explained.     

Cascade of phase transitions in GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Cascade of phase transitions in GdFe₃(BO₃)₄ at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd³⁺ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.     

Phase transitions in xPbZr0.53Ti0.47O3-(1 − x)Mn0.4Zn0.6Fe2O4 magnetoelectric composites

The elastic, inelastic, and dielectric properties of the magnetoelectric composite xPbZr_0.53Ti_0.47O₃-(1 ? x)Mn_0.4Zn_0.6Fe₂O₄ (PZT-MZF) are studied in the temperature range from room temperature to 673 K. The influence of the ferroelectric PZT phase on the magnetic phase transition and the magnetic MZF phase on the ferroelectric phase transition is revealed. It is established that, as the PZT content increases, the degree of diffuseness of the phase transition decreases and a gradual crossover from a pronounced relaxor behavior to a more ordered ferroelectric behavior occurs.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

Two new low-temperature phase transitions in the Li(NH4)1 − x Na x SO4 system

Dielectric measurements, X-ray diffraction and Raman scattering techniques have been used to investigate Li(NH₄)_0.99Na_0.01SO₄ (LASS1%) and Li(NH₄)_0.80Na_0.20SO₄ (LASS20%) crystals in the temperature range from 200 to 10?K. These are interesting systems because of the presence of hydrogen bonds and structural defects and because the mother crystal, LiNH₄SO₄, is ferroelectric at room temperature. From the analysis of the results we can conclude that the substitution of Na⁺ ions in the structure of LiNH₄SO₄, even at quantities as low as 1%, is sufficient to induce additional phase transitions to the LiNH₄SO₄ system. The new phase transitions are observed at 181 and 115?K for LASS1% and at 208 and 133?K for LASS20% and both are reversible.     

Photostimulated luminescence in PbHPO4 near TC

Optical properties (absorption and photoluminescence) of PbHPO₄ single crystals have been performed in the range 100 – 350 K, spanning the transition temperature (T_C=310 K). An intrinsic luminescene band centered at 450 nm has been observed. The excitation spectrum for this emission shifts with temperature in the ferroelectric phase. The temperature dependence for this shift has been determined as E(T) T, for T<T_C. Similar behaviour had been observed in birrefringence measurements (performed by other authors), which yield n T P_S² (spontaneous polarization). The results show that the flourescence is very useful, as an alternative technique, to study the dynamics of the phase transitions.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)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 (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m 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 FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Phase transitions in the oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript>

(NH₄)₃NbOF₆ single crystals were grown, polarization-optical studies were performed, and birefringence was measured over the temperature range 90–500 K. A sequence of first-order structural phase transitions was found at temperatures T _1↓ = 259.7 K and T _2↓ = 257.7 K with temperature hysteresis δT ₁ = 0.9 K and δT ₂ = 1.9 K. The transitions are accompanied by twinning and the following change in the crystal symmetry: cubic ? tetragonal ? monoclinic. Optical second harmonic generation is found to occur at room temperature, which indicates that the cubic phase is not centrosymmetric. It is assumed that the phase transitions are ferroelastic and ferroelectric in nature.     

Observation of87Rb NMR satellite transitions in the commensurate and incommensurate phases of Rb2ZnCl4

It is shown that the first order quadrupole split NMR satellite transition frequencies of the⁸⁷Rb nucleus can be detected in the paraelectric, incommensurate and ferroelectric phases of Rb₂ZnCl₄. From rotation patterns the electric field gradient tensor at the Rb sites is determined for the paraelectric phase. The data demonstrate a considerable influence of the structural changes in the incommensurate and ferroelectric phases on the observed NMR transition frequencies. For some crystal orientations the satellite transitions are followed through the incommensurate into the ferroelectric phase. Whereas in the former typical quasi continuous spectra are observed in the latter several sharp lines appear. The results are discussed in relation to the structural changes at the phase transitions.     

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C3H7NH3)2CdCl4

The structural phase transitions of (C₃H₇NH₃)₂CdCl₄ (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ? Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis (Acta CrystB34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.     

Interrelation of antiferrodistortive and ferroelectric phase transitions in Sr1−xAxTiO3 (A=Ba, Pb)

A dielectric and ultrasonic velocity study of antiferrodistortive and ferroelectric phase transitions in Sr_1−xA_xTiO₃ (A=Ba, Pb) is reported. It is shown that both phase transitions co-exist at x<0.03 but at the higher concentration the ferroelectric phase transition entirely suppresses the antiferrodistortive phase transition. The experimentally obtained phase diagrams are discussed in the framework of the Landau phenomenological theory.