Observation of potassium hyperfine interactions in x-irradiated KH2AsO4 through the method of electron spin echo envelope modulation

Hyperfine interaction frequencies of ¹H and ³⁹K nuclei near the AsO^4-₄ radical in X-ray irradiated KH₂AsO₄ (KDA) have been observed through the method of electron spin echo envelope modulation (ESEEM). This method enabled us to record nuclear hyperfine interaction (ENDOR-like) spectra around the ferroelectric phase transition of KDA for the first time. The ESEEM spectrum of ³⁹K exhibits a clear change when passing the ferroelectric phase transition temperature, but that of close protons does not. The result for close protons is in agreement with the symmetry breaking of the AsO^4-₄ site as observed via the EPR spectrum [5]. Finally, at 4.2 K the hyperfine interaction parameters of a ³⁹K nucleus near the AsO^4-₄ unit could be determined through the ESEEM method.     

Optic-mode coupling in ferroelectric gadolinium molybdate

Separate measurements of the A₁(TO) and A₁(LO) Raman spectra of ferroelectric gadolinium molybdate at 80°K and above have elucidated the origin of the anomalous temperature dependence of the two lowest frequency lines in the A₁(TO) spectrum. The observed behavior is postulated to be the result of coupling among modes at 44.5, 51.5, and 83 cm^?1 (at 80°K). The 44.5 and 83 cm^?1 modes become the degenerate, soft zone-boundary modes of the paraelectric phase while the 51.5 cm^?1 mode changes to B₂ symmetry. The two lowest frequency lines are the same as those observed previously in i.r. absorption.     

Raman spectra of cadmium titanate

Polarized Raman spectra of CdTiO₃ single crystals are recorded for the first time over the frequency range 5 < ν < 1000 cm^?1 at temperatures of 10 to 1200 K. The emphasis was on the low-frequency range, where an anomalous temperature dependence of a few phonon modes was observed. At high temperatures, four phonon modes exhibiting a behavior typical of soft modes were found to exist. These phonon modes are assumed to restore the cubic symmetry of the lattice. Their extrapolated temperature dependences suggest that there exists a sequence of three hypothetical high-temperature phase transitions analogous to those observed in the genuine perovskite CaTiO₃. At temperatures below 78 K, the Raman spectrum exhibits new lines associated with polar distortions of the unit cell. At low frequencies, three lines are observed whose parameters exhibit an anomalous behavior typical of soft modes in a ferroelectric phase. Several different polar states are assumed to exist at low temperatures.     

Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the A ₁(z) type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the A ₂ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm^–1 at 123 K to 106 cm^–1 at T = 513 K. A fair agreement of the experimental data for the A ₁(z) mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the A ₁(z) polar mode and the dispersion curves for axinons has been plotted.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ 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 Pb_0.50Sr_0.50TiO₃ 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     

Far-infrared reflectivity spectrum of potassium dihydrogen phosphate,polarized along the c axis,reinvestigated in the ferroelectric phase

The infrared (IR) reflectivity spectrum of KH₂PO₄, polarized along the c axis, has been carefully reinvestigated in the vicinity of the phase transition temperature and in the ferroelectric phase, between 10 and 600 cm⁻¹. Two new results are displayed by the present study of the A₁, spectrum in the ferroelectric phase of KH₂PO₄: first, the activity of the expected A¹ librational lattice mode is evidenced, the lattice mode parameters of this mode were never previously determined by IR spectroscopy in KH₂PO₄; and second, the peculiar behaviour of the broad reflection band corresponding to the A₁ translational lattice mode is observed for the first time in KH₂PO₄, and discussed.     

Far-infrared spectrum and the soft mode of ferroelectric betaine phosphite

The far-infrared reflection spectra of betaine phosphite single crystals in the three crystallographic directions are investigated in the range from 5 to 600 cm^?1 at temperatures between 203 and 323 K using a dispersive Fourier spectrometer. The dielectric function is evaluated from the complex amplitude reflection coefficient measured. In the spectrum polarized along the ferroelectric direction a soft mode of relaxation type is observed which accounts for a large part of the relevant static dielectric constant at T_c = 210 K. Among the oscillator modes, which generally exhibit normal temperature dependence, there is a heavily overdamped mode at 58 cm^?1, polarized along the crystallographia a₁ axis, with anomalous damping behaviour. The damping constant decreases nearly linearly with temperature up to 323 K which is close to T = 355 K where a second phase transition is known to exist.     

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.     

EPR study of the ferroelectric phase transition in chromium-doped DMAAS

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr³⁺-doped dimethylammonium aluminium sulphate hexahydrate are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from the ferroelastic to the ferroelectric phase. The spin-Hamiltonian parameters have been determined for both phases. The spin-Hamiltonian parameters in the ferroelectric phase are:g=1.980±0.003,b ₂ ⁰ =(1140±15)·10^?4 cm^?1,b ₂ ² =(214±10)·10^?4 cm^?1. Remarkable EPR line width changes confirm the order-disorder character of the ferroelectric phase transition on a microscopic level and demonstrate that the dimethylammonium reorientation freezing-out is the prime reason for this transition.     

A Raman and dielectric study of a diffuse phase transition in (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb_1-xCa_xTiO₃ thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca²⁺ content. On the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE–PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. PACS 77.80.Bh; 77.55.+f; 78.30.-j; 77.80.-e; 68.55.-a     

23Na nuclear magnetic resonance study of the phase transition in ammonium rochelle salt,NaNH4C4H4O6 • 4H2O

The satellite NMR spectrum of ²³Na in ammonium Rochelle salt (NaNH₄C₄H₄O₆ ? 4H₂O) is investigated near the transition temperature. Each line in the paraelectric phase splits discontinuously into four lines at the transition temperature; this fact is compatible with the first-order phase transition and reveals the existence of the superstructure in the ferroelectric phase.     

Specific heat behaviour of K2SeO4 in the 48K–800K temperature range a new phase transition

A calorimetric study performed on the ferroelectric K₂SeO₄ in the 48K–800K temperature range confirmed the existence of three phase transition and revealed the occurrence of a new one at 56.03±0.05K (T^IV_c). The critical measurements for the commensurate-incommensurate phase transition seem to show a cross-over behaviour.     

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.     

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     

Magnetic and ferroelectric properties of exchange-frustrated multiferroics (Ni1 ? xTx)3V2O8 (T = Co, Mn, Zn)

The effect of the substitution of Co²⁺, Mn²⁺, and Zn²⁺ ions for Ni²⁺ ions on the magnetic, dielectric, and ferroelectric properties of vanadate single crystals (Ni_{1 − x} T _x )₃V₂O₈ has been analyzed. It has been found that the low-level (x ≤ 0.1) substitution of both magnetic and nonmagnetic ions stabilizes the ferroelectric state with a cycloidal magnetic structure. The existence region of this state is expanded to low temperatures down to 3 K for Zn²⁺ and below 1.8 K for Co²⁺ and Mn²⁺ owing to the suppression of a low-temperature weak ferromagnetic phase. At the same time, the ferroelectric phase disappears completely at large concentrations of Co and Mn. The effect of magnetic fields on the magnetic and ferroelectric states has been analyzed. It has been shown that the magnetic field along the c axis suppresses the ferroelectric state, whereas the magnetization along the antiferromagnetism axis (a axis) induces the reentrant phase transition from a paraelectric weak ferromagnetic structure to a ferroelectric structure. The corresponding H-T phase diagrams have been drawn.     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

Ferroelectric phenomena in CdSnO<Subscript>3</Subscript>: A first-principles studies

The phonon spectrum of cubic cadmium metastannate and parameters of the crystal structure of its distorted phases were calculated from first principles within the density functional theory. It is shown that the phonon spectrum and the energy spectrum of the distorted phases in α-CdSnO₃ resemble surprisingly the corresponding characteristics of CdTiO₃. The ground state of α-CdSnO₃ is the ferroelectric Pbn2₁ phase, the energy gain from the phase transition to this phase from the nonpolar phase Pbnm is ∼30 meV, and the spontaneous polarization is 0.25 C/m². The analysis of the eigenvector of the ferroelectric mode in α-CdSnO₃ and the partial densities of states indicates that the ferroelectric instability in this crystal, which does not contain transition d-element atoms, is associated with the formation of a covalent bonding between Cd and O atoms.     

Symmetry breaking at the phase transitions in ferroelectric pyridinium salts

The phase transitions in ferroelectric pyridinium tetrafluoroborate (PyBF₄) and pyridinium perchlorate (PyClO₄) have been characterized by structural studies. The continuous ferroelectric phase transition at 238.7 K in PyBF₄ appears to be a unique case for multiaxial ferroelectrics, while the first order phase transition in PyClO₄ at 248 K is consistent with the Landau theorem for deducing the character of phase transitions from symmetry considerations. The phase transition in both materials is caused by ordering of the pyridinium cation as well as tetrahedral anions.     

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.     

Raman spectroscopy of smithsonite

Raman spectroscopy at both 298 and 77 K has been used to study a series of selected natural smithsonites from different origins. An intense sharp band at 1092 cm⁻¹ is assigned to the CO₃²⁻ symmetric stretching vibration. Impurities of hydrozincite are identified by a band around 1060 cm⁻¹. An additional band at 1088 cm⁻¹ which is observed in the 298 K spectra but not in the 77 K spectra is attributed to a CO₃²⁻ hot band. Raman spectra of smithsonite show a single band in the 1405–1409 cm⁻¹ range assigned to the ν₃ (CO₃)²⁻ antisymmetric stretching mode. The observation of additional bands for the ν₃^g modes for some smithsonites is significant in that it shows distortion of the ZnO₆ octahedron. No ν₂ bending modes are observed for smithsonite. A single band at 730 cm⁻¹ is assigned to the ν₄ in phase bending mode. Multiple bands be attributed to the structural distortion are observed for the carbonate ν₄ in phase bending modes in the Raman spectrum of hydrozincite with bands at 733, 707 and 636 cm⁻¹. An intense band at 304 cm⁻¹ is attributed to the ZnO symmetric stretching vibration. Copyright © 2007 John Wiley & Sons, Ltd.