Centrosymmetric Phase Research Articles

Competing Patterns of Weak Directional Forces in Pressure-Frozen CH2ClI and CH2I2

Isostructural relations and phase transitions of dihalomethanes have been rationalized by the competing patterns of CH...halogen hydrogen bonds and halogen...halogen interactions, the common weak directional interactions in soft organic matter. Pressure-frozen chloroiodomethane, CH(2)ClI, at 295 K and 0.72 GPa forms centrosymmetric phase III, which at ca. 400 K and 1.6 GPa disproportionates into CH(2)Cl(2) and CH(2)I(2). The directional character of intermolecular contacts between halogen atoms results from the characteristic anisotropic charge distribution on molecular surface.

Polymorphism of lead(ii) benzenethiolate: a noncentrosymmetric new allotropic form of Pb(SPh)2

Depending upon its conditions of crystallization, lead(II) benzenethiolate can exist in two forms: a low-temperature centrosymmetric phase α-Pb(SPh)2 which can be converted by heating into the noncentrosymmetric, 2nd order NLO strongly active and room temperature metastable β-Pb(SPh)2 phase, thus affording an example of transition towards noncentrosymmetry induced by a rise of temperature.

Experimental evidence for an intermediate phase in the multiferroicYMnO3

We have studied YMnO3 by high-temperature synchrotron x-ray powder diffraction, and have carried outdifferential thermal analysis and dilatometry on a single crystal sample. Theseexperiments show two phase transitions at about 1100 K and 1350 K, respectively. Thisdemonstrates the existence of an intermediate phase between the room temperatureferroelectric and the high-temperature centrosymmetric phase. This studyidentifies for the first time the different high-temperature phase transitions inYMnO3.

Piezoelectric and Ultrasonic Studies of Mixed CuInP2(SXSe1 - X)6 Layered Crystals

Results of ultrasonic and piezoelectric properties investigation of mixed CuInP 2 (S X Se 1 − X ) 6 layered ferroelectric single crystals are presented. The piezoelectric detection of longitudinal ultrasonic signal by thin plates of CuInP 2 (S x Se 1− x ) 6 crystals (x = 0.05, 0.1, 0.2 and 0.25) was observed and the temperature dependencies of piezoelectric properties have been studied. The piezoelectric sensitivity decreased upon heating and vanished at higher temperature, where crystals exhibit phase transition (PT) to centrosymmetric phase. At transition temperatures the anomalies of ultrasonic velocity and attenuation have been observed. At room temperature, above PT, the electrostriction-induced piezoelectricity was observed under DC bias electric field applied along c-axis of the CuInP 2 (S x Se 1− x ) 6 crystals.

Noncentrosymmetric Lamellar Phase in ABCD Tetrablock Copolymers

The phase behavior of ABCD tetrablock copolymer melts is studied using self-consistent mean-field theory. In particular, the noncentrosymmetric (NCS) lamellar phase (with sequence ...ABCDABCD...) and the centrosymmetric (CS) lamellar phase (...ABCDDCBA...) are examined in two cases. First, an enthalpically stabilized NCS lamellar phase is investigated by varying the interaction between the A and D blocks from repulsive to attractive while keeping all block lengths equal. In the second case, an entropically stabilized NCS lamellar phase is investigated by changing the lengths of the two end blocks, which are taken to be chemically identical (ABCA tetrablock melt). In both cases, the NCS phase is found in regions of the phase diagram, and direct disorder to NCS transitions are possible. In addition, two partially mixed centrosymmetric (MCS) lamellar phases are observed between the disordered and fully ordered (NCS and CS) phases. This study extends our previous work on triblock−diblock blends which revealed...

A new centrosymmetric modification of synthetic hilgardite Pb2[B5O9](OH) · 0.5H2O

A new centrosymmetric modification of Pb-hilgardite Pb2[B5O9](OH) · 0.5H2O is synthesized hydrothermally under conditions close to natural. This modification is intermediate between the centrosymmetric monoclinic phase studied earlier and the polar orthorhombic phase. The occupancies of the framework voids with water molecules, OH-groups, and halogen atoms are considered for all the known varieties of hilgardite. The correlation between the conditions of hilgardite formation and types of structure is established.

Structure–property correlations in the new ferroelectric Bi5PbTi3O14Cl and related layered oxyhalide intergrowth phases

The crystal structure and electrophysical properties of the layered intergrowth phase Bi5PbTi3O14Cl have been studied using powder neutron diffraction, ac impedance and second harmonic generation methods. This phase adopts a structure based on a regular intergrowth of Aurivillius-like and Sillen-like blocks in the layer sequence ⋯[M2O2][M2Ti3O10][M2O2][Cl]⋯, and is therefore closely structurally related to the well-known ferroelectric Bi4Ti3O12. The phase adopts the polar orthorhombic space group P2an at temperatures below 590 °C, and transforms directly to a centrosymmetric tetragonal phase, space group P4/mmm, at that temperature. This abrupt structural phase transition is consistent with a sudden loss of SHG signal and a dielectric maximum at the same temperature. The structural features and electrophysical behaviour of this phase in relation to Bi4Ti3O12 and related layered oxides/oxyhalides are discussed.

2–5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications

The weak-field dielectric dispersion (100 Hz–1.8 GHz) studies both of pure and Fe2+/Fe3 modified Cd2Nb2O7 ceramics over the temperature range of 90–380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intolerant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffraction analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the compositions Cd1.5Fe0.52+Nb2O7 and Cd1.5Fe0.53+Nb2O7 give CdNb2O6 columbite doped with Fe2+ or Fe3+ on the Cd sites (<8 and <2 mol %, respectively), except for minor amount of parasitic hematite. The novel CdNb2O6 type compounds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at TC=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior (like PMN). Near the characteristic freezing temperature of the zero-field-cooled state (Tf=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below Tf the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below TC (around 150 K) and that of Fe2+/Fe3+ doped CdNb2O6 between 90 and 380 K are typical of glass-forming systems at temperature far above Tglass. The relaxational process is characterized by (i) a significant frequency dependence of the peak permittivity position, (ii) non-Arrhenius behavior, and (iii) increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation (1 MHz–1.8 GHz) observed both in Cd2Nb2O7 and its isostructural analog Cd2Ta2O7 at RT and higher (a centrosymmetric phase) is attributed to fluctuations in polarization of the dynamically reoriented O(7th)–Cd–O(7th) dipoles due to dynamical off-center location of Cd ions.

Structural change of the ferroelectric-semiconductor at excitation of electronic subsystem and photovoltaic effect

A model for the photovoltaic effect mechanism is proposed. In this model the charge carrier shift takes place as a result of spontaneous polarization change associated with the inner photoemission effect. This shift is fixed due to the annihilation of carriers or antrapment of carriers on deep traps before the equilibrium state of the lattice is reached. The repeated recurring of such shifts of carriers at constant illumination can result in photovoltaic effects even greater in the magnitude than the measured ones. This model can explain the abnormal increase of a photovoltaic current at rising temperature up to the temperature Tc of transition into a centrosymmetric phase.

Dielectric properties and structure of Bi4NbO8Cl and Bi4TaO8Cl

The layered intergrowth phases Bi4MO8Cl (M = Nb, Ta), structurally related to the well-known Aurivillius phase ferroelectrics, have been studied using SHG and a.c. impedance techniques. Both undergo a polar to centrosymmetric phase ferroelectric-type transition (Tc = 765 and 640 K for M = Nb and Ta, respectively). The crystal structure of the low temperature phase of Bi4NbO8Cl has been re-analysed, and a tentative structure for Bi4TaO8Cl determined, from powder neutron diffraction data. The basic structural features are confirmed, but significant details differ from the previously reported X-ray structure of Bi4NbO8Cl.

Two-dimensional photorefractive spatial solitons in centrosymmetric paraelectric potassium–lithium–tantalate–niobate

We report the observation of steady-state two-dimensional photorefractive self-trapping and screening spatial soliton formation in a sample of potassium–lithium–tantalate–niobate in the centrosymmetric paraelectric phase.

Optical activity in the incommensurate and ferroelectric phases of Rb2ZnCl4

The birefringence and optical activity of Rb2ZnCl4 have been measured in the normal, incommensurate and ferroelectric phases. The measurements have been carried out using a high-accuracy universal polarimeter (HAUP). In contrast to previous observations, an essentially null g13 gyration coefficient (setting Pcmn) has been obtained in the incommensurate phase (g13<5*10-6), and no measurable change has been detected on entering the ferroelectric phase. Optical parameters, estimated in this phase using a classical point dipole-dipole polarizability theory, are consistent with the observed behaviour. In addition, a reprocessing of the previously reported measurements also shows that, within the experimental error, the resulting optical activity value is in agreement with ours. The conclusions deduced from the present study, together with some recently published results on gyration effects in centrosymmetric incommensurate phases, support the view that optical activity, like any other macroscopic property, is symmetry restricted by the point group associated with the structure. Possible release from this rule when considering point defects in real incommensurate structures is also discussed.

Critical behavior of hyper-rayleigh scattering in the centrosymmetric phase of hexagonal barium titanate

Abstract Critical behavior of hyper-Rayleigh (second-harmonic) scattering was investigated for the first time for a new type of second-order displacive phase transition at Ta = 222 K in the hexagonal barium titanate, which is neither ferroelectric nor proper-ferroelastic in nature. It is found that the scattering intensity in a 90° scattering configuration increases strikingly in such a manner as I2ω ∼ (T-Ta)−2, the square of the generalized susceptibility also observed, as Ta is approached in the centrosymmetric phase, only for those polarizations enabling one to observe the relevant nonpolar soft phonon, while that for other polarizations the 90 intensity remains almost constant. These aspects newly found can be well accounted for with a model that residual point defects acting as strongly localizing “field” should induce microclusters in their vicinity such that development of a locally condensed order-parameter, the square of which I2ω is proportinal to, follows the generalized susceptibility.

Femtosecond dynamics of laser-induced phase transition of the GaAs surface layer to a centrosymmetric phase

Recently we observed ultrafast laser-induced phase transition (on a time scale less than 100 fs) to a centrosymmetric semiconductor-like phase at the GaAs surface [1,2] by using time-resolved second harmonic generation in reflection. A phenomenological model describing this phase transition is developed. The new phase exists during the first 1 ps after laser excitation due to high plasma carrier density and lattice stress, the lattice temperature remaining well below the melting threshold.

Lack of optical activity in the incommensurate phases of Rb2ZnBr4 and

The optical activity of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ has been measured in their centrosymmetric incommensurate phases using a high-accuracy universal polarimeter (HAUP). The results obtained show a zero value of this quantity along all the measured directions B(Rtwo in the case of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and one for the [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ crystalB)R. This behavior differs from results published previously for these or other similar incommensurate materials. The present measurements are compared with those reported in these previous studies, analyzing, in view of the differences, the possible sources of systematic errors in the HAUP technique. Our experimental results are in accordance with the idea that gyration tensors in incommensurate structures must be symmetry restricted, in a conventional manner, by the point group associated to their superspace group. Recent models allowing the appearance of gyration effects in incommensurate crystals with inversion symmetry should be reconsidered in the light of our results.

Raman scattering and soft mode in TlTiOPO4

Abstract TlTiOPO4 (TTP) and its isomorphous compounds are ferroelectric (space group: Pna21; Z = 8) at room temperature and belong to a centrosymmetric (presumably Pnam) space group above a second-order transition temperature T c which is 1254 K for KTP (KTiOPO4), the most well known member of this family. TTP is characterized by a lower temperature of transition (T c = 852 K) and a smaller ionic conductivity than KTP. For these reasons, TTP has been chosen as the first candidate to a comprehensive Raman study of the ferroelectric to centrosymmetric phase transition of the KTP family.

Intra‐ and inter‐molecular interactions in non‐centrosymmetric and centrosymmetric phases of crystalline m‐nitrophenol as studied by polarized Raman and IR spectra. Comparison with internal vibrational spectra of m‐nitroaniline, m‐aminophenol and α‐resorcinol

AbstractThe polarized Raman and IR spectra of m‐nitrophenol (mNPh) crystals in two modifications and of mNPh and its deuterated analogue powders in a KBr pellet, nujol mull and CCl4 solution have been measured in the region 200–4000 cm−1. The results are discussed and compared with earlier published data for α‐resorcinol, m‐nitroaniline and m‐aminophenol (mAPh). For the last compound new results, i.e. the IR spectra of the deuterated analogue, are presented, and reinterpretation of some previous assignments is proposed. The assignments of the bands in the mNPh spectra and the re‐examination of the mAPh spectra were facilitated by ordering the spectra of the four compounds into two classes exhibiting common features.It has been found that the differences between the spectra of two phases of the mNPh crystal are small and reflect the differences in interactions between the chains of molecules linked by hydrogen bonds. These interactions perturb particularly the NO2 group vibrations which cannot be treated using the oriented gas model. On the basis of INDO/CI calculations of electronic absorption spectra vibronic couplings in the IR and Raman spectra of the mNPh crystals have been proposed to account for many intensity enhancements.

Phase transitions in [Ni(NH 3) 6](NO 3) 2

Electric permittivity ε ∗ = ε′ − iε″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at T k1 = 247 K and T k2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at T c = 63 K is a continuous phase transition (glass?).

The far-infrared spectrum of ice VIII

The far-infrared spectrum of ice VIII as recovered at ∼100 K and zero pressure has translational and rotational bands at 162 and 455 cm−1, respectively. They are undoubtedly due to the Eu translational vibration in the x, y plane and the Eu rotational vibration about the x, y axes. The correlation of the vibrations of ice VIII with the zone-center and -boundary vibrations of the centrosymmetric phase ice X is summarized.

Phase Transition in Thiourea under High Pressure

The phase transition of thiourea under high pressure has been investigated by means of second harmonic generation (SHG), pyroelectric and Raman scattering measurements. The high pressure phase VI turns out to be centrosymmetric as the results of the SHG and pyroelectric charge measurements. At high pressure some of new Raman-active modes were observed. On the basis of these experimental results it is guessed that the centrosymmetric high pressure phase VI of thiourea is induced by instability of a zone-boundary mode of the atmospheric pressure phase V.