Temperature Dependence Of Birefringence Research Articles

Washing soda (natron), Na 2 CO 3 �10H 2 O, revised: crystal structures at low and ambient temperatures

¶Crystals of natron, Na2CO3·10H2O, were grown by slow evaporation of a saturated sodium carbonate solution at room temperature. A single-crystal fragment was measured on a CCD X-ray four-circle diffractometer at 110, 270 and 295 K. Whereas the crystal structure of natron at room temperature had first been published in 1969 with R = 18%, in the present investigation the non-centrosymmetric structure was solved by direct methods and refined subsequently to R1 = 1.9% (110 K), 2.5% (270 K), and 3.5% (295 K). In contrast to the previous structure description, even the hydrogen atoms were located and refined with isotropic displacement parameters. Space group is Cc, lattice parameters are a = 12.740(1)/12.763(1)/12.750(2) A, b = 8.816(1)/8.955(1)/9.001(2) A, c = 12.571(1)/12.593(1)/12.590(2) A, β = 115.97(1)/115.89(1)/115.83(1)° at 110/270/295 K. The structure is composed of pairs of edge-sharing Na(H2O)6 octahedra forming Na2(H2O)10 units, and CO3 molecules. These units are connected by hydrogen bonds (d O⋯O > 2.73 A) in a NaCl-like arrangement. The structure is pseudo-centric, only the carbonate groups and four H atoms violating a centric symmetry. The CO3 molecules are perfectly ordered at 110 K and show almost isotropic ADPs of the carbonate O atoms. The CO3 position is constrained by hydrogen bonds, involving at least three H bonds per acceptor oxygen atom. At 270/295 K the carbonate groups still hold the former low-temperature position to 60/46%, whereas 40/54% are disordered into at least two different positions with large and strongly smeared anisotropic displacement parameters of the carbonate O atoms. However, a centrosymmetric equivalent of the low-temperature CO3 position is occupied to only ∼ 10%. A definite phase transition from ordered to partially disordered CO3 groups was neither observed in differential scanning calorimetry, nor in temperature-dependent birefringence, IR and Raman spectroscopic measurements, as the process takes place over a wide temperature interval and without any change of the space group symmetry Cc.

High-resolution fast response fiber-optic laser calorimeter using a twin interferometric temperature sensor

We describe a calorimeter for laser power measurements of the conduction type with twin fiber-optic temperature sensor. The sensor employs two fiber-optic polarization insensitive loop interferometers with twisted sections of high-birefringence fiber. The temperature dependent birefringence of these sections is monitored by a white light interferometric technique with signal processing in the spectral domain. One interferometer performs the measurements of the absorber's temperature that depends on the power of incident laser radiation, whereas the other is for monitoring the temperature inside the probe head. A thermal model of the calorimeter has been analyzed. It has been shown that it is possible to reduce the calorimeter's response time, if the laser power is measured using the difference ΔT= T 1− bT 2, where T 1 and T 2 are the readings of the twin temperature sensor, and the factor b depends on the thermal conduction path between the two interferometers. Experimental results are presented for optical power measurements between 0 and 3 W with a resolution of 0.3 mW and a response time of a few seconds.

POLARIZATION MEASUREMENT IN NEMATIC LIQUID CRYSTAL BASED ON THE PYROELECTRIC RESPONSE TO LASER IRRADIATION

The polarization measurement of a nematic liquid crystal has been performed by a pyroelectric technique using short pulse of a Nd:YAG laser to create temperature increment. The temperature increment has been evaluated independently by monitoring temperature dependent birefringence for a He-Ne laser beam. The sign and absolute magnitude of surface polarization have been evaluated for both the planar and homeotropic orientation of the nematic liquid crystal at a solid substrate. The flexoelectric polarization in a hybrid cells has also been measured using the same pyroelectric technique. The value of the sum of the flexoelectric coefficients (e 1 +e 3 ) for 5CB is negative and about—13pC/m at room temperature.

Spectroscopic evidences of the Jahn–Teller phase transition in the mixed crystals CsDy 1− xBi x(MoO 4) 2

Layered rhombic double molybdates of the family M IM III(MoO 4) 2 reveal the sequences of phase transitions of different types which take place in the temperature region from 300 K down to helium temperatures. The aim of this work is to investigate the phase diagram of caesium–dysprosium and caesium–bismuth double molybdates and mixed CsDy 1− x Bi x (MoO 4) 2 crystals ( x=0.05–0.8). We have studied the low-temperature Jahn–Teller phase transition which takes place in CsDy(MoO 4) 2 at 42 K and the influences of Bi doping on this feature. The polarized far-infrared spectra (40–500 cm −1), polarized reflectance spectra (400–1000 cm −1), absorption spectra (13,000–13,500 cm −1), temperature dependence of birefringence and visual observations of the domain structure at phase transition were measured. Hysteresis loops at the phase transition of the first order type were obtained too. The results were discussed in terms of the simplified one-dimensional model of layered crystals.

Surface and flexoelectric polarization in a nematic liquid crystal 5CB

The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel time-resolved “optical thermometer” technique monitoring temperature-dependent birefringence by a He- Ne laser beam. In accordance with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude ranges from -4 to +2pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid cells. The sum of the flexoelectric coefficients is e1 + e3 = - 13pC/m at 25°C.

Special temperature features of the birefringence of lithium tetraborate crystals

Using the Senarmont polarimetric method, the optical birefringence of lithium tetraborate crystals Li2B4O7 in the 290–480 K range was measured. The steplike temperature dependence of birefringence, the thermal hysteresis, and the thermooptic memory effect were observed, which are most likely associated with the presence of incommensurate phase modulation in lithium tetraborate.

Polarized Photoluminescence and Measurement of Degree of Order in Mixed Oriented Polymer Liquid Crystal Films

Optical anisotropy in a new form of cross-linked liquid crystal network incorporating a fluorescent chromophore has been demonstrated. Linearly polarized absorption and fluorescence is observed in oriented thin films. From measurement of absorption and fluorescence anisotropy, the order parameter was found to be 0.65 and 0.80 respectively. Temperature dependent birefringence measurements reveal that there is no appreciable loss of alignment up to 200°C. Raman spectroscopy was used to investigate the discrepancy between the absorption and fluorescence measurements. It is found that the fluorescent chromophore is less well ordered than the liquid crystal. The reasons for this are discussed.

Phase transitions in mixtures of a side-on-side chain liquid crystalline polymer and low molar mass nematic liquid crystals

Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.

Polarised photoluminescence from oriented polymer liquid crystal films

Optical anisotropy in a new form of crosslinked liquid crystal network incorporating a fluorescent chromophore has been demonstrated. Linearly polarised absorption and fluorescence is observed in oriented thin films. From measurement of absorption and fluorescence anisotropy, the order parameter was found to be 0.65 and 0.80, respectively. Temperature dependent birefringence measurements reveal that there is no appreciable loss of alignment up to 200 °C.

The temperature dependence of the optical anisotropy in magnetic fluids: birefringence and dichroism

The temperature dependence of birefringence, δ, and dichroism, Δ A, in Fe 3O 4 Isopar-M based magnetic fluids have been investigated in the temperature range 100 < T < 320 K and in magnetic fields up to 3 kOe. The results show that birefringence and dichroism for a given concentration and at a given field are zero below a certain temperature T s, and then both increase with temperature until they reach a maximum at a temperature T m, and then decrease with temperature for T > T m. The values of T m and T s are found to vary with concentration and the applied field. The results also show that δ- H 2 and Δ A-H 2 curves deviate from linearity. This deviation is attributed to interparticle interactions, the orientation of pre-existing clusters, and the field-induced chain formation.

Effect of the uniaxial compression on the phase transitions in Sn2P2S6‐type ferroelectrics

AbstractThe effect of uniaxial compression on the temperature dependences of birefringence and dielectric constant is studied for the proper uniaxial ferroelectrics Sn2P2S6 and Sn2P2Se6. The influences of compression on the ferroelectric phase transition (PT) in Sn2P2S6 and on the PT which restricts the incommensurate (IC) phase in Sn2P2Se6 are compared. The observed variations of PT temperature and IC‐phase range with pressure agree with those calculated in an approximation which takes into account the presence of a tricritical point on the phase diagram of the crystals under study along with the Lifshits point. The variation of the ratio of the rates of PT temperature change for different uniaxial mechanical stresses on the replacement of sulphur by selenium as well as the weak sensitivity of the IC‐phase to compression along the structure modulation axis are found. It is suggested that these manifestations of the anisotropy are related to the special features of the linear interaction of soft optical and acoustic phonon branches.

Optical activity of Rb2ZnBr4

Abstract All the gyration tensor components of Rb2ZnBr4 were measured by using the high accuracy universal polarimeter (HAUP). We found that only g23 component appeared in the incommensurate phase, the other components always zero in the measured temperature range. Simultaneously we also revealed temperature dependences of birefringence.

Critical behaviour of uniaxial Sn2P2S(Se)6ferroelectrics

Abstract Critical behaviour of proper uniaxial Sn2P2(SexS1-x)6 ferroelectrics whose phase diagram contains a Lifshitz point and a “virtual” tricritical point is studied by means of measured temperature dependences of birefringence and heat capacity. An anomalous heat capacity change in paraphase is described by an exponential function δC p ∼ τ−α. Far from the phase transition temperature, α = 0.5 and this corresponds to the first fluctuational correction. In the vicinity of the phase transition to ferroelectric phase for Sn2P2S6, α = 3/2, which corresponds to dipoled effects while in the vicinity of phase transition (PT) to incommensurate phase for Sn2P2Se6, α = −0.02, which satisfies the Heisenberg double-component model. The crossover from tricritical behaviour to critical one is displayed in ferroelectric and incommensurate phases. In the vicinity of Lifshiftz point, β = 0.2 for a compound with x = 0.29 within a wide temperature range.

Birefringence Study on Structural and Magnetic Phase Transitions of RbMnBr3

The temperature dependence of birefringence Δ n ( T ) of a triangular lattice antiferromagnet RbMnBr 3 has been measured together with comparative measurements of Δ n 's of related compounds CsMgCl 3 , CsMgBr 3 , RbMgBr 3 , CsMnI 3 , CsMnBr 3 . A structural phase transition of RbMnBr 3 at T S2 =220 K with symmetry-breaking in the hexagonal c -plane has been found. The low-temperature structure below T S2 is suggested as orthorhombic, which can be the origin of an incommensurate spin structure of RbMnBr 3 . Critical exponents of magnetic specific heat are determined as α ′ =0.22±0.06, α=0.42±0.16, which are in fair agreement with the values of the Z 2 × S 1 universality class.

Optical properties and phase diagram of (TMA)2FeCl4

Abstract Single crystals of [N(CH3)4]2FeCl4 have been prepared from water solution. For platelets perpendicular to the three orthorhombic axes of the high-temperature phase Pnma we present data on the temperature dependence of birefringence, orientation of the indicatrix, permittivity and domain structure. On the basis of these results and of previous studies we suggest the following phase sequence to take place on decreasing temperature: Phase I Pnma—modulated phase II with k i along the a-axis—phase III' presumably monoclinic with k = (3/7)a∗—lock-in phase IV P21/n11—lock-in phase PT. This differs from the previously published phase diagram in the absence of a reentrant incommensurate phase, in the presence of a monoclinic phase before the lock-in monoclinic P21/n11 is reached, and in the fact that the last phase is triclinic.

Electrooptic voltage sensor: birefringence effects and compensation methods

Crystals of Bi(4)Ge(3)O(12)from two different sources exhibited linear birefringences of 1.7 x 10(-5)-5.4 x 10(-5)5 (or, phase retardations of 1.3-4.1 degrees /cm) at a wavelength of 830 nm. These birefringences, however, are sensitive to temperature. The temperature variation of the birefringence dB/(B(0)dT) normalized by the room temperature birefringence B(0) was -1 to -7 x 10(-3)/ degrees C. The effects of the temperature dependent birefringence and the birefringence induced by pressure on an electrooptic voltage sensor were measured and quantitatively compared to the predictions. To remove the temperature dependent birefringences, the crystals were annealed over two days. The birefringences were reduced to about half of their original values after a first annealing process, but the values remained unchanged after a second annealing process. To eliminate the effects of the birefringences, a compensation method was used. After applying this compensation method to an electrooptic voltage sensor, the temperature stability of the sensor was improved to +/-0.75% from +/-7.0% in the temperature range between -2 and 65 degrees C, and the pressure stability was improved to +/-0.2% from +/-2% under pressure as high as 1 x 10(5) N/M(2).

Temperature dependence of birefringence and dichroism in magnetic fluids

Measurements of the birefringence and dichroism in Fe 3O 4 magnetic fluids were undertaken in the temperature range 100–320 K in magnetic fields up to 0.9 kOe. Our results show that both the birefringence and dichroism are not noticeable for temperatures below 170 K. Above 170 K there is an increase with temperature exhibiting a peak at ∼ 190 and ∼ 270 K respectively. Moreover, our results show that for temperatures above that at which the peak occurs the birefringence follows a generalized Curie-Weiss law yielding positive ordering temperatures.

Electrooptics and electrogyration of ferroelectric Ca2Pb(C2H5CO2)6crystal under smeared phase transition

Abstract Temperature dependence of birefringence, optical rotatory power and electrooptical coefficient of the DLP crystal were measured over the wide range. Temperature region of smearing for the ferroelectric phase transition was evaluated. Obtained data suggest, that crystals of the DLP group could be regarded as “weak (dilute) ferroelectrics.”

Symmetry-breaking birefringence in hexagonal antiferromagnetic systems ANiX 3

Birefringence study was performed on triangular lattice antiferromagnets CsNiBr 3, CsNiCl 3 and CsCoCl 3. A cusp in the temperature dependence of birefringence was observed at T N of the Ni 2+ systems. This singularity is attributed to the breaking of axial magnetic symmetry in the ordered state on their lattice.

Crystal-optical analysis of the low-temperature phases in KLiSO4

Temperature dependences of birefringence and optical activity in KLiSO4 are studied and used for analysis of symmetries of the phases in temperature region 150-300 K. Monoclinic (class 2) and triclinic (class 1) symmetries were found for phases IV and V. Kinetics of the phase transitions is discussed