Ferroelectric phase transitions in KNO3 embedded into porous glasses

This paper presents the results of dielectric and dilatometric measurements of porous glasses filled with KNO₃ from water solution and from the melt. It was shown that ferroelectric phase transition is observed during the cooling process only in KNO₃ nanocrystals introduced into the porous matrix. The phase diagram of transition temperatures versus average values of pore dimensions was obtained. The phenomenological model describing the phase transition and size effects in KNO₃ crystals is proposed.     

Phase Diagram and Dielectric Studies in Hydrogen-Bonded Liquid Crystal System

The thermal and dielectric behaviors of mixtures were investigated for systems of p-n-hexyloxybenzoic acid (6OBA) and p-n-octyloxybenzoic acid (8OBA) by differential scanning calorimetry, polarized optical microscopy, and dielectric measurements. The T-X phase diagram was obtained for this system. Eutectic composition was calculated and received experimentally. Enthalpies of mixing were calculated to describe phase transitions physically. All mixtures show enantiotropic smectic and nematic phases. Dielectric permittivity has higher value for the mixture with fractional proportion of 30 to 70% mol for 8OBA than that for initial acid. It is assumed that co-crystallization of initial compound occurs when the ratio of components is close to equimolar ratio.     

Crystal structure,thermal analysis,and IR spectroscopic investigation of bis(2-amino-6-methyl) pyridinium sulfate

Synthesis, crystal structure refinement, phase transitions studied by thermal analysis, and IR spectroscopic investigation of 2C₆H₉N₂ < eqid2 > ⋅SO₄²⁻ are reported. The title compound crystallizes in the monoclinic space group C2/c (no. 15) with a = 10.5068(4) Å, b = 10.2225(5) Å, c = 14.0422(7) Å, and β = 104.489(3)^∘. A crystal packing diagram shows layers built by all the components of the structure and centered by planes z = 1/4 and 3/4. The pyridine substituents stack forming channels parallel to the c direction with dimensions of 4.163(1) Å and 5.148(4) Å. Thermal analysis shows that the anhydrous compound possesses an irreversible weak phase transition.     

Porous silicon grains in SiO2 matrix: Ultrafast photoluminescence and optical gain

We search for the presence of stimulated emission in samples of porous silicon embedded in the sol–gel derived SiO₂ matrix. By modifying the etching conditions of the porous silicon using hydrogen peroxide, we decrease substantially the nanocrystal size and produce a significant blue shift of the PL emission. Femtosecond variable-stripe length experiments combined with the shifting-excited spot technique demonstrates positive optical gain (modal gain ∼25 cm⁻¹) in the range 550–730 nm. Ultrafast photoluminescence dynamics indicates the origin of the stimulated emission as possibly due to recombination of excitonic states inside silicon nanocrystals.     

Phase Transition and Structure Change of Urea Adducts with n-Paraffins and Paraffin-type Compounds

The phase transition temperatures of urea adducts with many n-paraffins and several paraffin-type compounds were measured by differential thermal analysis and the general structure change causing the phase transitions was revealed by X-ray structure analysis. The ordinary hexagonal adducts (high-temperature form) transform commonly into orthorhombic adducts (low-temperature form) except for a few adducts with guest molecules possessing cylindrical shapes. The phase transition can be regarded as an order-disorder transition with respect to the orientation of the guest molecules about the urea channel axis with a cooperative deformation of the urea channels resulting in the orderly orientation of guest molecules below the transition temperatures. The changes in orientation and motion of the guest molecules through the phase transitions are discussed on the basis of X-ray analysis, potential energy and broad-line NMR.     

Structural Phase Transition of an Intercalation Compound Mn1/4NbS2

Abstract

A structural phase transition of an intercalation compound Mn_1/4NbS₂ has been investigated by X-ray diffraction at high temperatures. The lattice parameter c exhibited a discontinuous change at 640K. The superlattice reflections observed below 640K disappeared suddenly above 640K. The phase transition at 640K took an aspect of the first-order phase transition. The precise structure analyses were performed at various temperatures above and below the phase-transition temperature. It was revealed that Mn atoms were arranged in disorder in the high-temperature phase, while the Mn atoms were ordered forming the 2a ₀ × 2a ₀ × c ₀ superlattice in the low-temperature phase. The Nb and S atoms around the ordered Mn atoms slightly shifted from the high-symmetry position in the low-temperature phase. The order parameters were the degree of order of the Mn atoms and the degree of displacement of the Nb and S atoms.     

Synthesis,structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

A series of novel liquid crystalline monomers (M₁?M₈) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and ¹H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (T_m) and isotropic temperature (T_i) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, T_m and T_i of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.     

Growth and melting of metallic nanoclusters in glass: A review of recent investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi-or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T _m on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T _m is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established. The text was submitted by the authors in English.     

Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT

The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (N_D) and calamitic (N_C) nematic phases. A biaxial nematic (N_B) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The N_B phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.     

A neutron diffraction study of the phase transition of fully deuterated triglycine sulphate (ND2CD2COOD)3.D2SO4

Using neutron single crystal and powder diffraction, the first thorough investigation of the structure of fully deuterated triglycine sulphate, (ND₂CD₂COOD)₃.D₂SO₄ is presented, including its evolution with T, through its structural phase transition. This includes new precise structural parameters determined at several key temperatures above and below T_C using single crystal diffraction, and for the first time a parametric study has been undertaken over a wide temperature range — from 4 to 500 K in 2 K steps. It was found that fully deuterated TGS shows a structure consistent with hydrogenous TGS and partially deuterated TGS. The evolution of several key hydrogen bond lengths suggests that weakening of the H‐bond network with T is crucial in decoupling the polarising glycine molecules from the other glycines and allowing the long‐range ferroelectric order to break down. A new parameterisation of the phase transition is demonstrated. Contrary to results of physical properties measurements, there is no evidence of a second low temperature phase transition in TGS – no low temperature anomalies were observed in the crystal structure.     

Bipolar-homogeneous structural phase transition in nematic droplets formed in the polymer matrix in a magnetic field

The phase transition from the bipolar structure to the homogeneous structure in droplets formed from the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in poly(vinyl butyral) in the presence of a magnetic field has been investigated. The phase transition is associated with the expansion of the regions with a tilted orientation of the director in the vicinity of defects (the disappearance of boojums) and with the alignment of the nematic director lines in the bulk of the droplet in the direction of the magnetic field. In the temperature range T = 24−34°C, cyclic mutual transformations between the bipolar configuration of the nematic director and the homogeneous structure, which have a period of ∼0.5–3.5 s and result from thermal fluctuations of the order parameter, are observed in droplets 3–15 μm in size after the forming field is switched off.     

Effect of annealing temperature on the structural and optical properties of amorphous Sb2Te2Se thin films

Thin films of Sb₂Te₂Se were prepared by conventional thermal evaporation of the presynthesized material on Corning glass substrates. The chemical composition of the samples was determined by means of energy‐dispersive X‐ray spectrometry. X‐ray diffraction studies on the as‐deposited and annealed films revealed an amorphous‐to‐crystalline phase transition. The as‐deposited and annealed films at T _a = 323 and 373 K are amorphous, while those annealed at T _a= 423 and 473 K are crystalline with a single‐phase of a rhombohedral crystalline structure as that of the source material. The unit‐cell lattice parameters were determined and compared with the reported data. The optical constants (n , k ) of the investigated films were determined from the transmittance and reflectance data at normal incidence in the spectral range 400–2500 nm. The analysis of the absorption spectra revealed non‐direct energy gaps, characterizing the amorphous films, while the crystalline films exhibited direct energy gaps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polymorphy of monoclinic terbium triborate,TbB3O6

TbB₃O₆, that suffers peritectic decomposition below its melting point, was obtained as single crystals by growth from borate flux. Depending on the borate flux, orthorhombic (space group Pnma) TbB₃O₆ belonging to the isomorphic series of rare earth triborates with small lanthanides, or monoclinic (space group C 2/c) TbB₃O₆, belonging the corresponding isomorphic series with large lanthanides, was synthesized. Within a broad range at subambient temperatures monoclinic TbB₃O₆ shows a structural phase transition with a large thermal hysteresis. The phase transition was characterised by means of differential scanning calorimetry, temperature‐dependent polarization microscopy and single‐crystal X‐ray diffraction. According to the observed features the transition can be classified as a non‐ferroic, martensitic phase transformation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural characterization of silicon nanocrystals from amorphous silicon oxide materials

Silicon nanocrystals have been produced by disproportionation reaction of bulk silicon monoxide at temperatures higher than 1073 K. More specific, samples annealed at 1123, 1173, 1223 and 1323 K as well as the starting material SiO have been examined. X-ray diffraction, high-resolution transmission electron microscopy and infrared spectroscopy have been employed in order to investigate the structure of the produced silicon nanocrystals. Photoluminescence measurements reveal a three band emission with maxima positioned at 1.33, 1.52 and 1.67 eV. The intensity of the photoluminescence emission increases with the annealing temperature exponentially. This fact can be directly correlated with the disproportionation reaction which results in bigger amounts of silicon nanocrystals by increasing the annealing temperature and is discussed here. Also a possible explanation is given for the origin of each emission band.     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO₂ matrix were synthesized by phase separation and thermal crystallization of SiO_x/SiO₂ supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.     

X-ray scattering by porous silicon modulated structures

A multilayered porous structure formed as a result of the anodization of a Si(111)(Sb) substrate in an HF:C₂H₅OH (1: 2) solution with a periodically alternating current has been investigated by high-resolution X-ray diffraction. It is established that, despite 50% porosity, a thickness of 30 μm, and significant strain (4 × 10⁻³⁾, the porous silicon structure consists mainly of coherent crystallites. A model of coherent scattering from a multilayered periodic porous structure is proposed within the dynamic theory of diffraction. It is shown that the presence of gradient strains of 5 × 10⁻⁴ or higher leads to phase loss upon scattering from porous superlattices and the suppression of characteristic satellites in rocking curves.     

Growth of Pb1 ? xSmxX (X is Se or Te and x = 0.01–0.04) films on porous silicon

Films with the composition Pb_{1 − x} Sm _x X (X is Se or Te and x = 0.01–0.04) have been prepared and their structural properties have been investigated. A method for growing a buffer porous silicon layer is described. It is established that, for all samples (except for an amorphous film on the surface of porous silicon), the real film structure consists of blocks 10–50 μm in size and is independent of the porous layer morphology.     

Room,low, and high temperature dehydration and phase transitions of kernite in vacuum and in air

Kernite Na₂B₄O₆(OH)₂·3H₂O dehydration in air at high temperature and in vacuum at room temperature has been studied. It was found that kernite easily dehydrates forming a new phase‐I both on heating and in vacuum. The chemical formula Na₂B₄O₆(OH)₂·1.5H₂O of the new phase‐I has been estimated on the basis of thermogravity analysis. It is triclinic with the unit cell parameters a = 7.047(8), b = 8.76(1), c = 13.08(2) Å, α = 93.40(9), β = 95.32(9), γ = 90.28(9)° changing slightly on pressure reduction. Due to the relatively low temperature (353 K) and reversibility of the kernite ⟷ phase‐I transition an anion of the new phase‐I likely consists of the same chains [B₄O₆(OH)₂]^2– like in kernite structure. The high anisotropy of kernite thermal expansion was explained by approaching of NaO chains due to the initial removing of water molecules from kernite crystal structure. The behaviour of the new phase‐I at low temperatures in vacuum was also investigated. A formation of an additional new phase II has been detected at the temperature of 93 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and phase transition of single crystalline CsNH2SO3

Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on cesium sulfamate CsNH₂SO₃ single crystal. Two distinct endothermic peaks in the DSC curves are observed at 330 and 436 K. It is pointed out that the peak at 330 K is attributed to the structural phase transition, and the other peak at 436 K is associated with the thermal decomposition of the crystal. The structures in room‐ and high‐temperasture phases are determined, and the space group of the sample crystal is found to change from monoclinic P 2₁/c to orthorhombic Pnma. The structure of the room‐temperature phase consists of two different types of N‐H···O hydrogen bond, but in the high‐temperature phase there is no specific hydrogen bond between the NSO₃ pseudo‐tetrahera. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)