The influence of water steam on the direct phase transformation of zeolite NaA to nepheline by thermal treatment

Heating of zeolite NaA leads to the consecutive formation of amorphous phase, carnegieite and nepheline in the high temperature region. These phase transformations are influenced not only by temperature but also by the presence of water (inside of the crystals). Water causes destruction of zeolite framework by hydrolytic splitting of Si-O-Al-bonds in addition to thermal effects. Thus, the amorphous phase as the first product of phase transformation occurs in the presence of water steam at a lower temperature. The formation of the crystalline phases carnegieite and nepheline was observed in a temperature range, 300 K lower than usually, and without occurrence of the amorphous phase.     

Crystal structures of the new diphosphates,K2NiP2O7 and K6Sr2Ni5(P2O7)5

Crystalline K₂NiP₂O₇ (I): monoclinic, P2₁,a=9.230(2),b=17.540(8),c=8.319(9)Å, =91.44(2)°,Z=8,d _calc=3.067 g cm^–3, MoK =0.71069Å,R/R _w=6.5/9.4%, is characterized by the existence of face sharing NiO₆ octahedra with Ni–Ni separation of 2.827 Å (Ni–O; 1.93(2)–2.17(2)Å). K⁺ is seen in sites of seven, six, and fivefold coordination (K–O, averages; 2.83(2), 2.81(2), and 2.77(2)Å, respectively) P₂O₇ ^4– groups are observed in semieclipsed conformation. K₆Sr₂Ni₅(P₂O₇)₅ (II) crystallizes in monoclinic space group P2₁/c;a=11.038(7),b=9.533(13),c=7.438(2)Å, =100.13(4)°,D _calc=3.309 g cm^–3,Z=2,R/R _w=6.4/8.1%. Nickel atoms are distributed on planes parallel to the crystallographic (100) plane at =0 and =0.5. On the planes, =0, Ni²⁺ ions form successive linked trimers of edge sharing NiO₆ octahedra (Ni–Ni, 3.018(1), 3.008(1)Å; Ni–O, av 2.07(2)Å). The intermediate planes (=0.5) contain isolated and six coordinate Ni²⁺ ions (Ni–O av. 2.09(2)Å). Seven-coordinate potassium ions (K–O, av. 2.74(2)Å) are located on intermediate planes at =0.25 and 0.75. P₂O₇ ^4– groups are found in eclipsed conformation. Strontium atoms are located between nickel and potassium planes and are surrounded by seven oxygen atoms (Sr–O, av. 2.586(2)Å).     

Elastic properties of the dithionates Na2S2O6 · 2 H2O,K2S2O6, Rb2S2O6, CaS2O6 · 4H2O and SrS2O6 · 4 H2O

Single crystals of the title compounds having optical quality and dimensions of several cm were grown from aqueous solutions. The elastic and thermoelastic constants were determined from ultrasonic resonance frequencies of thick plates. The true point symmetry of K₂S₂O₆ and Rb₂S₂O₆, which is screened by a hexagonal hypermorphy, could be clearly revealed to be trigonal (32) by the existence of the elastic constant c₁₄. In the case of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O the constant c₁₄ of the specimens appeared too small to confirm the trigonal symmetry group required from electrooptic and non-linear optic effects unambiguously. The isotypy of K₂S₂O₂ and Rb₂S₂O₆ as well as that of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O is confirmed by their elastic behaviour. The mean elastic stiffness of dithionates is closely related to that of the corresponding sulphates. In the vicinity of the second-order phase transition of K₂S₂O₆ near 235 K weak anomalies of the temperature derivatives of the longitudinal elastic stiffnesses are observed.     

The Characteristics of the Mixed Crystals of the K2SO4-(NH4)2SO4-H2O System at 298 K

Equilibrium studies of the K₂SO₄-(NH₄)₂SO₄-H₂O system at 298 K have been conducted. The isotherm of solubility and the curve of distribution have been mathematically described. The parameters of the crystal lattice and the enthalpy of crystallization of the [K_x(NH₄)_1-x]₂SO₄ solid solutions as a function of their composition have been presented.     

Hydrothermal crystallization in the KF-ZrO2-SiO2-H2O system at 40°C: Phase equilibria and modeling of ZrSiO4, K2ZrSi6O15, K2ZrSi3O9, K3ZrF7, and ZrO2 crystal structures

The phase equilibria in the KF-ZrO₂ (nanocrystals)-SiO₂-H₂O system are studied for the ZrO₂: SiO₂ molar ratio ranging from 2: 1 to 1: 6 and the KF concentration C _KF ranging from 3 to 36 wt % under 0.1 GPa at 40°C. We established crystallization of three silicates—ZrSiO₄ (zircon), K₂ZrSi₆O₁₅ (deleite), K₂ZrSi₃O₉ (wadeite)—and K₃ZrF₇ fluoride and ZrO₂ and SiO₂ oxides. The structures of K-and Zr-based silicates obtained can be represented as an open framework formed by M octahedra (ZrO₆) that share vertices with T tetrahedra (SiO₄). With the increase of KF concentration, silicates “substitute” each other in the following sequence: ZrSiO₄ ?K₂ZrSi₆O₁₅ ? K₂ZrSi₃ O₉. Depending on the ZrO₂: SiO₂ ratio and C _KF concentration, K₃ZrF₇ fluoride is formed simultaneously with K₂ZrSi₃O₉ and K₂ZrSi₃O₉ + K₂ZrSi₆O₁₅. The characteristic features of formation of Zr-containing phases are discussed in the framework of the model of the matrix assembly of crystal structures from subpolyhedral structural units—clusters of cyclic type. The features of the phase formation in the system are compared with the characteristics determined earlier for the KOH-ZrO₂ (nanocrystals)-SiO₂-H₂O system at 40°C.     

Control of the optical band gap of hydrogenated amorphous boron: The use of the B2H6–BF3 gas mixtures

Amorphous boron thin films have been produced by the glow discharge decomposition of diborane, boron trifluoride, and hydrogen gas mixtures. The optical band gap of these films was controlled by changing the relative proportions of diborane and boron trifluoride. Films with band gaps of approximately 1.45 eV can be obtained by using a B₂H₆ : BF₃ gas volume ratio of 8 : 1 and a substrate temperature of 225°C. The electron paramagnetic resonance spectrum shows a pronounced shift in the g factor, giving evidence of fluorine incorporated into the amorphous matrix.     

The Characteristics of the Mixed Crystals of the KCl-KBr-H2O System at 298 K

The equilibrium studies of the KCl-KBr-H₂O system at 298 K have been conducted. The isotherm of solubility and the curve of distribution have been mathematically described. The parameters of the crystal lattice and the enthalpy of crystallization of the K[Cl_xBr_1-x] solid solutions as a function of their composition have been presented.     

The growth of single crystals of langbeinites Rb2Cd2(SO4)3, Tl2Cd2(SO4)3, K2Co2(SO4)3, and their phase transitions

Single crystals of dicadmium dirubidium sulphate and dicadmium dithallium sulphate have been grown from non-stoichiometric aqueous solutions by various methods. Optical and thermal properties in the vicinity of their phase transitions have been studied. Polycrystalline samples of dicobalt dipotassium sulphate have been prepared using Bridgman method. Optical and specific heat measurements have shown a new phase transition at −148°C.     

Molecular aspects of the main phase transition in lipid systems as a weak first-order phase transition: 2. Fluctuation phenomena and the role of molecule conformation in the behavior of lipid films

The influence of molecule “softness” on the thermodynamic behavior of membranes, on the character of the main phase transition, and on an anomalous increase in the distance between bilayers in the film (swelling) above the phase-transition temperature is studied. The formation of the so-called ripple phase in multilayer films (modulation in the membrane plane) below the phase-transition temperature and the role of such factors as the molecule parameters and the lipid-film thickness are considered. It is shown that swelling of a lipid film is caused by the fluctuation-induced tilt of the molecules and the corresponding decrease in the number of “soft” molecules, whereas the formation of the ripple phase is caused by fluctuations of the order parameter in the gel phase and the corresponding increase in the number of soft molecules.     

The use of an electric analogue to solve the lamellar eutectic diffusion problem

An electrical analogue technique has been developed to obtain numerical solutions to the diffusion equation in the liquid ahead of a growing lamellar eutectic. Using the solutions it has been possible to examine certain of the approximations made in earlier treatments of the problem. A modification has been made to the Jackson and Hunt model of eutectic growth which becomes important when composition differences in the interfacial liquid composition become large (i.e. at very high growth velocities). The modified treatment no longer predicts υλ² = constant although the variation of υλ² with growth velocity is not likely to be detected easily.     

The Thermal Conductivity of Monocrystalline Gallium Garnets Doped with Rare-Earth Elements and Chromium in the Range 6–300 K

The effect of the additions of rare-earth elements Nd, Sm, Eu and Tb on the thermal conductivity of gadolinium-gallium garnets and besides the influence of the joint doping with Nd and Cr on the thermal conductivity of gadolinium-gallium and more compound gadolinium-scandium-gallium, gadolinium-magnesium-zirconium-gallium and yttrium-scandium-gallium garnets have been investigated.     

The Phase Diagram of the Pb–Mo–S System at 1250 K and Some Properties of the Superconducting PbMo6S8

Pb_xMo₆S_y arises as the sole ternary compound in the System Pb–Mo–S. At 1250 K it is surrounded by 4 phase regions with 3 solid phases. It has a homogeneity range of between 0.9 ≤ x < 1.1 and 7.6 ≤ y > ≤ 7.9. T_c changes within the homogeneity range. Chemical transport is possible within a limited section of the phase diagram in the presence of lead halides.     

The morphology of Zn3P2 single crystals grown from the vapour phase

The morphology of Zn₃P₂ single crystals, grown from the vapour phase by sublimation of bulk Zn₃P₂ in a sealed quartz tube pulled through a high temperature gradient, has been investigated. X-ray diffraction examination showed that the main direction of crystal growth with a tetragonal unit cell is the [021] direction which corresponds to the [111] direction in the pseudocubic system. Transverse and longitudinal cross sections of the single crystals were mechanically polished and chemically etched. Optical examination of the cross sections indicated that the growth of Zn₃P₂ begins most frequently in the (011) plane. Striae which appeared in a cross section after chemical etching are attributed to the existence of strain fields generated in the sample as a result of temperature fluctuations or as a result of crystal structure defects in the core region.     

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al-2 at.% Zn-1 at.% Mg alloy

The decomposition behaviour of an Al-2.0 at.% Zn-1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, T_h) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η-particles yields a maximum value at pre-ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop-formation and clustering model was introduced.     

Stokes to anti-Stokes intensity ratio in Raman spectra of the soft mode in KH2PO4 near the phase transition temperature

We investigate the Stokes to anti-Stokes intensity ratio for the Raman spectrum of a central peak in a potassium dihydrogen phosphate (KDP) crystal. The ratio deviates from the Boltzmann factor as the temperature approaches the phase transition temperature. The response functions evaluated by three ways assuming the fluctuation–dissipation theorem do not agree in a frequency region below ～30 cm^?1. This means that the usual relation based on the fluctuation–dissipation theorem are not directly applicable to the spectral analysis of the soft mode. Further, we compare the result of KDP with that of the cooperative rotational dynamics in liquid carbon disulfide.     

The thermodynamic equilibrium of the formation process of High-Tc superconducting phase YBa2Cu3O7−x

Analysing the equilibrium state of the formation reaction of the high-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) on the base of the pressure-temperature-oxygen content (P-T−x) correlation research and using the Van't-Hoff equation, the Gibbs free energy (ΔG_m) of the reaction has been calculated for the first time. It is seen that the absolute value of ΔG_m is not large and of the order of 2.5 kcal · mole⁻¹. This leads to an instability of the 1:2:3 phase. Also, from the calculated values of ΔG_m relative stability regions of the 1:2:3 phase are determined for different values of x. Combinatively analysing the relation lg P = A−B/T (where A – the x-dependent constant, and value of B was determined to be equal to 10518 atm. K) and the value of ΔG_m, we come to the conclusion that the synthesis of YBa₂Cu₃O_{7 − x} when x → 0 must be carried out under high oxygen pressure and low temperature for a lasting time, because the reaction rate is limited by slow diffusion in the solid phase.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

The Investigation of Relation Between the Transition Temperature and the Unit Cell Volume in (K1-xCsx)2ZnCl4 Mixed Crystals

The relation between the phase transition temperature and unit cell volume in the (K_1-xCs_x)₂ZnCl₄ mixed crystals was studied. The phase transition temperature of A₂BX₄ family is dependent on the size of cation and anion (FABRY and PEREZ-MATO). That is, the transition temperature of crystal decreases with increasing unit cell volume. In this study we investigated this property of mixed crystals with the increasing mixture ratio. From the current study, we obtained the result that in the mixed crystals (K_1-xCs_x)₂ZnCl₄, the increase of x induces the increase of unit cell volume, so that T_I decreases with increasing unit cell volume. In order to determine the distribution of the substituted Cs⁺, using the near IR and UV spectrophotometer, we investigated both the band gap energy and the type of transition.