Thermodynamic properties of LiZr2(PO4)3 crystal phosphate

The temperature dependence of the heat capacity of LiZr₂(PO₄)₃ crystal phosphate is studied in an adiabatic vacuum calorimeter in the temperature range of 6 to 358 K. A phase transition caused by the transition of a low-temperature (triclinic) modification to a high-temperature (rhombohedral) modification is observed in the temperature range of 290–338 K and its standard thermodynamic characteristics are estimated and analyzed. Standard thermodynamic functions are calculated from experimental data: heat capacity, enthalpy, entropy, and Gibbs function in the range of T → 0 to 358 K. Fractal dimensionality D is calculated from the data on low-temperature (20 K ≤ T ≤ 50 K) heat capacity and the topology of the phosphate’s structure is estimated.     

Neue Beobachtungen zum chemischen Transport von GeO2. VI. Temperaturabhängigkeit mit dem Transportmittel Chlor

New Observations on the Chemical Transport of GeO₂. VI. Temperature Dependence with the Transport Agent Chlorine In a temperature gradient T₂ ? T₁ = 100 K the chemical transport of tetragonal GeO₂ is kinetically controlled at an average transport temperature T ≤ 1080 K. The same is true for the metastable hexagonal modification at T ≤ 1150 K. An Arrhenius equation describes the rate of deposition in a satisfactory way. The activation energy amounts to 21 kcal/Mol. At higher temperatures diffusion determines the rate of transport whereby GeCl₄, Cl₂, and O₂ are looked upon as prominent gaseous molecules and the formation of a solid solution of hexagonal GeO₂ with SiO₂ is taken into account. For a transport temperature T < 1100 K GeO₂ is deposited at T₁ only if seeds of the specific modification are present. The deposition of GeO₂(hex.) ceases at temperatures lower than T ≈ 1000 K. The formation of GeO₂(tetr.) requires not only seeds but also NaCl as mineralizer and the temperature should not be lower than T ≈ 900 K.     

Kinetic investigation of the textile bleaching process

Volumetric H₂-uptake measurements on an Mo₂N (79 m²g^–1) sample reduced at 673 K have been carried out and the uptake isotherms in the temperature range of 308–623 K have been determined. Both the total and reversible hydrogen uptake increased with the uptake temperature. The irreversible hydrogen uptake increased abruptly when the uptake temperature was raised up to 423 K. The maximum of irreversible hydrogen uptake was measured at 473 K. The H_IR/Mo ratio calculated from the uptakes obtained in the temperature range of 308–623 K varies in the range of 0.0010–0.0202. One possible mechanism for hydrogen adsorption is proposed to be heterolytic dissociation on Mo-N paris, in which the molybdenum atoms are in unsaturated coordination.     

Raman Investigation of the Low Temperature Phase Transitions in Monoclinic TlReO4

The Raman spectrum of monoclinic TlReO₄ is presented together with a factor group analysis and assignment of the bands. The phase transitions of monoclinic TlReO₄ is investigated below room temperature, and two phase transitions were identified between room temperature and 90 K. The room temperature monoclinic phase was converted to the orthorhombic phase at 170 K upon cooling, and to the tetragonal phase at 150 K. Upon heating back to room temperature the tetragonal phase persisted until the orthorhombic phase could be identified at 210 K and 220 K. From 230 K to room temperature the monoclinic phase could again be observed.     

Direct temperature measurement in alkali halide cluster beam

The temperature of a sodium fluoride cluster beam produced by laser vaporization is estimated from the rotationally-resolved laser spectroscopy of the Na₂ dimer. The cluster beam is obtained by laser vaporization of a sodium rod in a mixture of helium containing a small amount (1%) of SF₆. Both rotational and vibrational temperatures (respectively 170 K and 400 K) are much higher than expected from a simple theoretical model of a supersonic beam. These temperatures can be lowered to 70 K and 250 K respectively by cooling the nozzle to liquid nitrogen temperature.     

Temperature dependence of trapped electrons in crystalline D2O ice from 77 to 6 k

The temperature dependence of the abundance of trapped electrons which absorb in the visible (e^?_vis) and infrared (e^?_IR) in crystalline D₂O ice has been studied by pulse radiolysis between 77 and 6 k. The yield (G) and decay of e^?_vis show little dependence on temperature or doping with NH₄F. At 6 K G(e^?_vis) is 0.54 and the electron decays by half within 5 μs. These observations are consistent with e^?_vis being mainly located in spurs. The yield and decay of e^?_IR, on the other hand, show a more marked dependence on temperature. In the pure crystal G(e^?_IR) increases more than tenfold from ≈ 0.1 at 77 K to 1.3 at 6 K and its decay rate is greatly decreased at the lower temperature. Doping with NH₄F increases G(e^?_IR) to 0.85 to 77 K and to 1.8 at 6 K and some decay is observed at 6 K but not at 77 K. These results are interpreted on the basis that geminate recombination between electrons and holes is very fast at 77 K but becomes sufficiently slow for the electrons to be observed at 6 K. It is also inferred that the hole is more mobile than e^?_IR. The mechanisms causing the decays of e^?_vis and e^?_IR are discussed.     

Features of the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation

¹⁹F, ¹H NMR and DSC methods are used to study the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation. A transformation of the NMR spectra observed in a temperature range 77(130)-450 K is found to be associated with a change in the type of ion motions in the anion and cation sublattices of the studied compounds. In a temperature range 170-450 K the main types of ion motions are isotropic reorientations of TaF₆, NbF₆, TiF₆ octahedra and tetramethylammonium ions. Two endothermic effects with the maxima at 232.5 K and 256.5 K for [N(CH₃)₄]TaF₆ and 235 K and 250 K for [N(CH₃)₄]NbF₆, which are observed in the DSC curve in the range 170-400 K correspond to phase transitions that have almost no effect on the NMR spectral parameters the a temperature range 230-260 K. For the [N(CH₃)₄]₂TiF₆ compound, an endothermic effect at 422 K is observed, corresponding to the phase transition from the rhombohedral to the cubic modification.     

Thermodynamic performance of the NaNO3–NaCl–NaNO2 ternary system

Calculations of phase diagram for additive ternary molten salt system are carried out by the conformal ionic solution theory. The liquidus temperatures of the system NaNO₃–NaCl–NaNO₂ are determined according to different types of solid–liquid equilibrium and different values of the binary interaction coefficients. For the system NaNO₃–NaCl–NaNO₂, the calculated and experimental temperatures differ are very small below 673 K, but the oxidation and decomposition of the mixed salts are found when the temperature is higher than 673 K. Meanwhile, the eutectic point is obtained from calculated phase diagram and the eutectic temperature is 507 K. Thermal stability of this eutectic mixture is investigated by thermo-gravimetric analysis device. Experimental results show this kind of molten salt has a lower melting point (501.28 K), similar to solar salt (493 K). It is thermally stable at temperatures up to 819 K, and may be used up to 827 K for short periods.     

Molecular motions and λ phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the λ-phase transition. Site splittings on two intramolecular modes of HMB-h₁₈ (but on only one corresponding HMB-d₁₈ mode) are observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the λ-phase transition takes place at 113°K (± 2°K) for HMB-h₁₈ crystal, the transition temperature is 133°K (± 2°K) for HMB-d₁₈. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D_3d and reduces to C_i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.     

Untersuchungen zur SiF4-Sorption an Kieselglaspulvern

Studies on the Absorption of SiF₄ at Silica Powders The absorption of SiF₄ at silica powder was investigated in dependence on temperature and pressure. It starts at about 800 K. In the range of 800 K up to 1160 K the amount of SiF₄ absorbed within 24 hours increases with temperature, and at a constant temperature (880 K) with the filling pressure of SiF₄ (2 kPa–30 kPa). Desorption of SiF₄ was proved experimentally at temperature above 1273 K. It is connected with crystallization of silica into β-cristobalite.     

NMR and impedance spectroscopy data on the ionic mobility and conductivity in PbSnF4 doped with alkali metal fluoride

NMR and impedance spectroscopy are used to study the ionic mobility and conductivity in crystalline samples in PbSnF₄-MF systems (M = Li, Na, K) in a 150?C473 K temperature range. The ¹⁹F NMR spectral parameters, types of ionic motion, and ionic conductivity value in the PbSnF₄ compound doped with alkali metal fluoride is found to be determined by the temperature, nature, and concentration of an alkali cation. The specific conductivity of the crystalline samples in PbSnF₄-MF systems (M = Li, Na, K) is rather high at room temperature and hence, it seems possible to apply them in the development of functional materials with high ionic (superionic) conductivity.     

High temperature dielectric loss of h-BN at X band and its dependence on the electron structure,defects and impurities

The dielectric loss of hexagonal boron nitride (h-BN) at high temperature ranging from 291 K to 2023 K at X band is investigated by experimental and theoretical analyses. The dielectric loss is relatively low below 1273 K, nevertheless, above this temperature, much rapid increase in the dielectric loss is observed with the increase of temperature. At 2023 K, the dielectric loss is found to be 40 times more than that at room temperature. Our calculation reveals that the rapid increase in the dielectric loss from 1273 K to 1800 K is mainly contributed by the boron vacancy (V_B) defects but from that 1800 K to 2023 K is mostly caused by the hopping phenomenon of the electrons. It is also found that the reason that V_B has the smallest energy among the defect is that its electronegativiities are smaller and its bonding ability is weaker compared to other defects. And the co-existence of defects and impurities is found to enhance the conduction loss by first principle calculations, which is due to that the bond of B1 atom and N1 atom becomes weak and the jumping distance of the impurity ions is increased when V_B and Na co-doped in the structure.     

Solid–liquid interface growth of Cu50Ni50 under deep undercoolings

Molecular dynamics simulations have been performed to explore interface growth of liquid Cu₅₀Ni₅₀ alloy by using an embedded atom potential, namely due to Zhou. The simulated melting temperature is 1585 K in agreement well with the experimental value of 1600 K. The calculated interface velocity increases with decreasing temperature ranging from 1585 K to 1100 K, where the calculated values are a little higher than the experimental ones at higher temperatures, and in agreement with the experimental ones at lower temperatures; while the calculated interface velocity decreases with decreasing temperature lower than 1100 K. The activation energy of atom is 0.0048 eV, almost close to zero under deep undercoolings, although the crystal growth still proceeds with the speed ranging from 50 m s^?1 to 10 m s^?1. The crystal growth of Cu₅₀Ni₅₀ is not controlled by diffusion mechanism under deep undercoolings.     

Heat capacity and thermodynamic functions of thulium orthophosphate TmPO<Subscript>4</Subscript> in the range of 10–1350 K

The heat capacity of TmPO₄ in temperature ranges of 9.11–346.05 and 304.6–1344.6 K is measured via adiabatic and differential scanning calorimetry, respectively. The measurement data are used to calculate the temperature dependences of the heat capacity, entropy, change in enthalpy, and reduced Gibbs energy of TmPO₄ in the range of 10–1344 K. The Gibbs energy of formation of thulium orthophosphate from elements Δ_fG⁰(298.15 K) is determined.     

A study on the dehydriding kinetics in the Mg2Ni-Ni (< 5 wt.%)-H2 system

The dehydriding kinetics of the Mg₂Ni-Ni (< 5 wt.%)-H₂ system is studied at temperatures between 553 and 593 K under hydrogen pressures from 0.8 to 3.2 bar. In the temperature range from 553 to 563 K the nucleation of α-solid solution of hydrogen in Mg₂Ni is found to be the rate-controlling step for the dehydriding reaction. In the temperature range from 573 to 593 K the rate-controlling steps are Knudsen flow and ordinary gaseous diffusion of hydrogen molecules through pores, interparticle channels and cracks.     

Heat capacity and thermodynamic properties of Mg(Fe<Subscript>0.6</Subscript>Ga<Subscript>0.4</Subscript>)<Subscript>2</Subscript>O<Subscript>4</Subscript> in the 0–800 K temperature range

The heat capacity of Mg(Fe_0.6Ga_0.4)₂O₄ in the temperature range of 4.56–804.9 K is measured by adiabatic and differential scanning calorimetry. The temperature dependence of the heat capacity of Mg(Fe_0.6Ga_0.4)₂O₄ in the 0–800 K range is determined by generalizing the experimental data. The temperature dependences of thermodynamic functions (entropy, enthalpy change, and the reduced Gibbs free energy) are calculated. The abnormal contribution to the heat capacity C _p ^an(T) in the temperature range of 5–52 K is estimated.     

Hydration heat capacities of hydrophobic solutes at 248–373 K

A new equation is suggested to define the temperature dependence of the Gibbs energy of hydration of hydrophobic substances: ΔG ⁰ = b ₀ + b ₁ T + b ₂lnT. According to this equation, the hydration heat capacity is in inverse proportion to temperature. Consistent values of hydration heat capacity of nonpolar solutes have been obtained for different temperatures using data on solubility and dissolution enthalpy. The contributions of the hydrocarbon radicals and OH group to the heat capacity of hydration of the compounds were found for the temperature range 248–373 K. The hydration heat capacity of the hydroxyl group has a weak dependence on temperature and increases by only 12 J/(mol·K) in the specified temperature interval. Changes in the hydration entropy of hydrophobic and OH groups are calculated for the temperature increasing from 248 K to 373 K.     

Low temperature DC electrical conductivity studies of Pb1-xZnxS semiconductor compounds

Pb_1-xZn_xS (x = 0 to 0.4 in steps of 0.1) ternary semiconductor material shave been prepared by co-precipitation method. The structural and electrical studies have been carried out to understand the influence of Zinc on activation energy. X-ray diffractograms showed that the Pb_1-xZn_xS compounds are polycrystalline in nature with cubic phase. The temperature-dependent conductivity measured in the range 300–4 K showed a change in the conduction mechanism at 62 K and 31 K. The activation energies evaluated at low and very low temperature regions. Activation energy increases and conductivity decreases with the increase of Zn in Pb_1-xZn_xS compounds. The samples are exhibiting low freezing temperature points indicating their capability to use in low temperature applications.     

Magnetic susceptibility of CfCl3 and its dependence on crystal structure

Magnetic measurements are presented for polycrystalline samples of californium trichloride in both a hexagonal (UC1₃ type; californium(III) CN = 9) and an orthorhombic (PuBr₃ type; californium(III) CN = 8) crystal form. In the Curie-Weiss temperature regions (orthorhombic, 100–340 K, hexagonal, 60–340 K) the Weiss constant θ_p is significantly larger for the orthorhombic form. As the temperature decreases, the susceptibility of the orthorhombic form begins to deviate from Curie-Weiss behavior at a higher temperature than the hexagonal form indicating a larger crystal-field splitting in the orthorhombic form. At temperatures below 15 K a field-dependent transition is observed. In a field of 5 kG an antiferromagnetic maximum in the susceptibility is observed at 7 K and 13 K for the hexagonal and orthorhombic forms respectively. For 30 kG both types exhibit monotonically increasing susceptibilities with decreasing temperature and the hexagonal form shows saturation behavior in the susceptibility vs. temperature plots with a saturation moment of about 6_μB atom⁻¹.     

Temperature evolution of structural and magnetic properties of stoichiometric LiCu2O2: Correlation of thermal expansion coefficient and magnetic order

Temperature-dependent crystallographic and magnetic studies on stoichiometric single crystals of LiCu₂O₂ are reported. The temperature dependence of the lattice parameters was extracted from X-ray powder diffractograms collected on crushed single crystals, from 12 K to 295 K. The magnetic properties are similar to earlier findings demonstrating antiferromagnetic ordering below 25 K. Evidence of magnetoelastic coupling is observed in the thermal expansion along the c-direction; not only at the low temperature antiferromagnetic transitions, but an anomalous behavior of the thermal expansion indicate magnetoelastic coupling also to the magnetic ordering related to a weak spontaneous magnetic moment appearing at 150 K. Ac-susceptibility measurements at different frequencies and superposed dc-fields are employed to further characterize this magnetic anomaly.