Thermodynamic characteristics of thermal dissociation of platinum trichloride

Single crystals of platinum trichloride were grown for the first time. The IR spectrum of single-crystal PtCl₃ was recorded. The pressure of thermal dissociation of PtCl₃ was measured by the static method with a quartz membrane-gauge zero-pressure manometer. An approximating equation for the dissociation pressure vs. temperature (540 K ≤ T ≤ 775 K) for the reaction 2 PtCl₃(s) → 2 PtCl₂(s) + Cl₂(g) was found. The enthalpy (123.1±1.7 kJ mol⁻¹) and entropy (183.6±2.8 J mol⁻¹ K⁻¹) for the dissociation of PtCl₃(s) were calculated at 298.15 K.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2032–2034, October, 2004.     

Crystal structure, spectroscopy and thermodynamic properties of MVWO6(M – Li, Na)

In the present work lithium (sodium) vanadium tungsten oxides with brannerite structure is refined by the Rietveld method (space group C2/m, Z=2). IR and Raman spectroscopy was used to assign vibrational bands and determine structural particularities. The diffuse reflectance spectra allow to calculate bandgap for M^IVWO₆(M^I – Li, Na). The temperature dependences of heat capacity have been measured first in the range from 7 to 350 K for these compounds and then between 330 and 640 K, respectively, by precision adiabatic vacuum and dynamic calorimetry. The experimental data were used to calculate standard thermodynamic functions, namely the heat capacity C_p^o(T), enthalpy H^o(T)−H^o(0), entropy S^o(T)−S^o(0) and Gibbs function G^o(T)−H^o(0), for the range from T→0 to 640 K. The differential scanning calorimetry was applied to measure decomposition temperature of compounds under study.     

Electrical Conductance and Volumetric Studies in Aqueous Solutions of DL-Pyroglutamic Acid

Conductivity measurements of DL-pyroglutamic acid and sodium pyroglutamate in dilute aqueous solutions were performed in the 288.15–323.15 K temperature range. The limiting molar conductances of pyroglutamate anion, λ^o(pGlu⁻, T) and the dissociation constants of pyroglutamic acid, K(T) were derived from the Onsager, and the Quint and Viallard conductivity equations. Densities of aqueous solutions with molalities lower than 0.5 mol-kg⁻¹ were determined at 5 K intervals from T = 288.15 K to 333.15 K. Densities served to evaluate the apparent molar volumes, V_2,φ(m, T), the cubic expansion coefficients, α (m,T) and the changes of the isobaric heat capacities with respect to pressure, (∂ C_P/∂ P)_T,m. They were correlated qualitatively with the changes in the structure of water when pyroglumatic acid is dissolved in it.     

Electrical Conductance and Volumetric Studies in Aqueous Solutions of Nicotinic Acid

Conductivity measurements of nicotinic acid and sodium nicotinate in dilute aqueous solutions were performed in the (288.15 to 323.15) K temperature range. The limiting equivalent conductances of the nicotinate anion, λ⁰(Nic⁻, T), and the dissociation constants of nicotinic acid, K(T), were derived by the use of the Onsager and the Quint and Viallard conductivity equations. Densities of aqueous solutions with molalities lower than 0.2 mol-kg⁻¹ were determined at 5 K temperature intervals, from T = (288.15 to 333.15) K. The measured densities were used to evaluate the apparent molar volumes, V_{2, φ}(m, T), the cubic expansion coefficients, α(m, T), and the changes of isobaric heat capacities with respect to pressure, (∂C_P/∂p)_{T, m}. They were qualitatively correlated with the changes in the structure of water when nicotinic acid is dissolved in it.     

Theoretical study of the stereodynamics of the reactions O(D) + H2, D2 and HD

Calculations of the dynamics of the reactions O(¹D) + H₂ → OH + H, O(¹D) + HD → OH + D, O(¹D) + HD → OD + H and O(¹D) + D₂ → OD + D have been performed using the quasi-classical trajectory (QCT) method with symplectic integration. The theoretical calculations were carried out on the ground state 1A′ potential energy surfaces (PES) by Dobbyn and Knowles. The distributions of the dihedral angle P(_r), the angle between k and j′, P(θ_r), and the product vibrational state are presented. The results show that the intermediate geometrical structures and lifetimes of the reactive collisions play a vital role in these reactions.     

The solubility and isotopic fractionation of gases in dilute aqueous solution. IIa. solubilities of the noble gases

A method for the analysis of precise gas solubility data is presented and applied to new determinations of the Henry constant, k₂, for He, Ne, Ar, Kr, and Xe. The values of k₂ are fitted to the same sets of temperature functions which we have tried for oxygen. Our previously proposed power series in 1/T, ln(k_2/P )=a₀+a₁/T+a₂/T² (Mark I), gives the best 3-term fit within the temperature range 0–60°C. For use over the full range to the critical temperature of water, we have discovered a new function given by (T^*)²ln(k_2/P )=A₀(T^*)²+A₁(1-T^*)^1/3+A₂(1-T^*)^2/3(Mark II), where T^*T/T _c1 . It fits our data from 0–60°C nearly as well as Mark I; it fits high temperature data from other sources; and at the critical temperature of water it satisfies theoretical requirements. Expansion of Mark II reveals the relationship between Mark II and Mark I and leads to a 4-term smoothing function, ln(k_2/P )=a_–2(T^*)^–2+a_–1(T^*)^–1+a₀+a₁T^* (Mark III), which we believe gives the best values only for the 0–60°C range. Mark III is used to calculate values for , and , 0–60°C, and a procedure is empolyed to estimate the errors. Agreement is excellent between these results and those obtained from precise microcalorimetric measurements made by others. With the inclusion of pressure correction terms, Mark II yields the four thermodynamic function changes for use at high temperatures. With increasing temperature, these changes suddenly turn upward toward plus infinity as T _c1 is approached. Essentially direct determinations of for argon by other workers are in excellent agreement with our results. The symmetrical activity coefficient at infinite dilution, ₂ ^° is examined and the hypothetical properties of k₂ are explored below 0°C. Mark II can be expressed in the reduced form (T^*)²ln(k ₂ ^* )=A₁(1-T^*)^1/3+A₂(1-T^*)^2/3, where k ₂ ^* k ₂/(p _c12c1). A₂ is a very good linear fit to A₁, which suggests a characteristic temperature for water at 287.3 K.     

Low-Temperature Electronic Properties of the Lan+1NinO3n+1 (n = 2, 3, and ∞) System: Evidence for a Crossover from Fluctuating-Valence to Fermi-Liquid-like Behavior

Measurements of the temperature dependence of the electrical resistivity ρ(T), magnetic susceptibility χ(T), and Seebeck coefficient S(T) have been carried out on the n = 2, 3, and ∞ members of the homologous lanthanum nickel oxide systems La_n+1Ni_nO_3n+1 that were annealed in air. With increasing n, a progressive decrease in the electrical resistivity and a gradual change from insulating to metallic behavior are observed. La₃Ni₂O₇ is nonmetallic, showing a gradual increase in ρ when T decreases (dp/dT < 0) from 300 to 4.2 K, whereas La₄Ni₃O₁₀ and LaNiO₃ exhibit metallic resistivity (dp/dT > 0). A minimum in ρ(T) near 140 K is observed for La₄Ni₃O₁₀, while LaNiO₃ exhibits a T² dependence for ρ(T) below 50 K. The magnetic susceptibility of LaNiO₃ is Pauli-like, but the χ(T) data for La₃Ni₂O₇ and La₄Ni₃O₁₀ below 350 K show a decrease with decreasing temperature. The Seebeck coefficient of all these compounds is negative at high temperatures; La₃Ni₂O₇ and La₄Ni₃O₁₀ exhibit a sign change in S at low temperatures. These results suggest a crossover from a fluctuating-valence to a Fermi-liquid-like behavior with increasing n.     

Determination of Erbium by Two-Color Three-Photon Resonance Ionization Mass Spectrometry

Excited states and autoionization states of the erbium atom were investigated by the use of multicolor resonance ionization mass spectrometry. Among the observed first excited states, a level [4f¹²(³H₆)6s6p(³P₁)] located 17,348 cm⁻¹from the ground state is regarded as the most efficient state for excitation within the wavelength range investigated (560–600 nm), while a level located 17,080 cm⁻¹from this first excited state (E= 34,458 cm⁻¹) is identified as the best second excited state for the optimal photoionization scheme. Many ionization schemes adopting an autoionization state are also investigated, and the most efficient scheme is identified as 4f¹²6s²(³H₆) → 4f¹²(³H₆)6s6p(³P₁⁰), 17,348 cm⁻¹→ 34,458 cm⁻¹→ continuum state, which corresponds to the two-color (ω₁+ ω₂+ ω_1,2) scheme. Various concentrations of standard solutions for erbium are determined and the minimum amount detectable by two-color three-photon ionization was determined to be 20 pg.     

Molecular dynamics study of phase transition and nucleation in supercooled clusters of potassium iodide

In a series of molecular dynamics (MD) runs on (KI)₁₀₈ clusters, the Born–Mayer–Huggins potential function is employed to study structural, energetic, and kinetic aspects of phase change and the homogeneous nucleation of KI clusters. Melting and freezing are reproducible when clusters are heated and cooled. The melted clusters are not spherical in shape no matter the starting cluster is cubic or spherical. Quenching a melted (KI)₁₀₈ cluster from 960 K in a bath with temperature range 200–400 K for a time period of 80 ps both nucleation and crystallization are observed. Nucleation rates exceeding 10³⁶ critical nuclei m⁻³ s⁻¹ are determined at 200, 250, 300, 350, and 400 K. Results are interpreted in terms of the classical theory of nucleation of Turnbull and Fisher and of Buckle. Interfacial free energies of the liquid–solid phase derived from the nucleation rates are 7–10 mJ m⁻². This quantity is 0.19 of the heat of transition per unit area from solid to liquid, or about two-thirds of the corresponding ratio which Turnbull proposed for freezing transition. The temperature dependence of σ_sl(T) of (KI)₁₀₈ clusters can be expressed as σ_sl(T)∝T^0.34.     

Heat capacities of molten salts with polyatomic anions

The molar heat capacities at constant pressure, C_P, of molten salts with polyatomic anions, obtained from the literature, are examined. As a rule, the C_P values are independent of the temperature T, but the molar heat capacities at constant volume, C_V, derived from them, depend on T. The latter were obtained, as far as the required density, expansibility and compressibility data are available, for 1.1T_m, presumed to be the corresponding state, T_m being the melting temperature. Their ratio γ = C_P/C_V is linear with the cation–anion distance in the molten salt, d_C–A. The communal, quasi-lattice, heat capacity ΔC_P = C_P − C_P(i.g.) is obtained by subtraction of the sum of published ideal gas heat capacities of the constituent ions at 1.1T_m, C_P(i.g.). This communal heat capacity ΔC_P is proportional to the packing fraction of the ions in the melt, y = πN_Aνd_C−A³/6V. Here N_A is Avogadro's number, ν the number of ions per formula unit, and V the molar volume at 1.1T_m. Some models for the heat capacities of molten salts are shown not to be well applicable to the set of salts discussed here, but no alternative could be suggested.     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

Thermodynamic analysis of the phase separation during the polymerization of a thermoset system into a thermoplastic matrix. I. Effect of the composition on the cloud‐point curves

The cloud‐point curves of polystyrene (PS) mixed with reactive epoxy monomers based on diglycidyl ether of bisphenol A with stoichiometric amounts of 4,4′‐methylenebis(2,6‐diethylaniline) were experimentally studied. A thermodynamic analysis of the phase‐separation process in these epoxy‐modified polymers was performed that considered the composition dependence of the interaction parameter, χ(T,Φ₂) (where T is the temperature and Φ₂ is the volume fraction of polystyrene), and the polydispersity of both polymers. In this analysis, χ(T,Φ₂) was considered the product of two functions: one depending on the temperature [D(T)] and the other depending on the composition [B(Φ₂)]. For mixtures without a reaction, the cloud‐point curves showed upper critical solution temperature behavior, and the dependence of χ(T,Φ₂) on the composition was determined from the threshold point, that is, the maximum cloud‐point temperature. During the isothermal reactions of mixtures with different initial PS concentrations, the dependence of χ(T,Φ₂) on the composition was determined under the assumption that, at each conversion level, the D(T) contribution to the χ(T,Φ₂) value had to be constant independently of the composition. For these mixtures, it was demonstrated that the changes in the chemical structure produced by the epoxy–amine reaction reduced χ(T,Φ₂). This effect was more important at lower volume fractions of PS. Nevertheless, the decrease in the absolute value of the entropic contribution to the free energy of mixing was the principal driving force behind the phase‐separation process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1351–1360, 2004     

Recognizing Kekuléan benzenoid systems byC-P-V path elimination

In this paper, we define the concept of a canonicalP-V pathP(p _i– _i ) on the boundary of a benzenoid systemH, and prove thatH has a Kekulé structure if and only ifH-P(p _i–v _i) has a Kekulé structure, whereH-P(p _i–v _i) is the graph obtained fromH by deleting the vertices onP(p _i–v _i) . It is also proved that there are at least two canonicalP-V paths in a benzenoid system. By the above results, we give an efficient and simple algorithm, called the canonicalP-V (C-P-V) path elimination, for determining whether or not a given benzenoid systemH has Kekulé structures. IfH is Kekuléan, the algorithm can find a Kekulé structure ofH.Supported by NSFC.     

Structural characterization, thermal, dielectric, vibrational properties and molecular motions in

[C₃N₂H₅]₆[Bi₄Br₁₈] has been synthesized and characterized by the X-ray (at 293 and 110 K), calorimetric, dilatometric and dielectric measurements. At room temperature it crystallizes in the monoclinic space group, C2/m. A crystal structure consists of disordered imidazolium cations and ordered discrete tetramers of [Bi₄Br₁₈]^6-. This compound reveals a rich polymorphism in a solid state. It undergoes three solid–solid phase transitions: from phase I to II at 426/423 K (heating–cooling), II→III at 227 K and III→IV at 219.5/219 K. A clear dielectric relaxation process is found in the room temperature phase II. Infrared studies of the polycrystalline [C₃N₂H₅]₆[Bi₄Br₁₈] showed that the ν(N–H), δ(ring) and δ(C–H) modes of the imidazolium cations appeared to be very sensitive to the IV→III phase transition. ¹H NMR measurements confirmed a key role of the imidazolium cations in the phase transitions mechanisms at low temperatures.     

Synthesis,crystal and molecular structure and magnetic properties of an extended two-dimensional fumarato-bridged polymeric cobalt(II) complex [Co(μ-fumarato)(γ-methylpyridine)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>]<Subscript>∞</Subscript>

An extended hydrogen-bonded two-dimensional fumarato-bridged complex [Co(μ-fumarato)(γ-methylpyridine)₂(H₂O)₂]_∞ has been synthesized and its crystal structure determined by single crystal X-ray analysis. The X-ray analysis reveals that the cobalt atom is positioned in distorted octahedral surroundings. An extended twodimensional structure was formed through interchain hydrogen bonding. Magnetic measurements showed the presence of weak antiferromagnetic exchange interactions between the cobalt(II) ions within the chain, based on the spin Hamilitonian H = −2J ∑ S_i S_{i+ 1} (J = − 0.22 cm⁻¹).     

Thermodynamic properties of the first to fifth generations of carbosilane dendrimers with allyl terminal groups

Temperature dependences of the specific heats, characteristic temperatures, and enthalpies of physical transformations of the first to fifth generations of carbosilane dendrimers with allyl terminal groups were studied using an adiabatic vacuum calorimeter in the temperature range 6—340 K. The error of measurements was, as a rule, about 0.2%. Thermodynamic characteristics of physical transformations of the dendrimers were determined and their thermodynamic functions C _p°(T), H°(T)—H°(0), S°(T)—S°(0), and G°(T)—H°(0) were calculated for the temperature range 0—340 K. The thermodynamic functions of the dendrimers are linearly related to their molecular weights, the number of allyl groups on their outer spheres, and the number of moles of diallylmethylsilane per mole of the dendrimers formed. Additive dependence of the properties of the dendrimers on their chemical composition and structure indicates that the energy of interaction between structural fragments of the dendrimers is independent of the dendrimer generation number. The fractal dimensions, D, of all dendrimers studied in this work are 1.2—1.3 in the temperature range 30—50 K, thus indicating a chain-layered structure of the dendrimer glasses.     

The Superstructure of Semiconducting SmTe2−x

SmTe_1.84was synthesized and the crystal structure was studied by the single-crystal technique. The substructure was isostructural with LaTe_2−x, where corrugated rock salt LaTe slabs alternate with planar tellurium square lattices. The substructure of SmTe_1.84is tetragonal anti-Cu₂Sb type and the superstructure is √5×√5 of the tetragonal subcell. The superstructure is tetragonal, withP4₂/nsymmetry,a=9.709(1) Å andc=18.007(7) Å. There are both ordered and disordered defects in the Te sheet. The superstructure obtained consists of the three possible stable solutions suggested by Lee and Foran, and all three solutions were found in a single crystal. The resistivity dependence on temperature indicates that SmTe_1.84is semiconducting, which seems due to structural modulation. The structural stability of the other phases of SmTe_n(n=1–2) is discussed in terms of temperature and ionic radius ratio.     

Recombination of HCO and DCO ions with electrons

Recombination of HCO⁺ and DCO⁺ ions with electrons was studied in afterglow plasma. The flowing afterglow with Langmuir probe (FALP) apparatus was used to measure the recombination rate coefficients and their temperature dependencies in the range 150–270 K. To obtain a recombination rate coefficient for a particular ion, the dependencies on partial pressures of gases used in the ion formation were measured. The variations of α_HCO⁺(T) and α_DCO⁺(T) seem to obey the power law: α_HCO⁺(T) = (2.0 ± 0.6) × 10⁻⁷ (T/300)^−1.3 cm³ s⁻¹ and α_DCO⁺(T) = (1.7 ± 0.5) × 10⁻⁷ (T/300)^−1.1 cm³ s⁻¹ over the studied temperature range.     

Critical point of the competition between different orbital- and Bi lone pair-orderings in the Bi-rich part of the BiCaMnO system

The present paper deals with the evidence of an unprecedented ordering in the Bi-rich part of the BiCaMnO diagram, where the spin glass state is observed at low temperature. The χ⁻¹(T) curves of three compounds, corresponding to different Ca/Bi + Ca ratio varying between 0.37 and 0.25, have been characterized and discussed in correlation with the structural transitions as a function of T. Three different behaviours have been observed for Bi_0.63Ca_0.37MnO₃, Bi_0.67Ca_0.33MnO₃ and Bi_0.71Ca_0.24MnO_3−δ. The anomalies in the χ⁻¹(T) curves of Bi_0.71Ca_0.24MnO_3−δ and Bi_0.63Ca_0.37MnO₃ appear to be associated to two different mechanisms, which are not observed in the intermediate Bi_0.67Ca_0.33MnO₃. For the Bi-rich compound, an unprecedented [111]_p-type ordering of the perovskite cell is reported, which is associated to a change of the slope in the magnetic curve. Bi_0.71Ca_0.24MnO_3−δ appears at the convergence point of a complex competition between the complementary lone pair ordering/orbital ordering of the ferromagnetic BiMnO₃ and the charge ordering/orbital ordering of the pseudo CE antiferro-magnetic Bi_0.63Ca_0.37MnO₃.     

Sodium and silver phosphate glasses doped with chlorides of Fe,Mn and Zn

A number of samples of sodium and silver phosphate glasses doped with various compositions of some transition metals viz. iron, manganese and zinc chlorides alongwith undoped samples of sodium and silver phosphate glasses were synthesized and characterized by X-ray diffraction, IR spectral, electrical conductivity and differential scanning calorimetry (DSC). The glass transition temperature (T _g) and crystallization temperature (T _c) values obtained from DSC curves were found to increase with increasing concentration of the dopant Fe/Mn/Zn chlorides in both sodium and silver phosphate glasses and the following sequence is observed: T _g(–FeCl₃)>T _g(–MnCl₂)>T _g(–ZnCl₂) T _c(–FeCl₃)>T _c(–MnCl₂)>T _c(–ZnCl₂) The increase in T _g and T _c values indicate enhanced chemical durability of the doped glasses. The electrical conductivity values and the results of FTIR spectral studies have been correlated with the structural changes in the glass matrix by the addition of different transition metal cations as dopants.