Dynamical stability and high pressure phases of platinum carbide

Using the density-functional linear response method, we study the dynamical properties of ground state zinc-blende and high pressure NaCl phases of platinum carbide (PtC). The calculated phonon dispersion curve does not show any soft modes for all wave vectors, indicating the dynamic stability of the ground state zinc-blende phase. The high pressure rock-salt phase exhibits imaginary frequencies, practically along all directions of the Brillouin zone, which means that PtC cannot exist in the NaCl phase at least up to a high pressure of 100 GPa.     

Calorimetry in nonstandard conditions: The noncrystalline phases of linear polyethylene

A linear Union Carbide PE (UC) has been analyzed by nonstandard calorimetry with a common DSC calorimeter and a Setaram C80 calorimeter. Nonstandard calorimetry entails using a low rate of heating (0.5–1 K/min), a small mass (0.5–1.5 mg), and an open cell (O‐cell) instead of the standard C‐cell. Events in O‐cells overlap less and occur with a faster kinetics than in C‐cells. PE crystals are nascent, solution‐grown (S‐grown), press‐grown (P‐grown), and strained by extrusion. In Part A, the traces show that the phase‐changes in the melt, previously observed in a C80 calorimeter (slow T‐ramp) and characterized by ΔH_network, can be observed with a common DSC in nonstandard conditions. In Part B, the difference between the C‐ and O‐cells and the changes in the main peak enthalpy (ΔH_ortho) are of interest. The main result is that, in O‐cells, the value of ΔH_ortho around T_ortho, exceeds unambiguously in certain conditions ΔH_ortho found for perfect orthorhombic crystals. The main endotherm contains then another contribution, namely ΔH_network. Crystal reorganization during the slow T‐ramp is followed in the C‐ and O‐cells on S‐grown crystals. In O‐cells, lamellar thickening observed in the slow‐ramp is more extensive. The ease of phase‐change depends on the sample history. It is as follows: strained‐part extruded > nascent > S‐grown > P‐grown. Co‐operative chain motions are more hindered in the standard C‐cells than in the O‐cells. In Part C, lower values of m succeed in bringing phase‐changes in P‐grown (O‐cells) samples. The origin of the events is discussed: three processes are thought to contribute to the phase‐changes namely, melting of strained short‐range order, activation of vibrations in the CH₂ groups, and fast decay of chain orientation which occurs simultaneously with melting. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1932–1949, 2007     

Electronic properties and band structure of IrSe2

Transport coefficient measurements (DC conductivity and thermoelectric power) were performed on compacted bars of polycrystalline IrSe₂ in the 100–570 K temperature range. The experimental results are interpeted on the basis of an n-type compensated semiconductor model. At low temperatures, a narrow band which originates from non-stoichiometry mainly participates in the conduction by thermally activated hopping of small polarons. The charge balance adopted by anionic groups is discussed. The schematic band model which is retained takes into account all the transport properties and the ion charge balance Ir³⁺Se^2-(Se₂)^2-_{1/ 2}. Moreover, IrSe₂ has recently been shown to function as a cathode material in lithium batteries.     

Fundamental and transport properties of ZnX,CdX and HgX (X = S,Se, Te) compounds

We have performed ab initio self-consistent calculations based on full potential linear augmented plane-wave method (FP-LAPW) with the local density approximation (LDA) and generalised gradient approximation (GGA) to investigate the relativistic effects on the structural, electronic, transport and optical properties of II–VI compounds. We mainly show that the stabilisation (destabilisation) of s, p*(p) orbital energies reduces the lattice parameters of II–VI compounds, the band gaps and the effective masses. This, however, induces strong spin–orbit splitting of heavier II–VI compounds.     

High pressure structural phase transitions of PbPo

First-principles calculations have been performed to investigate the high pressure phase transitions and dynamical properties of the less known lead polonium compound. The calculated ground state parameters for the NaCl phase show good agreement with the experimental data. The obtained results show that the intermediate phase transition for this compound is the orthorhombic Pnma phase. The PbPo undergoes from the rocksalt to Pnma phase at 4.20 GPa. Further structural phase transition from intermediate to CsCl phase has been found at 8.5 GPa. In addition, phonon dispersion spectra were derived from linear-response to density functional theory. In particular, we show that the dynamical properties of PbPo exhibit some peculiar features compared to other III–V compounds. Finally, thermodynamics properties have been also addressed from quasiharmonic approximation.     

Some characteristic behavior of mixed spin-1/2 and spin-1 Ising nano-tube

The magnetization of a cylindrical Ising nano-tube is investigated by the use of the effective field theory with correlations. The effects of the crystal field couplings at the surface shell to the order parameters, susceptibility, internal energy, specific heat and free energy are investigated. Some characteristic phenomena are examined in the thermal variations, depending on crystal field term. Moreover, tricritical and critical points are found on the (D/J,kT/J) plane, where D/J and kT/J are reduced crystal-field and temperature, respectively.     

A new technique to characterize mono-molecular micelles in random ethylene–propylene copolymers

A new technique to investigate the nano-structure of ethylene–propylene (EP) random copolymers has been developed. It consists in the measurement of the turbidity which develops at a lower critical solution temperature (LCST) in pentane solutions. The information on the solution comes from different types of turbidity obtained during a step-by-step temperature increase. The transient turbidity (h_i) is associated with random coils (I) and structured coils (II) while the stable turbidity comes from aggregates (III). The proportion of (I), (II) and (III) depends on the solution history and on the solvent. The M_w distribution can be obtained from the set h_i (T_i) of (I). Turbidity (II) has an unexpected gap in the h_i (T_i) trace. The gap (10–20 K) is explained by the presence of two entities in solution. Their temperatures of phase separation permit their identification as monomolecular micelles, whose outer core is either E-rich or P-rich. This nano-structure is thought to exist in the solid and also in solution as a metastable state. The technique can differentiate between mobile chains in solutions (I, II) and attached chains in a network (III) through the sedimentation behaviour of the concentrated phase. Three samples with a similar (EP) content (0.75) made with different catalysts have been analysed by LCST and slow calorimetry.     

Isolation and X-ray structure analysis of citreohybridonol from marine-derived Penicillium atrovenetum

The anti-neuroinflammatory meroterpenoid citreohybridonol was isolated for the first time from a sponge-derived fungus Penicillium atrovenetum. In this study, in addition to isolation and structure featuring, its unambiguous absolute configuration was determined exclusively by single crystal X-ray diffraction. The C-17-keto tautomer was clearly observed in X-ray analysis. The substance crystallises in the monoclinic space group P2₁ with a = 10.7496(5) Å, b = 14.3286(7) Å, c = 17.4909(8) Å, β = 103.235(2)°, V = 2622.5(2) Å³, Z = 2, D_calcd = 1.280 g/cm³. The chirality of the asymmetric carbon atoms was as follows: C3 (S), C5 (R), C6 (S), C8 (S), C9 (R), C10 (R), C13 (R), C14 (R).     

Electrical properties in compressed poorly crystalline apatite

A poorly crystallized apatite is prepared by co-precipitation at room temperature and dried by lyophilization. The electrical properties, in terms of ionic conductivity, improve after a second compression of the sample under 15 t/cm². Characterization by X-ray diffraction, infrared spectroscopy and conductivity spectra show that this improved conductivity following the compression processing could be related to the establishment of order within the apatitic network. Electronic Publication     

Notable chenodeoxycholic acid oligomers—synthesis, characterization, and 7α-OR steric hindrance evaluation

The steric hindrance of the 7α-OR group in bile acid derivatives is very different from the 12α-OR group. The effect of this difference in steric hindrance on the synthesis of cyclocholates has been evaluated with support from AM1 and MM2 calculations. The ¹³C NMR parameters for cheneodeoxycholic acid-based cyclocholates have been determined.