On the pressure dependence and the Grüneisen parameters of the B1u-lattice vibration in high density polyethylene

Far infra-red spectroscopic measurements have been made on a high density (0.973 g cm?3) sample of linear polyethylene to determine the frequency of the zone-centre B1u translational lattice mode (vibrating in the b-axis direction) as a function of pressure up to 7 kbar. The variation of frequency over this range is found to exceed, considerably, its total variation with temperature up to the melting point. These measurements and X-ray data from the literature give a volumetric mode Grüneisen parameter γB1uV varying from 3.15 at low pressures to about 5.5 at 7 kbar. Theoretical calculations using the potential functions of Hägele et al. give, for the special (anisotropic) Grüneisen mode parameters, γB1ua = 6.1, γB1ub = 2.2, and predict a pressure variation of $\text{v}\text{?}\text{(B}{}_{\mathrm{1u}}\text{)}$ similar to but greater than that observed experimentally. The results are compared with other data in the literature.     

First‐Principle Calculation and Assignment for Vibrational Spectra of Ba(Mg1/2W1/2)O3 Microwave Dielectric Ceramic

Ba(Mg_1/2W_1/2)O₃ ceramic was synthesized using a conventional solid‐state reaction method at 1500°C for 4 h. The face‐centered cubic crystal structure of the material was confirmed by Rietveld refinement of X‐ray diffraction (XRD) data, and vibrational modes were obtained by Raman and Fourier transform far‐infrared (FTIR) reflection spectroscopies. First‐principle calculations based on density functional theory with local density approximation were used to calculate Gamma‐point modes and dielectric properties of Ba(Mg_1/2W_1/2)O₃. The Raman spectrum with nine active modes can be fitted with Lorentzian function, and the modes were assigned as F_2g⁽¹⁾ (126 cm^?1), F_2g⁽²⁾ (441 cm^?1), E_g(O) (538 cm^?1), and A_1g(O) (812 cm^?1). Far‐infrared spectrum with 12 infrared active modes was fitted using both the Lorenz three‐parameter classical and four‐parameter semiquantum models. Consequently, the modes were assigned as F_1u⁽¹⁾ (144 cm^?1), F_1u⁽²⁾ (284 cm^?1), F_1u⁽³⁾ (330–468 cm^?1), and F_1u⁽⁴⁾ (593–678 cm^?1). The active modes were represented by linear combinations of symmetry coordinates that were obtained by group theory analyses. The Raman mode A_1g, which has the highest wave number (812 cm^?1) is dominated by the breath vibration of the MgO₆ octahedron. The infrared modes F_1u⁽²⁾, that can be described as the inverted vibrations of Mg atoms in the MgO₆ octahedron along the x_i, y_i, and z_i axes have the most contributions to the microwave permittivity and dielectric loss.     

Pulsed cathodoluminescence of two-alkali sodium potassium silicate glasses

Alkali silicate glasses of variable composition 22xNa₂O · 22(1?x)K₂O · 3CaO · 75SiO₂ with equimolecular replacement of sodium ions by potassium ions are investigated using pulsed cathodoluminescence. It is revealed that localized electronic states interact with vibrations of two types, namely, polarization vibrations of the silicon-oxygen network with the frequency v₀ = 820 cm^?1 and bending vibrations of the modifier sublattice. At low concentrations of one of the alkali components (x < 0.1), bending vibrations are observed at two frequencies. These frequencies coincide with those of the corresponding vibrations in one-alkali systems containing Na (530 cm^?1) and K (520 cm^?1). At higher concentrations (x in the range ～0.14–0.86), there occur bending vibrations of the cationic sublattice with a frequency of 420 cm^?1. This can be interpreted as a luminescence analog of two-alkali (mixed-alkali) effect.     

Investigation of layered perovskite NdBa0.5Sr0.25Ca0.25Co2O5+δ as cathode for solid oxide fuel cells

Layered perovskite oxides with and without Ca-doped NdBa_0.5Sr_0.25Ca_0.25Co₂O_5+δ (NBSCaCO) and NdBa_0.5Sr_0.5Co₂O_5+δ (NBSCO) are studied to investigate the effects of Ca doping on the crystal structure, thermal behavior, electrical and electrochemical properties. Both NBSCO and NBSCaCO are tetragonal structure with P4/mmm space group. The average thermal expansion coefficient (TEC) value is reduced from 23.3?×?10^?6 K^?1 to 19.8?×?10^?6 K^?1 during 30–800?°C. The electrical conductivities are increased by Ca doping. Both electrical conductivities of NBSCO and NBSCaCO are higher than 600?S·cm^?1 over 30–800?°C. Substitution of Sr with Ca can effectively improve the electrochemical properties of NBSCaCO. From 650?°C to 800?°C, the area specific resistance (ASR) of NBSCaCO are decreased from 0.62 to 0.062?Ω?cm² and the corresponding output power density are increased from 258 to 812?mW?cm^?2. On the basis of these results, Ca doped layered perovskite NBSCaCO can be a good cathode candidate material for SOFC application.     

Raman spectra of ground natural graphite

Structural changes in Ceylon natural graphite with grinding were studied by Raman spectroscopy along with X-ray diffraction. The natural graphite shows a single Raman band at 1580 cm^?1, but the ground graphite samples exhibit two Raman bands at 1360 and 1620 cm^?1 in addition to the 1580 cm^?1 graphite band. The 1360 cm^?1 band increases in intensity with increasing grinding time, and becomes much stronger than the 1580 cm^?1 band after 200-hr grinding. Raman results are compared with structural parameters such as effective Debye parameter and C₀ spacing obtained from X-ray diffraction measurements, and discussed in terms of structural defects introduced into the crystal lattice of natural graphite. A linear relationship was obtained for the ground graphite when the relative intensity of the 1360 cm^?1 band is plotted as a function of effective Debye parameter. The slope of the linear plot is different for the ground graphite from that for the graphitized cokes, indicating a difference in the type of structural defects involved.     

Higher permittivity of Ni-doped lead zirconate titanate,Pb[(Zr0.52Ti0.48)(1-x) Nix]O3, ceramics

The paper reports highest obtained dielectric constant for Ni-doped Lead Zirconate Titanate [PZT, Pb(Zr_0.52Ti_0.48)O₃] ceramics. The Ni-doped PZT ceramic pellets were prepared via conventional solid-state reaction method with Ni content chosen in the range 0–20?at%. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy were employed to investigate the crystal structure of the prepared ceramics. The X-ray diffraction analysis indicated that the ceramic pellets had crystallized into tetragonal perovskite structure. A minute displacement of XRD peaks was detected in the diffraction spectra of Ni-doped PZT ceramic samples which when examined by size-strain plot (SSP) method revealed presence of homogenous strain that decreased with increase in concentration of Ni. In FTIR the maximum absorption at 597?cm^?1, 608?cm^?1, 611?cm^?1, 605 and 613?cm^?1 for Ni?=?0, 5, 10, 15 and 20?at%, respectively, confirmed the formation of perovskite structure in all the compositions and the slight shift suggests decrease in cell size on doping. The values of dielectric constant (ε′) & tanδ as a function of frequency and temperature were measured for the prepared ceramics and it revealed highest ever reported dielectric constant for Ni - doped PZT with Ni?=?5?at%. The dielectric variation with temperature exhibited a diffused type ferroelectric–paraelectric phase transition for the doped samples. Also, the maximum dielectric constant value (ε′_max) decreased while the phase transition temperature increased with increase in doping concentration of Ni. The estimated activation energy of different compositions was found to increase from 0.057 to 0.068?eV for x?=?0.00 to x?=?0.20 in ferroelectric phase. The piezoelectric, ferroelectric and magnetic properties were also investigated.     

Low frequency excitation in amorphous acrylic polymers

Low frequency Raman depolarized spectra from poly(methyl methacrylate) have been determined at temperatures between 293 K and 85 K in the frequency region $\text{5}\text{cm}{}^{\mathrm{?1}}\text{<}\text{v}\text{?}\text{<2000}\text{cm}{}^{\mathrm{?1}}$. All the reduced intensity spectra resolve into two bands: a broad and intense band at 91 cm^?1 and a narrower and weaker band at 22 cm^?1. The Raman intensities do not depend on temperature which implies first order Raman scattering. Room temperature low frequency Raman spectra were also recorded from poly(ethyl methacrylate) and poly(-n-butyl methacrylate). In the two PMMA substitutes the high frequency band is shifted to lower values as the side group mass is increased and show a Debye-like character. The lower frequency band cannot be resolved in the spectrum of PEMA and PnBMA. The two bands at 91 cm^?1 and 22 cm^?1 are attributed to side group and backbone motions respectively. By studying PMMA samples of different tacticity no dependence of the band frequency on chain configuration has been detected. The density of states g(Ω) obtained through the low frequency Raman spectrum has been used to calculate the specific heat C_v. The values of C_v obtained are lower than those measured calorimetrically but varied with temperature in a similar manner in the region 0.8<T<4K.     

Crystal structure and microwave dielectric properties of NaPb2B2V3O12(B=Mg,Zn) ceramics

Two low temperature sintered NaPb₂B₂V₃O₁₂ (B?=?Mg, Zn) ceramics with garnet structure were synthesized through conventional solid state reaction route and their crystal structure and microwave dielectric properties were investigated for the first time. Rietveld refinements of XRD patterns show both the compounds belong to cubic symmetry with space group Ia-3d. Observed number of Raman bands and group theoretical predictions also confirm cubic symmetry with space group Ia-3d for both NPMVO and NPZVO. At the optimum sintering temperature of 725?°C NPMVO has a relative permittivity of 20.6?±?0.2, unloaded quality factor (Q_uxf) of 22,800?±?1500?GHz (f?=?7.7?GHz) and temperature coefficient of resonant frequency +25.1?±?1?ppm/°C while NPZVO has relative permittivity of 22.4?±?0.2, Q_uxf of 7900?±?1500?GHz (f?=?7.4?GHz) and near zero temperature coefficient of resonant frequency of -6?±?1?ppm/°C at 650?°C. The relative permittivity of the compounds is inversely related to the corresponding Raman shifts.     

Raman spectroscopy, 19F and 31P MAS-NMR of a series of fluorochloroapatites

We report the Raman spectra of a series of fluorochloroapatites Ca₅(PO₄)₃F_1?xCl_x, where x = 0.0, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 0.9 and 1.0 (i.e. from pure fluorapatite to chlorapatite). The series did not appear to exhibit any immiscibility, however peak broadening and additional Raman and NMR spectral features on chlorine addition could be interpreted as reduction in symmetry and ordering in the ion channels. All Raman bands showed significant broadening with chlorine substitution, indicating disordering of the crystal lattice or ordering into fluorine and chlorine rich environments. There was also a general trend of the Raman bands to shift to lower wavenumber linearly with chlorine substitution with the ν₁ phosphate band decreasing from 966 cm^?1 for x = 0.0 to 961 cm^?1 for x = 1.0. The full width half maximum (FWHM) of this band increases linearly with chlorine addition from 5 cm^?1 for x = 0.0 to 10 cm^?1 for x = 1.0. The shift in a component in the ν₃ phosphate band with chlorine addition at around 1035 cm^?1 agrees with data in the literature on chlorine containing geological apatites. An additional component was seen at around 586 cm^?1 in the ν₄ phosphate region for chlorapatite and the area of this band decreased linearly to zero as fluorine replaced chlorine. This could be due to E_g symmetry phonons splitting into A_g and B_g modes due to a symmetry change such as hexagonal to monoclinic with chlorine addition. ¹⁹F magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra showed a shift to lower ppm values with chlorine addition and a broadening of resonances, consistent with the Raman data. The ³¹P spectra developed additional shoulders (at around 2 and 4 ppm) with chlorine addition with the main peak position (3.3 ppm for x = 0) decreasing for compositions moving away from x = 0.5 indicating maximum phosphorous nuclear shielding occurs at an approximate F:Cl ratio of 1:1. This discontinuity is a possible indication of a structural transition at x = 0.5 related to local short scale phosphate order ? disorder or a change in crystal symmetry.     

Faradaic rectification study of the metal/metal ion reactions

The kinetic parameters for Ag⁺/Ag and Cu²⁺/Cu reactions at the equilibrium potential and in the absence of dc polarisation have been obtained using the faradaic rectification method at audio frequencies. The values of transfer coefficients, ion exchange current densities and apparent rate constants, obtained for the two reactions at 27°C using 1·0mM of each of the Ag⁺ and Cu²⁺ in 1·0M KNO₃, are respectively 0·22; 7·3mA/cm²; 3·6·10^?3 cm/s and 0·45; 10·7 mA/cm²; 1·1×10^?4cm/s. These data are comparable to those reported in the literature. For obtaining reliable and reproducible results for the studies with metal/metal ion reactions suitable experimental conditions have been described.     

On the ionic conductivity of some zirconia-derived high-entropy oxides

The reported High Entropy Oxides (HEOs) up to now exhibit lower ionic conductivity values than those of classical SOFC electrolytes. Multi-cations oxides, stabilized with the fluorite-type structure are investigated here in order to examine whether the high entropy is relevant to enhance the anionic conductivity of such HEOs, or not. The two synthesis routes that are used do not show significant impact on material properties. Based on configurational entropy (≥ 1.5 k_B/f.u. for HEOs) and ionic radius difference calculations, new compositions are designed and prepared: i) the (Hf_1/3Ce_1/3Zr_1/3)_1-x(Gd_1/2Y_1/2)_xO_2-x/2 series in which the x ratio is increased so as to promote a high vacancy concentration, ii) the (Hf_xCe_yZr_1-x-y)_0.85Yb_0.15O_1.93 series based on the critical radius concept. The ionic conductivity of these HEOs is slightly improved compared to previously reported data but does not exceed 4 × 10^?4 S.cm^?1 at 600 °C. Possible causes of such a low ionic conductivity value are discussed.     

Raman and dielectric spectra of the glass and single crystal of Li2Ge7O15 in the frequency range 3–1000 cm−1: I. Comparison of the structures of the crystal and initial glass

The Raman and dielectric spectra of vitreous and single-crystal lithium heptagermanates are measured over a wide range of frequencies from 3 to 1000 cm^?1. It is revealed that the spectra of the Li₂O · 7GeO₂ glass and the Li₂Ge₇O₁₅ crystal are similar to each other because the intense but broad bands in the spectrum of the glass correspond to the most intense groups of bands in the spectrum of the crystal. The similarity is observed over the entire frequency range, including the range of the boson peak. This indicates that the shortrange orders in the structures of the glass and the crystal are similar to each other and that the medium-range order in the glass resembles the main structural motif of the crystal.     

Observation of specific optical phonon modes dominating Li ion diffusion in γ-LiAlO2 ceramic

In γ-LiAlO₂ ceramic, Li ion diffusion plays a key role both in tritium release as a solid tritium breeder material in nuclear fusion and in phase stability as a matrix material in molten carbonate fuel cells (MCFC). Yet fundamental understanding of the diffusive process in γ-LiAlO₂ is still missing, especially considering the interaction of the lattice dynamics and Li ion motion. Herein, we demonstrated that two specific optical phonon modes are coupled with Li ion diffusion in γ-LiAlO₂, by investigating the temperature-dependent lattice dynamics via Raman scattering experiments, as well as first-principles calculations and neutron diffraction experiments (ND). The high-temperature ND experiments showed that γ-LiAlO₂ undergoes partial Li ion disordering in Li sublattice at high temperature. Notably, the lattice dynamics studies demonstrated that B₁ mode at 226 cm^?1 and A₁ mode at 263 cm^?1 are responsible for Li ion motion through the oscillation of Li–O in <010> and <001> direction, respectively. Our results further suggest that the behavior of tritium release and phase stability in γ-LiAlO₂ at elevated temperature may be controlled by tuning the two specific phonon modes through photon/electron-phonon coupling.     

Investigation of Raman Modes and Born‐Effective Charges in AgNb1/2Ta1/2O3: A Density‐Functional and Raman Scattering Study

Using ab initio density‐functional theory, the Born‐effective charge tensors and zone‐center phonon mode frequencies are computed for AgNb_1/2Ta_1/2O₃ in monoclinic P2/m and orthorombic Pbcm symmetries. The experimental mode frequencies are obtained from deconvolution of Raman spectrum of prepared AgNb_1/2Ta_1/2O₃ samples and are compared with computed mode frequencies. The Raman modes with high (>350 cm^?1) and low frequencies (<90 cm^?1) correspond to Ag and O vibrations, respectively. The modes in intermediate frequency band (120–350 cm^?1) are dominated by Nb(Ta)–O vibrations. The computed effective charge tensors of cations at A (Ag) and B (Nb, Ta) sites are found to be diagonal. The off‐diagonal components of charge tensor are found sizeable only for O ions in orthorhombic AgNb_1/2Ta_1/2O₃ with Pbcm symmetry. Further, charge tensor structure of O ions is found to depend on site symmetry in the unit cell. Charge tensor components for Nb, Ta, and O ions differ significantly from their nominal ionic values suggesting (1) large local dipole moments induced by off‐centering of Nb(Ta) ions and tilting(rotations) of Nb(Ta)O₆ octahedra, (2) hybridization between d‐orbitals of Nb(Ta) and p‐orbitals of O atoms. Furthermore, the electronic structure, directional dependence of effective charges and performance of LDA (GGA) exchange‐correlation functionals with regard to computed values are also discussed.     

A new far infra-red absorption band at 39 cm−1 in the spectrum of high-pressure crystallized polyethylene

High-pressure annealed samples of polyethylene (PE), currently thought to contain the extended chain form, show a new band at 39 cm^?1 in the absorption spectrum when the sample temperature falls below 170K. This band shows a strong temperature dependence of intensity but no shift in frequency as the temperature varies. Possible assignments of the band are discussed. The preferred interpretation, with some reservations, is that it arises from a mode analogous to the forbidden A_u mode of orthorhombic PE but which is now active in the monoclinic form produced by the annealing.     

Fourier-transform infrared assay of bile salt-stimulated lipase activity in reversed micelles

A Fourier-transform infrared (FT-IR) spectroscopic method has been developed for assaying the bile salt-stimulated human milk lipase (BSSL, EC3.1) catalyzed hydrolysis of triolein in AOT reversed micelles in iso-octane. At 37°C in 50 mmol dm^?3 AOT the molar absorbtivities for the carbonyl stretching frequencies for triolein (at 1751 cm^?1) and oleic acid (at 1714 cm^?1) were 1646 dm³ mol^?1 cm^?1 and 743 dm^?3 mol^?1 cm^?1, respectively. The rate was linearly dependent upon the concentration of enzyme in the water pool up to 10 mg cm^?3 and maximum activity was observed at a ratio (w₀) of [H₂O]:[AOT] = 16·7. Using these conditions, and in the presence of 10 mmol dm^?3 sodium taurocholate (TC), the derived Michaelis–Menten parameters V_max and K_m were 57·5 μmol min^?1 mg^?1 and 5·53 mmol dm^?3, respectively. These results are compared with those obtained in an oil-in-water microemulsion system and are discussed in terms of the relative partitioning of the enzyme and the substrate in the aqueous and oil phases and the interfacial concentration of the substrate in the two systems.     

Electrical properties of nanoporous F doped SiO2 low-k thin films prepared by sol-gel method with catalyst HF

Using hydrofluoric acid as acid catalyst, F doped nanoporous low-k SiO₂ thin films were prepared through sol-gel method. Compared with the hydrochloric acid catalyzed film, the films showed better micro structural and electrical properties. The capacitance-voltage and current-voltage characteristics of F doped SiO₂ thin films were then studied based on the structures of metal-SiO₂-semiconductor and metal-SiO₂-metal, respectively. The density of state (DOS) of samples deposited on metal is found to decrease to a level of 2 × 10¹⁷ eV^?1 cm^?3. The values of mobile ions, fix positive charges, trapped charges and the interface state density between the SiO₂/Si interfaces also decrease obviously, together with the reduction of the leakage current density and the dielectric constant, which imply the improvement of the electrical properties of thin films. After annealing at a temperature of 450^°C, the lower values of the leakage current density and dielectric constant could be obtained, i.e. 1.06 × 10^?9 A/cm² and 1.5, respectively.     

Infrared spectral studies of nanostructured Co2+-substituted Li-Ni-Zn ferrites

Polycrystalline Li_0.5Ni_0.25–0.5x Co_0.5x Zn_0.5Fe₂O₄ ferrites (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were prepared by standard auto-combustion method and characterized by XRD and IR spectra. The XRD results suggest that all samples were single-phase with a cubic spinel-type structure. The lattice constant, particle size, metal-oxygen bond length (R _A and R _B), cation radii (r _A and r _B) on A and B-sub lattices were found to increase while the X-ray density and porosity, decrease with increasing x. The IR spectra show two absorption bands, at ν₁ about 600 cm^?1 and ν₂ at about 425 cm^?1, which were attributed to tetrahedral (A) and octahedral (B) sites of the spinel structure, respectively. The band positions were found to depend on x. The force constants, K _t and K _o, were calculated and plotted against cobalt concentration x. Compositional dependence of the force constants is explained in terms of cation-oxygen bond distances and redistribution of cations.     

Influence of doping with various cations on electrical conductivity of apatite-type neodymium silicates

Apatite-type neodymium silicates doped with various cations at the Si site, Nd₁₀Si₅BO_27?δ (B=Mg, Al, Fe, Si), were synthesized via the high-temperature solid state reaction process. X-ray diffraction and complex impedance analysis were used to investigate the microstructure and electrical properties of Nd₁₀Si₅BO_27?δ ceramics. All Nd₁₀Si₅BO_27?δ ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd₂SiO₅. Neodymium silicates doped with Mg²⁺ or Al³⁺ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd₁₀Si₆O₂₇ ceramic at all temperature levels. However, doping with Fe³⁺ cations at the Si site has a little effect on improving the total conductivity above 873 K. The enhanced oxide-ion conductivity in a hexagonal apatite-type structure depends upon the diffusion of interstitial oxide-ion through oxygen vacancies induced by the Mg²⁺ or Al³⁺ substitution to the Si⁴⁺ site and through the channels between the SiO₄ tetrahedron and Nd³⁺ cations. At 773 K, the highest total conductivity is 4.19×10^?5 S cm^?1 for Nd₁₀Si₅MgO₂₆ ceramic. At 1073 K, Nd₁₀Si₅AlO_26.5 silicate has a total conductivity of 1.55×10^?3 S cm^?1, which is two orders of magnitude higher than that of undoped Nd₁₀Si₆O₂₇.     

Critical current density and mechanical performance of MgB2 superconductors prepared with different magnesium sources

In this study, the critical current density and mechanical performance of MgB₂ superconductors prepared using different magnesium sources (Mg and MgH₂ with 99.9% and 96.5% purities, respectively) were analyzed. When the samples were characterized structurally and electrically, we found that the grain sizes were about 40?nm and transition temperature was around 30?K being lower than the literature. Critical current densities of all samples were calculated using Bean's model and our calculations yielded critical current density values higher than 10⁶?A?cm^?2 in self-field. The highest critical transition temperature value belongs to M800 (Mg with 99.9% purity) sample and when the critical current density and transition temperature values of this sample are considered, this sample appears to have the potential for practical use. Vicker's microhardness measurements were performed and yield stress (Y), elastic modulus (E), brittleness (B_i), ductility (D) and fracture toughness (K_IC) were calculated. All samples exhibited Indentation Size Effect (ISE) and microhardness measurements were compared with some microhardness models with were compatible with ISE behavior.