Synthesis of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al₂O₃ substrate, ZnAl₂O₄/α-Al₂O₃ complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl₂O₄ changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl₂O₄ transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl₂O₄ prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl₂O₄/α-Al₂O₃ substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl₂O₄ layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl₂O₄ layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.     

NMR study and electrical properties investigation of Zn<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The α-Zn₂P₂O₇ compound was obtained by conventional solid-state reaction. The sample was characterized by X-ray powder diffraction, solid state ³¹P NMR MAS, and electrical impedance spectroscopy. The solid state ³¹P MAS NMR, performed at 121.49 MHz, shows three isotropic resonances at −21.1, −18.8, and −15.8 ppm, confirming the non-equivalency of the three PO₄ groups in the α-Zn₂P₂O₇ form. They are characterized by different chemical shift tensor parameters with the local geometrical features of the tetrahedra. Electrical impedance measurements of β-Zn₂P₂O₇, form stable for temperature greater than 403 K, were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The AC conductivity obeys the universal power law. The approximation type correlated barrier hopping model explains the universal behavior of the n exponent. The impedance plane plot shows semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The simulated spectra show a good correlation with the experimental data.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

Ultra-sharp α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoflakes: growth mechanism and field-emission

We report the synthesis of single-crystalline α-Fe₂O₃ nanoflakes from a simple Fe–air reaction within the temperatures range of 260–400 °C. The nanoflakes synthesized at the lowest temperature (260 °C) in this work show an ultra-sharp morphology: 5–10 nm in thickness, 1–2 μm in length, 20 nm in base-width and around 5 nm at the tips; successfully demonstrate the promising electron field emission properties of a large-scaled α-Fe₂O₃ nanostructure film and exhibit the potential applications as future field-emission (FE) electron sources and displays (FEDs). The formation and growth of α-Fe₂O₃ nanostructures were discussed based on the surface diffusion mechanism. PACS 79.60.Jv; 79.70.+q; 77.84.Bw     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

Thermal lensing study and athermal directions in flashlamp-pumped Nd:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> laser crystal

Thermal lensing has been studied by a probe beam technique in flashlamp-pumped Nd:KGdW laser rods with rod axes positioned in the N _p–N _g plane and making different angles with the N _g optical indicatrix axis. It was found that the orientation of principal meridional planes of the astigmatic thermal lens in Nd:KGdW corresponds to the directions with maximum and minimum values of thermal-expansion coefficient in the plane perpendicular to the rod axis. The athermal Nd:KGdW laser crystal orientation defined as orientation with weak positive near spherical thermal lens has been determined to make an angle of 30^○–40^○ with the N _g axis for light polarization E∥N _m.     

Magnetic phases in Mn<Subscript>1</Subscript>_<Subscript>x</Subscript>Fe<Subscript>x</Subscript>WO<Subscript>4</Subscript> studied by neutron powder diffraction

The magnetic structures of Mn_1-xFe_xWO₄ with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn²⁺- and Fe²⁺-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO₄ and FeWO₄. Received 9 October 2002 Published online 14 March 2003     

Solvent Effect on Absorption and Fluorescence Spectra of Three Biologically Active Carboxamides (C<Subscript>1</Subscript>, C<Subscript>2</Subscript> and C<Subscript>3</Subscript>). Estimation of Ground and Excited State Dipole Moment from Solvatochromic Method Using Solvent Polarity Parameters

The absorption and fluorescence spectra of three Carboxamides namely (E)-2-(4-Chlorobenzylideneamino)-N-(2-chlorophenyl)-4, 5, 6, 7-tetrahydrobenzo[b]thiophene-3-carboxamide (C₁), (E)-N-(3-Chlorophenyl)-2-(3, 4-dimethoxybenzylideneamino)-4, 5, 6, 7-tetrahydrobenzo[b]thiophene-3-carboxamide (C₂) and (E)-N-(3-Chlorophenyl)-2-(3, 4, 5-trimethoxybenzylideneamino)-4, 5, 6, 7-tetrahydrobenzo[b]thiophene-3-carboxamide (C₃) have been recorded at room temperature in solvents of different polarities using dielectric constant (ε) and refractive index (n). Experimental ground (μ_g) and excited (μ_e) state dipole moments are estimated by means of solvatochromic shift method and also the excited dipole moments are estimated in combination with ground state dipole moments. It was estimated that dipole moments of the excited state were higher than those of the ground state of all three molecules. Further, the changes in dipole moment (Dm \Delta \mu ) were calculated both from solvatochromic shift method and on the basis of microscopic empirical solvent polarity parameter (E_T^N E_T^N ) and the values are compared.     

Structural and magnetic phase transitions in Pr<Subscript>0.15</Subscript>Sr<Subscript>0.85</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of Pr_0.15Sr_0.85MnO₃ manganite have been studied by means of powder X-ray and neutron diffraction in the temperature range 10–400 K at high external pressures up to 55 and 4 GPa, respectively. A structural phase transition from cubic to tetragonal phase upon compression was observed, with large positive pressure coefficient of transition temperature dT _ct /dP = 28(2) K/GPa. The C-type antiferromagnetic (AFM) ground state is formed below T _N ≈ 260 K at ambient pressure. While at ambient pressure the structural and magnetic transition temperatures are close, T _ct ~ T _N , upon compression they become decoupled with T _N ≪ T _ct due to much weaker T _N pressure dependence with coefficient dT _N /dP = 3.8(1) K/GPa.     

Two-phase reaction mechanism during chemical lithium insertion into α-MoO<Subscript>3</Subscript>

The phase transition during chemical lithium insertion into α-MoO₃ was investigated by chemical analysis, X-ray diffraction (XRD) and electrochemical characterisation. The samples have been prepared by reaction of various amounts of water-free lithium iodide with fine-particulate orthorhombic molybdenum trioxide in n-hexane (non-aqueous media), which yielded materials with different Li/Mo ratio. XRD investigations of these materials proved that the crystal structure of the layered α-MoO₃ has been changed after the chemical lithiation. The phase transition ranged from 0.25 < x < 0.5 in Li _x MoO₃ upon chemical lithium insertion into α-MoO₃. The XRD lines of lithium inserted phase Li _x MoO₃ grew at the expense of the XRD lines of the pristine α-MoO₃ as lithium ions were chemically inserted until the disappearance of lines related to α-MoO₃. The electrochemical performance of the lithiated samples is improved in comparison with the starting material (non-lithiated α-MoO₃).     

Multichannel calculation of the very narrow D<Subscript>s0</Subscript><Superscript>*</Superscript>(2317) and the very broad D<Subscript>0</Subscript><Superscript>*</Superscript>(2300-2400)

The narrow D _s0 ^* (2317) and broad D ₀ ^* (2300-2400) charmed scalar mesons and their radial excitations are described in a coupled-channel quark model that also reproduces the properties of the light scalar nonet. All two-meson channels containing ground-state pseudoscalars and vectors are included. The parameters are chosen fixed at published values, except for the overall coupling constant λ, which is fine-tuned to reproduce the D _s0 ^* (2317) mass, and a damping constant α for subthreshold contributions. Variations of λ and D ₀ ^* (2300-2400) pole postions are studied for different α values. Calculated cross-sections for S-wave DK and Dπ scattering, as well as resonance pole positions, are given for the value of α that fits the light scalars. The thus predicted radially excited state D _s0 ^*′(2850), with a width of about 50MeV, seems to have been observed already.     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.     

Electrical characterization of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>][N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]ZnCl<Subscript>4</Subscript> compound

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P2₁2₁2 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M ^* for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) ^β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.     

Soft X-ray resonant scattering study of single-crystal LaSr<Subscript>2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Soft X-ray resonant scattering studies at the Mn L_{II, III}- and the La M_{IV, V}- edges of single-crystal LaSr₂Mn₂O₇ are reported. At low temperatures, below T_N ≈ 160 K, energy scans with a fixed momentum transfer at the A-type antiferromagnetic (0 0 1) reflection around the Mn L_{II, III}-edges with incident linear σ and π polarizations show strong resonant enhancements. The splitting of the energy spectra around the Mn L_{II, III}-edges may indicate the presence of a mixed valence state, e.g., Mn³⁺/Mn⁴⁺. The relative intensities of the resonance and the clear shoulder-feature as well as the strong incident σ and π polarization dependences strongly indicate its complex electronic origin. Unexpected enhancement of the charge Bragg (0 0 2) reflection at the La M_{IV, V}-edges with σ polarization has been observed up to 300 K, with an anomaly appearing around the orbital-ordering transition temperature, T_OO ≈ 220 K, suggesting a strong coupling (competition) between them.     

Extraction of G<Subscript>M</Subscript><Superscript>n</Superscript> from inclusive electron scattering on D, <Superscript>4</Superscript>He

We use the relation between Structure Functions (SFs) of nuclei A and nucleons N in order to fomulate a criterion which isolates the QE part out of the total inclusive cross-section. From data points around the QEP we extract the reduced neutron magnetic form factor 〈α _n = G _M ⁿ/μ _n G _d〉. The latter shows an unexpected decrease up to Q ² = 10GeV^2, the largest measured.     

The determination of <Emphasis Type="Italic">α</Emphasis><Subscript><Emphasis Type="Italic">S</Emphasis></Subscript> from <Emphasis Type="Italic">τ</Emphasis> decays revisited

We revisit the determination of α _S (m _τ ²) using a fit to inclusive τ hadronic spectral moments in light of (1) the recent calculation of the fourth-order perturbative coefficient K ₄ in the expansion of the Adler function, (2) new precision measurements from BABAR of e⁺e⁻ annihilation cross sections, which decrease the uncertainty in the separation of vector and axial-vector spectral functions, and (3) improved results from BABAR and Belle on τ branching fractions involving kaons. We estimate that the fourth-order perturbative prediction reduces the theoretical uncertainty, introduced by the truncation of the series, by 20% with respect to earlier determinations. We discuss to some detail the perturbative prediction of two different methods: fixed-order perturbation theory (FOPT) and contour-improved perturbative theory (CIPT). The corresponding theoretical uncertainties are studied at the τ and Z mass scales. The CIPT method is found to be more stable with respect to the missing higher order contributions and to renormalization scale variations. It is also shown that FOPT suffers from convergence problems along the complex integration contour. Nonperturbative contributions extracted from the most inclusive fit are small, in agreement with earlier determinations. Systematic effects from quark-hadron duality violation are estimated with simple models and found to be within the quoted systematic errors. The fit based on CIPT gives α _S (m _τ ²)=0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution to M _Z we obtain α _S (M _Z ²)=0.1212±0.0005±0.0008±0.0005, where the errors are respectively experimental, theoretical and due to the evolution. The result is in agreement with the corresponding N³LO value derived from essentially the Z width in the global electroweak fit. The α _S (M _Z ²) determination from τ decays is the most precise one to date.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Evolution of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> crystals in Al-Mg-SiO<Subscript>2</Subscript> composites

The present study analyzes the morphological transformations of reaction products i.e., MgO, MgAl₂O₄ occurring during the reaction between SiO₂ and Al-Mg alloy in Al-Mg-SiO₂ composite processed by the liquid metallurgy technique. Different phases of platelet and hexagonal morphologies are detected and their composition analysis by EDS has confirmed them as being transition phases existing between MgO, MgAl₂O₄ and Al₂O₃. This study has also revealed the gradual transformation of (i) MgO needles to octahedral MgAl₂O₄ through Mg-Al-Si-O and Mg-Al-O transition phases having platelet morphologies and (ii) MgAl₂O₄ to Al₂O₃ through hexagonal transition phases on holding of Al-5Mg-SiO₂ and Al-1Mg-SiO₂ composites respectively at 1023K. Fully developed α-Al₂O₃ crystals are not observed under the present experimental conditions, wherein the Mg content is well above the equilibrium Mg content required for the formation of stable Al₂O₃ (<0.05 wt. %). PACS 05.70.Np     

Thermo-optic coefficients of monoclinic KLu(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The three thermo-optic coefficients of the biaxial laser host KLu(WO₄)₂ are measured at 633 nm by a deflection method. Their values at 300 K amount to ∂ n _g /∂ T=−7.4×10⁻⁶ K⁻¹; ∂ n _m /∂ T=−1.6×10⁻⁶ K⁻¹ and ∂ n _p /∂ T=−10.8×10⁻⁶ K⁻¹. Nearly athermal propagation directions are found for polarizations along the N _m and N _p dielectric axes.     

Pulsed laser deposition of BaGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper reports the first results obtained on monobarium gallate thin films grown on silicon and platinum coated substrates by pulsed laser deposition. The influence of oxygen background pressure and substrate (or post-annealing) temperature on the film properties was studied. The films were characterized by XRD, RHEED, AFM, photoelectron and electrical impedance spectroscopy. The structure analysis showed that the films crystallized into a hexagonal phase, most probably into (metastable) α-BaGa₂O₄. Depending on deposition conditions, films with different (from nearly epitaxial to polycrystalline) textures were obtained.