Polarization-dependent electrolyte electroreflectance study of Cu2ZnSiS4 and Cu2ZnSiSe4 single crystals

Polarization-dependent electrolyte electroreflectance (EER) measurements were carried out on the oriented Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ single crystals at room temperature. Thin blade single crystals of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ were grown by chemical vapor transport technique using iodine as a transport agent. Laue pattern normal to the basal plane of the as-grown crystal revealed the formation of orthorhombic structure with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The polarized EER spectra in the vicinity of the direct band edge of Cu₂ZnSiS₄ displayed distinct structures associated with transitions from two upper-most valence bands to the conduction band minimum at Γ point. In the E⊥c configuration, the feature designated as E_A ∼ 3.345 eV was detected and for Е‖c, only E_B ∼ 3.432 eV appeared. For Cu₂ZnSiSe₄, three features denoted as E_A, E_B, and E_C at around 2.348, 2.406 and 2.605 eV, respectively, were recorded for E⊥c polarization, whereas in the Е‖c, only E_B and E_C were the allowed transitions. Based on the experimental observations and a recent band-structure calculation by Chen et al. [Phys. Rev. B 82 (2010) 195203], plausible band structures near the direct band edge of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ have been proposed.     

Luminescent properties of UV excitable blue emitting phosphors MSr4(BO3)3:Ce (M = Li and Na)

A series of Ce³⁺ doped novel borate phosphors MSr₄(BO₃)₃ (M = Li or Na) were successfully synthesized by traditional solid-state reaction. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. The optimal concentrations of dopant Ce³⁺ ions in compound MSr₄(BO₃)₃ (M = Li or Na) were determined through the measurements of photoluminescence spectra of phosphors. Ce³⁺ doped phosphors MSr₄(BO₃)₃ (M = Li or Na) show strong broad band absorption in UV spectral region and bright blue emission under the excitation of 345 nm light. In addition, the temperature dependences of emission spectra of M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors with optimal composition x = 0.05 for Li and x = 0.09 for Na excited under 355 nm pulse laser were also investigated. The experimental results indicate that the M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors are promising blue emitting phosphors pumped by UV light.     

Spectroscopic analysis of Nd:Ca3Gd2(BO3)4 crystal and laser operating at 1.06 μm

Laser crystal Ca₃Gd₂(BO₃)₄ of high quality activated with Nd³⁺ has been grown successfully by Czochralski technique. The product was characterized by means of X-ray diffraction (XRD) powder investigation. The absorption spectra along a-, b- and c-axes were measured and investigated according to the Judd-Ofelt theory. The fluorescence properties were pursued and the factors influencing the fluorescence quantum efficiency were indicated. Xe lamp-pumped pulsed laser results were also obtained and the highest output power was up to 62 mJ.     

Growth and spectroscopic properties of Nd-doped Na2Gd4(MoO4)7 crystal

This paper reports the growth and spectral properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystals. An Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal with dimensions of Ø20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal were investigated. The Judd-Ofelt theory was applied to calculate the spectral parameters. The polarized absorption cross-sections of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal are 4.25 × 10⁻²⁰ cm² with full width at half maximum (FWHM) of 14.6 nm for the π-polarization and 2.87 × 10⁻²⁰ cm² with FWHM of 16.2 nm for the σ-polarization, respectively. The emission cross-sections are 10.0 × 10⁻²⁰ cm² at 1060 nm for π-polarization and 13.6 × 10⁻²⁰ cm² at 1067 nm for σ-polarization, respectively. The fluorescence quantum efficiency has been estimated to be 90.0%. Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal may be considered as a potential laser gain medium for the diode laser pumping.     

Spectroscopic investigation of Dy-doped Li2Gd4(MoO4)7 crystal for potential application in solid-state yellow laser

Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal with dimensions of ∅20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal have been investigated. Based on the analysis of polarized absorption spectra in the framework of the Judd-Ofelt theory, the main spectroscopic characteristics, including the phenomenological intensity parameters, spontaneous transition probabilities, fluorescence branching ratios and radiative lifetimes of Dy³⁺ in the crystal, have been determined. The emission cross-sections for the ⁴F_9/2 → ⁶H_13/2 transition of special interest for laser application have been calculated using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence and radiative lifetimes of ⁴F_9/2 manifold are equal to 126.5 μs and 201.1 μs, respectively, which mean the quantum efficiency is 62.9%. The results propose the possibility of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal for solid-state yellow laser pumped by commercially available blue laser diodes.     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

Anisotropy compensation and high low-field magnetostriction of epoxy/Tb1−xHox(Fe0.8Co0.2)2 composites (0.60 ≤ x ≤ 1.0)

The anisotropy compensation and magnetostrictive properties of Tb_1−xHo_x(Fe_0.8Co_0.2)₂ (0.60 ≤ x ≤ 1.0) alloys have been investigated. The easy magnetization direction (EMD) at room temperature rotates from the 〈1 1 1〉 axis (x ≤ 0.75) to the 〈1 0 0〉 axis (x ≥ 0.90) through an intermediate state 〈1 1 0〉, subjected to the anisotropy compensation between Tb³⁺ and Ho³⁺ ions. Composition anisotropy compensation is realized near x = 0.75. The Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ alloy has a minimum anisotropy and a large spontaneous magnetostriction coefficient λ₁₁₁ (≈740 ppm) at room temperature. The strong 〈1 1 1〉-oriented 1-3 epoxy-bonded composite has been fabricated by curing under a moderate magnetic field. A high low-field magnetostriction of about 400 ppm at 3 kOe is obtained for the 1-3 epoxy/Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ composite with 40-vol% alloy particles, which can be attributed to the low magnetic anisotropy, EMD lying along 〈1 1 1〉 direction, the strong 〈1 1 1〉-textured orientation and the chain structure.     

Blue-emitting phosphor M2B5O9Cl: Eu (MSr, Ca) for white LEDs

Eu²⁺ doped M₂B₅O₉Cl (MCa, Sr) phosphors were prepared by a solid-state reaction method. Luminescent properties of as-synthesized phosphors were investigated under the excitation of near-ultraviolet (n-UV) light. Photoluminescence (PL) spectra revealed a blue emission band, and the emission band shifted to a longer wavelength region as Ca²⁺ substituted Sr²⁺. The photoluminescence excitation (PLE) spectra of M₂B₅O₉Cl: Eu²⁺ (MCa, Sr) showed broadband absorptions in the n-UV region. Moreover, the as-prepared phosphors were also compared with commercial BaMgAl₁₀O₁₇: Eu²⁺ (BAM) blue phosphor. The present phosphors showed a narrower full width at half-maximum and higher PL intensity than the reference BAM under the n-UV light excitation.     

First-principles study of lattice dynamics and thermodynamic properties of LiInX2 (X = S, Se, Te)

First-principles calculations of lattice dynamics and thermodynamic properties of orthorhombic LiInS₂ and LiInSe₂ and chalcopyrite LiInTe₂ have been performed within density functional perturbation theory using norm conserving pseudopotentials. Theoretical values of phonon mode frequencies are in good agreement with the experimental data available for these crystals obtained by methods of Raman spectroscopy and infrared one. In the whole frequency range a significant decrease of the vibrational frequencies is observed going from LiInS₂ to LiInSe₂ and LiInTe₂, which is a consequence of the anion radius increase. The lattice vibrations of In-X (X = S, Se, Te) bonds are mainly located in the low-frequency and mid-frequency ranges, and the Li-X bond vibrations are dominated in the higher frequency range. The mixed covalent-ionic nature of the three compounds is manifested by Born effective charge data. The vibration patterns of orthorhombic LiInS₂ and chalcopyrite LiInTe₂ were discussed in detail. The temperature dependences of thermodynamic quantities (including the internal energy, free energy, heat capacity, entropy and the Debye temperature Θ_D) of all the three compounds were also presented in this paper. It is proved that Debye stiffness increases from LiInTe₂ to LiInSe₂ and LiInS₂.     

Properties of the GdTX (T = Mn, Fe, Ni, Pd, X = Al, In) and GdFe6Al6 intermetallics

The magnetic and magnetocaloric properties of GdTX (T = Mn, Fe, Ni, Pd, X = Al, In) and GdFe₆Al₆ ternary compounds for possible applications in magnetic refrigeration have been investigated. Magnetization measurements have been performed in the temperature range of 2-400 K and magnetic field range of 0-7 T. The magnetic entropy changes ΔS_m have been calculated indirectly from the magnetization measurements. The calculated values of entropy change ΔS_m for examined compounds amount −13.63 J/K kg, −13.05 J/K kg, −6.13 J/K kg, −3.72 J/K kg, −1.38 J/K kg and −0.94 J/K kg, respectively, for GdNiAl, GdPdAl, GdPdIn, GdFeAl, GdFe₆Al₆ and GdMnAl at 7 T.     

Dielectric investigation of MM(PO4)2 double orthophosphates (M = Ca, Sr, Ba, Pb; M = Ti, Zr, Hf, Ge, Sn)

M^IIM^IV(PO₄)₂ (M^II = Ca, Sr, Ba, Pb; M^IV = Ti, Zr, Hf, Ge, Sn) ceramics were prepared by solid state reaction method and consolidated by spark plasma sintering in order to study their electrical properties. The dielectric study performed at room temperature was aimed to determine the basic electrical properties of these compounds. Maxwell-Wagner polarization contributions, which are active at low frequencies cause a strong extrinsic increase of both real and imaginary part of permittivity as well of the dc-conductivity for BaZr(PO₄)₂, BaTi(PO₄)₂, BaHf(PO₄)₂, and SrGe(PO₄)₂ double orthophosphates. Moderate real (ε^′r=20-150) and imaginary low-frequency permittivity (ε^′r=7-300) and typical Debye relaxation with relaxation time in the range of ms is typical for: PbHf(PO₄)₂, PbZn(PO₄)₂, and CaGe(PO₄)₂. At high-frequency (f = 10⁹ Hz), the ceramics have permittivities of 2.29÷8.02 and tangent loss of 0.003÷0.153. The compounds SrGe(PO₄)₂, BaGe(PO₄)₂, BaZr(PO₄)₂ and BaSn(PO₄)₂ have excellent high-frequency dielectric characteristics, with losses of 3÷6% and permittivity slightly above 2 and are possible candidate as microwave ceramics.     

Magnetic annealing effects on multiferroic BiFeO3/CoFe2O4 bilayered films

In situ magnetic annealing effects on c-axis-preferred multiferroic BiFeO₃/CoFe₂O₄ bilayered by chemical solution deposition route are investigated. It is observed that magnetic annealing can enhance the crystallization quality, texture and densification as well as dielectric properties. In addition, the magnetolosses decrease with increasing the magnetic fields. Moreover, both increase of the polarization and decrease of the leakage current due to magnetic annealing are beneficial for potential applications of BiFeO₃ films.     

Perpendicular magnetic anisotropy in 70 nm CoFe2O4 thin films fabricated on SiO2/Si(1 0 0) by the sol-gel method

Cobalt ferrite CoFe₂O₄ films were fabricated on SiO₂/Si(1 0 0) by the sol-gel method. Films crystallized at/above 600 °C are stoichiometric as expected. With increase of the annealing temperature from 600 °C to 750 °C, the columnar grain size of CoFe₂O₄ film increases from 13 nm to 50 nm, resulting in surface roughness increasing from 0.46 nm to 2.55 nm. Magnetic hysteresis loops in both in-plane and out-of-plane directions, at different annealing temperatures, indicate that the films annealed at 750 °C exhibit obvious perpendicular magnetic anisotropy. Simultaneously, with the annealing temperature increasing from 600 °C to 750 °C, the out of plane coercivity increases from 1 kOe to 2.4 kOe and the corresponding saturation magnetization increases from 200 emu/cm³ to 283 emu/cm³. In addition, all crystallized films exhibit cluster-like structured magnetic domains.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Mechanical and phonon properties of the superhard LuB2, LuB4, and LuB12 compounds

We have studied structural, elastic, and lattice dynamical properties of the LuB_2, LuB_4, and LuB₁₂ compounds by using the plane-wave pseudopotential approach to the density-functional theory within the generalized gradient approximation. We have considered three different crystal structures of LuB_x: LuB₂ (P6/mmm), LuB₄ (P4/mbm), and LuB₁₂ (Fm-3m). The most stable structure is found to be tetragonal (P4/mbm) structure. The comparative results on the basic physical parameters such as lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, and Poison's ratio are reported. Also, we have predicted that LuB₄ and LuB₁₂ compounds are potential superhard materials. Furthermore, the phonon dispersion curves and corresponding phonon density of states (DOS) are computed for considered phases. Our structural and some other results are in agreement with the available experimental and other theoretical data.     

Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.     

Polyacrylamide gel synthesis and photocatalytic performance of Bi2Fe4O9 nanoparticles

In this report, a polyacrylamide gel route is introduced to synthesize Bi₂Fe₄O₉ nanoparticles. It is demonstrated that high-phase-purity Bi₂Fe₄O₉ nanoparticles can be prepared using different chelating agents. Interestingly, however, the particle size of the products is found to be dependent on the choice of chelating agent. The use of EDTA as the chelating agent allows the production of Bi₂Fe₄O₉ nanopowder with a relatively smaller particle size. The photocatalytic experiments reveal that the as-prepared Bi₂Fe₄O₉ nanoparticles possess excellent photocatalytic activity for oxidative decomposition of methyl red under ultraviolet and visible light irradiation. Magnetic hysteresis loop measurement shows that the Bi₂Fe₄O₉ nanoparticles exhibit a weak ferromagnetic behavior at room temperature.     

Optical properties of CeNi5 and CeNi4M (M = Al, Cu) compounds

The experimental study of the optical properties of CeNi₅, CeNi₄Cu and CeNi₄Al compounds was carried out in the 0.083-5.64 eV energy range using the ellipsometry method. The optical constants, dielectric functions and electronic parameters (plasma and relaxation frequencies) were determined. The energy dependencies of the optical interband conductivities are discussed by using the available information on the electronic band structure of these compounds. In the ternary alloys the optical spectra show the presence of peculiarities related to effect of Cu or Al substitution at Ni sites.     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.