Electronic and optical properties of iron pyrite

We have computed for the first time the energy bands, density of states and Compton profiles of FeS₂ using linear combination of atomic orbital approach. To interpret the theoretical Compton profiles, we have measured the first ever experimental Compton profiles along [1 1 0] and [1 0 0] directions using 100 mCi ²⁴¹Am Compton spectrometer. The absolute profiles and the anisotropies in momentum densities are well explained by the hybridisation of Hartree-Fock scheme and density functional theory. In addition, to explore the utility of FeS₂ in photovoltaics, we have also discussed the optical properties using full potential linearised augmented plane wave method.     

Synthesis, structure, and properties of Li2Pb2CuB4O10 with isolated [CuB4O10] units

The copper borate Li₂Pb₂CuB₄O₁₀ has been synthesized in air by the standard solid-state reaction at temperature in the range 550-650 °C and the structure of Li₂Pb₂CuB₄O₁₀ was determined by single-crystal X-ray diffraction. Li₂Pb₂CuB₄O₁₀ crystallizes in the monoclinic space group C2/c (no. 15) with a = 16.8419(12), b = 4.7895(4), c = 13.8976(10) Å, and β = 125.3620(10)°, V = 914.22(12) Å³, and Z = 4, as determined by single-crystal X-ray diffraction. The Li₂Pb₂CuB₄O₁₀ structure exhibits isolated units of stoichiometry [CuB₄O₁₀]⁶⁻ that are built from CuO₄ distorted square planes and triangular BO₃ groups. The IR spectroscopy and thermal analysis investigations of Li₂Pb₂CuB₄O₁₀ are also presented.     

Laser operated borate polymer nanocomposites

We have found that UV polarized treatment of the nanocrystals (non-centrosymmetric Li₂B₄O₇ and centrosymmetric α-BaB₂O₄) embedded into the olygoether acrylate photopolymer samples cause substantially different response depending on the laser features. For the Li₂B₄O₇ NC, during treatment by pulsed 7 ns laser polarized nitrogen laser pulses at 337 nm we observed substantial red spectral shift of the energy gap from 7.3 eV up to 6.3 eV. The spectral shift is disappeared during the 15 min after the interruption of optical treatment. The effect exists only in the nanocrystallites with sizes below 200 nm. At the same time for the centrosymmetrical α-BaB₂O₄ nanocrystallites this effect was absent. However the photoinduced second order optical effects induced by coherent bicolor treatment by wavelengths 532 nm and 266 nm are enhanced for the α-BaB₂O₄ NC and are absent for the Li₂B₄O₇ ones.     

Synthesis and characterization of in situ carbon-coated Li2FeSiO4 cathode materials for lithium ion battery

Li₂FeSiO₄/C composites with in situ carbon coating were synthesized via sol-gel method based on acid-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS) with sucrose and l-ascorbic acid as carbon additives, respectively. As-obtained Li₂FeSiO₄/C composites prepared with l-ascorbic acid as a carbon additive are composed of nanoparticulate Li₂FeSiO₄ in an intimate contact with a continuous thin layer of residual carbon and exhibit large specific surface area up to 395.7 m² g⁻¹. The results indicate that structure of the residual carbon is graphene-rich with obviously lower disordered/graphene (D/G) ratio. These as-obtained Li₂FeSiO₄/C composites exhibit first discharge capacity of 135.3 mAh g⁻¹ at C/16 and perform cycling stability, which are superior to those of Li₂FeSiO₄/C composites synthesized with sucrose as a carbon additive.     

Investigation on temperature dependence of photoluminescence in Sr1.7Eu0.3MxCeO4.15+x/2 (M = Li, Na, K, x = 0, 0.3) red phosphors

The temperature-dependent luminescence of Sr_1.7Eu_0.3M_xCeO_4.15+x/2 (M = Li⁺, Na⁺, K⁺, x = 0, 0.3) samples was investigated and discussed in the temperature range from 303 to 573 K. It is found that the thermal quenching temperature of samples decreases with Li⁺-/Na⁺-doping but increases with the incorporation of K⁺. We suggest that these observations are resulted from two factors. One is that the incorporation of Li⁺/Na⁺/K⁺ ions reduces the strength of potential field at the O²⁻ sites, and then results in a red-shift of the Eu-O charge transfer band. The other is that Δr expands with Li⁺-/Na⁺-doping but shrinks with K⁺-doping. We consider that it is a feasible way to adjust the temperature-dependent luminescence properties of materials by introducing appropriate impurities.     

Structural, electronic, optical and thermodynamic properties of SrxCa1−xO, BaxSr1−xO and BaxCa1−xO alloys

The structural, electronic, optical and thermodynamic properties of Sr_xCa_1−xO, Ba_xSr_1−xO and Ba_xCa_1−xO ternary alloys in NaCl phase were studied using pseudo-potential plane-wave method within the density functional theory. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependence of the lattice parameters, band gaps, dielectric constants, refractive indices, Debye temperatures, mixing entropies and heat capacities on the composition x were analyzed for x = 0, 0.25, 0.50, 0.75 and 1. The lattice constant for Sr_xCa_1−xO and Ba_xSr_1−xO exhibits a marginal deviation from the Vegard's law, while the Ba_xCa_1−xO lattice constant exhibits an appreciable upward bowing. A strong deviation of the bulk modulus from linear concentration dependence was observed for the three alloys. The microscopic origins of the gap bowing were detailed and explained. The composition dependence of the dielectric constant and refractive index was studied using different models. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds which may be a support for the results of the ternary alloys reported here for the first time.     

Antiferromagnetic ordering below 1.7 K in PrIr2Ge2

We present our investigations on magnetic and transport properties of polycrystalline PrIr₂Ge₂ which forms in CaBe₂Ge₂-type primitive tetragonal structure (space group P4/nmm). The ac magnetic susceptibility data exhibit two well pronounced peaks at 2.08 K and 0.76 K due to the onset of magnetic order. The specific heat also exhibits a sharp λ-type anomaly at 1.7 K confirming the onset of bulk antiferromagnetic order. The temperature dependence of magnetic part of entropy suggests a quasi-triplet ground state in this compound. The onset of magnetic order is also confirmed by the electrical resistivity data.     

Thermo-electrochemical activation of Cu3Sn negative electrode for lithium-ion batteries

A Cu₃Sn film electrode (thickness = ca. 3 μm) is prepared by DC magnetron sputtering deposition of Sn on Cu substrate and subsequent annealing at 300 °C for 30 h. At 25 °C, this Cu-Sn binary intermetallic compound is inactive for lithiation, but becomes active at elevated temperatures due to facilitation of Cu-Sn bond cleavage for the conversion-type lithiation. The lithiated product at 120 °C is the most Li-rich Li-Sn alloy (Li₁₇Sn₄). Upon de-lithiation, the Cu-Sn intermetallics of different compositions are generated by the reaction between the metallic Sn that is restored from Li₁₇Sn₄ and the idling metallic Cu. The nature of the resulting intermetallics is dependent on the de-lithiation temperature: Cu₁₀Sn₃ at 120 °C and Cu₆Sn₅ at 25 °C. Only the latter is active for lithiation in the subsequent room-temperature cycling. That is, Cu₃Sn is thermo-electrochemically activated to be Cu₆Sn₅ by lithiation at 120 °C and subsequent de-lithiation at 25 °C. The higher lithiation activity observed with the more Sn-rich phase (Cu₆Sn₅) compared to the initial one (Cu₃Sn) has been accounted for by the higher equilibrium lithiation potential (thermodynamic consideration) and smaller number of Cu-Sn bonds to be broken (kinetic consideration).     

Synthesis, optical and dielectric studies on novel semi organic nonlinear optical crystal by solution growth technique

Bisthiourea lead acetate Pb[SC(NH₂)₂]₂(CH₃COO)₂), a novel semi organic non linear optical crystal having dimensions 17 mm × 2 mm × 2 mm were grown using slow evaporation technique. The lattice parameters for the grown crystals were determined using single crystal XRD. The presence of functional groups for the grown crystals was confirmed using Fourier transform infrared (FT-IR) spectroscopy. The optical absorption studies show that the material has wide optical transparency in the entire visible region. The dielectric constant and dielectric loss has been studied as a function of frequency for various temperatures and the results were discussed in detail. The second harmonic generation was confirmed by Kurtz powder method and it is found to be 5 times than that of potassium dihydrogen phosphate (KDP) crystal.     

Effect of increasing tellurium content on the electronic and optical properties of cadmium selenide telluride alloys CdSe1−xTex: An ab initio study

An all electron full potential linearized augmented plane wave method, within a framework of GGA (EV-GGA) approach, has been used for an ab initio theoretical study of the effect of increasing tellurium content on the band structure, density of states, and the spectral features of the linear and nonlinear optical susceptibilities of the cadmium-selenide-telluride ternary alloys CdSe_1−xTe_x (x = 0.0, 0.25, 0.5, 0.75 and 1.0). Our calculations show that increasing Te content leads to a decrease in the energy band gap. We find that the band gaps are 0.95 (1.76), 0.89 (1.65), 0.83 (1.56), 0.79 (1.44) and 0.76 (1.31) eV for x = 0.0, 0.25, 0.5, 0.75 and 1.0 in the cubic structure. As these alloys are known to have a wurtzite structure for x less than 0.25, the energy gaps are 0.8 (1.6) eV and 0.7 (1.55) eV for the wurtzite structure (x = 0.0, 0.25) for the GGA (EV-GGA) exchange correlation potentials. This reduction in the energy gaps enhances the functionality of the CdSe_1−xTe_x alloys, at least for these concentrations, leading to an increase in the effective second-order susceptibility coefficients from 16.75 pm/V (CdSe) to 18.85 pm/V (CdSe_0.75Te_0.25), 27.23 pm/V (CdSe_0.5Te_0.5), 32.25 pm/V (CdSe_0.25Te_0.75), and 37.70 pm/V (CdTe) for the cubic structure and from 12.65 pm/V (CdSe) to 21.11 pm/V (CdSe_0.75Te_0.25) in the wurtzite structure. We find a nonlinear relationship between the absorption/emission energies and composition, and a significant enhancement of the electronic properties as a function of tellurium concentration. This variation will help in designing better second-order susceptibility materials by manipulating of the electronic structures of these materials with different compositions to achieve more delocalized atomic bonds.     

Porous Li4Ti5O12 anode material synthesized by one-step solid state method for electrochemical properties enhancement

A porous Li₄Ti₅O₁₂ anode material was successfully synthesized from mixture of LiCl and TiCl₄ with 70 wt% oxalic acid by a modified one-step solid state method. The anode material Li₄Ti₅O₁₂ exhibited a cubic spinel structure and only one voltage plateau occurred around 1.5 V. The initial capacity of porous Li₄Ti₅O₁₂ was 167 and 133 mAh g⁻¹ at 0.5 and 1C charge/discharge rate, respectively, and the capacity retention maintained above 98% after 200 cycles. The porous Li₄Ti₅O₁₂ structure showed promising rate performance with a capacity of 70 mAh g⁻¹ at charge/discharge 10C rate after 200 cycles. It was demonstrated that the porous structure could withstand 50C charge/discharge rate and exhibited excellent cycling stability.     

Magnetic properties of the 1/1 approximant Ag42In42Gd16 to the icosahedral quasicrystal Ag-In-Gd

The structural, magnetic, and ¹⁵⁵Gd Mössbauer spectral properties of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ to an icosahedral quasicrystal Ag-In-Gd are reported. Based on dc magnetic susceptibility measurements, it is shown that the studied compound develops no long-range magnetic order in the temperature range 1.8-300 K. The dc zero-field-cooled and field-cooled susceptibility data indicate that the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is a spin glasss with freezing temperature T_f = 3.6(1) K. This is further confirmed by the analysis of the frequency dependence of T_f using the Vogel-Fulcher law and the dynamic scaling behavior near T_f. It is argued that the spin freezing process is a true equilibrium phase transition rather than a nonequilibrium phenomenon. The large frustration parameter of the studied compound indicates that it belongs to a category of strongly geometrically frustrated magnets. The ¹⁵⁵Gd Mössbauer spectra of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. The Debye temperature of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is 200(1) K.     

Structure, magnetic, and magnetocaloric properties of amorphous and crystalline La0.4Ca0.6MnO3+δ nanoparticles

We report a systematic study of the effects of size reduction on the magnetic and magnetocaloric properties of amorphous and crystalline La_0.4Ca_0.6MnO_3+δ nanoparticles. The materials were synthesized using a modified wet chemical Pechini route, starting with nitrate precursors to produce the perovskite structure. Phase purity, structure, size, and crystallinity were investigated using XRD and TEM. Thermal treatments resulted in nanocrystals with average diameters of 25 nm, 50 nm, and 130 nm, as well as amorphous particles ∼10 nm in diameter. Magnetic measurements revealed broad, second order ferromagnetic transitions in the nanocrystals. As particle size increased from 10 nm to 130 nm, the Curie temperature shifted from 40 K to 255 K. Magnetization, magnetic entropy change (ΔS_M), and refrigerant capacity (RC) also increased with size in the nanocrystalline samples. For a field change of 5 T, the 130 nm particles exhibit a magnetic entropy change of 2.8 J/kg K and a large refrigerant capacity of ∼240 J/kg at 250 K. Interestingly, the 10 nm amorphous particles undergo the sharpest magnetic transition, leading to a larger value of ΔS_M than in the 25 nm or 50 nm crystalline particles. These results reveal that size reduction has a significant impact on the magnetic and magnetocaloric properties of La_0.4Ca_0.6MnO_3+δ.     

Oriented crystallization of NLO organic materials

A new methodology to study the second order NLO coefficients of organic materials was developed. We have demonstrated that the combination of oriented crystal growth on structured surfaces, grazing incidence X-ray diffraction and second harmonic ellipsometry gives an insight of the relations between the molecular hyperpolarizability and the crystalline non-linear susceptibility tensor. Thin organic layers of 2-methyl-4-nitroaniline (MNA), N-(4-nitrophenyl)-l-prolinol (NPP) and 2-(N,N-dimethylamino)-5-nitroacetanilide (DAN) were grown on poly(tetrafluoroethylene) (PTFE) substrates prepared by the friction transfer method. From the observed crystallographic orientations, a growth mechanism based on a “soft-epitaxy” was proposed. At low substrate temperature, the overlayer-substrate interactions dominate and induce a two-dimensional epitaxial growth which corresponds to a specific physisorption of aromatic molecules on the polymer surface. At high substrate temperature, the aromatic crystals nucleate via a grapho-epitaxy mechanism which occurs on topological defects of the surface. The formation of specific interactions with the PTFE substrates is not a requisite to orient the crystals in this model. Finally, we will show that the ratio between the non-vanishing components of the first non-linear susceptibility tensor (χ_ijk or d_ij) can be estimated from second harmonic ellipsometry measurements.     

Effect of deposition temperature on structural, surface, optical and magnetic properties of pulsed laser deposited Al-doped CdO thin films

The objective of this work is to study the influence of deposition temperature on structural, surface, optical and magnetic properties of the Al doped CdO thin films prepared by pulsed laser deposition (PLD) technique. KrF excimer laser (λ = 248 nm, τ_l = 20 ns, ν = 10 Hz, ?_l = 2.5 J/cm²) was employed for the deposition of thin films. It is observed by XRD results that films grown at room temperature and 100 °C show preferential growth along (1 1 1) and (2 0 0) directions while high temperatures (200-400 °C) lead to preferential growth along the (2 0 0) direction only. The optical constants (n, k, α, and optical band gap energy) of films measured by spectroscopic ellipsometry show strong dependence upon deposition temperature. M-H loop of films, measured by vibrating sample magnetometer, deposited at 25 °C and 100 °C show paramagnetic nature while films deposited at temperatures (200-400 °C) exhibit ferromagnetic character. Scanning electron micrographs show degraded elongated grains at lower deposition temperatures, while smooth and compact surface is observed for films deposited at higher deposition temperatures.     

Chemical synthesis, structural and magnetic properties of nano-structured Co-Zn-Fe-Cr ferrite

Nanoparticles of Co_1−xZn_xFe_2−xCr_xO₄ (x = 0.0-0.5) ferrites were prepared by chemical co-precipitation technique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and AC susceptibility measurements. X-ray diffraction patterns indicate that the samples possess single phase cubic spinel structure. The lattice constant initially increases for x ≤ 0.3 and thereafter for x > 0.3 it decreases with increasing x. The saturation magnetization (Ms), magneton number (n_B) and coercivity (Hc) decreases with increasing Cr-Zn content x. Curie temperature deduced from AC susceptibility data decreases with increasing x.     

Anisotropic magnetocaloric effect in TbNiAl

We present study of the anisotropic magnetocaloric effect in TbNiAl crystallizing in the hexagonal ZrNiAl-type structure. TbNiAl orders antiferromagnetically below T_N = 47 K and undergoes second magnetic phase transition to another antiferromagnetic structure at T₁ = 23 K. Magnetization and specific heat measurements on single crystal revealed strongly anisotropic magnetocaloric effect. The large effect occurs for field applied along the hexagonal c-axis whereas the entropy change is almost zero for the perpendicular field direction. Plateau-like character of the determined temperature change is observed between T_N and T₁.     

Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La_1−xCa_xFe_11.5Si_1.5 (x = 0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature T_C increases from 183 to 208 K, and the maximum magnetic entropy changes |ΔS| at the respective T_C with a magnetic field change from 0 to 5 T are 21.3, 19.5, 16.9, and 11.2 J/kg K for x = 0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La_1−xCa_xFe_11.5Si_1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher T_C and the smaller hysteresis in comparison with those of the parent compound suggest that the La_1−xCa_xFe_11.5Si_1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.     

Optical properties of sputtered hexagonal CdZnO films with band gap energies from 1.8 to 3.3 eV

Cd_xZn_1−xO films in a wide range of Cd contents have been grown by reactive direct-current magnetron sputtering. At x ≤ 0.66, the sputtered Cd_xZn_1−xO films are of single hexagonal phase. The optical band gap energies of the Cd_xZn_1−xO films decrease from ∼3.3 eV at x = 0 to ∼1.8 eV at x = 0.66. Correspondingly, the near-band-edge photoluminescence is tuned in a wide visible region from ∼378 to 678 nm. Moreover, the ultraviolet irradiation enhanced PL from the Cd_xZn_1−xO films has been observed.