A DFT Study on Structural,Electronic Mechanical and Thermodynamic Properties of 5f-Electron System BaAmO<Subscript>3</Subscript>

The structural, electronic, mechanical and thermodynamic properties of the perovskite oxide BaAmO₃ have been predicted using the full-potential linearized augmented plane wave (FP-LAPW) method. The equilibrium lattice constant, bulk modulus and pressure derivative were computed using different exchange correlations. The optimization of structure was carried out in ferromagnetic, anti-ferromagnetic and non-magnetic states, and the compound was found to be stable in the ferromagnetic state. A systematic study on the band structure and density of states was accomplished using generalized gradient approximation (GGA), Hubbard approximation (GGA + U) and modified Becke–Johnson exchange potential (mBJ),and the compound was found to have a half-metallic nature in all the approximations. The calculated total spin magnetic moment was found to be 5 μ _B in all the approximations used. The second-order elastic constants, Young modulus, shear modulus, Poisson ratio and anisotropic factor have also been calculated. In order to have a complete understanding of BaAmO₃, the thermodynamic properties were studied in the pressure range of 0 to 40 GPa and the temperature range extending from 0 to 600 K.     

Magnetic,Optoelectronic, and Thermodynamic Properties of Sr<Subscript>2</Subscript>Cr<Emphasis Type="Italic">X</Emphasis>O<Subscript>6</Subscript> (<Emphasis Type="Italic">X</Emphasis> = La and Y): Half-Metallic and Ferromagnetic Behavior

The effects of spin polarization on the structure, magnetic, and optoelectronic properties of Cr-based series of double perovskites Sr₂CrXO₆ (X = La and Y) have been studied by using the full-potential linearized augmented plane-wave method (FP-LAPW), based on the density functional theory (DFT) as implemented in the Wien2k code, within the generalized gradient approximation (GGA), GGA + U, and GGA plus Trans-Blaha-modified Becke–Johnson (TB-mBJ) as the exchange correlation. Our results show a similar half-metallic ferromagnetic ground state for both materials. From the electronic properties, it is found that Sr₂CrYO₆ has a direct band gap at (Γ-Γ) direction and Sr₂CrLaO₆ has an indirect band gap at (Γ-W) direction. Furthermore, we have computed the optic and thermodynamic properties which are investigated for the first time. Consequently, the magnetic, optoelectronic, and thermodynamic properties show these compounds are promising for high technological applications, namely spintronic materials.     

First-Principles Prediction of Structural,Magnetic, Electronic,and Elastic Properties of Full-Heusler Compounds Co<Subscript>2</Subscript>YIn (Y = Ti,V)

Density functional theory based on the full-potential linearized augmented plane wave (FP-LAPW) method is used to investigate the structural, magnetic, electronic, and elastic properties of Heusler alloys Co₂YIn (Y = Ti, V). It is shown that the calculated spin magnetic moments using the local spin-density approximation (LSDA), generalized gradient approximation (GGA), LSDA + U, and Tran–Blaha (TB)-modified Becke–Johnson (mBJ)-local density approximations (LDA) are in good agreement with the Slater–Pauling rule. The obtained results with LSDA, GGA-PBE, and LSDA + U of the density of states illustrate that both compounds have a metal behavior; however, mBJ-LDA predicts Co₂VIn alloy to be a half metal. The band structure obtained with mBJ-LDA has an indirect band gap along the Γ–X symmetry with energy of 0.4 eV for Co₂VIn, and E _F lies in the middle of the gap; the electrons at the Fermi level are fully spin-polarized. The calculation of elastic properties indicates the stability of these compounds, and they have a ductile behavior. The 3D dependences of Young’s modulus exhibit a strong anisotropic character. The high values of the elastic constant C ₁₁ reflect the strength of the bonding Ti (V)–In.     

Synthesis,crystal structure and luminescence in Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript>

Bluish green emitting phosphor, Ca₃Al₂O₆:Ce³⁺, is prepared by low-temperature combustion method. X-ray diffraction, photoluminescence, scanning electron microscopy techniques are used to characterize the synthesized phosphor. The most efficient bluish green (483 nm) emission is observed under the excitation by near UV light. The emission characteristics are credited to 5d → 4f type transitions in Ce³⁺. The luminescence properties of Eu²⁺ are predicted for the first time from those of Ce³⁺. Also, photoluminescence of Eu³⁺ is studied in the same host. The emission spectrum of Ca₃Al₂O₆:Eu³⁺ shows the peak at 592 (orange) and 614 nm (red) wavelengths. Ca₃Al₂O₆:Ce³⁺phosphor can be a potential blue phosphor for field emission display, solid-state lighting and LED.     

Microstructure evolution and thermoelectric properties of Te-poor and Te-rich (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript> prepared via solidification

This paper explores in detail, the microstructures and thermoelectric properties of Te-rich and Te-poor (Bi,Sb)₂Te₃ alloys. We show that tuning the composition of ternary Bi–Sb–Te type alloys allows us to synthesize a range of microstructures containing a primary solid solution of (Bi,Sb)₂Te₃ with varying amounts of Te solid solution or a (Bi,Sb)Te compound. Te exists as a constituent of the multilayer domain while (Bi,Sb)Te appears in the thin intercellular regions of the (Bi,Sb)₂Te₃ dendritic cells. The presence of Te imparts an n-type behavior to the composite while the (Bi,Sb)₂Te₃ with a small amount of (Bi,Sb)Te exhibits p-type properties. A maximum ZT value of ≈0.4 at 425 K was achieved, opening up the possibility of using these alloys for thermoelectric device applications.     

Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>-4 mol % Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals with Graded Carrier Concentration

A procedure is developed for the preparation of Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ single crystals with a graded longitudinal carrier concentration profile. The thermoelectric power gradient in the crystals is 40–70 μV/K over a distance of 1–1.5 mm. The shape of the growth interface is determined. Graded legs are produced that have optimal carrier concentrations for different working temperatures of thermoelectric coolers (the thermoelectric power at the hot end is from 220 to 270 μV/K). Regions with low carrier concentration (α > 220 μV/K) are shown to contain Te-based eutectic precipitates along the cleavage plane. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1200–1205. Original Russian Text Copyright ? 2005 by Ivanova, Granatkina, Petrova, Korzhuev.     

Thermoelectric materials based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions with optimal performance in the range 100–400 K

We have optimized the compositions of thermoelectric materials based on Sb₂Te₃-Bi₂Te₃ solid solutions using Czochralski-grown single crystals. The thermoelectric performance of Sb₂Te₃-Bi₂Te₃ solid solutions containing 0–100 mol % Bi₂Te₃ and Bi₂Te₃-Sb₂Te₃-Bi₂-Bi₂Se₃ solid solutions containing 2, 4, or 7 mol % Bi₂Se₃ has been investigated. The Bi₂Se₃-doped crystals are found to have higher thermoelectric figures of merit compared to the undoped crystals. The optimal crystal compositions are selected for different temperatures in the range 100–400 K.     

Isotropic Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> compound prepared by solid-state reaction and ball milling combined with spark plasma sintering

Mg₃Sb₂ compounds were synthesized via low-temperature solid-state reaction (SSR) and ball milling (BM), respectively, followed by spark plasma sintering (SPS) process. The effects of possible sintering pressure-induced orientation in the SPS process have been investigated in terms of the microstructure and thermoelectric transport properties. The results indicate that BM technique causes more severe Mg loss than pure SSR method, leading to distinct Sb phase existing in the product after SPS consolidation process. On the contrary, a single phase of Mg₃Sb₂ is easily obtained with the combination of SSR and SPS techniques. Besides, these BM–SPS and SSR–SPS samples exhibit the similar microstructure as well as the same electrical and thermal transport properties parallel or perpendicular to the direction of sintering pressure. The study suggests that SSR method embodies the advantages of both the composition control and the orientation elimination in Mg₃Sb₂ compound as compared to BM method with the specific parameters in the current work. This investigation is quite favorable for this material fabrication and the future application of thermoelectric modules and devices.     

Photoelectrochemical properties of lead-free ferroelectric ceramic Ba(Ti<Subscript>0.96</Subscript>Mg<Subscript>0.013</Subscript>Nb<Subscript>0.026</Subscript>)O<Subscript>3</Subscript>: application to solar conversion of eosin

The lead-free Ba(Ti_0.96Mg_0.013Nb_0.026)O₃ composition has been prepared by solid state reaction. The room temperature X-ray diffraction revealed a perovskite phase with a tetragonal symmetry. The complex dielectric permittivity measured on cooling from 470 to 150 K in the frequency range (10²–10⁶ Hz) indicated a ferroelectric behavior and exhibited a large electromechanical response. This ferroelectric perovskite showed photoelectrochemical properties with an optical gap of 2.90 eV, n-type conduction and a flat band potential of ?0.57 V _SCE . As application, the oxide is successfully tested for the eosin oxidation under solar light. At pH ~ 6.3, 90 % of eosin (15 mg L^?1) disappeared after 6 h of illumination for a catalyst dose of 2.5 g L^?1.     

Structural and optical characterization of RE (Eu<Superscript>2+</Superscript>, Ce<Superscript>3+</Superscript>) doped SrMg<Subscript>2</Subscript>Al<Subscript>6</Subscript>Si<Subscript>9</Subscript>O<Subscript>30</Subscript> nanocrystalline phosphor

This article present the reports on optical study of Eu²⁺ and Ce³⁺ doped SrMg₂Al₆Si₉O₃₀ phosphors, which has been synthesized by combustion method at 550 °C. Here SrMg₂Al₆Si₉O₃₀:Eu²⁺ emission band observed at 425 nm by keeping the excitation wavelength constant at 342 nm, whereas SrMg₂Al₆Si₉O₃₀:Ce³⁺ ions shows the broad emission band at 383 nm, under 321 nm excitation wavelength, both the emission bands are assigned due to 5d–4f transition respectively. Further, phase purity, morphology and crystallite size are confirmed by XRD, SEM and TEM analysis. However, the TGA analysis is carried out to know the amount of weight lost during the thermal processing. The CIE coordinates of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor is observed at x?=?0.160, y?=?0.102 respectively, which may be used as a blue component for NUV-WLEDs. The critical distance of energy transfer between Ce³⁺ ions and host lattice is found to be 10.65 Å.     

In-situ elevated-temperature TEM study of (AgSbTe<Subscript>2</Subscript>)<Subscript>15</Subscript>(GeTe)<Subscript>85</Subscript>

(AgSbTe₂)₁₅(GeTe)₈₅ (TAGS-85) is a p-type semiconductor characterized by a maximum dimensionless thermoelectric figure of merit of 1.4–1.7 at elevated temperature. In this study, the microstructure of as-solidified TAGS-85 at room temperature and elevated temperature (160 °C) was investigated using TEM. At room temperature, pervasive twinning was observed throughout the specimen. Upon heating to above 120 °C, some of the twins dissolved and new point defects began to nucleate. The mechanisms responsible for formation of high temperature defects are discussed.     

A novel blue emitting phosphor NaBa<Subscript>0.98</Subscript>Eu<Subscript>0.02</Subscript>PO<Subscript>4</Subscript> and the improvement of its luminescence properties

A novel blue-emitting phosphor NaBa_0.98Eu_0.02PO₄ was synthesized by conventional solid state reaction, and it exhibits efficient blue emission under near-ultraviolet (n-UV) excitation. The emission spectrum shows a single band centered at about 440 nm, which corresponds to the 4f⁶5d¹-4f⁷ transition of Eu²⁺. The excitation spectrum is a broad band in the wavelength range between 200 and 450 nm, which can match the emission of white light emitting diodes (LEDs) by the method of n-UV conversion. The Ca²⁺, Sr²⁺ and Mg²⁺ were co-doped into NaBa_0.98Eu_0.02PO₄ respectively. Special attention was paid to the sample co-doped with Ca²⁺ that could possess a higher luminous efficacy than the analogs co-doped with Sr²⁺ and Mg²⁺. With the co-doping of Ca²⁺, the enhanced intensity of the excitation and emission band appears. The optimum co-doping concentration of Ca²⁺ is 7 mol.%. The emission intensity of NaBa_0.91Ca_0.07Eu_0.02PO₄ phosphoris about 1.68 times than that of NaBa_0.98Eu_0.02PO₄ phosphor. The as-prepared phosphors are the potential blue phosphors for application in white LEDs.     

Investigation of Sb<Subscript>65</Subscript>Se<Subscript>35</Subscript>/Sb multilayer thin films for high speed and high thermal stability application in phase change memory

Sb₆₅Se₃₅/Sb multilayer composite thin films were prepared by depositing the Sb₆₅Se₃₅ and Sb layers alternately. In situ resistance vs. temperature was measured and the crystallization temperature increased with thickening the Sb₆₅Se₃₅ layer in Sb₆₅Se₃₅/Sb thin films. The data retention temperature of 10 years increased greatly from 14 °C of pure Sb to 103 °C of [Sb₆₅Se₃₅(3 nm)/Sb(7 nm)]₃. Also, the band gap was broadened and the surface became smoother. X-ray diffraction patterns for the studied materials revealed that Sb and Sb₂Se₃ phases coexisted in Sb₆₅Se₃₅/Sb thin films. Absorbing the advantages of the fast phase change for Sb, the [Sb₆₅Se₃₅(1 nm)/Sb(9 nm)]₅ multilayer thin film had an ultrafast amorphization speed of 1.6 ns. The results indicated that Sb₆₅Se₃₅/Sb multilayer thin film was a potential phase change material for fast speed and good stability.     

High-performance multilayer WSe<Subscript>2</Subscript> field-effect transistors with carrier type control

In this study, high-performance multilayer WSe₂ field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe₂ FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe₂ thickness. The carrier type evolves with increasing WSe₂ channel thickness, being p-type, ambipolar, and n-type at thicknesses <3, ～4, and >5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe₂ as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe₂ FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.

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Crystallization and microstructure of a glass seal for rapid laser sealing in the system CaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript>

A glass with the composition 61 CaO·30 Al₂O₃·9 SiO₂ was studied with respect to its crystallization behavior and its suitability as a rapidly crystallizing material for laser sealing. The glass was studied by differential scanning calorimetry; from the profiles recorded, Avrami activation energies and Avrami coefficients were calculated. The latter are in the range between 0.99 and 1.55 which is supposedly attributed to sole surface crystallization. During thermal treatment as well as during laser sealing, Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₃Al₂O₆ are formed. These phases were also observed in SEM micrographs as evidenced by electron backscatter diffraction from the attributed Kikuchi patterns. Transmission electron microscopy showed a crystallized CaO- and SiO₂-enriched interface which strongly adhered to the Al₂O₃ ceramic. The porosity of the crystallized seal was in the order of few percent. The studied glass proved suitable as crystallizing seal for rapid laser sealing.     

Single step electrochemical synthesis of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films: effect of molarities of precursor solution

In the present investigation, we have successfully synthesized polycrystalline Sb₂Se₃ thin films by single-step electrochemical method. Effect of concentration of precursor solution on structural, morphological, optical, and wettability properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, and contact angle measurement have been investigated. It is evident from XRD pattern that Sb₂Se₃ thin films are polycrystalline having orthorhombic crystal structure. Also, as precursor concentration increases the diffraction peak intensity also increases. Scanning electron micrographs reveal that the increase in precursor concentration causes the formation of soap foam like microstructure which is spread in the form of ellipsoids over whole substrate surface. The optical band gap decreases from 1.49 to 1.35 eV and contact angle decreases from 40° to 13°, i.e., the surface of Sb₂Se₃ thin films converts from hydrophilic to superhydrophilic nature due to increase in precursor concentration. In addition, the holographic interferometric properties have been studied. The thickness, stress to substrate and deposited mass of the thin films is determined using double exposure holographic interferometry (DEHI) technique.     

Structural,electronic, and optical properties of orthorhombic and triclinic BiNbO4 determined via DFT calculations

We performed ab initio calculations using the FPLAW method with the local density approximation (LDA) implemented in the WIEN2 k code for the orthorhombic (α) and triclinic (β) phases of BiNbO₄. The modified Becke–Johnson exchange potential (mBJ)-LDA approach was also used to improve the electronic properties. The lattice constants calculated for both structures using the LDA are in good agreement with the experimental values. For the band structure calculations, the mBJ-LDA approach provides reasonable agreement for the band gap value compared with the LDA. The estimated (mBJ)-LDA band gap values are 2.89 eV (3.73 eV) and 2.62 eV (3.15 eV) for the α and β phases of BiNbO₄, respectively. Significant optical anisotropy is clearly observed in the visible-light region. We also calculated and evaluated the electron energy loss spectrum for BiNbO₄. This work provides the first quantitative theoretical prediction of optical properties and electron energy loss spectra for both the orthorhombic and triclinic phases of BiNbO₄.     

Influence of the concentration of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> on the electrical properties of SnO<Subscript>2</Subscript> varistors

Varistors are electronic materials with nonohmic behavior. In traditional SnO₂ varistors, CoO acts as a densifying agent, Nb₂O₅ increases the electrical conductivity of SnO₂ grains, and Cr₂O₃ produces a more uniform microstructure and acts as an oxygen retaining agent at the grain boundaries. The present work involved a systematic study of the substitution of Nb₂O₅ for Sb₂O₃ in the composition of a ternary varistor system. The compositions were prepared by conventional wet ceramic processing using deionized water, and the resulting slips were dried by spray-drying. Pellets were produced under a pressure of 330 MPa and sintered at 1,350 °C for 2 h. Similar to the behavior of Nb₂O₅, increasing the concentration of Sb₂O₃ reduced the nonlinear behavior of the ceramic and its breakdown electric field while increasing its leakage current. The samples’ microstructure showed greater porosity, suggesting that higher concentrations of Sb₂O₃ reduce the sintering rate, probably in response to the higher concentration of tin vacancies in the structure.     

Luminescence properties and preparation of Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> powder doped with Ce and Pr ions

Lu₃Al₅O₁₂:Ce³⁺ phosphor powder, which exhibits green emission band, was synthesized by the high-temperature solid-state reaction method with a flux BaF₂. X-ray diffraction (XRD), photoluminescence (PL) spectra, and fluorescent lifetime spectra were used to characterize the structure and luminescent properties of the sample. The XRD patterns indicated that when prepared at 1550 °C for 3 h with 4 wt% flux, Lu₃Al₅O₁₂:Ce³⁺ phosphors powder is the garnet cubic crystal system structure. Photoluminescence (PL) spectra showed that the Lu₃Al₅O₁₂:Ce³⁺ phosphor powder can be effectively excited by near ultraviolet and blue light, emitting broad band peaking at 505 nm, which is attributed to ²F_5/2?→?²D_5/2 transition. The self-concentration quenching mechanism of Ce³⁺ is the dipole–dipole interaction. Small amount of Pr³⁺ increased red light emission at 610 nm. Photoluminescence (PL) spectra and fluorescent lifetime spectra indicated that there was an efficient energy transfer process between Ce³⁺ and Pr³⁺.     

Preparation of Sb<Subscript>2</Subscript>S<Subscript>3</Subscript> film on functional organic self-assembled monolayers by chemical bath deposition

In this work, self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) were applied to induce the nucleation and growth of the antimony sulfide (Sb₂S₃) films on the functional ITO glass substrate at low temperature. The structure, morphology, and optical properties of the Sb₂S₃ films were investigated by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, and UV–vis spectroscopy. After thermal treatment at 200 °C for 1 h in air, the orthorhombic Sb₂S₃ was formed as a predominant phase in the deposited thin films. When the deposited films were thermally treated at 400 °C for 1 h in air, the orthorhombic Sb₂S₃ was decomposed and a cubic Sb₂O₃ was formed. The optical band energies of the as-deposited and thermally treated Sb₂S₃ films at 200 °C for 1 h in air and nitrogen were found to be 2.05 eV, 1.77, and 1.76 eV, respectively. As chemical templates, the OTS-functionalized SAMs played an important role in controlling the nucleation and growth of Sb₂S₃ films at low temperature. The results obtained from different preparation parameters applied in the present work will allow controlling the growth of the Sb₂S₃ films with uniform surface.