Polycrystalline Compounds Research Articles

Dy3+:Ca3Gd2(BO3)4 single crystal with disordered structure: A promising yellow laser candidate

In this article, a series of DyxCa3Gd2-x(BO3)4 (x = 0.02–0.2) polycrystalline compounds were synthesized by solid-state reaction to select the optimum concentration for single crystal growth, after then a 6.36 at.% Dy3+:Ca3Gd2(BO3)4 (Dy:CGB) single crystal with a disordered structure was successfully grown by the Czochralski method. The spectroscopic properties and Judd-Ofelt analyses of the crystal were carried out. Focusing on the absorption cross-section at 453 nm corresponding to the emission bands of InGaN laser diodes, the Dy:CGB crystal has a comparatively large value of 0.95 × 10-21 cm2 with a full width at half maximum (FWHM) of 10.2 nm. The yellow light emission cross-section at 576 nm is 2.2 × 10-21 cm2 with a FWHM of 14.0 nm. The Judd-Ofelt parameters Ω2, Ω4 and Ω6 were obtained to be 2.79 × 10-20 cm2, 2.10 × 10-20 cm2 and 2.28 × 10-20 cm2, respectively. The fluorescence lifetime of 4F9/2→6H13/2 transition was measured to be 0.26 ms. These results reveal that the Dy:CGB crystal is a promising yellow laser candidate.

Synthesis and luminescence studies of Dy3+ doped Li3Sc(BO3)2 polycrystalline powder for warm white light

Li3Sc(BO3)2, an orthoborate polycrystalline compound, was synthesised using the solid-state synthesis method as a pure and Dy2O3-doped material. Dy3+ ions were used in various concentrations (from 1 to 6 mol. %). The experimental powder X-ray diffraction data and calculated Rietveld refined data are found to be in good agreement, verifying the effectiveness of the synthesis procedure. The luminescence studies were performed under the excitations of X-ray, proton beam, and UV light. The success of radioluminescence studies verifies the prospects of the Dy3+-doped compound for its application as a radiation scintillator in imaging. The experimental study of cross-relaxation processes confirms the lack of potential evidence of the quenching of the emission peak at 765 nm through the absorption via 6H15/2 → 6F3/2 electronic transition cross-relaxation channel −1. The CIE 1931 colour coordinates and correlated colour temperatures were determined for all doped samples under X-ray, proton beam, and photoluminescence excitations. The obtained combined emission colour appeared in the warm white region, unlike many other Dy3+-doped phosphors that suffer suppression of luminescence through the cross-relaxation process.

Magnetic and magnetocaloric properties of Ho-=SUB=-1-x-=/SUB=-Y-=SUB=-x-=/SUB=-(Co-=SUB=-0.84-=/SUB=-Fe-=SUB=-0.16-=/SUB=-)-=SUB=-2-=/SUB=- compounds

Crystal structure, temperature and field dependences of magnetization, high-field susceptibility, and magnetocaloric effect (MCE) of Ho1-xYx(Co0.84Fe0.16)2 polycrystalline compounds (x=0-1) in magnetic fields up to 90 kOe in a temperature range of 5-400 K are investigated. An analysis of temperature dependences of magnetization (sigma) showed that, depending on the yttrium content (x), up to three "critical" temperatures can be simultaneously present in the studied samples: spin-reorientation (Tsr), magnetic compensation point (Tcomp), and Curie temperature (TC). The temperature dependence of the high-field susceptibility (chihf) of all samples with x<1 has an extremum in the vicinity of 100 K, where a significant MCE is observed, associated with a sharp change in the magnetization of the rare-earth sublattice. The temperature dependences of the magnetic entropy change (Delta Sm) and the adiabatic temperature change (Delta Tad) have a complex shape and reflect all the above-mentioned "critical" temperatures observed at different x. Keywords: magnetic properties, direct and inverse magnetocaloric effect, adiabatic temperature change, magnetic moment, Laves phases, Curie temperature, compensation point, spin-reorientation transition.

Physical properties of {Ti,Zr,Hf}2Ni2Sn compounds.

Physical properties, i.e. electrical resistivity (4.2-800 K), Seebeck coefficient (300-800 K), specific heat (2-110 K), Vickers hardness and elastic moduli (RT), have been defined for single-phase compounds with slightly nonstoichiometric compositions: Ti2.13Ni2Sn0.87, Zr2.025Ni2Sn0.975, and Hf2.055Ni2Sn0.945. From X-ray single crystal and TEM analyses, Ti2+xNi2Sn1-x, x ∼ 0.13(1), is isotypic with the U2Pt2Sn-type (space group P42/mnm, ternary ordered version of the Zr3Al2-type), also adopted by the homologous compounds with Zr and Hf. For all three polycrystalline compounds (relative densities >95%) the electrical resistivity of the samples is metallic-like with dominant scattering from static defects mainly conditioned by off-stoichiometry. Analyses of the specific heat curves Cpvs. T and Cp/T vs. T2 reveal Sommerfeld coefficients of γTi2Ni2Sn = 14.3(3) mJ mol-1 K-2, γZr2Ni2Sn = 10(1) mJ mol-1 K-2, γHf2Ni2Sn = 9.1(5) mJ mol-1 K-2 and low-temperature Debye-temperatures: θLTD = 373(7)K, 357(14)K and 318(10)K. Einstein temperatures were in the range of 130-155 K. Rather low Seebeck coefficients (<15 μV K-1), power factors (pf < 0.07 mW mK-2) and an estimated thermal conductivity of λ < 148 mW cm-1 K-1 yield thermoelectric figures of merit ZT < 0.007 at ∼800 K. Whereas for polycrystalline Zr2Ni2Sn elastic properties were determined by resonant ultrasound spectroscopy (RUS): E = 171 GPa, ν = 0.31, G = 65.5 GPa, and B = 147 GPa, the accelerated mechanical property mapping (XPM) mode was used to map the hardness and elastic moduli of T2Ni2Sn. Above 180 K, Zr2Ni2Sn reveals a quasi-linear expansion with CTE = 15.4 × 10-6 K-1. The calculated density of states is similar for all three compounds and confirms a metallic type of conductivity. The isosurface of elf shows a spherical shape for Ti/Zr/Hf atoms and indicates their ionic character, while the [Ni2Sn]n- sublattice reflects localizations around the Ni and Sn atoms with a large somewhat diffuse charge density between the closest Ni atoms.

Polycrystalline Compound of Co2+-doped Zn2SnO4: Structural and Photoluminescent Properties

In this work, Zn2SnO4:Co2+ (0.1%) was obtained through a solid-state reaction at high temperature using ZnO, SnO2, and CoCO3. The sample was investigated using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and photoluminescence spectroscopy. Rietveld refinements of the X-ray diffraction data showed two crystalline phases, Zn2SnO4 and SnO2, with proportions of 97.24% and 2.76%, respectively, while Zn and Sn atoms were observed by X-ray fluorescence. Scanning electron microscopy images showed the polycrystalline nature of the material. Photoluminescence spectroscopy showed two emission bands in the red and near-infrared regions, with the broader and more intense band having a barycenter at 694 nm. The intensity of the emission changed with the excitation wavelength, whereas the barycenter remained unchanged. The intensity, shape, and position of the bands, as well as the calculated crystal field parameters, indicate the insertion of Co2+ ions in the tetrahedral sites of the Zn2SnO4 material.

Construction of corrosion resistant and osteogenic multiphase reinforced Titanium/hydroxyapatite nanocomposites prepared by spark plasma sintering

ABSTRACT Poor corrosion resistance and osseointegration of titanium/hydroxyapatite (Ti/HA) composites or coatings limit their application of long-term implants. In this study, spark plasma sintering was employed to fabricate Ti/HA nanocrystalline composites, in which HA was proposed to improve osseointegration of Ti alloys and surface-functionalized nanocarbons (graphene nanoflakes (GNFs) and carbon nanotubes (CNTs)) were proposed to improve corrosion resistance and osteogenicity of Ti/HA nanocomposites. The microstructures indicated that phase transition of Ti, HA decomposition and chemical reaction occurred during sintering and typical microstructural characteristics such as porosity, dislocations, polycrystalline compounds or secondary compounds were present in the Ti/HA nanocomposites. The electrochemical behavior showed that Ti/HA nanocomposites containing graphenes (0.5 GNFs and 0.4CNT/0.1GNFs) had better corrosion resistance compared to other composites (0.5 CNTs and nonreinforced). The embedded graphene acted as an efficient barrier against ionic diffusion, thus causing an effective shielding against corrosion. In vitro cytocompatibility and osteogenesis evaluation demonstrated that 0.4CNT/0.1GNF-reinforced Ti/HA nanocomposites led to optimal osteoblast adhesion and proliferation, support of osteogenic differentiation and osteogenic activity. In summary, overall, considering the corrosion resistance, cytocompatibility and osteogenesis of the Ti/HA nanocomposites, 0.4CNT/0.1GNF-reinforced Ti/HA nanocomposites had acceptable corrosion resistance and optimal cytocompatibility and osteogenicity, making it a potentially feasible bone implant biomaterial.

High-pressure preparation and structural analysis of Se-S-As ternary system compounds

Se-S-As ternary polycrystalline compounds were prepared at a synthesis temperature of 900 K under a pressure of 4 GPa. The samples were analyzed by XRD, SEM and EDS. The experimental results showed that the high-pressure conditions promoted the reaction between the ternary compound Se-S-As, and the prepared samples had fairly crystallinity with the sample grain diameter around 50 μm. The results of Se-S-As elemental composition analysis showed that the atomic ratio Se:S: As was close to 8:5:8.

Magnetic properties and magnetocaloric effect of [formula omitted

We report the structural, and magnetic properties as well as magnetocaloric effect of a polycrystalline compound of Tb2Rh3Ge. This compound crystallizes with Mg2Ni3Si-type of rhombohedral Laves phases (space group R3‾m, hR18). The magnetic properties and magnetocaloric effect of Tb2Rh3Ge are explored through dc-magnetization measurements. Temperature dependence of magnetization revealed that the compound exhibits ferromagnetic behavior with TC=56 K. The field dependence of magnetization indicates that Tb2Rh3Ge is a soft ferromagnet. The obtained isothermal magnetic entropy changes (ΔSm) and refrigeration capacity (relative cooling power) for a change of magnetic field 0–9 T are 12.74 J kg−1K−1 and 497(680) J/kg respectively. The Arrott plots and universal curve of normalized ΔSm indicate that this compound undergoes a second order ferromagnetic phase transition.

Complex magnetic properties associated with competing local and itinerant magnetism in {text {Pr}}_2 {text {Co}}_{0.86} {text {Si}}_{2.88}

Ternary intermetallic compound {text {Pr}}_2 {text {Co}}_{0.86} {text {Si}}_{2.88} has been synthesized in single phase and characterized by x-ray diffraction, scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX) analysis, magnetization, heat capacity, neutron diffraction and muon spin rotation/relaxation (muSR) measurements. The polycrystalline compound was synthesized in single phase by introducing necessary vacancies in Co/Si sites. Magnetic, heat capacity, and zero-field neutron diffraction studies reveal that the system undergoes magnetic transition below sim4 K. Neutron diffraction measurement further reveals that the magnetic ordering is antiferromagnetic in nature with an weak ordered moment. The high temperature magnetic phase has been attributed to glassy in nature consisting of ferromagnetic clusters of itinerant (3d) Co moments as evident by the development of internal field in zero-field muSR below 50 K. The density-functional theory (DFT) calculations suggest that the low temperature magnetic transition is associated with antiferromagnetic coupling between Pr 4f and Co 3d spins. Pr moments show spin fluctuation along with unconventional orbital moment quenching due to crystal field. The evolution of the symmetry and the crystalline electric field environment of Pr-ions are also studied and compared theoretically between the elemental Pr and when it is coupled with other elements such as Co. The localized moment of Pr 4f and itinerant moment of Co 3d compete with each other below sim20 K resulting in an unusual temperature dependence of magnetic coercivity in the system.

A new look at the ground state properties of Ce2Ir3Al9: Coexistence of two competing energy scales

We present an extended research on characterization of the ground state in a polycrystalline compound Ce2Ir3Al9 using powder x-ray diffraction, magnetic susceptibility χ(T), isothermal magnetization M(B), specific heat Cp(T), electrical resistivity ρ(T), magnetoresistivity ρ(B), thermoelectric power S(T) and thermal conductivity κ(T) measurements, together with a theoretical description of the obtained data to develop a more profound understanding of the physics in this compound. Ce2Ir3Al9 crystallizes in the orthorhombic Y2Co3Ga9-type of structure (space group Cmcm, No. 63, oC56, Z = 4). The χ(T) and Cp(T)/T data reveal the signatures of intermediate valence behaviour on Ce ions with the approximate value of the Kondo temperature TK = 96 K and the Sommerfeld coefficient γ = 31 mJ/mol ⋅ K2. In contrast to the magnetometric and thermodynamic measurements, the transport data (ρ(T) and S(T)) indicate the development of the Kondo lattice state. Accordingly, we expect that for Ce2Ir3Al9 the most likely scenario, which develops in a wide T range is the coexistence and competition of two energy scales of interactions between conduction electrons and 4f spins on Ce ions, located in one unique crystallographic site in the unit cell, in consequence carrying out our system from Ce4+Temp.→ Ce3+. At higher temperatures, where the Kondo interaction becomes dominant the influence of the crystal electric field effect seems to play an influential role in the ground state of this compound too.

Electronic structure and chemical bonding in transition-metal-mixed gallium oxide (Ga2O3) compounds

Transition metal (TM)-mixed gallium oxide (Ga2O3) ceramic compounds have potential applications in optical, optoelectronic, and photovoltaic devices. Herein, we report on the salient electronic structures and chemical properties of a series of such compounds, with a view to optimizing performance. The TM-mixed Ga2O3 (Ga2-xMxO3; TM-GO; 0.0 ≤ x ≤ 0.3, M = Fe, Ti, W) polycrystalline compounds were synthesized by a conventional, high-temperature solid-state reaction method. The effects of Fe-, Ti-, and W-doping on the electronic structures of the TM-GO compounds have been studied. The chemical states of Fe, Ti, and W ions in the TM-GO compounds vary as a function of TM concentration. The electronic structures and chemical states of the respective ions are greatly modified when Fe, Ti, or W ions are mixed in Ga2O3. For Fe and W, mixed chemical states (Fe2+/Fe3+, W4+/W6+) are evident in single-phase TM-GO compounds for lower x. On the contrary, Ti ions are invariably in their highest oxidation state (Ti4+) for all x in Ti-mixed-Ga2O3 compounds. Irrespective of the TM dopant, Ga ions exist in their highest oxidation state (Ga3+). The fundamental scientific understanding of the electronic properties, and tunability thereof, of TM-GO compounds presented in this detailed study should be useful when considering them for application in electronic, optoelectronic, or energy devices, or related technological fields.

Double magnetic phase transitions and magnetotransport anomalies in a new compound Gd2AgSi3

Dc and ac–magnetic susceptibility (χ), specific heat (CP), electrical resistivity (ρ) and magnetoresistance measurements performed on the new polycrystalline compound Gd2AgSi3, crystallizing in the α–ThSi2 tetragonal structure, are reported. Two magnetic phase transitions were observed in dc and ac susceptibility, specific heat and resistivity measurements at temperatures TN1=11 K and TN2=20 K, despite a single site occupied by Gd atom, which is an indication of the complex magnetic behavior. Gd2AgSi3 turns out be one of the rare Gd compound in which a minimum is observed in the temperature dependence of resistivity in the paramagnetic state and also negative magnetoresistance over a wide temperature range (above TN2), mimicking the behavior of exotic Gd2PdSi3, in this ternary family. The isothermal magnetic entropy change, adiabatic temperature change, and refrigerant capacity reach a value of 9.5 J/kg-K, 7.5 K and 302 J/kg, respectively for the change of magnetic field 0-9 T.

SYNTHESIS AND X-RAY INVESTIGATION OF NOVEL NANOSTRUCTURED COPPER-ZINC MANGANITES OF LANTHANUM AND ALKALI METALS

The aim of this work is to synthesize new nanostructured copper-zinc lanthanum and alkaline metal manganites. Polycrystalline copper-zinc manganites of lanthanum and alkali metals were synthesized by the method of ceramic technology from lanthanum (III), copper (II), zinc (II), manganese (III) oxides, and lithium, sodium, and potassium carbonates in the range of 800-1200 oC. Nanostructured particles were obtained by grinding the synthesized polycrystalline compounds at the «MM301» vibration mill of «Retsch» (Germany). By indexing X-ray images of nanostructured copper-zinc lanthanum and alkaline metal manganites, it was found that they crystallize in cubic syngony. Their lattice parameters are determined. There is a pattern in the change of the lattice parameters from the ionic radii of alkaline metals.

Effect of Mn substitution on crystal structure and electrical behaviour of YFeO3 ceramic

Polycrystalline ceramic samples of Mn substituted Yttrium ortho-ferrite have been synthesized using the conventional high temperature solid-state reaction method. X-ray diffraction analysis confirmed the formation of the polycrystalline compound. Rietveld analysis of X-ray diffraction patterns revealed the monophasic orthorhombic structure with space group Pnma. Average crystallite size has been calculated which is showing the decreasing trend with increase of Mn content. The three-dimensional crystal structures have been generated using the Vesta software and different structural parameters have been calculated using these structures. The dielectric measurement as a function of frequency and temperature indicates that the relative permittivity and loss factors are increasing with the increase in substitution concentration. Similarly, the impedance has been measured as a function of frequency and temperature which reflects overall decreasing trend. In all cases the AC conductivity is showing the increasing trend with temperature. Further, the DC conductivity has also been measured and the material constant (B) has been calculated which reflect the NTCR (negative temperature coefficient of resistance) effect for these materials. The I–V characteristic curves recorded at 573 K and 673 K show the enhancement in current and linearity with temperature as well as with substitution concentration.

A novel yellow laser candidate: Dy3+ doped Ca3NbGa3Si2O14 crystal

In this article, a series of polycrystalline compounds of DyxCa3-xNbGa3Si2O14 (x = 0.03–0.45) were synthesized to obtain the proper Dy3+ ions doping concentration for the crystal growth. After that, a 5.58 at.% Dy:Ca3NbGa3Si2O14 (Dy:CNGS) single crystal was successfully grown by Czochralski method. Its spectral properties and Judd-Ofelt theory analysis were performed. The crystal has a relatively strong and wide absorption peak at about 453 nm with the absorption cross-section of 1.3 × 10−21 cm2 and a full width at half maximum (FWHM) of 10.2 nm. The Judd-Ofelt parameters Ω2, Ω4 and Ω6 were calculated to be 4.51, 2.93 and 0.19 × 10−20 cm2, respectively. The emission cross-section at 572 nm is 1.8 × 10−21 cm2 with a FWHM of 16.3 nm. The fluorescence time of the 4F9/2→6H13/2 transition is 0.24 ms. The results mean that Dy:CNGS crystal is a potential solid-state yellow laser material suitable for the commercial InGaN laser diode pumping.

Mechanochemically Assisted Synthesis of Ca/K 1144-Type Iron Pnictides

The 1144 family of Iron Based Superconductors (IBSCs) is characterized by critical currents among the highest for single crystalline IBSCs. The synthesis of polycrystalline compounds is usually carried out through high temperature (T > 900 °C) processes. These methods, justified by a supposed thermodynamic instability of the 1144 phase at low temperature, are complicated by the presence of highly volatile elements and require fast heating and cooling steps. Here, it is shown how a High Energy Ball Milling (HEBM) treatment is effective to promote the reaction among the starting elements, thus favoring the synthesis of the 1144 phase at lower temperature. The evolution of the powders has been assessed for different milling times, observing the consumption of the starting reactants and the formation of crystalline pnictide phases. Different thermal treatments have been coupled to the HEBM treatment, allowing the study of the effect of time and temperature of the thermal step on the materials properties. It has been observed how the formation of the superconducting phase occurs on the HEBM samples at 500 °C and 600 °C. Critical temperatures comparable with data reported in the literature were obtained. Granularity phenomena and critical current density of sintered samples are highly dependent on the processing variables.

Dielectric properties, relaxation and study of conduction process of Ba0.5Sr0.5Ti0.99Sn0.01O3 ceramics

Polycrystalline compound of Ba0.5Sr0.5Ti(1-x)SnxO3 (x = 0.01) prepared via solid-state reaction route was analyzed by XRD, SEM, dielectric and CIS examination of the sample. Study of XRD data confirmed the material to crystallize in cubic structure for the produced sample. The SEM micrograph depicted distributed grains with clear grain boundary. Loss tangent and dielectric constant variations were studied for frequency ranging from 1 kHz to 1 MHz and thermal range of room temperature (RT) to 400 °C. The loss tangent was found to be significantly less than Ba0.5Sr0.5TiO3. The presence of peak in loss tangent spectroscopy plot suggested dielectric relaxation occurring in the material within studied frequency range. Impedance spectroscopy data were analyzed in different formalism for the contribution of grain boundary and grain in conduction/relaxation process and the activation energies of charge carriers involved were estimated. These values suggested that conduction process is primarily dependent on oxygen vacancies. Jonscher’s power law defines the frequency dependant AC conductivity at different temperature values.

Colossal magnetoresistance of Nd-doped LaCaMnO3 polycrystalline ceramics

The cation doped polycrystalline compound La0.5Nd0.15Ca0.35MnO3 has been synthesized by a conventional solid state reaction method. The Rietveld analysis of the powder XRD of the composites reveals an orthorhombic structure with Pbnm space group symmetry. The cell parameters are a = 5.4505 ?, b = 5.4457 ? and c = 7.6876?. The AC magnetization studies show a semiconducting paramagnetic to metallic ferromagnetic transition at a temperature below the Curie temperature TC which is found to be 206.3K. It is also observed that metal to semiconductor transition temperature TMS is very close to TC. The sample also possesses high magnetoresistance, which is revealed in MR studies.

Solid-phase crystallization of gallium arsenide thin films on insulators

• The grain size and crystal orientation of polycrystalline GaAs on insulators are discussed for the first time. • The precursor conditions dramatically influence the subsequent solid-phase crystallization of GaAs layers. • The GaAs layer crystallizes at 400 °C, developing polycrystalline GaAs layer on a flexible plastic sheet. • The findings can be key parameters for synthesizing polycrystalline compound semiconductors.

Structure and properties of nano- and polycrystalline Mn-doped CuCr2Se4 obtained by ceramic method and high-energy ball milling

Chemical compounds with a general formula CuCr2–xMnxSe4 (x = 0.1, 0.2) were successfully synthesized in polycrystalline form, using solid state reaction method, and as nanoparticles, using high-energy ball milling. The influence of manganese and grain size on their structure, magnetic and thermal properties was investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), magnetic measurements (SQUID, high magnetic fields) and DSC (Differential Scanning Calorimetry). Electronic structures of obtained compounds were determined using XANES (X-ray Near Edge Absorption Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy). X-ray studies confirmed that the spinel phase forms only for x = 0.1, 0.2. The obtained compounds crystallize with the space group Fd3¯m. The results of magnetic measurements for poly- and nanocrystalline compounds showed that the polycrystalline compounds are ferromagnets with the Curie temperature of TC =352 K for x = 0.1 and TC =342 K for x = 0.2, and the Curie-Weiss temperature of θCW =362 K for x = 0.1 and θCW =354 K for x = 0.2. Whereas the nanoparticles CuCr2–xMnxSe4 (x = 0.1, 0.2) revealed a superparamagnetic character with blocking temperature of TB ≈ 25 K for both compounds.The XANES and XPS data indicate the Cr ions are trivalent (Cr3+). Mn ions are trivalent and tetravalent (Mn3+ and Mn4+), which implies small amount of the Cu ions change the valency from divalency to monovalency (Cu2+ and Cu1+). DSC/TG analysis indicates that the presence of Mn ions in the CuCr2Se4 crystal lattice affects the stability and resistance of the doped compounds in comparison with the pure CuCr2Se4. Decreasing the grain size leads to a decrease in melting temperature.