Tetragonal Crystals Research Articles

Fe(Se,Te) system crystallized in molten chlorides flux: The obtained materials and their characterization

In this work we report the crystallization of iron chalcogenides compounds starting from a Fe(Se0.5Te0.5) solid pellet. The synthesis process is carried out at 675 °C using a molten chloride salts mixture and permitted to obtain well defined and easily separated hexagonal and tetragonal crystals, characterized by Fe3(Se0.43Te0.57)4 and Fe1.02Se0.41Te0.59 compositions respectively. The obtained tetragonal compound exhibits superconductivity at 14 K with critical current density of ~105 A/cm2 at 5 K.

First-principles studies on the structural, electronic and mechanical properties of L1o and L12 FexPt1-x alloys

Bimetallic FexPt1-x alloys with the L10 and L12 structures have recently gained a lot of consideration in practical applications for solid-state devices, storage of ultra-high density magnetic data and biomedicine. This is due to their high magnetic and magnetocrystalline anisotropy, density, and coercivity. In order to gain knowledge on the structural, electronic and mechanical properties of the cubic and tetragonal FexPt1-x alloys, we have calculated their equilibrium lattice constants, density of states, and elastic constants at 0 K, employing first-principles calculations. The calculated equilibrium lattice constants were found to be in good agreement with the experimental data to within 3 %. All independent elastic constants satisfy the necessary stability conditions for both cubic and tetragonal systems, suggesting mechanical stability. The shear anisotropic factors predict that the tetragonal FexPt1-x crystals are highly anisotropic along the {001} plane than {100}. Moreover, the percentage of bulk (AB) and shear (AG) anisotropies revealed completely isotropic systems in the bulk and slightly anisotropic in shear modulus.

On the photoelastic constants for anisotropic stressed crystals

Abstract The need for high-quality and high-performance crystals for high-energy physics (e.g. scintillators) and biomedical applications require a good knowledge of their mechanical and optical properties, amongst the others. This is a mandatory step in many quality control process aimed at the improvement of the technological processes for crystals growth. Following the first analysis already done for Tetragonal crystals, in this paper we study the elasto-optic behavior of optically biaxial (Monoclinic and Orthorhombic groups) and uniaxial crystals (Trigonal and Hexagonal groups). We evaluate the photoelastic constants, the optic angle and the optic plane associated to various states of stress, in terms of the components of the Piezo-Optic tensor.

Synthesis and luminescence properties of single-phase Ca2P2O7:Eu2+, Eu3+ phosphor with tunable red/blue emission

A series of Eu2+ and Eu3+ coexisting Ca2P2O7 phosphors with tunable red/blue (R/B) emission have been successfully synthesized by a two-step method. Phase composition, luminescence properties and surface states are investigated by the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) emission spectra, fluorescence quantum efficiency, fluorescent lifetime and X-ray photoelectron spectroscopy (XPS). It can be concluded from XRD and FTIR results that pure Ca2P2O7 phase can be obtained at 900 °C and Ca3(PO4)2 impurity phase occurs at an increasing preparation temperature (1100 °C). The XRD results confirm that the Eu2+or Eu3+ doped samples all exhibit pure tetragonal Ca2P2O7 crystal with space group P41. Under excitation of 330 nm, Eu2+ singly-doped Ca2P2O7 phosphors showed a broad blue emission band at 422 nm, while the Eu3+ single-doped samples emitted a strong orange-red light with wavelength of 591, 597 and 618 nm excited by 395 nm. In particular, oxygen vacancies in pure Ca2P2O7 crystal are found due to the strong PL emission at 420 nm. Furthermore, Eu2+/Eu3+ co-doped Ca2P2O7 phosphors prepared by this two-step synthesis show the blue emission at 422 nm (Eu2+ ions) and the orange-red emission (Eu2+ ions) at 591 nm, 597 nm and 618 nm under near-ultraviolet (NUV) excitation of 380 nm. It was detected that the quantum efficiency of the Ca1.88P2O7:0.12Eu phosphor was 37.7%. The tunable R/B emission ratio from 0.13 to 1.56 can be achieved by controlling the recalcined temperature in the second step when excited by 380 nm. The XPS demonstrate the existence of Eu3+ ions on the surface of Eu2+/Eu3+ co-doped Ca2P2O7 phosphors. Under NUV excitation in the range from 350 to 400 nm, the optimum Commission International de I’Eclairage (CIE) coordinate of Ca2P2O7:0.12Eu2+/Eu3+ phosphors is (0.3648, 0.215), which can be potentially used as an emitting source to meet the needs of illumination devices.

Synthesis and multicolor luminescence of Tb3+ and Sm3+ co-doped LiGd(MoO4)2 phosphor

Herein, Tb3+ and Sm3+ single doped and co-doped LiGd(MoO4)2 phosphors were prepared via a sol–gel method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) fluorescence spectroscopy were utilized to investigate the structure, morphology and luminescent properties of the samples, respectively. XRD and FESEM analyses show that the samples are tetragonal phase crystals with an average size of 3 μm. In the photoluminescence excitation spectra, the MoO42− group plays a dominant role in absorbing near ultraviolet light and transferring energy to Tb3+ and Sm3+. Under the excitation of 287 nm ultraviolet light, both the green emission of Tb3+ and the orange–red emission of Sm3+ could be observed from the emission spectra of LiGd(MoO4)2: 0.01 Tb3+, xSm3+ phosphors. Accordingly, LiGd(MoO4)2: 0.01 Tb3+, 0.01Sm3+ phosphor shows an emission color that can be modulated from yellow to red, and then to yellow–green light when excited by 287, 365, 377, 405, 417, 465, 483, and 488 nm, respectively. As a result, the tunable emission color of the phosphors can be provided by changing the doping concentration of Sm3+ and the excitation wavelength.

Characterization and electrical properties of tausonite (SrTiO3) in nano ceramic composites

Tausonite phase was crystallized in SrTiO3 ceramic/borosilicate glass composite upon heat treatment at 1000–1100 °C range. Four samples containing 100/0, 90/10, 70/30 and 50/50 ratios from SrTiO3 ceramic/borosilicate glass were prepared. XRD, SEM/EDX, HR-TEM, SAED, ac conductivity (σac), dielectric constant (e′), porosity and bulk density were measured. The sintering process permits the precipitation of tausonite phase solely in case of SrTiO3 ceramic ratio between 100 and 70%; whereas, rutile and cristobalite were developed in case of equal ratios of SrTiO3 ceramic and borosilicate glass. Investigating the microcrystalline structure revealed the spread of cubic tausonite crystals in micro- and nano-scale size in the samples containing 100, 90 and 70% SrTiO3 ceramic. The decrease of SrTiO3 content to 50% led to formation tetragonal crystals of both rutile and cristobalite phases. The dielectric constant and ac conductivity of the samples have been studied in the temperature range from room temperature up to 300 °C. The activation energy attained values between 0.12 and 0.74 eV. The sample with the highest percentage of SrTiO3 phase (100 wt%) exhibited the utmost dielectric constant and that with 30 wt% glass fraction showed dielectric constant mainly temperature independent, which permits its application in electronic devices in wide range of temperatures.

Cascade χ(3)-Nonlinear Processes in a New SRS-Active Ca2MgSi2O7 Crystal

Stimulated Raman scattering in magnesium silicate Ca2MgSi2O7, which enters the family of $$D_{{2d}}^{3}$$ ‑tetragonal acentric crystals with a melilite structure, has been observed. The recorded components of Stokes and anti-Stokes χ(3)-nonlinear lasing are identified with two SRS-active modes, ωSRS1 ~ 908 cm–1 and ωSRS2 ~ 668 cm–1, which also manifest themselves in the generation of combined phonon modes ωSRS3C ~ 170 cm–1 and ωSRS4C ~ 1270 cm–1.

Physical properties of conventional and monolithic yttria-zirconia materials after low-temperature degradation

Abstract This study compared the physical properties of conventional and monolithic yttria-zirconia (Yz) materials after low-temperature degradation (LTD). Bar-shaped specimens (2.0 mm thick × 4.0 mm wide × 16.0 mm in length) were produced. Autoclave aging was executed at 134°C under 2 bar pressure for 5 h. The surface characteristics were assessed using a confocal laser microscope, microstructural phase alterations via an X-ray diffractometer (XRD), 3-point flexural strength using a universal testing machine, Vickers microhardness via a digital microhardness tester, and toughness using a durometer. Fractured specimens and indented surfaces was observed via a scanning electron microscope for fractography characterization. All of the data underwent specific statistical tests considering their distribution and homogeneity. Regarding topography, different Yz materials were very similar after LTD. Conventional Yz materials were smoother. The XRD data support that both materials were mainly composed of tetragonal crystals, whereas slight monoclinic peaks were detected. Monolithic Yz showed higher flexural strength than conventional Yz. All of the fractures started on surface defects at the side where tensile stress was concentrated (opposite to load application) and propagated to the opposite side. Vickers microhardness and toughness analysis found that conventional Yz had higher performance than monolithic Yz. This study has found that there are slight differences in strength, hardness, and toughness between conventional and monolithic yttria-stabilized zirconia ceramics, although both materials demonstrate adequate performance for clinical use.

Diffraction Resonances of Acoustic Stresses in the Crystal Layer

The six-beam scattering of elastic waves by a crystal film is investigated. Cases of symmetric diffraction of waves inside the crystal layer are analyzed. In this scattering geometry, the projections of the wave vectors on the normal to the film surface are pairwise equal to each other up to the sign. Under certain conditions, quasi-shear waves modulate the structure in the path of quasi-longitudinal waves. If the period of the modulated structure is close to the length of a quasi-longitudinal wave, the amplitude of oscillations of the latter is amplified. The explanation of this effect is similar to the description of parametric resonance.This effect is illustrated by examples of calculations of the scattering spectra of elastic waves by cubic and tetragonal crystals in a three-layer model. Calculations are based on the transfer matrix method.

Elastic Properties of a FeGe2 Single Crystal

The report presents the results of studying the temperature dependences of the velocities of propagation of longitudinal and torsion waves and the internal friction in a tetragonal FeGe2 single crystal along crystallographic axes [100], [110], and [001]. At temperatures of magnetic phase transitions of T1 ≈ 263 K and T2 ≈ 289 K an abrupt decrease of sound velocities is detected. A high anisotropy of the internal friction is observed in the region of existence of an incommensurate magnetic structure (T1 ≤ T ≤ T2).

High-Resolution Spectroscopy of Low-Temperature Phase Transitions in Copper Metaborate CuB2O4

The linear magnetic dichroism in the crystallographically isotropic ab plane of a tetragonal CuB2O4 single crystal has been investigated by high-resolution Fourier spectroscopy at temperatures of 1.4–22 K. Magnetic phase transitions are found at T < 2.0 K and their nature is discussed.

Fast and Accurate Artificial Neural Network Potential Model for MAPbI3 Perovskite Materials.

Hybrid organic–inorganic perovskite materials are promising materials for photovoltaic and optoelectronic applications. Nevertheless, the construction of a computationally efficient potential model for atomistic simulations of perovskite with high fidelity to ab initio calculations is not a trivial task given the chemically complex nature of perovskite in terms of its chemical components and interatomic interactions. In the present study, we demonstrate that artificial neural network (ANN) models can be employed for efficient and accurate potential energy evaluation of MAPbI3 perovskite materials. The ANN models were trained using training sets composed of thousands of atomic images of tetragonal MAPbI3 crystals, with their respective energies and atomic forces obtained from ab initio calculations. The trained ANN models were validated by predicting the lattice parameters and energies/atomic forces of cubic MAPbI3 perovskite and had excellent agreement with ab initio calculations. The phonon modes could also be extracted using the trained ANN model with good agreement with ab initio calculations, provided that the atomic forces were incorporated into the training processes. Finally, we demonstrate that for a given system size, the trained ANN model offers 104 to 105 faster time consumption per energy evaluation relative to ab initio calculations using Vienna Ab initio Simulation Package, demonstrating the potential of the ANN model for exhaustively sampling the configuration spaces of chemically complex materials for predictions of thermodynamic properties and phase stabilities.

Synthesis and Characterization of TiO2 Nanoparticle as Adsorbent on The Treatment of Methylene Blue Dye Pollutant

The dye of methylene blue (MB) is organics pollutant from chemical industries. The parameter of MB content are available as methylene blue active substance (MBAS) do not exceed 200 µg/L. The treatment of MB dye pollutant can be done by adsorption process with TiO2 nanoparticle. This aim of this research was synthesized of TiO2 nanoparticle using PVA as surfactant and titanium tetraisoproxide (TTIP) as titanium source through sol-gel preparation to hydrothermal process at temperature 400 and 600oC. The result of XRD spectrum was shown the tetragonal crystal, the phase of anatase, and the size of particle was 1.66 to 3.15 nm. Infrared spectroscopy of Ti-O functional group was at 467 cm-1 and there were hydroxyl groups of the surface TiO2. DSC analysis was indicated the thermal stability of TiO2 at 250oC. Application of TiO2 nanoparticle as an adsorbent to photodegradation of MB with Hg lamp 150 watt during 60 minutes. The highest of adsorption capacity at the sample TiO2-600 was 217 mg/g and effectiveness adsorption was 81%. Isoterm adsorption process of MB by TiO2 nanoparticle can be obtained with Langmuir and Freundlish equation, with the determined coefficient value were close to 1. These results suggest of TiO2 nanoparticle was indicated highly potential to apply for the treatment of pollutant industries.

Room-Temperature Synthesis of Metal-Organic Framework Isomers in the Tetragonal and Kagome Crystal Structure.

Two metal-organic framework (MOF) isomers with the chemical formula Zn2(X)2(DABCO) [X = terephthalic acid (BDC), dimethyl terephthalic acid (DM), 2-aminoterephthalic acid (NH2), 2,3,5,6-tetramethyl terephthalic acid (TM), and anthracene dicarboxylic acid (ADC); DABCO = 1,4-diazabicyclo[2.2.2]octane] have been synthesized via a fast, room-temperature synthesis procedure. The synthesis solvent was found to play a vital role in directing the formation of the Kagome lattice (ZnBD) versus tetragonal topology (DMOF-1). When N, N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) was used as the synthesis solvent, the reaction resulted in the formation of ZnBD, whereas methanol, ethanol, acetone, N, N-diethylformamide (DEF), and acetonitrile each produced DMOF-1. Water adsorption isotherms of ZnBD and DMOF-1 were collected, and the materials were found to have similar adsorption characteristics and stabilities. Both MOFs degraded upon exposure to water at a relative pressure ( P/ Po) of 0.5 at 25 °C, but both are hydrophobic below a P/ Po of 0.4, displaying very little water adsorption. Additionally, CO2 adsorption isotherms of ZnBD were collected and compared to those previously reported for DMOF-1. ZnBD adsorbs less CO2 at low pressure compared to DMOF-1 but reaches a similar capacity at 20 bar. This adsorption behavior can be explained by the structural features of the materials, where ZnBD possesses large hexagonal pores (15 Å) compared to the smaller pore opening (7.5 Å) in DMOF-1. The heat of adsorption of CO2 on ZnBD was calculated to be ∼22 kJ/mol at zero coverage. Attempts to functionalize the Kagome lattice proved to be unsuccessful but instead resulted in a new method for producing functionalized DMOF-1 at room temperature. This was hypothesized to be a result of the steric effects imposed by the functional groups that prevent the formation of the Kagome lattice.

Improved phase-change properties of Sn–Zn–Sb alloys with a two-step crystallization process for multi-level data storage applications

Abstract We investigated Te-free ZnSb films for the potential applications in multi-level data storage. It was found that, the films could exhibit three phases during crystallization process, e.g., high amorphous phase, intermediate metastable ZnSb phase, and low stable ZnSb phase. However, the formation of the voids in the metastable ZnSb phase, high crystallization temperature and large amorphous resistance have an influence on the structure stability and SET power consumption during the repeated switching process. Interestingly, doping of Sn with a concentration of 11.1 at% into ZnSb can prevent the growth of metastable ZnSb grains, and different crystals like SnSb and ZnSnSb2 can be precipitated out during heat treatment, but the two-step crystallization process is kept unchanged. Moreover, the chemical environment of Sn in Zn–Sb studied by X-ray photoelectron spectroscopy indicates the formation of the large number of Sn–Sb and Sn–Zn bonds, which in turn reduces the crystallization temperature, lowers crystallization activation energy as well as decreases the amorphous resistance. The in situ transmission electron microscopy measurements on the optimized (ZnSb)88.9Sn11.1 composition confirms a two-step phase transition process from amorphous to rhombohedral SnSb crystals first, and then to the stable tetragonal ZnSb2Sn crystals. These excellent properties make Zn–Sb–Sn materials useful as a phase-change layer in low-power and high-density memory.

Temperature-Dependent Ambipolar Charge Carrier Mobility in Large-Crystal Hybrid Halide Perovskite Thin Films

Perovskite-based thin-film solar cells today reach power conversion efficiencies of more than 22%. Methylammonium lead iodide (MAPI) is prototypical for this material class of hybrid halide perovskite semiconductors and at the focal point of interest for a growing community in research and engineering. Here, a detailed understanding of the charge carrier transport and its limitations by underlying scattering mechanisms is of great interest to the material's optimization and development. In this article, we present an all-optical study of the charge carrier diffusion properties in large-crystal MAPI thin films in the tetragonal crystal phase from 170 K to room temperature. We probe the local material properties of individual crystal grains within a MAPI thin film and find a steady decrease of the charge carrier diffusion constant with increasing temperature. From the resulting charge carrier mobility, we find a power law dependence of μ ∝ T m with m = -(1.8 ± 0.1). We further study the temperature-dependent mobility of the orthorhombic crystal phase from 50 to 140 K and observe a distinctly different exponent of m = -(1.2 ± 0.1).

Enhanced photocatalytic activity of $$\hbox {SnO}_{2}$$ NPs by chromium (Cr) concentration

This study reports the synthesis of un- and chromium (Cr)-doped tin dioxide \((\hbox {SnO}_{2})\) nanoparticles (NPs) through chemical precipitation and their application for photodegradation of methylene blue dye. The obtained NPs were characterized by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, elemental mapping and ultraviolet–visible (UV–Vis) spectroscopy. The TEM and XRD results demonstrated that the \(\hbox {SnO}_{2}\) NPs are spherical and contain polycrystalline tetragonal crystals. UV–Vis spectra showed that increasing the concentration of the Cr dopant enhances the light absorption potential of the \( \hbox {SnO}_{2} \) NPs. Finally, the findings of the photocatalytic experiments performed for 120 min in ultraviolet irradiation under ambient conditions revealed an improvement in the degradation efficiency of the \( \hbox {SnO}_{2} \) NPs by Cr doping.

Ferroelectricity of the Orthorhombic and Tetragonal MAPbBr3 Single Crystal.

Hybrid organic-inorganic halide perovskites (HOIPs) MAPbBr3 and their ramifications have emerged because of the photovoltaic, optical, and other fascinating performances of HOIPs in recent years. However, many intrinsic properties, such as crystal structure and ferroelectricity, are still controversial. In this work, the ferroelectricity of the orthorhombic and tetragonal MAPbBr3 single crystal was confirmed through the dielectric behavior versus bias electric field ε( E), the temperature-dependent pyroelectric current with positive/negative poling, and the positive-up-negative-down (PUND) measurements. The electric field dependence of dielectric constant curves shows a butterfly type shape in the orthorhombic and tetragonal phase. The pyroelectric current shows two maxima at 155 and 245 K, corresponding to ferroelectric-ferroelectric and ferroelectric-paraelectric phase transitions, respectively. In particular, the direction of the pyroelectric current can be reversed by a positive or negative poling electric field, which is the assertive evidence of ferroelectricity. The PUND measurements act as the most convincing proof of the ferroelectricity of the MAPbBr3 single crystal. This work reports new evidence of the ferroelectric properties of the MAPbBr3 single crystal, which provides the intrinsic property when considering their high power conversion efficiencies.

Study of Structural Properties of BiOI on ZnO Thin Films and ZnO Nanorods

We report the study of ZnO nanorods (ZnO-NR) decorated with BiOI nanoplatelets by the simple precipitation of BiOI in aqueous solution containing ZnO-NR powders. Likewise, as a reference for the decoration of ZnO-NR, BiOI thin films were prepared by the SILAR technique on RF magnetron-sputtered ZnO thin films. The scanning electron microscopy analysis showed that BiOI grew on the ZnO-NR surfaces in nanoplatelet shape with an order of magnitude smaller in size, with respect to the platelets observed in BiOI thin films. Analysis by X-ray diffraction exhibited the hexagonal P63mc phase of ZnO and the tetragonal P4nmm crystal phase of BiOI. On the other hand, BiOI thin films are nonuniform and strongly adherent and, besides they exhibited the P4nmm tetragonal crystal structure of BiOI, they have highly preferential orientation along (001) direction (9.66°). Optical characterization of ZnO-NR/BiOI by absorbance showed a widened peak slightly shifted to infrared compared with the ZnO-NR absorbance peak, suggesting the increase of the absorption wavelength ranges from the ultraviolet to visible for ZnO-NR. Similarly, the results of the optical characterization by absorbance, for BiOI thin films, indicated the bandgap energy of the samples was between 1.78 eV and 1.88 eV, values previously reported for BiOI.

Highly Hydrophilic TiO₂ Nanotubes Network by Alkaline Hydrothermal Method for Photocatalysis Degradation of Methyl Orange.

High-density and highly cross-coated anatase TiO2 nanotubes networks have been successfully prepared on the surface of Ti foil by alkaline hydrothermal using NaOH and Ti foil as the precursors. The nanotubes networks were analyzed using X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), transmission electron microscope (TEM), scanning electron microscopy (SEM), optical contact angle tester, and ultraviolet (UV) fluorescence spectrophotometer, respectively. The results showed that the nanotubes network with diameters of 30–50 nm were obtained on the Ti foil surface. The morphology of the nanotubes network possessed the three-dimensional network structure, The TiO2 nanotubes network grew along the (101) direction of the tetragonal anatase crystal. The morphology and crystal phase of the TiO2 nanotubes network were better at the conditions of NaOH concentration 7–10 mol/L and temperature 160–170 °C. The best contact angle of TiO2 nanotubes network after UV-light irradition was only 5.1 ± 2.9°. Under the irradiation of mercury lamp, the nanotubes network exhibited excellent photocatalytic performance and the degradation ratio of methyl orange solution reached to 80.00 ± 2.33%. Thus, the anatase TiO2 nanotubes network has great potential in applications for pollution photocatalytic degradation.