Stoichiometric Sr Research Articles

Solar Light Photocatalytic Degradation of Malachite Green by Hydrothermally Synthesized Strontium Arsenate Nanomaterial through Response Surface Methodology

Nanostructured Sr2As2O7 samples were synthesized by hydrothermal crystal growth reactions between Sr(NO3)2 and As2O3 with different stoichiometric Sr:As molar ratios as 1:1 (S1), 1:2 (S2), and 2:1 (S3). The synthesized nanomaterials were characterized by powder X‐ray diffraction (PXRD) technique and fourier‐transform infrared (FT‐IR) spectroscopy. Rietveld analysis showed that the obtained materials were crystallized well in the monoclinic crystal structure with the space group P21. The morphologies of the synthesized materials were studied by field emission scanning electron microscopy (FESEM) technique. TEM images verified formation of the particles with nanometer size. Ultraviolet‐visible spectra analysis showed that the synthesized Sr2As2O7 nanomaterials had strong light absorption in the ultraviolet light region. Photocatalytic performance of the synthesized nanomaterials was also investigated for the degradation of pollutant Malachite Green (4‐{[4‐(dimethylamino)phenyl](phenyl)methylidene}‐N′N‐dimethylcyclohexa‐25‐dien‐1‐iminium chloride) (MG) in aqueous solution under solar light condition. The optimal conditions were obtained by design expert software for S1. It was found that the optimum condition was 0.14 mL H2O2, 20 mg catalyst, and 33 min time. The volume and concentration of the as prepared MG solution were 70 mL and 100 ppm, respectively, for obtaining the optimum conditions. The degradation yield in the optimized conditions was 97 % for S1.

Strontium magnesium phosphate, Sr2+xMg3−xP4O15(x∼ 0.36), from laboratory X-ray powder data

The previously unknown crystal structure of strontium magnesium phosphate, Sr(2+x)Mg(3-x)P(4)O(15) (x ~ 0.36), determined and refined from laboratory powder X-ray diffraction data, represents a new structure type. The title compound was synthesized by high-temperature solid-state reaction and it crystallizes in the orthorhombic space group Cmcm. It was earlier thought to be stoichiometric Sr(2)Mg(3)P(4)O(15), but our structural study indicates the nonstoichiometric composition. The asymmetric unit contains one Sr (site symmetry ..m on special position 8g), one M (= Mg 64%/Sr 36%; site symmetry 2/m.. on special position 4b), one Mg (site symmetry 2.. on special position 8e), two P (site symmetry m.. on special position 8f and site symmetry ..m on special position 8g), and six O sites [two on general positions 16h, two on 8g, one on 8f and one on special position 4c (site symmetry m2m)]. The nonstoichiometry is due to the mixing of magnesium and strontium ions on the M site. The structure consists of three-dimensional networks of MgO(4) and PO(4) tetrahedra, and MO(6) octahedra with the other strontium ions occupying the larger cavities surrounded by ten O atoms. All the polyhedra are connected by corner-sharing except the edge-sharing MO(6) octahedra forming one-dimensional arrangements along [001].

Sr 2CoMoO 6 anode for solid oxide fuel cell running on H 2 and CH 4 fuels

The double perovskite Sr 2CoMoO 6− δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H 2 atmosphere with the introduction of a limited concentration of oxide–ion vacancies. Stoichiometric Sr 2CoMoO 6 has an antiferromagnetic Néel temperature T N ≈ 37 K, but after reduction in H 2 at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H 2, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr 2CoMoO 6− δ a good mixed oxide–ion/electronic conductor (MIEC). With a 300-μm-thick La 0.8Sr 0.12Ga 0.83Mg 0.17O 2.815 (LSGM) oxide–ion electrolyte and SrCo 0.8Fe 0.2O 3− δ as the cathode, the Sr 2CoMoO 6− δ anode gave a maximum power density of 1017 mW cm −2 in H 2 and 634 mW cm −2 in wet CH 4. A degradation of power in CH 4 was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).

Hydrothermally-mediated preparation and photoluminescent properties of Sr 3Al 2O 6:Eu 3+ phosphor

Hydrogarnet Sr 3Al 2(OH) 12 (SAH) has been obtained from hydrothermal systems with a wide range of pH values under mild conditions. The influence of reaction conditions on the formation of various phases has been considered. The addition of sodium hydroxide can favor the yield of pure SAH phase. Stoichiometric Sr 3Al 2O 6 (SAO) has been prepared from SAH through thermal decomposition over 600 °C for 4 h. SAO can act as a red phosphor host with doping of Eu 3+ ions. The Eu 3+ ions as activators can be introduced directly under hydrothermal conditions before thermal decomposition.

Raman study of phonons in Sr 3Mn 2O 6.54 and Sr 3Mn 2O 7 layered manganites

Polycrystalline Sr 3Mn 2O 6.54 and Sr 3Mn 2O 7 layered manganites were studied at room temperature using Raman spectroscopy. While the phonon spectrum of the stoichiometric Sr 3Mn 2O 7 sample is consistent with the group theoretical analysis for the I4/mm structure, three additional modes are observed in the phonon spectrum of the oxygen-deficient Sr 3Mn 2O 6.54 sample. Based on an analysis of the temperature dependence of the Raman spectrum of Sr 3Mn 2O 6.54 and a comparison with the Raman spectrum of LaSr 2Mn 2O 7, we conclude that the ‘extra’ modes should be viewed as disorder-induced Raman scattering.

Composition Dependence of Dielectric Anomaly in Strontium Bismuth Tantalate Thin Films

AbstractEffects of off-stoichiometry and Nb substitution on the dielectric anomaly and ferroelectric properties have been investigated for SrBi2Ta2O9(SBT) thin films. Local atomic environment for the stoichiometric and off-stoichiometric SBT films has been also measured. The features of the dielectric anomaly, the Curie temperature (Tc), and the temperature dependence of the spontaneous polarization (Ps) are independent of the film thickness, and are governed by the nature of the crystal. For the stoichiometric Sr content SBT films, the grain size increases and the temperature dependence of the remanent polarization (Pr) decreases with increasing thickness. For the films with Sr/Bi/Ta = 0.8/2/2 and 0.8/2.2/2, Sr deficient local structure was observed, and such lattice structure probably leads to higher Tc than the stoichiometric crystal. Nb substitution for Ta also raises Tc, and makes the dielectric anomaly sharper. The phase transition associated with the dielectric anomaly for these films is thought to be first-order.

Temperature dependence of ferroelectric properties of SrBi2Ta2O9 thin films

Temperature dependence of ferroelectric properties was studied on SrBi2Ta2O9 thin films with stoichiometric composition and Sr deficient and/or Bi excess compositions. Decreasing rate of remanent polarization Pr with increasing temperature was fairly small for the 20% Sr deficient compositions compared to the stoichiometric Sr content compositions. The large Pr decrease for the stoichiometric compositions was attributed to the increase of fast polarization relaxation with increasing temperature. Fatigue-free property was confirmed even at 150 °C for all compositions.

Mössbauer characteristics of the Sr 2CoFe 6 perovskite obtained by electrochemical oxidation

The Sr 2CoFeO 6 cubic perovskite was prepared using electrochemical oxidation of the Sr 2CoFeO 5 brownmillerite - type compound. The starting material was prepared by quenching in liquid nitrogen from 1473 K. This preparation method produces stoichiometric Sr 2CoFeO 5 with random distribution of Fe and Co cations in both crystallographic sites. The starting material was characterized by powder X-ray diffraction and 57Fe Mössbauer resonance spectroscopy. The electrochemical oxidation was carried out in the galvanostatic mode ( I = 250 μA), at room temperature, using a three electrode system with a solution of IM NaOH in equilibrium with air. The material obtained after this oxidation, Sr 2CoFeO 6, showed a cubic structure ( a = 383.7 pm) and its Mössbauer resonance spectra revealed the presence of Fe(IV) only. The temperature dependence of the spectra between 293 and 341 K characterizes the coexistence of magnetically ordered and paramagnetic phases up to 325 K, which corraborates the ferromagnetic behaviour found at 325 K from magnetization measurements.

Spin ladders with spin gaps: A description of a class of cuprates.

We investigate the magnetic properties of the Cu-O planes in stoichiometric Sr$_{n-1}$Cu$_{n+1}$O$_{2n}$ (n=3,5,7,...) which consist of CuO double chains periodically intergrown within the CuO$_2$ planes. The double chains break up the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with $n_r=\frac{1}{2}(n-1)$ rungs and $n_l=\frac{1}{2}(n+1)$ legs and described by the usual antiferromagnetic coupling J inside each ladder and a weak and frustrated interladder coupling J$^\prime$. The resulting lattice is a new two-dimensional trellis lattice. We first examine the spin excitation spectra of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless spectra when $n_r$ is even and $n_l$ is odd ( corresponding to n=5,9,...) and a gapped spectra when $n_r$ is odd and $n_l$ is even (corresponding to n=3,7,...). We use the bond operator representation of quantum $S=\frac{1}{2}$ spins in a mean field treatment with self-energy corrections and obtain a spin gap of $\approx \frac{1}{2} J$ for the simplest single rung ladder (n=3), in agreement with numerical estimates.

Influence of strontium on monofluorophosphate uptake by hydroxyapatite XPS characterization of the hydroxyapatite surface

The influence of Sr 2+ on fluoride uptake from monofluorophosphate (MFP) solutions by hydroxyapatite (HAp) has been investigated using adsorption measurements and X-ray photoelectron spectroscopy (XPS). XPS analysis of Sr 2+/MFP treated HAp powders indicates that a stoichiometric Sr 2+/Ca 2+ cation exchange occurs on the HAp surface. Complete exchange of surface Ca 2+ occurs at Sr 2+ concentrations >0.02 mol dm −3 forming a Sr HAp layer. Fluoride adsorption measurements reveal an increase in fluoride uptake with increasing Sr 2+ concentration in the treatment solution. XPS data also show that fluorapatite (FAp) formation increases with increasing Sr 2+ concentration and that a linear correlation exists between FAp formation and surface Sr 2+. The data further suggest the formation of MFP-like particles or precipitates on the HAp surface.

The formation of nonequilibrium microdomains during the oxidiation of Sr 2CoFeO 5

The phase Sr 2CoFeO 5+ y has been studied by Mössbauer spectroscopy, X-ray powder diffraction, and magnetic susceptibility techniques. Quenching in air from 1200°C produces a partially oxidized material which contains small nonequilibrium microdomains of the brownmillerite Sr 2CoFeO 5 with excess oxygen accommodated in the domain walls. Quenching in liquid nitrogen from 1200°C produces stoichiometric Sr 2CoFeO 5 with a random distribution of cobalt and iron on the tetrahedral and octahedral sites. Annealing under argon at 800°C causes preferential occupation of the tetrahedral sites by cobalt. Both the nitrogen-quenched and the annealed sample are antiferromagnetic at room temperature.