Sr Atoms Research Articles

Improving coarsening resistance of dilute Al-Sc-Zr-Si alloys with Sr or Zn additions

We study the effect of a small addition of Zn (0.5 at.%) or a micro-addition of Sr (0.005 at.%) to a dilute Al-0.05Sc-0.07Zr-0.02Si (at.%) on precipitation of Al3(Sc,Zr) nanoprecipitates. Upon aging at 400 °C, both the Zn- and Sr-modified alloys retain peak-aged hardness (600 ± 10 MPa) for at least two weeks, while the Zn/Sr-free control alloy shows lower peak hardness (475 ± 8 MPa), which starts dropping after only 3 days. Similarly, peak hardness values achieved at 475 °C during isochronal aging, are much higher for the Zn/Sr-modified alloy (610 ± 10 and 590 ± 14 MPa) than for the Zn/Sr-free alloy (480 ± 5 MPa). Local electrode atom probe tomography shows that Sr and Zn atoms segregate to the Al3(Sc,Zr) nanoprecipitates: up to 0.1 at. % for Sr, and up to 1.1 at.% for Zn, with a peak of ∼5 at.% at the interface with the matrix. This may affect their coarsening rate by (i) reducing the diffusivity of Sc and Zr in the matrix and (ii) altering the precipitate/matrix interfacial energy. In the peak-aged condition, the Zn-modified alloy has the same creep resistance at 300 °C than the Zn-free control alloy, but its higher overaging resistance allows for longer creep operating times.

Local structure of relaxor ferroelectric SrxBa1−xNb2O6 from a pair distribution function analysis

Pair distribution function analysis of neutron-scattering data and of ab initio molecular dynamics results have been employed to study short-range structural correlations and their temperature dependence in a heavily disordered dielectric material ${\mathrm{Sr}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}\phantom{\rule{4pt}{0ex}}(x=0.35,\phantom{\rule{0.28em}{0ex}}0.5$, and 0.61). Intrinsic disorder caused by a partial occupation of the cationic sites by differently sized Sr and Ba atoms and their vacancies introduces important local strains to the structure and directly influences the Nb-${\mathrm{O}}_{6}$ octahedra tilting. The resulting complex system of tilts is found to be both temperature and Sr-doping sensitive with the biggest tilt magnitudes reached at low temperatures and high strontium contents, where ferroelectric relaxor behavior appears. We find evidence for two Nb-${\mathrm{O}}_{6}$ subsystems with different variations of niobium-oxygen bond lengths, distinct dynamics, and disparate levels of deviation from macroscopic polarization direction. These findings establish a detailed picture of the local structure of ${\mathrm{Sr}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}$ and provide a deeper insight into the origins of the materials dielectric properties.

Effect of Sr doping on structure, morphology, and transport properties of Bi2Se3 epitaxial thin films

We report molecular beam epitaxy growth of Sr-doped Bi2Se3 films on the (111) BaF2 substrate, aimed to realize unusual superconducting properties inherent to SrxBi2Se3 single crystals. Despite the wide range of the compositions, we do not achieve superconductivity. To explore the reason for this, we study the structural, morphological, and electronic properties of the films and compare them with the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on the Sr content appears to be more than an order of magnitude stronger than that in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies arise from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for the design of superconducting SrxBi2Se3 thin films.

Effects of Solute Atoms on 9R Phase Stabilization in High-Performance Al Alloys: A First-Principles Study

Recent experiments have proven that the 9R phase can dramatically improve the mechanical properties of Al alloys. This work aims to investigate the effects of solute atoms on 9R phase stabilization in Al alloys through first-principles calculations. Based on two distribution models of solute atoms, namely, the uniform distribution and the Fermi–Dirac distribution models, we discuss the influences of solute concentrations and finite temperatures on the increments of intrinsic stacking fault energy in the 9R phase structure. The results reveal that high-concentration solute atoms (Ga, Ge, Sc, Si, Sn, Sr, and Y) can promote 9R phase stabilization. Among them, the Sr atom shows the highest performance, but its stabilization can be disrupted by high temperature. These findings serve as a valuable guidance for designing and using high-performance Al alloys with the 9R phase structure.

Heteroatom-induced band-reconstruction of metal vanadates for photocatalytic cyclohexane oxidation towards KA-oil selectivity

Structure-function correlation constitutes a central theme in the selective photocatalytic oxidation of cyclohexane with molecular oxygen, as a result of precise tailoring band-structure towards KA-oil selectivity. Herein, we comparatively and systematically investigate the correlation among optical properties, electronic structure, and photocatalytic performance of two d°-vanadate photocatalysts (BiVO4 and Cu3V2O8) using theoretical calculations and experimental techniques. Results show that BiVO4 promotes the selective oxidation pathway under visible-light irradiation with higher efficiency (TOF = 7.5 h−1, based on the molar amount of V sites) and chemoselectivity (cyclohexanone/cyclohexanol molar ratio = 3.0) than the Cu3V2O8. Meanwhile, the photocatalytic performance of BiVO4 demonstrates high photocatalytic selectivity towards KA-oil (95%), while Cu3V2O8 seems to facilitate the accommodation of chlorine promoters, leading to very different chlorocyclohexane selectivity (46%). Such the distinct photocatalytic performances of BiVO4 and Cu3V2O8 can be ascribed to their unique electronic structures to facilitate the e−−h+ charge carriers and generation of reactive species during photoirradiation. The Bi3+6p states of BiVO4 are predominant components of the conduction band minimum (CBM) and hybridize with Bi3+6 s, and O2−2p and V5+3d orbitals at the valence band maximum (VBM); whereas the Cu2+3d and V5+3d orbitals contribute to the CBM of Cu3V2O8, and Cu2+3d, O2−2p and V5+3d orbitals at the VBM. So BiVO4 with s − p matrix elements is more effective than the Cu3V2O8 in producing OH, h+ and H2O2, which can rapidly oxidize the C − Cl band cleavage for chlorocyclohexane intermediate to regenerate newly Cl radicals in the next cycle reactions. Heteroatom-tuning of photocatalytic performance is further supported by universal evidences from experimental results by replacing Bi or Cu of the above vanadates with Ag, La, Sr, Fe or Ti atom and the study of the electrons, holes and radicals mediated reaction pathway by scavengers, respectively.

Towards generation of millihertz-linewidth laser light with 10−18frequency instability via four-wave mixing

Laser light with spectral purity and frequency stability is pursued in precision spectroscopy and precision measurements. We propose a scheme to generate millihertz-linewidth laser light with a frequency instability of 10−18 via optical four-wave mixing in alkaline-earth atoms. We show that the linewidth of the mixing laser light is ultimately limited by the natural linewidth of the atomic transition rather than by the linewidth of the input lasers. The frequency stability of the mixing laser light depends largely on the intensity stability of the input lasers. It is possible to generate a millihertz-linewidth laser light with a frequency instability of 10−18 and a power of 10−12 W when the input lasers with a relative intensity instability of 10−4 and a spectral width of 1 Hz interact with strontium (Sr) atoms with a density of 1 × 1011 cm−3.

Diode-pumped solid-state laser platform for compact and long-lasting strontium-based optical clocks.

Optical clocks based on strontium (Sr) atoms and ions are currently used to provide precise standards for measuring time. These devices require several narrow-band visible laser sources at specific wavelengths to operate. We report on the diode-pumped continuous-wave laser operation at 674, 688, 689, 698, and 707nm of a single crystal of Pr3+:LiGdF4 (Pr:GLF). To the best of our knowledge, Pr:GLF is the only active medium suitable for solid-state laser operation at all the wavelengths of interest for Sr-based clocks in the deep-red region. We also investigated the spectral tunability of this source, achieving an uninterrupted tuning range of 17nm, between 697 and 714nm.

Study of the relationship between the local geometric structure and the stability of La0.6Sr0.4MnO3−δ and La0.6Sr0.4FeO3−δ electrodes

Sr-substituted ABO3 perovskite oxides such as La0.6Sr0.4MnO3−δ (LSM) and La0.6Sr0.4FeO3−δ (LSF) are widely used as oxygen electrode materials in solid oxide cells. The substituted Sr is not adequately stable under the operating conditions, because of the surface segregation of Sr. Herein, we focused on investigating the relationship between the local geometric structure due to Sr substitution and stability of LSM and LSF. We characterized the local geometric structure of Sr atoms via X-ray absorption spectroscopy. A greater Debye–Waller factor and a longer bond length of both the second and third Sr–O shells were observed in LSF, which demonstrates that LSF has a higher local structural disorder and that Sr in LSF requires less energy to segregate. After 20 h of heat treatment in the presence of a Fe–Cr alloy interconnect, the Sr/La molar ratio on LSF was observed to be much larger than that on LSM. This result unequivocally suggests that Sr in LSF is not as stable as in LSM, and the reaction between Sr and Cr accelerates the Sr surface segregation in LSF.

Structure and microstructure characterization of the La2SrB10O19 glass-ceramics

The crystal structure of new lanthanum strontium borate glass-ceramics was refined by the Rietveld method. The results showed that La2SrB10O19 has the monoclinic crystal structure, space group C2 (No. 5) with a = 11.1170(6), b = 6.5667(3), c = 9.2363(3) ?, ? = 91.481?, V = 674.04(7) ?3, two formula units per unit-cell, and density 3.830 g cm?3. The main building units of the crystal structure are BO4 tetrahedra and BO3 triangles. Clusters composed of [B5O12] form an infinite double layer running perpendicular to the c-axis. Lanthanum atoms, situated in [B5O12]n layers, are coordinated with 10 oxygen atom in a form of LaO10 polyhedra, while Sr atoms, located between [B5O12]n layers, are coordinated with 8 oxygen atoms in a form of SrO8 polyhedra. Microstructural measurements contain both crystallite domain sizes and microstrain calculations obtained by the Warren-Averbach and the simplified integral-breadth methods.

Magnetocaloric properties in the Pr0.7Sr0.3-xCaxMnO3: Direct and indirect estimations from thermal diffusivity data

Magnetization, specific heat, thermal diffusivity, and magnetocaloric effect were studied in Pr0.7Sr0.3-xCaxMnO3 system (x = 0; 0.1; 0.2; 0.3) in the temperature range of 80–320 K and in a magnetic field of 18 kOe. A correlation was found between the coefficients under study and microscopic characteristics (〈rA〉 – average ionic radius and σ2 – cationic disorder). The decrease in σ2 and 〈rA〉, caused by the substitution of Sr atoms by Ca atoms, causes a weakening of the exchange interactions and a decrease in ТС. The observed minima in the temperature dependence of thermal diffusivity η(T) near ТС are associated with the appearance of an additional channel of phonon relaxation in fluctuations of the magnetic order parameter. A relationship was found between the MCE value and the change in thermal diffusivity in a magnetic field near ТС. This relationship can be applied as another MCE evaluating indirect method. It is shown that MCE value estimation from thermal diffusivity data is consistent with the results of caloric measurements.

First-principles study on optic-electronic properties of doped formamidinium lead iodide perovskite**Project supported by the National Natural Science Foundation of China (Grant No. 11164004), the Industrial Research Project of Guizhou Province, China (Grant No. GY[2012]3060), the Project of Education Department of Guizhou Province, China (Grant No. [2016]215), and the Special Laboratory Fund of Education Department of Guizhou Province, China (Grant No.

We have discussed the materials of solar cell based on hybrid organic–inorganic halide perovskites with formamidinium ( or FA) lead iodide. Firstly, we build the structure of formamidinium lead iodide (FAPbI3) by using the material studio. By using the first-principles calculations, the energy band structure, density of states (DOS), and partial DOS (PDOS) of the hydrazine-iodide lead halide are obtained. Then, we theoretically analyze a design scheme for perovskite solar cell materials, published in [Science 354, 861 (2016)], with the photoelectric conversion efficiency that can reach 20.3%. Also, we use non-toxic elements to replace lead in FAPbI3 without affecting its photoelectric conversion efficiency. Here in this work, we explore the energy band structure, lattice constant, light absorption efficiency, etc. After the Ca, Zn, Ge Sr, Sn, and Ta atoms replacing lead (Pb) and through comparing the spectral distributions of the solar spectrum, it can be found that FAGeI3, FASnI3, and FAZnI3 have better absorbance characteristics in the solar spectrum range. If the band gap structure is taken into account, FAGeI3 will become an ideal material to replace FAPbI3, although its performance is slightly lower than that of FAPbI3. The toxicity of Pb is taken into account, and the Ge element can be used as a substitute element for Pb. Furthermore, we explore one of the perovskite materials, i.e., FA0.75Cs0.25Sn0.25Ge0.75I3 whose photovoltaic properties are close to those of FA0.75Cs0.25Sn0.5Pb0.5I3, but the former does not contain toxic atoms. Our results pave the way for further investigating the applications of these materials in relevant technologies.

Photoassociative spectroscopy of a halo molecule in Sr86

We present two-photon photoassociation to the least-bound vibrational level of the X$^1\Sigma_g^+$ electronic ground state of the $^{86}$Sr$_2$ dimer and measure a binding energy of $E_b=-83.00(7)(20)$\,kHz. Because of the very small binding energy, this is a halo state corresponding to the scattering resonance for two $^{86}$Sr atoms at low temperature. The measured binding energy, combined with universal theory for a very weakly bound state on a potential that asymptotes to a van der Waals form, is used to determine an $s$-wave scattering length $a=810.6(12)$\,$a_0$, which is consistent with, but substantially more accurate than the previously determined $a=798(12)\,a_0$ found from mass-scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable $^1S_0-{^3P_1}$ potential. Large sensitivity of the dimer binding energy to light near-resonant with the bound-bound transition to the intermediate state suggests that $^{86}$Sr has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states.

Theoretical study of 4p5nln'l'LSJstates of Sr ion excited by electron impact on Sr atom

Calculations of energy levels and electron-impact excitation cross sections of the Auger autoionizing states 4p5nln'l'(nl= 4d, 5s, 5p; n'l'= 4d, 5s, 5p, 5d, 6s, 6p, 6d, 7s, 7p) of Sr ion were performed. A large-scale configuration interaction method on the basis of the solutions of Dirac–Fock–Slater equations was used. The cross sections of electron-impact simultaneous ionization and excitation, and Auger decay of the electron-impact excited states of Sr atom to the 4p-core excited autoionizing states of Sr ion were calculated for the first time and used to estimate the intensity of ejected electron lines. Tentative identification of the Auger electron lines of Sr ion registered in a number of experiments is presented.

Syntheses, modulated crystal structures of Ba6−2xU2+xAg4Se12 (x = 0 and 0.5), and crystal structure and spectroscopy of Sr4Th2.78Cu4S12

Single crystals of Ba6−2xU2+xAg4Se12 (x = 0 and 0.5) and Sr4Th2.78Cu4S12 were obtained by standard solid-state chemistry methods at 1173 K. The Ba6−2xU2+xAg4Se12 structures are modulated. They were solved and refined in the monoclinic C2(α0γ)0 super space group. The structure consists of layers formed by the connection of USe6 and AgSe4 polyhedra. The incommensurate nature of the structure arises from the disorder among Ba atoms and U atoms in the layers. The crystal structure of the Sr4Th2.78Cu4S12 was solved in space group Pmn21 of the orthorhombic system. The refinement of the structure reveals three partially occupied Th sites. The connection of ThS7 and CuS4 polyhedra form a channel structure filled by Sr atoms. From optical measurements Sr4Th2.78Cu4S12 is a semiconductor with an indirect band gap of 2.27(2) eV.

First-principles study of phase-transition temperature and optical properties of alkaline earth metal (Be, Mg, Ca, Sr or Ba)-doped VO2

Vanadium dioxide (VO2) is an attractive material for energy-saving smart windows due to its metal-to-insulator reversible phase transition near ambient temperature, accompanied by large changes in its optical properties. We conducted first-principles calculations to study the phase-transition temperature and optical properties of alkaline earth metal (Be, Mg, Ca, Sr or Ba)-doped VO2. The results show that the Be atom prefers to locate at the octahedral interstitial site, while Mg, Ca, Sr and Ba atoms prefer to substitute for the V atom in VO2. Be, Mg, Ca, Sr and Ba doping reduces the phase-transition temperature of VO2 0by 51.4, 59.7, 61.5, 58.4 and 58.3 K, respectively, when the doping concentration is set at one atomic percentage. In addition, the introduction of alkaline earth metal scales the band structures of VO2, which enhances the ability to block the infrared light (in the order of Be > Mg > Ca > Sr > Ba) and promotes the transmission of visible light (in the order of Be > Mg ≈ Ca > Sr > Ba).

The Formation of Nanoparticles and Their Competitive Interaction with Twins during Eutectic Si Growth

In order to investigate the competitive interaction between nanoparticles and twin, the eutectic Si microstructures in Al–10Si (wt. %) base alloys with exclusive and combined addition of Sr and Sb are characterized by combined TEM and atom probe tomography (APT). The chemical short range order in Sb–Sb and Sb–Sr pairs is revealed by ab initio molecular dynamics simulation, which promotes the formation of clusters and nanoparticles. The coexistence of nanoparticles and twins is observed in Sb containing alloys, with a negative correlation in the corresponding number density, owing to the competitive stacking of precursors and individual atoms at the solid–liquid interface. Large size particles around 70 nm with a uniform distribution of Sr atoms are formed in Al–10Si–0.35Sb–0.015Sr (wt. %) alloys, due to the precursor aggregation and homogeneous nucleation in the droplets that nucleation are depressed. A model for the formation of nanoparticles and their interaction with twins is proposed.

Operational and Spectral Characteristics of a Sr–Ne Glow Discharge Plasma

We have used emission spectroscopy to determine the plasma parameters of strontium in a glow discharge lamp. The excitation temperature is determined from the Boltzmann’s plot method, and the electron density is measured from the Stark broadening of the observed spectral lines. Sr I line at 640.846 nm corresponding to $$4d5p~ ^{3}{\mathbf{F}^{0}}_{4}\rightarrow 5{s}4{d}~ ^{3}{} \mathbf{D}_{3}$$ transition has been used for the determination of electron number density. Further, the electron number densities in the plasma are determined at different discharge currents. Spectral emission intensities of some selected lines display Frank–Hertz-type dips at two different discharge currents. The possible role of near-resonant collisions between long-lived metastable levels of Ne and Sr atoms in creating ionization of Sr II is highlighted. The competing collisional ionization and radiative decay of the levels such as $${4p}^{6}{7p}$$ in Sr II and doubly excited 4d5p levels of Sr I is considered significant in this case. Our observations show that the electron number density changes linearly with the electric field for low-discharge currents.

Atomic visualization of copper oxide structure in the infinite-layer cuprate SrCuO2

We report the atomic-scale structure of epitaxial films of the parent infinite-layer compound $\mathrm{SrCu}{\mathrm{O}}_{2}$ prepared on ${\mathrm{SrTiO}}_{3}$ by molecular beam epitaxy. In situ scanning tunneling microscopy study reveals a stoichiometric copper oxide (${\mathrm{CuO}}_{2}$) -terminated surface featured by $2\ifmmode\times\else\texttimes\fi{}2$ reconstruction, caused primarily by structural distortions of four adjacent ${\mathrm{CuO}}_{2}$ plaquettes. Furthermore, the subsurface Sr atoms have been rarely discernible, showing intra-unit-cell rotational symmetry breaking. These observations can be reasonably modeled by a periodic up-down buckling of oxygen ions on the ${\mathrm{CuO}}_{2}$ plane. Further postannealing leads to the removal of surface oxygens and an incommensurate stripe phase. Our study provides indispensable structural information to help understand the exotic properties of cuprate superconductors.

Photoinduced Molecule Formation of Spatially Separated Atoms on Helium Nanodroplets.

Besides the use as cold matrix for spectroscopic studies, superfluid helium droplets have served as a cold environment for the synthesis of molecules and clusters. Since vibrational frequencies of molecules in helium droplets exhibit almost no shift compared to the free molecule values, one could assume the solvated particles move frictionless and undergo a reaction as soon as their paths cross. There have been a few unexplained observations that seemed to indicate cases of two species on one droplet not forming bonds but remaining isolated. In this work, we performed a systematic study of helium droplets doped with one rubidium and one strontium atom showing that besides a reaction to RbSr, there is a probability of finding separated Rb and Sr atoms on one droplet that only react after electronic excitation. Our results further indicate that ground-state Sr atoms can reside at the surface as well as inside the droplet.

Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1- xSb xHfSe3 Orthorhombic Perovskite.

Single-phase polycrystalline powders of Sr1- xSb xHfSe3 ( x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe3, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe3 revealed low values ranging from 0.9 to 1.3 W m-1 K-1 from 300 to 700 K, which is further lowered to 0.77 W m-1 K-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe3 is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1- xSb xHfSe3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe3 is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe3 as absorbing layer in photovoltaic devices.