Surrounding Oxygen Ions Research Articles

Systematic First-Principles Calculations of Ce3+-Dopant in Garnet-Type Oxide Phosphors

A study of Ce3+-doped oxide phosphors was performed using first-principles calculations based on the relativistic DV-Xα method. In particular, we focused on the Ce3+-doped garnet-type oxides, which have potential applications in solid-state lighting and display devices. The electronic structure and optical properties of the phosphors were analyzed. The energy levels of the Ce3+ ions were affected by the coordination number and the crystal field strength of the surrounding oxygen ions. In addition, the crystal structure of the material had a significant impact on the energy levels of the Ce3+ ions, with different crystal structures leading to different energy levels.The Ce3+ ion is substitutionally incorporated in the garnet lattice, and the 4f electron of Ce3+ plays an important role in the luminescence of the material. The emission wavelengths of the 5d-4f transition energy of Ce3+ can be tailored by varying the crystal structure and the composition. The 5d orbitals of Ce3+ are substantially affected by the crystal field splitting while the amplitude of crystal field splitting (εcfs) is caused by many parameters: coordination number, symmetry of Ce3+ site, and bond lengths between the Ce3+ ion and the oxygen ions (O2-). This study incorporated the following computational conditions: i) lattice relaxation, ii) the choice of the exchange-correlation potential, and iii) Slater’s transition state method. The effects of these conditions were analyzed. In addition, the correlations among physical properties and electronic structure parameters were also investigated in detail.The first-principles calculation results were compared to the experimental data to validate the accuracy of the theoretical models, which showed a good agreement between the experimental and predicted values.

Microscopic probing of the doping effects of In ions in Fe3O4

Minute examination of local lattice structures in matter affected by impurity doping is of special importance for the development of functional materials. In order to obtain microscopic information on spinel ferrites, in the present work, we introduced nonmagnetic In3+ ions in Fe3O4 and probed their site selectivity and the doping effect on the local lattice structures and bulk magnetism by means of 57Fe Mössbauer spectroscopy and positron annihilation spectroscopies. The Mössbauer parameters of the area intensity and isomer shift (IS) show that In3+ ions predominantly reside in the tetrahedral A site, especially at low doping level. With increasing concentration of In ions, however, they gradually occupy the octahedral B site replacing Fe3+ ions. Along with the site information, the IS values confirmed that the introduced In ions squeeze the B-site Fe ions at their nearest neighbors. Supporting results were obtained from positron annihilation lifetime spectroscopy; positron lifetimes become shorter with increasing In concentration, signifying that the oxygen ions are pressed by the introduced In ions resulting in lowering the volume of the adjacent lattice vacancies. The results of Doppler broadening spectroscopy also support the squeezing effect; the positrons in the vacancies adjacent to In ions are more likely to annihilate with the inner shell electrons of the surrounding oxygen ions as a result of a reduction in the vacancy volume.

Electrocatalytic activity of nano-flowered yavapaiite anchored on magnetic graphite oxide for nitrate selective reduction

• Nano-flowered yavapaiite anchored on graphite oxide was firstly fabricated by one-pot. • Electrocatalytic conversion of NO 3 − to N 2 was achieved on cathode with yavapaiite. • Reduction of NO 3 − occurred preferentially at yavapaiite (0 0 1) surface sites. • Nano-flowered yavapaiite showed stability and reusability as an electrocatalyst. Nano-flowered yavapaiite anchored on magnetic graphite oxide (YMGO) was firstly fabricated by one-pot with K 2 FeO 4 as both oxidant and iron source to selectively reduce nitrate to N 2 . The assembling cathode of carbon black and YMGO on carbon cloth (YMGO/CB-CC) exhibited complete nitrate conversion with excellent reusability in a wide pH range of 3–11 and Na 2 SO 4 of 25 mM (3.55 g L −1 ). The corresponding N 2 selectivity was found to be 60.9% and 96.3% in the absence or presence of NaCl. Iron in nano-flowered yavapaiite with empty d -orbital electrons and surrounding oxygen ions provided high-active sites and boosted electron transfer. It was found that Fe(III) active sites and chemical bonding between graphite oxide and yavapaiite (Fe-O-C) reduced iron leaching, which critically dominated reusability and electrocatalytic activity. The catalytic activity of yavapaiite for spontaneously adsorbed nitrate on the Fe(III) activity sites through two-fold coordination and NO 3 − reduction to N 2 pathway were revealed by density functional theory calculations. Experimental results of nitrate removal from domestic wastewater demonstrated that yavapaiite catalyst may have potential for practical application in wastewater treatment for nitrate reduction.

Metal-insulator transition in Ba3−xSrxNb5O15

We investigated single crystals of ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ with a tetragonal tungsten bronze (TTB) structure, which exhibits a metal-insulator transition with $x$. We found that there is a one-dimensional anisotropy with a higher dc conductivity along the $c$ axis in the metallic phase, but it disappears in the insulating phase. The optical conductivity spectra of the metallic samples show similar anisotropy to the dc conductivity, but the anisotropy persists even in the insulating phase. These experimental results, together with the result of magnetic susceptibility, indicate that the disorder caused by the Sr ions loosely bound to the surrounding oxygen ions plays an important role in the metal-insulator transition.

Uniaxial magnetic anisotropy of transition metal oxides: role of local lattice deformation

We study the effects of the local lattice structure around magnetic ions, on the uniaxial magnetic anisotropy (MA) in transition metal oxides, particularly ferrites, using the electron theory. We address M-type hexagonal ferrites, tetragonally distorted spinel ferrites, and an ilmenite (CoMnO3). The tight-binding scheme with spin–orbit interaction (LS coupling) is applied to calculate the electronic structure and MA energy of small clusters composed of a transition metal (TM) ion and surrounding oxygen ions. The results of uniaxial MA for M-type ferrites agree with those calculated using the first principles as well as those obtained experimentally, indicating the validity of the present scheme. The high numerical accuracy enables us to conclude that the p − d mixing between Fe3+ and O2− ions is crucial for the uniaxial MA of M-type ferrites and that a change in the local lattice structure around TM ions may cause a sign change in the local uniaxial MA of Fe2+ or Co2+ doped in M-type ferrites. The results of the uniaxial MA in tetragonally distorted spinel ferrites agree well with the experimental ones. These observations may indicate a feasible method to enhance the magnitude of the uniaxial MA in ferrites.

Structural defects characterization of silver-phosphate glass nanocomposites by positron annihilation and related experimental studies

The structural defects and their role in the formation and properties of the metal oxide glass nanocomposite xAgI–(1-x) (0.65Ag2O–0.35P2O5) have been studied using several sensitive spectroscopic techniques. The nanocomposite samples have been prepared by conventional melt-quenching method and the formation of nanocrystallites over the amorphous glassy matrices has been verified from X-ray diffraction and transmission electron microscopy studies. The optical band gap energies were determined from Tauc's plots of UV–Vis absorption spectra taken from a spectrophotometer. Special emphasis on the defect characterization and defects-mediated stages of evolution is made possible through positron annihilation lifetime (PAL) and coincidence Doppler broadening spectroscopic (CDBS) measurements, which further helped in the investigation of the growth of defects, especially the Ag+ ion vacancies, within the nanocomposites. A defect-specific positron lifetime is identified to represent positron trapping in the interfacial gaps between nanocrystallites and the amorphous glass matrices besides the vacancies of Ag+ ions. The longest positron lifetime component occurs due to the formation of orthopositronium in free volume holes of small sizes and it increased as an effect of agglomeration of the free volume holes within the amorphous glassy matrices at increased incorporation of AgI. The CDB spectra revealed the changing proximities of the Ag+ ion vacancies to the surrounding oxygen ions. The positron annihilation characteristics were especially indicative of the importance of the choice of stoichiometry in the formation of the nanocomposite and its exhilarating properties.

Puzzling Lack of Temperature Dependence of the PuO2 Magnetic Susceptibility Explained According to Ab Initio Wave Function Calculations.

The electronic structure and the magnetic properties of solid PuO2 are investigated by wave function theory calculations, using a relativistic complete active space (CAS) approach including spin-orbit coupling. The experimental magnetic susceptibility is well reproduced by calculations for an embedded PuO812- cluster model. The calculations indicate that the surprising lack of temperature dependence of the magnetic susceptibility χ of solid PuO2 can be rationalized based on the properties of a single Pu4+ ion in the cubic ligand field of the surrounding oxygen ions. Below ∼300 K, the only populated state is the nonmagnetic ground state, leading to standard temperature-independent paramagnetism (TIP). Above 300 K, there is an almost perfect cancellation of temperature-dependent contributions to χ that depends delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling between them.

Energy transfer between Ni2+ sensitizers and Er3+ emitters in broadband-sensitive upconverters La(Ga,Sc,In)O3:Er,Ni,Nb

We have analyzed broadband-sensitive upconversion from 1.1–1.6 μm to 0.98 μm in La(Ga,Sc,In)O3 doped with Er, Ni, and Nb, which could significantly boost the conversion efficiency of crystalline silicon solar cells, in particular, energy transfer from the Ni2+ sensitizers to the Er3+ emitters and back transfer from the Er3+ to the Ni2+. We have compared these processes and the resultant upconversion emission intensities depending on the host material compositions. With increasing the bond length between the Ni2+ and surrounding oxygen ions, the Ni2+ emission band located at around 1.2–1.6 μm red-shifts and hence overlaps more significantly with the Er3+ absorption band ranging from 1.45 μm to 1.6 μm, resulting in more rapid energy transfer from the Ni2+ to the Er3+. However, back energy transfer from the Er3+ to the Ni2+ deteriorates the performance more considerably, because of more significant overlap between the Er3+ emission band and Ni2+ absorption band. This trade-off relationship strongly affects the upconversion emission intensity. The key of the material design for further efficient upconversion is to achieve narrower bands and a larger Stokes shift of the Ni2+ absorption/emission to suppress the back energy transfer while maintaining the efficient energy transfer in the forward direction.

White-light-induced disruption of nanoscale conducting filament in hafnia

Nanoscale conducting filament, which forms the basis of the HfO2 resistive memory, is shown to exhibit a “negative photoconductivity” behavior, in that, electrical conduction through it can be disrupted upon white-light illumination. This behavior should be contrasted against the positive photoconductivity behavior commonly exhibited by oxides or perovskites having narrower bandgaps. The negative photoconductivity effect may be explained in terms of a photon-induced excitation of surrounding oxygen ions, which leads to migration and subsequent recombination with vacancies in the conducting filament. The finding suggests possible electrical-cum-optical applications for HfO2-based devices, whose functionality is limited to-date by electrical stimulation.

The yellow color center and trapped electrons in beryl

The yellow color of beryl has been related to charge-transfer between Fe 3+ ions substituting for octahedral Al 3+ ions in the crystal and the surrounding oxygen ions. This assumption is contradicted by results from Electron Paramagnetic Resonance (EPR) measurements. A strong EPR signal from octahedral Fe 3+ ions can be found in beryl of all colors and in colorless beryl. An EPR signal unique to yellow beryl comes from Fe 3+ ions at a tetrahedral site. I propose that the yellow color of beryl comes from charge-transfer of tetrahedral Fe 3+ ions. A simple model involving electrons trapped in the crystal structure explains the creation and the decay of the yellow color. The absence of trapped electrons in dark red beryl can explain the high temperature stability of its color. There is no evidence that iron ions are located in the channels of the beryl structure, which has often been assumed.

Structural changes in supercooled Al2O3–Y2O3 liquids

Structural changes in liquids between Al2O3 and Y2O3 are investigated as a function of the composition and during supercooling using high energy X-ray diffraction (HEXRD) techniques combined with containerless aerodynamic levitation. Many-body molecular dynamics simulation techniques utilizing potential models that incorporate anion polarization effects are applied to study the same liquid systems. The X-ray scattering experiments indicate a change in liquid structure during supercooling around a composition 20% Y2O3 (AlY20) that occurs over a narrow temperature interval. We have associated this change in structure with the onset of a liquid-liquid phase transformation. Analysis of the MD simulated structures has allowed the structure changes to be interpreted in terms of Al(3+) and Y(3+) coordination environments and particularly the Y(3+)-Y(3+) structural correlations. We show that the incipient liquid-liquid phase transition behaviour is correlated with local density fluctuations that represent different coordination polyhedra surrounding oxygen ions. The difference in energy and volume associated with this sampling of high and low density basins in the underlying energy landscape is consistent with independent verifications of the volume and enthalpy differences between different amorphous forms. The differences in the high- and low-density configurations match the difference in diffraction patterns observed experimentally.

Site-Selective Mott Transition in Rare-Earth-Element Nickelates

A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare-earth-element nickelate perovskites arises from a site-selective Mott phase, in which the d electrons on half of the Ni ions are localized to form a fluctuating moment while the d electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of charge order. A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.

Electronic correlations and crystal structure distortions in BaBiO3

BaBiO3 is a material where Bi4+ ions with half-filled 6s-states form an alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by breathing distortion of the BiO6 octahedra (extension and contraction of the Bi–O bond lengths). Standard density functional theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively an experimentally observed insulator with monoclinic crystal structure but strongly underestimates the breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA + U method using a Wannier function basis set for the Bi 6s-band. Due to the high oxidation state of bismuth in this material, the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part of the d-band Wannier function is concentrated on the metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA + U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA + U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting.

First-principles study of substitutional magnesium and zinc in hydroxyapatite and octacalcium phosphate

First-principles calculations are performed for Mg(2+) and Zn(2+) substitution in hydroxyapatite (HAp) and octacalcium phosphate (OCP), because the foreign ions are known to play an important role for bone formation. In order to study their possible location in the system of HAp in contact with the aqueous solution, OCP is considered as a structural model of the transition region between HAp and the solution. It is found that, when the foreign ions substitute for Ca sites, the surrounding oxygen ions undergo considerable inward relaxation, due to their smaller ionic sizes than Ca(2+), which results in the smaller coordination numbers with oxygen as compared with those of Ca in bulk HAp and OCP. From the calculated defect formation energies, it is likely that the substitutional foreign ions are quite difficult to dissolve into HAp whereas can be more easily incorporated in OCP. In particular, Zn(2+) can more favorably substitute for the specific Ca site of OCP, as compared to Mg(2+), which is attributed with covalent bond formation between Zn and the surrounding oxygen ions. It is thus considered that zinc may play its role to promote bone formation by being incorporated into the transition region between HAp and the surrounding solution.

Effects of x and R3+ on the luminescent properties of Eu3+ in nanocrystalline YVxP1-xO4:Eu3+ and RVO4:Eu3+ thin-film phosphors

Using inorganic oxides and salts instead of alkoxides as the main starting materials, we prepared nanocrystalline YVxP1-xO4:Eu3+ and RVO4:Eu3+ (0≤x≤1; R=Y,La,Gd) thin-film phosphors by the Pechini sol–gel dip-coating process. The resulting films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and photoluminescence excitation and emission spectra as well as luminescence decay. The results of XRD showed that a solid solution formed in the YVxP1-xO4:Eu3+ film series from x=0 to x=1 with zircon structure. The same structure also held for the GdVO4:Eu3+ film, but the LaVO4:Eu3+ film crystallized with a different structure, monazite. AFM and SEM studies revealed that the phosphor films consisted of spherical particles ranging from 90 to 400 nm depending on the film compositions. With the increase of x values in YVxP1-xO4:Eu3+ films, the integrated emission intensity and the red (5D0–7F2)-to-orange (5D0–7F1) intensity ratio of Eu3+ increase due to the increased energy-transfer probability from VO43- to Eu3+ and the increased polarizability of the surrounding oxygen ions, respectively. The x values also have an influence on the decay behavior of Eu3+. The YVO4:Eu3+ and GdVO4:Eu3+ films showed very similar luminescence properties due to their same crystal structures. However, the LaVO4:Eu3+ film exhibited a much different emission property from those of the YVO4:Eu3+ and GdVO4:Eu3+ films due to the structural effects.

Structural Study of Inhomogeneous Charge Distribution of Inequivalent CuO2Planes in Bi2.1Sr1.9Ca2Cu3O10+δSingle Crystals

Four-circle x-ray diffraction measurements were carried out on a single crystal of Bi 2.1 Sr 1.9 Ca 2 Cu 3 O 10+δ with doping levels from underdoped to overdoped. The excess charge of each inequivalent CuO 2 plane was estimated using the obtained inter-atomic distances in two ways by means of the bond–valence-sum method. The copper valence and the valence sum of the copper and the surrounding oxygen ions gave contradictory results for the inhomogeneous charge distribution of the inequivalent CuO 2 planes.

Structure change near the magnetic-transition temperature in the perovskite compound Ba(In 0.5Sb 0.5)O 3

Measurement of the Mössbauer effect of the 37.2 keV γ transition of 121Sb as well as magnetic measurements have been performed to study the structure change near the magnetic-transition temperature in the perovskite compound Ba(In 0.5Sb 0.5)O 3, which was synthesized by the hydrothermal method for the first time. The Mössbauer isomer shift falls in the region of the Sb(V) and reflects some hybridized-orbital behavior. The major component of the quadrupole coupling tensor (eQV zz ) changes with temperature, while the asymmetry parameter ( η) is equal to zero. This implies that the structure change occurs and that the central antimony cations move relative to the surrounding oxygen ions along the z-axis of the crystal.

Theoretical study on defects associated with lead vacancies in PbWO 4 crystals

The defects associated with lead vacancies (VPb) in lead tungstate crystals (PbWO4) are investigated by the relativistic self-consistent discrete variational embedded cluster method. We focus on the density of states and the effect of VPb on surroundings. The results show that the existence of VPb can diminish the bandwidth of WO42- group, however, it can neither produce O- and Pb3+ ions nor result in absorptions at 350 and 420 nm. The charge balance of VPb may be evenly compensated by the surrounding oxygen ions.

Trends in the Stability of Ternary Oxides: Systems M-Pb-O (M = Ca, Sr, Ba)

Recent experimental investigations of phase equilibria and thermodynamic properties of the systems M-Pb-O, where M = Ca, Sr or Ba, indicate a regular increase in thermodynamic stability of ternary oxides, MPbO3 and M2PbO4, with increasing basicity of the oxide of the alkaline-earth metal. Number of stable interoxide compounds at 1100 K in the systems M-Pb-O (M = Mg, Ca, Sr, Ba) increases in unit increments from Mg to Ba. In this paper, experimentally determined standard Gibbs energies of formation of M2PbO4 (M = Ca, Sr, Ba) and MPbO3 (M = Sr, Ba) from their component binary monoxides and oxygen gas are combined with an estimated value for CaPbO3 to delineate systematic trends in thermodynamic stability of the ternary oxides. The trends are interpreted using concepts of tolerance factor and acid-base interactions. All the ternary oxides in these systems contain lead in the tetravalent state. The small Pb4+ ions polarize the surrounding oxygen ions and cause the formation of oxyanions which are acidic in character. Hence, the higher oxidation state of lead is stabilized in the presence of basic oxides of alkaline-earth group. A schematic subsolidus temperature-composition phase diagram is presented for the system BaO-PbO-O-2 to illustrate the change in oxidation states in binary and ternary oxides with temperature.

Synthese und Kristallstruktur des Lanthan-Titanat-Tellurats LaTi(Ti0,25Te0,75)O6 und seine Verwandtschaft mit PbSb2O6 und Sr(MnTe)O6/Synthesis and Crystal Structure of the Lanthanum Titanate Tellurate LaTi(Ti0.25Te0.75)O6 and its Relationship to PbSb2O6 and Sr(MnTe)O6

Abstract Single crystals of LaTi(Ti0.25Te0.75)O6 have been prepared by solid state reactions. X-ray investigations led to trigonal symmetry, space group C3 1-P3 , a = 5.141(10), c = 5.218(10) Å, Z = 1. The compound is characterized by a predominantly ordered distribution of Ti4+ and Te6+. Typical features of the crystal structure are staggered layers containing edge connected TiO6 and (Ti,Te)O6 octahedra. The layers are connected by La3+ ions receiving an octahedral coordination by the surrounding oxygen ions. The relationships to the PbSb2O6 type and the recently described compound Sr(MnTe)O6 are discussed.