Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi<Superscript>57</Superscript>Fe<Subscript>0.10</Subscript>Fe<Subscript>0.85</Subscript>Cr<Subscript>0.05</Subscript>O<Subscript>3</Subscript>

Results of Mössbauer investigations on ⁵⁷Fe nuclei in multiferroic material Bi⁵⁷Fe_0.10Fe_0.85Cr_0.05O₃ in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO₃ is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO₃.     

Visualization of 2-μm radiation by BiF<Subscript>3</Subscript>:Ho<Superscript>3+</Superscript> and BiF<Subscript>3</Subscript>:Ho<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> ceramics

A series of ceramic samples of the compositions BiF₃:1%Ho³⁺, BiF₃:4%Ho³⁺, BiF₃:1%Ho³⁺ + 1%Yb³⁺, and BiF₃:1%Ho³⁺ + 3%Yb³⁺ is synthesized and the conversion of Tm:YLF laser radiation (λ = 1908 nm) is studied. The luminescence spectra exhibit bands in the regions of 490, 545, and 650 nm. The kinetic measurements of the afterglow of the green and red bands show that the population of the ⁵S₂ and ⁵F₄ states in the BiF₃:1%Ho³⁺ samples occurs due to successive absorption of excitation photons, while the ⁵F₅ level of Ho³⁺ is populated due to the ion–ion interaction. Codoping with Yb³⁺ leads to a decrease in the visualization threshold power density to 2 W/cm².     

Visible upconversion and infrared luminescence investigations of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders doped with Er<Superscript>3+</Superscript>, Yb<Superscript>3+</Superscript> and Zn<Superscript>2+</Superscript> ions

Alumina (Al₂O₃) powders doped with Er³⁺, Yb³⁺ and Zn²⁺ ions have been prepared by a low-temperature combustion synthesis technique. The phase purity and crystalline structure of the combustion products are confirmed by powder X-ray diffraction. An efficient frequency upconversion in the visible region and the emission in the infrared (IR) region respectively corresponding to the ²H_11/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions upon direct excitation with a CW laser lasing at ∼980 nm are discussed. The enhancement observed in the intensity of the upconversion emission bands in the visible region and the emission band in the IR region due to the presence of Yb³⁺ and Zn²⁺ in Er³⁺:Al₂O₃ powders is reported and explained in detail.     

Photoluminescence of sol–gel-derived Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> thin-film phosphors with Mg<Superscript>2+</Superscript> and Al<Superscript>3+</Superscript> co-doping

Red-light-emitting Y₂O₃:Eu³⁺ thin-film phosphors were synthesized using a sol–gel process. The effect of Mg²⁺ and Al³⁺ co-dopants on the Y₂O₃:Eu³⁺ thin-film phosphor photoluminescence (PL) property was investigated. At a certain concentration, both Mg²⁺ and Al³⁺ co-dopants were found to further enhance the PL emission intensity of Y₂O₃:Eu³⁺ thin-film phosphors. The optimum PL emission intensity was observed in Y₂O₃:12%Eu³⁺, 7%Mg²⁺ and Y₂O₃:12%Eu³⁺, 2%Al³⁺ phosphor films. From our results, the enhancement of the emission intensity by the Mg²⁺ and Al³⁺ co-dopants is explained in terms of the creation of defect states near the Y(4d+5s) conduction band, which overlap with the Eu³⁺ charge-transfer state (CTS). The overlapping leads to CTS broadening and consequently induces higher absorption and hence an increase of the emission intensity. From X-ray diffraction results, we have found that there is no additional phase formed in the co-doped phosphor films. PACS 68.55.Ln; 78.55.-m; 81.20.Fw     

The single-purpose Mössbauer effect analyser for fast determination of Fe<Superscript>2+</Superscript>/Fe<Superscript>3+</Superscript> ratio in ilmenite

Ilmenite (FeTiO₃) is an initial raw material in titanium white manufacture. The Fe²⁺/Fe³⁺ ratio in ilmenite is measured before the start of sulfate technological process. Possibility of fast measurements of this ratio could help to control manufacturing process. The single-purpose Mössbauer effect analyzer for fast determination of Fe²⁺/Fe³⁺ ratio in ilmenite was built.     

Luminescent properties of solid solutions in the PbF<Subscript>2</Subscript>–Euf<Subscript>3</Subscript> system and lead fluoroborate glass ceramics doped with Eu<Superscript>3+</Superscript> ions

Lead fluoroborate glasses doped with Eu³⁺ are synthesized, studied, and used to produce glassceramics by heat treatment. The structure of glass-ceramics is determined by X-ray diffraction. The optical, mechanical, and luminescent properties of the glass-ceramics are studied. The structure and spectral-luminescent properties of solid solutions in the PbF₂–EuF₃ system obtained by both solid-phase reaction and coprecipitation from solutions are investigated.     

Frequency upconversion properties of Ag: TeO<Subscript>2</Subscript>–ZnO nanocomposites codoped with Yb<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Yb³⁺–Tm³⁺ codoped tellurite glasses containing silver nanoparticles (NPs) were synthesized and characterized using transmission electron microscopy and optical techniques. The samples’ composition and the nucleation of NPs were investigated using electron diffraction and energy dispersive spectroscopy. For the optical experiments, the samples were excited using a diode laser operating at 980 nm, in resonance with the Yb³⁺ transition ²F_7/2→²F_5/2. Photoluminescence (PL) bands corresponding to Tm³⁺ transitions were observed at 480, 650, and 800 nm due to the Yb³⁺→ Tm³⁺ energy transfer. PL enhancement was achieved by heat-treatment of the samples at 325°C during different time intervals. The growth of the PL bands correlates with the increase of the silver NPs concentration. The relevant mechanisms contributing for the PL characteristics are discussed.     

Electron paramagnetic resonance of Er<Superscript>3+</Superscript> ions in a polycrystalline α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The EPR spectra of rare-earth Er³⁺ ions in a polycrystalline corundum α-Al₂O₃ synthesized by the sol-gel technology were revealed. It is shown that the EPR spectra belong to the Er³⁺ ions in the ground state corresponding to the lower Stark sublevel of the ⁴ I _15/2 term and can be described by the spin Hamiltonian of axial symmetry with an effective spin S = 1/2 and the g tensor with components g _‖ = 12.176 and g _⊥ = 4.14. The average value of the g tensor (〈g〉 = 6.82) corresponds to the Γ₇ state in a cubic field. Erbium is assumed to substitute for aluminum in the Al₂O₃ corundum crystal. The local symmetry C ₃ of the Al³⁺ ion remains despite the pronounced expansion of the lattice around the Er³⁺ ion.     

Synthesis and electrochemical performance of amorphous nickel hydroxide codoped with Fe<Superscript>3+</Superscript> and CO<Stack><Subscript>3</Subscript><Superscript>2−</Superscript></Stack>

Amorphous nickel hydroxide codoped with Fe³⁺ and CO₃²⁻ was synthesized by micro-emulsion precipitation method combined with rapid freezing technique. The microstructure and composition of the sample were characterized by X-ray diffraction and IR analysis. The electrochemical performance of the sample was analyzed by cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests. The results showed that the Fe³⁺ and CO₃²⁻ codoping enhances the amorphous feature of the prepared nickel hydroxide. Moreover, the Fe³⁺ and CO₃²⁻ codoping could increase the specific capacity and improve the electrochemical reversibility of the amorphous nickel hydroxide electrode.     

Mössbauer study of copper ferrite diluted by Ga<Superscript>3+</Superscript> and Al<Superscript>3+</Superscript> ions

Mössbauer spectra of the CuGa _x Al_2x Fe_{2 ? 3x} O₄ system (x = 0.1, 0.2, 0.3, 0.4, 0.5) have been studied at a temperature of 295 K. The results obtained are compared with Mössbauer data for the CuGa _x Al _x Fe_2?2x O₄ system. It is established that the hyperfine magnetic fields H _B (for octahedral sites) and H _A (for tetrahedral sites) for ferrites in both systems with ferrimagnetic ordering decrease linearly depending on the total number of nonmagnetic ions in octahedral and tetrahedral sites of the two systems.     

Spectroscopic characteristics of anionic centers in α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystals bombarded by Cu<Superscript>+</Superscript> and Ti<Superscript>+</Superscript> ions

We have studied the effect of bombardment by Cu⁺ and Ti⁺ ions with energy 30 keV on the optical absorption and luminescence of F centers in oxygen-deficient aluminum oxide. We have shown that in the induced optical absorption spectra there are six components of gaussian shape, which can be assigned to absorption bands of F⁺, F², and F₂⁺ centers. We have established that bombardment of the samples by ion beams has a weak effect on the thermoluminescence parameters in the 3.0 eV and 2.4 eV bands, while in the 3.8 eV luminescence band for F⁺ centers, the thermoluminescent response increases considerably. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 422–424, May–June, 2008.     

F<Stack> <Subscript>3</Subscript> <Superscript>−</Superscript> </Stack> molecular ions in fluoride crystals

The UV absorption spectra of F₃^? molecular ions in LaF₃, SrF₂, CaF₂, and BaF₂ crystals doped with rare-earth elements are studied. Comparison of radiation-colored and additively colored crystals reveals the absorption bands of F₃^? hole centers in the region near 6 eV. Nonempirical calculations of optical transitions agree well with experimental results.     

Spectroscopic analysis of Pr<Superscript>3+</Superscript> crystal-field transitions in YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Yttrium aluminium borate single crystals, doped with 1 and 4 mol% of Pr³⁺, were analyzed in the wave number range 500–25000 cm⁻¹ and temperature range 9–300 K by means of high-resolution Fourier transform spectroscopy. In spite of the complex spectra, exhibiting broad and split lines, the energy level scheme was obtained for several excited manifolds. The careful analysis of the spectra as a function of the temperature allowed us to identify most of the sublevels of the ground manifold. The thermally induced line shift, well described by a single-phonon coupling model, could be exploited to provide information about the energy of the phonons involved. The orientation of the dielectric ellipsoid and of the dipole moments associated to a few transitions was also determined from linear dichroism measurements. The experimental data were fitted in the framework of the crystal-field theory, but the agreement was not satisfactory, as already reported for Pr³⁺ ion in other matrices. Additional discrepancies came from the dichroic spectra analysis and the line splitting, possibly associated to hyperfine interaction. Some causes which might be responsible for the difficulties encountered in the Pr³⁺ ion theoretical modelling are discussed.     

Mössbauer study of mechanical alloying in a Mo<Subscript>80</Subscript>Fe<Subscript>20</Subscript> system

The sequence of solid state reactions upon the mechanical alloying of Mo and Fe powders with an 80: 20 atomic ratio was established by means of Mössbauer spectroscopy and X-ray diffraction. At the first stage, a nanostructure and Mo₆₃Fe₃₇ hexagonal close packed (HCP) phase are formed in Mo body-centered cubic lattice (BCC) particles. At the second stage, a body-centered cubic lattice of Mo-Fe solid solution is formed. The process is accompanied by the formation of a minor amount (about 20%) of X-ray amorphous phase.     

XRD,impedance, and Mössbauer spectroscopy study of the Li<Subscript>3</Subscript>Fe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> composite for Li ion batteries

The synthesis procedure of the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite is presented. The monoclinic (A type) and hematite phases were detected by X-ray diffraction after the synthesis of the composite. The structural α–β (at a temperature of 460 K) and β–γ (at a temperature of 523 K) phase transitions in the composite were indicated by the anomalies of the electrical conductivity, dielectric permittivity, and changes of activation energies of conductivity. Two phase transitions have been detected in the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite by ⁵⁷Fe Mössbauer spectroscopy: the phase transition in Li₃Fe₂(PO₄)₃ from the paramagnetic to antiferromagnetic phase at temperature T _N?=?29.5 K and the Morin phase transition in Fe₂O₃ at temperature T _M?=?235 K.     

Hydrothermal synthesis,characterization, and luminescence of Ca<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>:RE (RE = Eu<Superscript>3+</Superscript>, Tb<Superscript>3+</Superscript>, Dy<Superscript>3+</Superscript>) nanofibers

Ca₂B₂O₅:RE (RE = Eu³⁺, Tb³⁺, Dy³⁺) nanofibers were synthesized by the hydrothermal reaction method. The structural refinement was conducted on the base of the X-ray powder diffraction (XRD) measurements. The surface properties of the Ca₂B₂O₅:RE (RE = Eu³⁺, Tb³⁺, Dy³⁺) nanofibers were investigated by the measurements such as the scanning electron microscope (SEM), transmission electron microscope (TEM), and the energy dispersive spectrum (EDS). The nanofiber has a diameter of about 100 nm and a length of several micrometers. The luminescence properties such as photoluminescence excitation (PLE) and emission spectra (PL), decay lifetime, color coordinates, and the absolute internal quantum efficiency (QE) were reported. Ca₂B₂O₅:Eu³⁺ nanofibers show the red luminescence with CIE coordinates of (x = 0.41, y = 0.51) and the luminescence lifetime of 0.63 ms. The luminescence of Ca₂B₂O₅:Tb³⁺ nanofibers is green color (x = 0.29, y = 0.53) with the lifetime of 2.13 ms. However, Dy³⁺-doped Ca₂B₂O₅ nanofibers present a single-phase white-color phosphor with the fluorescence decay of 3.05 ms. Upon near-UV excitation, the absolute quantum efficiency is measured to be 65, 35, and 37 % for Eu³⁺-, Tb³⁺-, Dy³⁺-doped Ca₂B₂O₅ nanofibers, respectively. It is suggested that Ca₂B₂O₅:RE (RE = Eu³⁺, Tb³⁺, Dy³⁺) nanofibers could be an efficient phosphor for lighting and display.     

Investigations on the local structure and the spin-Hamiltonian parameters for the tetragonal Cu<Superscript>2+</Superscript> centre in ZnGeF<Subscript>6</Subscript>⋅6H<Subscript>2</Subscript>O crystal

The spin-Hamiltonian parameters (g factors g_∥, g_⊥ and hyperfine structure constants A_∥, A_⊥) and the local structure for the tetragonal Cu²⁺ centre in trigonal ZnGeF₆?6H₂O crystal are theoretically studied using the perturbation formulae of these parameters for a 3 d⁹ ion in tetragonally elongated octahedra. In the calculations, the contributions to the spin-Hamiltonian parameters from ligand orbital and spin-orbit coupling are included on the basis of the cluster approach in view of moderate covalency of the studied systems, and the required crystal-field parameters are obtained using the superposition model and the local structures of the studied [Cu(H₂O)₆] ²⁺ cluster. According to the calculations, the ligand octahedra around Cu²⁺ suffer relative elongation τ (≈ 0.085 Å) along the [0 0 1] (or C₄) axis for the tetragonal Cu²⁺ centres in ZnGeF₆?6H₂O crystal, due to the Jahn–Teller effect. The calculated results show good agreement with the experimental data.     

Theoretical study of the structure of lead zirconate–titanate PbZr<Subscript>0.6</Subscript>Ti<Subscript>0.4</Subscript>O<Subscript>3</Subscript>

A model of the structure of the piezoelectric ceramic lead zirconate–titanate PbZr_1–x Ti _x O₃ (PZT) is proposed. The model is based on ab initio calculations for possible local structures using the density functional theory (DFT) approach. A comparison of the calculated neutron diffraction data for local structures and the measured diffraction data obtained for actual powder samples shows there is a partially established long-range crystalline order in the material, in the sublattice of Zr and Ti cations.     

Anomalous nuclear spin–lattice relaxation of <Superscript>3</Superscript>Не in contact with ordered Al<Subscript>2</Subscript>O<Subscript>3</Subscript> aerogel

Spin–lattice relaxation of ³Не in contact with the ordered Al₂O₃ fiber aerogel has been studied at the temperature of 1.6 K in fields of 0.1–0.5 T by the pulsed nuclear magnetic resonance (NMR) method. An additional mechanism of the relaxation of ³Не in aerogels is found and it is shown that this relaxation mechanism is not associated with the adsorbed layer. A hypothesis about the influence of intrinsic paramagnetic centers on the relaxation of gaseous ³Не is proposed.     

Mössbauer and X-ray study of biodegradation of <Superscript>57</Superscript>Fe<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript> magnetic nanoparticles in rat brain

Biodegradation of a ⁵⁷Fe₃ O ₄ - based dextran - stabilized ferrofluid in the ventricular cavities of the rat brain was studied by X-ray diffraction and Mössbauer spectroscopy. A two-step process of biodegradation, consisting of fast disintegration of the initial composite magnetic beads into separate superparamagnetic nanoparticles and subsequent slow dissolution of the nanoparticles has been found. Joint fitting of the couples of Mössbauer spectra measured at different temperatures in the formalism of multi-level relaxation model with one set of fitting parameters, allowed us to measure concentration of exogenous iron in the rat brain as a function of time after the injection of nanoparticles.