EXAFS of the superconducting oxide BaPb1−xBixO3

The two-valence state Bi³⁺ and Bi⁵⁺ of Bi in the semiconductor BaBiO₃ and in the BaPb_1?xBi_xO₃ (x = 0, 0.25, 0.6 and 1) system has been determined from the EXAFS spectra above the L_III-edges of Ba, Pb and Bi. Peaks in the radial distribution function up to 5 Å from the absorber have been identified through a model calculation using theoretical amplitudes and phase shifts and the interatomic distances from neutron diffraction measurements. We found indication of local disorder in the Pb-Bi sublattice for the mixed compound.     

Synthesis and luminescence properties of Eu3+,Bi3+-doped BaWO4 phosphors

BaWO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the conventional high-temperature solid-state reaction and chemical precipitation. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) technologies. When the phosphors are prepared by the high-temperature solidstate reaction, Bi³⁺ doping into BaWO₄:Eu³⁺ can increase the emission intensity of 613 nm. However, maximum emission at about 595 nm was observed in Eu³⁺,Bi³⁺-doped BaWO₄ phosphors prepared by the chemical precipitation. The decay constants (monitored at 595 and/or 613 nm) are within 45–100 s. The color purity of the Ba_0:865WO₄: Eu_0:11,Bi_0:025 phosphor (prepared by chemical precipitation) was 100%. The emission mechanism of Eu³⁺,Bi³⁺ in the BaWO₄ phosphors is briefly discussed.     

Studying local structural, valence, and magnetic states of iron ions in Bi0.9Ca0.1FeO3 perovskite

Mössbauer method was used to study a perovskite compound Bi_0.9Ca_0.1FeO₃ at T = 295 K and at temperature above T _N . It has been established that Bi_0.9Ca_0.1FeO₃ has a rhombohedral crystal structure similar to that of BiFeO₃. The substitution of Ca²⁺ for Bi³⁺ ions leads to the formation of three states of Fe³⁺ ions with an octahedral surroundings and one state with a tetrahedral oxygen surroundings with substantially different hyperfine magnetic fields. All Fe ions are in a trivalent state; the compensation of the charge deficit occurs via the formation of oxygen vacancies. Above T _N , two structurally nonequivalent states of Fe³⁺ ions exist in the Bi_0.9Ca_0.1FeO₃ sample, which correspond to the Fe³⁺ ions with an octahedral and tetrahedral oxygen coordination.     

Combustion synthesis and photoluminescence of ZnS:Mn particles

An efficient process based on a solid-state combustion technique has been developed to produce high crystalline and micrometer sized particles of ZnS:Mn⁺² phosphor with sphalerite structure. The precursor mixture of 0.915Zn+S+0.05Mn+0.035ZnCl₂+kNaCl composition (where k is the mole number of NaCl) was combusted under the argon atmosphere followed by post-heat treatment procedure at 700 °C. It was shown that photoluminescence (PL) intensity of ZnS sample can be easily controlled through adjusting NaCl concentration. In the optimized reaction conditions ZnS samples have showed PL intensity almost comparable to that of a commercial one, despite the relatively low purity of precursor materials used. Many interesting phenomena such as high luminescent efficiency, pure cubic ZnS formation after the post-heat treatment and strong influence of Cl⁻ ion on PL intensity have been observed and discussed.     

Temperature dependent photoconductivity and photoluminescence of Cd1 − xMnxTe

Photoconductivity (PC) and Photoluminescence (PL) studies of Cd_{1 − x}Mn_xTe are reported in the ranges of composition 0.15 ⪅ x ⪅ 0.40 and temperature 10 ⪅ T ⪅ 300 K. The results of this study show that p-d transitions make an important contribution to the optical properties of Cd_{1 − x}Mn_xTe in the above range of compositions. Based on UPS and an estimate of the spin-flip energy for the Mn d electrons in Cd_{1 − x}Mn_xTe from literature data we place the centers of the Mn d↑ and Mn d↓ bands contributing to the photoemission and p-d transition processes at -3.5 eV and +2.0 eV, respectively, relative to the Γ₈ valence band edge. Strong changes in both the PL and PC spectra are observed when the Γ₆ conduction band edge shifts through the Mn 3d↓ levels. This explains the specific temperature dependence of the PC for x = 0.32 and the strong increase in the normalized intensity of the PL band at 2.0 eV at x ⪆ 0.35. The shift in the binding energy of the excitonic contribution to the PC with temperature agrees well with the model of Golnik on interactions of the excitons with the Mn²⁺ spins.     

Photoluminescence characteristics of ZnTiO3: Bi nanocrystals

The photoluminescence properties of the Bi³⁺ in sol-gel derived ZnTiO₃ nanocrystals have been investigated. An ultra-violet emission at 360 nm and a visible emission band at 506 nm have been observed, originating from two kinds of emission centers. The former is ascribed to the ³P₁-¹S₀ transition of Bi³⁺ and the latter to the recombination of the electrons with the photo-generated holes trapped in the zinc vacancies. In all cases the latter contribution is predominant.     

Photoluminescence, optical absorption and XRD studies on X-ray irradiated terbium doped KBr crystals

This paper reports the optical absorption, photoluminescence (PL), XRD, SEM studies made on KBr:Tb³⁺ crystals. The integrated light intensity is enhanced by about two orders of magnitude as compared to the undoped samples in spite of some OH^- present in the samples. PL of these crystals exhibits characteristic Tb³⁺ emissions due to transitions from the ⁵D₃ to ⁵D₄ levels to various levels of the ⁷F septet on F-bleaching X-ray irradiated crystals Z₃ centers are observed. The bands observed in the emission spectra of Tb³⁺ doped KBr are found to be shifted to higher wavelength. The stokes-shift of KBr:Tb³⁺ is determined and it is found to have a large value. The XRD studies have been made to determine the crystalline structure of KBr and KBr:Tb³⁺ and also the Miller indices. The SEM studies made reveal the presence of microcrystalline structure.     

Luminescence of LaBr3: Ce,Hf crystals under photon excitation in the ultraviolet,vacuum ultraviolet,and X-ray ranges

This study has been carried out using synchrotron radiation, time-resolved luminescence ultraviolet and vacuum ultraviolet spectroscopy, optical absorption spectroscopy, and thermal activation spectroscopy. It has been found that, in scintillation spectrometric crystals LaBr₃: Ce,Hf characterized by a low hygroscopicity, along with Ce³⁺ centers in regular lattice sites, there are Ce³⁺ centers located in the vicinity of the defects of the crystal structure. It has also been found that the studied crystals exhibit photoluminescence (PL) of new point defects responsible for a broad band at wavelengths of 500–600 nm in the PL spectra. The minimum energy of interband transitions in LaBr₃ is estimated as E _g ～ 6.2 eV. The effect of multiplication of electronic excitations has been observed in the range of PL excitation energies higher than 13 eV (more than 2E _g ). Thermal activation studies have revealed channels of electronic excitation energy transfer to Ce³⁺ impurity centers.     

Enhanced optical activation of Er in Er-doped Al2O3 powders by Y codoping in a sol-gel method

Enhanced photoluminescence (PL) mechanism of Er³⁺-doped Al₂O₃ powders by Y³⁺ codoping at wavelength 1.53 μm has been investigated through PL measurements of 0.1 mol% Er³⁺- and 0-20 mol% Y³⁺-codoped Al₂O₃ powders prepared at a sintering temperature of 900 °C in a non-aqueous sol-gel method. PL intensity and lifetime of Er³⁺-Y³⁺-codoped Al₂O₃ powders composed of γ-(Al,Er,Y)₂O₃ and θ-(Al,Er,Y)₂O₃ phases increased with increasing Y³⁺-codoping concentration. The 10-20 mol% Y³⁺ codoping in 0.1 mol% Er³⁺-doped Al₂O₃ powders intensified the PL intensity by about 20 times, with a PL lifetime prolonged from 3.5 to 5.8 ms. A maximal increase of the optical activity of Er³⁺ in 0.1 mol% Er³⁺-Y³⁺-codoped Al₂O₃ powders about one order was achieved by 10-20 mol% Y³⁺ codoping. It is found that the improved PL properties for Er³⁺-Y³⁺-codoped Al₂O₃ powders are mainly attributed to enhanced optical activation of Er³⁺ in the Al₂O₃ by Y³⁺ codoping, and to the slightly increased radiative quantum efficiency of Er³⁺ in the Al₂O₃.     

Luminescence spectroscopy of Ln-doped Bi-containing phosphates and molybdates

The photoluminescence (PL) emission and excitation spectra of undoped and doped with rare-earth (RE = Eu, Tb) ions K₃Bi₅(PO₄)₆ and K₂Bi(PO₄)(MoO₄) crystals are studied in 3.7–14 eV region of the excitation photon energies at T = 8 and 300 K. The mechanisms of the host-related and RE-related luminescence in 3.7–7 eV region of the excitation photon energies are revealed in comparative analysis of the PL spectra of studied compounds. It is assumed that the excitation mechanisms of host luminescence of K₃Bi₅(PO₄)₆ and K₂Bi(PO₄) (MoO₄) crystals below 4.8 eV are related to Bi³⁺ ions in oxygen surrounding. An efficient energy transfer from the Bi³⁺-related luminescence centers to the emitting RE centers exists in crystals with low concentration of the RE dopants (1%). The PL excitation spectra of K₃Bi₅(PO₄)₆ crystals with high concentration of Eu dopants are formed by O – Eu CT transitions.     

First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6

First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA + U) scheme, show rhombohedral Bi₂FeTiO₆ is a potential multiferroic in which the magnetism and ferroelectricity coexist. A ferromagnetic configuration with magnetic moment of 4μB per formula unit has been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 μC/cm², caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical activity of long-pair and the ‘d⁰-ness’ criterion in off-centring of Ti were coexisting in the predicted new system. In view of the oxidation state of Bi³⁺, Fe²⁺, Ti⁴⁺, and O²⁻ from the orbital-resolved density of states of the Bi-6p, Fe-3d, Ti-3d, and O-2p states, the valence state of Bi₂FeTiO₆ in the rhombohedral phase was found to be Bi³⁺₂Fe²⁺Ti⁴⁺O₆.     

Photoluminescence characteristics of energy transfer between Er and Bi in Gd2O3: Er, Bi

The phosphors, Bi³⁺- activated Gd₂O₃:Er³⁺, were prepared by sol-gel combustion method, and their photoluminescent properties were investigated under ultraviolet light excitation. The emission spectrum exhibited sharp peaks at about 520, 535, 545, 550 and 559 nm due to (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions. The luminescent intensity was remarkably improved by the incorporation of Bi³⁺ ions under 340 nm light excitation, which suggested very efficient energy transfer from Bi³⁺ ions to Er³⁺ions. The introducing of Bi³⁺ ions broadened the excitation band of the phosphor, of which a new strong peak occurred ranging from 320 to 360 nm due to the 6s²→6s6p transition of Bi³⁺ ions. There is significant energy overlap between the emission band of Bi³⁺ ions and the excitation band of Er³⁺ ions. Under 340 nm light excitation, Bi³⁺ absorbed most of the energy and transferred it to Er³⁺. The energy transfer probability from Bi³⁺ to Er³⁺ is strongly dependent on the Bi³⁺ ion concentration. Also, the sensitization effectiveness was studied and discussed in this paper.     

Role of Bi ions for Er ions efficient 1.54 μm light emission in Er/Bi codoped SiO2 thin film prepared by sol-gel method

Er/Bi codoped SiO₂ thin films were prepared by sol-gel method and spin-on technology with subsequent annealing process. The bismuth silicate crystal phase appeared at low annealing temperature while vanished as annealing temperature exceeded 1000 °C, characterized by X-ray diffraction, and Rutherford backscattering measurements well explained the structure change of the films, which was due to the decrease of bismuth concentration. Fine structures of the Er³⁺-related 1.54 μm light emission (line width less than 7 nm) at room temperature was observed by photoluminescence (PL) measurement. The PL intensity at 1.54 μm reached maximum at 800 °C and decreased dramatically at 1000 °C. The PL dependent annealing temperature was studied and suggested a clear link with bismuth silicate phase. Excitation spectrum measurements further reveal the role of Bi³⁺ ions for Er³⁺ ions near infrared light emission. Through sol-gel method and thermal treatment, Bi³⁺ ions can provide a perfect environment for Er³⁺ ion light emission by forming Er-Bi-Si-O complex. Furthermore, energy transfer from Bi³⁺ ions to Er³⁺ ions is evidenced and found to be a more efficient way for Er³⁺ ions near infrared emission. This makes the Bi³⁺ ions doped material a promising application for future erbium-doped waveguide amplifier and infrared LED.     

Synthesis and photoluminescence properties of MZr2(PO4)3:Eu3+; Bi3+ (M=Na; K) phosphors

A series of new red phosphors, MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K), were synthesized using the solidstate reaction method, and their photoluminescence spectra were measured. The MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K) phosphors were efficiently excited by an ultraviolet (UV; 395 nm) source, and showed intense orange-red emission at 595 nm. Further investigation of the concentration-dependent emission spectra indicated that the MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K) phosphors exhibit the strongest luminescence intensity when y = 0.01 in NaZr_2(0:95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺ and y = 0.09 in NaZr_2(0.95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺, whereas the relative PL intensity decreases with increasing Bi³⁺ concentration due to concentration quenching. The addition of Bi³⁺ widens the excitation band of NaZr_2(0.95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺ around 320 nm, which provides the useful idea of broadening the excitation band around 300–350 nm to fit the ultraviolet chip.     

Photoluminescence and radioluminescence study of NaMgF3:Eu nanoparticles

Photoluminescence (PL) and radioluminescence (RL) measurements were made on small (∼25 nm) NaMgF₃ nanoparticles doped with Eu concentrations ranging from 0.1% to 5%. We find that they contained Eu³⁺, Eu²⁺, and an additional unidentified defect with a broad PL emission ∼470 nm. Similar to previous measurements on larger (57 nm–77 nm) NaMgF₃:Eu nanoparticles with 1% Eu and 5% Eu, we find that the PL lifetime decreases with increasing Eu concentration that can be attributed to Eu energy transfer to non-radiative recombination sites. However, there is no change in the fraction of Eu³⁺ distorted sites. The ∼470 nm PL defect peak was also reported for larger nanoparticles, which suggests that this peak arises from similar unidentified point defects. However, the activated non-radiative decay for the small nanoparticles has a significantly lower activation energy. The Eu³⁺ RL decreases by only 2.3% at 10 kGy for low Eu concentrations.     

Nd photoluminescence study of Nd-doped Si-rich silica films obtained by reactive magnetron sputtering

Nd³⁺-doped silicon-rich silicon oxide (SRSO) thin films have been fabricated by reactive magnetron sputtering of a pure silica target topped with Nd₂O₃ chips. The concentration of Nd ions in the deposited layers is controlled by the number of Nd₂O₃ chips, whereas the incorporation of silicon excess is monitored by the hydrogen partial pressure, P_H2, introduced in the Ar plasma, owing to the ability of hydrogen to reduce the oxygen released by the sputtering of the silica target. Photoluminescence (PL) experiments were made at room temperature using a nonresonant excitation line from an Ar laser. The influences of Nd³⁺ content and P_H2 have been studied to optimize the Nd³⁺ emission. PL spectra reveal a two order of magnitude enhancement of the Nd³⁺ emission around both 0.9 and 1.1 μm, when Si nanoclusters (Si-nc) are formed in the same Nd³⁺-doped matrix. The dependence of the Nd³⁺ PL with P_H2 and Nd concentration is indicative of the occurrence of an efficient energy transfer from the Si-nc to the rare earth ions. The radiative lifetime is also deduced and commented in the light of Nd³⁺-Si-nc coupling.     

The temperature‐sensitive luminescence of (Y,Gd)VO4:Bi3+,Eu3+ and its application for stealth anti‐counterfeiting

Anti‐counterfeiting technologies are desired to protect products far away from the violation of dummy, fake and shoddy goods. The phosphor of (Y,Gd)VO₄:Bi³⁺,Eu³⁺ was synthesized for the application of this purpose. Its photoluminescence was investigated by exciting with different wavelengths at variant temperatures. Wide emission color ranged from green through yellow to orange was tuned up by tailor‐ing Bi³⁺ and Eu³⁺ concentrations. The temperature dependent luminescence and wavelength selective excitation of (Y,Gd)VO₄:Bi³⁺,Eu³⁺ were observed, which provide different encryptions in anti‐counterfeiting. To verify the feasibility in application, two anti‐counterfeiting patterns were fabricated practically and excellent performance was obtained. Moreover, the physical mechanisms for the different phenomena of luminescence were elucidated from excitation spectra combining with the configuration coordinate model. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of Bi3+ doping on electronic transport properties of La0.5−xBixCa0.5MnO3 manganites

Electronic transport properties of La_0.5?xBi_xCa_0.5MnO₃ (x=0, 1/16, 1/8, 1/4, 3/8 and 1/2) compounds have been studied systematically to investigate their charge ordering (CO) behaviors. The results show that the CO temperature increases with the substitution of Bi³⁺ ion for La³⁺ ion, suggesting that the charge ordering is enhanced. This is attributed to the special role of the 6s² lone pair of Bi³⁺. It is found that for all the samples the adiabatic small polaronic conduction mechanism is responsible for the transport behavior above CO transition, whereas Mott's variable range hopping mechanism dominates below the CO transition. In addition, the electronic transport behavior of La_0.5?xBi_xCa_0.5MnO₃ compounds is high sensitive to an external magnetic field, which could raise fresh opportunities for application in magnetic sensors.     

Electric field effect on the mechanisms of recombination in KI doped with divalent ions

In KI crystals doped with divalent ions (Eu²⁺, Sr²⁺, Mn²⁺) a strong influence of the electric field is observed, after irradiation, on the carriers (electrons and holes) recombination kinetics. The phenomena are similar whether the electrons, distributed on traps bound to divalent ions, are excited by IR at 4 K, and recombine with trapped holes (V_k centers) or whether the holes are made thermally mobile at T>77 K. It is suggested that this is due to the recombination mechanism: the kinetics are simultaneously controlled by diffusion and tunneling. The tunneling range is a function of the applied field.     

Valence state of manganese ions in the La1−α BiLaβMnLa1−δ OLa3±γ ceramics

The valence state of the Mn ions in ceramic samples of the La_1?α BiLa_βMnLa_1?δ OLa_3±γ composition (LBMO) has been studied using the X-ray photoelectron spectroscopy. The presence of Mn³⁺ and Mn⁴⁺ ions in these compounds has been shown. The relative content Mn³⁺/Mn⁴⁺ has been determined by means of fitting the experimental Mn 2p spectra by the superposition of theoretical spectra of Mn³⁺ and Mn⁴⁺ ions. The elemental composition of the samples has been determined by the X-ray photoelectron spectroscopy and electron probe microanalysis. It has been established that the relative content of Mn⁴⁺ ions correlates with parameter δ, which characterizes the deviation of the actual elemental composition of the La_1?α BiLa_βMnLa_1?δ OLa_3±γ ceramics from stoichiometry La_{1 ? x} Bi _x MnO₃.