The influence of different alkaline earth oxides on the structural and optical properties of undoped,Ce-doped,Sm-doped,and Sm/Ce co-doped lithium alumino-phosphate glasses

Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO₂ concentration as a result of Ce³⁺/Ce⁴⁺ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg²⁺ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba²⁺ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity R_m and molar polarizability α_m values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm³⁺ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO₂, the absorption of Ce³⁺ hinders the high energy absorption of Sm³⁺ and this effect becomes more obvious with increasing optical basicity.     

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

Electron spin resonance studies of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ amorphous thin films have been made. For lower molar contents of B₂O₃, the co-evaporated V₂O₅/B₂O₃ films show poorly resolved hyperfine structure, whereas for higher content of B₂O₃ the hyperfine spectra are well resolved. This behaviour of films is attributed to the increase in the lifetime of a particular V⁴⁺ ion due to Anderson localization of charge, as the degree of disorder increases with increase in the molar content of B₂O₃. The unpaired electron at a given time is localized on a single ⁵¹V nucleus. The low intensity of ESR signal for higher concentration of V₂O₅ in the co-evaporated V₂O₅/B₂O₃ films has been related to the less effective concentration of V⁴⁺ ions due to antiferromagnetic coupling of the V⁴⁺ ions.     

Preparation and scintillation properties of YCl3:Ce crystals

Ce³⁺-activated YCl₃ crystals have been grown by the Bridgman-Stockbarger method, and their scintillation properties have been investigated. The luminescence spectrum of YCl₃:Ce has a complex character and consists of two overlapping peaks at 390 and 410 nm, characteristic of Ce³⁺ luminescence. The light output of YCl₃:Ce as a function of Ce³⁺ concentration has a maximum at 1 mol % Ce³⁺. Original Russian Text ? V.L. Cherginets, T.V. Ponomarenko, L.N. Trefilova, N.V. Rebrova, N.N. Kosinov, V.D. Alekseev, O.V. Zelenskaya, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 8, pp. 1017–1020.     

Effect of annealing temperature on growth of Ce-ZnO nanocomposite thin films: X-ray photoelectron spectroscopy study

Ce-doped ZnO nanocomposite thin films with Ce/Zn ratio fixed at optimum value (10 at.%) have been prepared via sol-gel method at different annealing temperatures varied from 180 to 500 °C. The synthesized samples were characterized employing atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) techniques. According to AFM analysis, the average grain size increased from about 70 nm to 150 nm by increasing the annealing temperature from 300 to 500 °C. Moreover, based on the XPS data analysis, it was found that three major metal ions namely Ce³⁺, Ce⁴⁺, and Zn²⁺ coexist on the surface of the nanocomposite films. XPS data analysis also revealed that Ce³⁺ ion is oxidized to Ce⁴⁺ ion with increasing annealing temperature. Due to oxidation, the ratio of [Ce^3±]/[Ce total] changed from 68.8 to 38.1% by increasing the annealing temperature from 180 to 500 °C. In addition, the Ce/Zn ratio increased from 0.21 to 0.42 when increasing the annealing temperature from 180 to 500 °C indicating migration of Ce ions toward the surface at higher temperatures. Finally, the XRD measurements determined that the ZnO thin films have a hexagonal wurtzite structure and CeO₂ crystallites are formed at 500 °C in the Ce-doped ZnO nanocomposite thin films.     

Photoemission study of the tin doped cerium oxide thin films prepared by RF magnetron sputtering

Ce-Sn-O mixed oxide films prepared by simultaneous Sn metal and cerium oxide magnetron sputtering were studied by high resolution photoemission. The analysis showed that the degree of reduction of the cerium oxide depends on the tin concentration in the film. Ce⁴⁺ → Ce³⁺ conversion is explained by a charge transfer from Sn atoms to unoccupied orbital Ce 4f⁰ of cerium oxide by forming Ce 4f¹ state. The X-ray Photoelectron Spectroscopy data were compared with study of the single-crystalline CeO₂ thin films and Sn/CeO₂(111) model system prepared and studied in situ excluding air exposure effects.     

Growth and luminescent properties of Lu2SiO5 and Lu2SiO5:Ce single crystalline films

Single crystalline films (SCF) of Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce (LSO:Ce) silicates with thickness of 2.5-15 μm were crystallized by liquid phase epitaxy method onto undoped LSO substrates from melt-solution based on PbO-B₂O₃ flux. The scintillation and luminescence properties of LSO:Ce SCF were compared with the properties of LSO:Ce single crystal. The peculiarities of luminescence properties of LSO:Ce SCF in comparison with crystal analog can be due to different distribution of Ce³⁺ over the Lu1 and Lu2 positions of LSO host and are further influenced by Pb²⁺ flux-originated contamination.     

Studies on blue and red photoluminescence from Al2O3:Ce:Mncoatings synthesized by spray pyrolysis technique

Al₂O₃:Ce³⁺:Mn²⁺ films deposited by the spray pyrolysis technique show blue and red emissions under ultraviolet light excitation. The blue emission is due to the de-excitation of Ce³⁺ ions from their excited state 5d to the split ground state ²F. The usually weak red emission attributed to 3d→3d de-excitation of Mn²⁺ is enhanced through an efficient energy transfer from Ce³⁺ to Mn²⁺ ions. The quantum efficiency of this transfer is near to 100%. SEM and RBS have been used to analyze the surface morphology and chemical composition of Ce- and Mn-doped Al₂O₃ films. The films were also characterized by the X-ray photoelectron spectroscopy technique, and it was found that a considerable amount of Mn ions remains linked to chlorine while Ce is mostly in an oxidized state.     

Photoluminescence studies on energy transfer processes in cerium-doped Gd3Al2Ga3O12 crystals

We have measured photoluminescence properties of cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce³⁺ 5d–4f emission exhibit prominent peaks at Gd³⁺ intra-4f absorption bands. The Gd³⁺ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd³⁺ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd³⁺ emission intensity is not correlated with that of the Ce³⁺ emission intensity. Decay curves of the Ce³⁺ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.     

Energy transfers in the potential laser materials LMA:Ce,Nd and LMA:Cr,Ce, Nd (LMA = LaMgAl11O19)

Single crystals of LaMgAl₁₁O₁₉ (LMA) coactivated with Ce³⁺, Nd³⁺ or Ce³⁺, Cr³⁺, Nd³⁺, have been grown by the Verneuil (flame-fusion) method. In LMA:Ce, Nd the Ce → Nd energy transfer occurs both radiatively and non-radiatively. The efficiency of the non-radiative Ce → Nd transfer reaches ∼47%, as deduced from Ce³⁺ fluorescence lifetime measurements. In LMA:Ce, Cr, Nd, besides the Ce → Nd energy transfer, Ce → Cr and Cr → Nd transfers occur. The Ce → Cr energy transfer is highly efficient. This results from the good resonance between Ce emission and Cr absorption, and also from the short Ce-Cr distance compared to the longer Ce-Nd one.     

Scintillation and luminescent properties of undoped and Ce3+ doped Y2SiO5 and Lu2SiO5 single crystalline films grown by LPE method

Single crystalline films (SCFs) of undoped and Ce³⁺ doped Y₂SiO₅ (YSO) and Lu₂SiO₅ (LSO) orthosilicates were crystallized for the first time by liquid phase epitaxy method onto undoped YSO substrates from melt-solution based on PbO–B₂O₃ flux. The scintillation and luminescent properties of YSO:Ce and LSO:Ce SCFs were compared with the properties of bulk single crystal counterparts. We show that the peculiarities of luminescent properties of YSO:Ce and LSO:Ce SCFs in comparison with the crystal analogues are caused by the different distribution of Ce³⁺ ions over Y1/Lu1 and Y2/Lu2 positions of YSO and LSO host and strong influence of Pb²⁺ flux-related impurity on luminescent properties of Ce³⁺ ions.     

Optical properties of translucent CaGa2S4:Ce films prepared by pulsed laser deposition

Translucent CaGa₂S₄:Ce thin films were prepared by pulsed laser deposition. Optical absorption associated with the 4f₁ (²F_5/2) to 5d₂ second-highest excited state transition of the Ce³⁺ ion was clearly observed at 3.60 eV as well as the 4f₁ (²F_5/2) to 5d₁ lowest excited state transition at 2.93 eV. Energy splitting of the 5d state by the crystal field is discussed in terms of a model of a square antiprism structure with D_4d symmetry.     

Absence of chromatic effect: crystal relaxation around CeIII in Y1−xCexPS4 (0≤x≤1)

Ce-containing chalcogenides may exhibit interesting chromatic properties due to the presence of a Ce^III-5f/Ce^III-4d gap. In such insulating materials, color varies from red to yellow depending on the covalent character of the Ce–S bond. Thanks to this correlation, a continuous modification of the Ce–S bond characteristics may be induced to control the hue. Attempts in modifying these bonds have been undertaken by synthesizing the solid solution Y_1−xCe_xPS₄ (x=0, 0.005, 0.01, 0.05 and 0.1), YPS₄ being colorless. The substitution of Y³⁺ cations (rY³⁺(CN8)=1.019 Å) for Ce³⁺ cations (rCe³⁺(CN8)=1.143 Å) let expect a regular shift of the physical parameters deemed to influence the chromatic properties. Nevertheless, diffuse reflectance measurements did not evidence any absorption threshold shift vs. x in the Y_1−xCe_xPS₄ series. The lack of sizeable influence of the Y/Ce substitution on the color is correlated not only to the intrasite, band to band nature of the Ce-5f→Ce-4d transition, but also to the occurrence of local relaxations around Ce in the YPS₄:Ce solid solution insuring a chemical environment of the rare earth very similar to that observed in CePS₄. EXAFS measurements performed at the Y–K and the Ce–L_III edges evidence the maintaining of the average Y–S and Ce–S distances in the Y_1−xCe_xPS₄ solution from x=0 to x=1 as they are in the non-substituted end compounds YPS₄ and CePS₄.     

Application of photoconductivity measurements to photodynamic processes investigation in LiYF4:Ce and LiLuF4:Ce crystals

Photoconductivity measurements are applied to the studies of photodynamic processes in LiYF₄:Ce³⁺ and LiLuF₄:Ce³⁺ crystals undergoing ultraviolet irradiation. Photoconductivity spectra were registered by means of microwave technique under irradiation at 220-320 nm at the room temperature. Photoconductivity signal appeared at ∼300 nm and monotonically increased with the shortening of wavelength. Pumping energy dependencies of photoconductivity within 225-305 nm spectral range were registered and revealed the change of the dependence degree from quadratic at longer wavelengths through linear to saturation-like at shorter ones. Numerical simulation based on four-level model of photodynamic processes was performed. Values and spectral distributions of probabilities of different types of photodynamic processes for 225-295 nm were estimated. Ce³⁺ ions excited-state photoionization cross-section spectra in both investigated materials revealed a band with a peak around 270 nm most probably corresponding to 5d-6s transition of Ce³⁺. Recombination cross-section in Ce:LiLuF₄ appeared to be two orders of magnitude higher than in Ce:LiYF₄. A complete energy level diagram of “Ce³⁺ ion-LiYF₄ crystal” system has been proposed.     

Structure and photocatalytic activity studies of TiO2-supported over Ce-modified Al-MCM-41

Ce-Al-MCM-41, TiO₂/Al-MCM-41 and TiO₂/Ce-Al-MCM-41 materials with varying contents of Ce (by impregnation) and TiO₂ loaded (by solid-state dispersion) on Al-MCM-41 support are prepared. The Ce modified and TiO₂ loaded composite systems are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS) techniques. The DRS and XPS of low Ce content (0.2-0.5 wt.%) modified Al-MCM-41 samples are showing more characteristic of Ce³⁺ species wherein cerium in interaction with Al-MCM-41 and that of high Ce (0.8, 3.0 wt.%) content modified samples are showing the characteristic of both Ce⁴⁺and Ce³⁺species. A series of Ce-modified Al-MCM-41 and TiO₂ loaded composite catalysts are evaluated for photocatalytic degradation of phenol under UV irradiation. Low Ce content in Ce³⁺ state on Al-MCM-41 is showing good photoactivity in comparison with high Ce content samples and pure ceria. The composite TiO₂/Ce-Al-MCM-41 is showing enhanced degradation activity due decreased rate of electron-hole recombination on TiO₂ surface by the redox properties of cerium. The photocatalyst TiO₂/Ce-Al-MCM-41 with an optimum of 10 wt.% TiO₂ and 0.3 wt.% Ce is showing maximum phenol degradation activity. The possible mechanism of phenol degradation on the composite photocatalyst is proposed.     

Incorporation of Si–O induced valence state variation of cerium ion and phase evolution in YAG:Ce phosphors for white light emitting diodes

The role of Si⁴⁺ in the valence state of Ce and phase control in YAG:Ce phosphors was investigated upon incorporation of Si–O in the form of SiO₂. By varying the amount of SiO₂ addition, the valence state of tetravalent and trivalent Ce ions was identified distinctly by X-ray photoelectron spectroscopy and the phase purity of YAG host phase was examined by X-ray diffraction patterns. The self-reduction phenomena from Ce⁴⁺ to Ce³⁺ in YAG:Ce samples was observed to be featured in a typical 5d → 4f yellow-green transition of Ce³⁺ ion upon firing in air, driven by charge compensation for imbalance substitution of Ce⁴⁺ for Y³⁺. The addition of SiO₂ promotes further reduction of Ce⁴⁺ to Ce³⁺ in an amount up to 7.0 wt%, owing to spontaneous charge compensation, and suppresses the formation of YAP (yttrium aluminate perovskite, YAlO₃) and YAM (yttrium aluminate monoclinic, Y₄Al₂O₉). The results reveal the role of SiO₂ addition in a proper amount to be able to achieve desired luminous center of Ce³⁺ and phase-pure YAG for a series of YAG hosted luminescence materials such as blue-excitable YAG phosphor or laser-pumped YAG-based transparent ceramics or glass ceramics for lighting and display purposes.     

Luminescence of nano- and macrosized LaPO4:Ce,Tb excited by synchrotron radiation

Comparing the luminescence properties of nanosized and macroscopic LaPO₄:Ce,Tb powders are performed in wide spectral range using synchrotron radiation. In the present study, LaPO₄:Ce,Tb nanopowder was produced by means of a microwave-induced synthesis in ionic liquids, whereas the bulk sample represents a commercial lamp phosphor. Emission and excitation of both, Ce³⁺ and Tb³⁺ luminescence, is observed to be different when comparing bulk and nanosized LaPO₄:Ce,Tb. In particular, it was shown that the fine structure of the Ce³⁺ as well as the Tb³⁺ related emission is poorly resolved for the nanomaterial. It is suggested that the nanoparticles surface plays a key role regarding the perturbation of rare-earth ions and changes their luminescence properties. Furthermore, it is demonstrated that allowed f-d transitions on Tb³⁺ at high energy are significantly suppressed for nanosized LaPO₄:Ce,Tb. Energy transfer is required to initiate Tb³⁺ emission even in the vacuum ultraviolet spectral range.     

Synthesis and structure of cerium-substituted hydroxyapatite

The aim of this study was to explore the effect of cerium ions on the formation and structure of hydroxyapatite (HAP). All particles, prepared by hydrothermal method, were synthesized at varied X_Ce = Ce/(Ca + Ce) (from 0 to 10%) with the atomic ratio (Ce + Ca)/P fixed at 1.67. Their morphology, composition and crystal structure were characterized by TEM, EPMA, XRD and FTIR. The results showed that in this composition range the apatite structure is maintained, Ce^{3 +} ions could enter the crystal lattice of apatite and substitute Ca^{2 +} ions. The doping of Ce^{3 +} ions resulted in the decrease of the crystallite size with increase in X_Ce. The HAP particles without doping were short rods having a diameter from 10 to 20 nm and a length from 30 to 50 nm. They grew into long needles upon increasing X_Ce.     

Adsorption and kinetic performance of Cs+, Co2+, and Ce4+ radionuclides on zirconium vanadate as a cation exchanger

Amorphous zirconium vanadate samples with different molar ratios have been prepared under optimum conditions. The experimental parameters such as the order of mixing and mixing ratios were established. The ion-exchange powders were characterized by X-ray diffraction, IR spectroscopy, and differential thermal analysis. The materials are thermally and chemically stable. The sorption behavior of Cs⁺, Co²⁺, and Ce⁴⁺ radionuclides was studied at varied values of Zr: V molar ratio, pH, and temperature. The thermodynamic equilibrium constants and the values of ΔG ₀, ΔH ₀, and ΔS ₀ for the exchange of Cs⁺, Co²⁺ and Ce⁴⁺ radionuclides at Zr: V = 1: 6 were calculated from the exchange isotherms. At Zr: V = 1: 6, the capability of the material to sorb Cs⁺, Co²⁺, and Ce⁴⁺ ions is the highest. The process kinetics can be descirbed by first- or second-order equations.     

Electrospinning preparation and photoluminescence properties of SrAl2O4:Ce3+ nanowires

One-dimensional monoclinic SrAl₂O₄:Ce³⁺ nanowires (NWs) were prepared by the electrospinning method for the first time. By annealing PVP/SrAl₂O₄:Ce³⁺ composite nanowires (NWs) with different ratio of PVP (M _w ≈ 1300000) to inorganic precursors of the electrospinning solutions, size-controllable NWs were obtained, ranging of 90–180 nm. Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, and dynamics were employed to characterize the NWs. The results demonstrate that the positions of excitation and emission bands varied uneven with the ratio of PVP to the inorganic precursors and the NWs diameter, which was attributed to the difference of the crystallinity, porosity, and local micro-structures surrounding Ce³⁺. Two decay time constants were observed for the PL of Ce³⁺, one faster (1–3 ns) and one slower (10–20 ns), which were attributed to the hole-electron capture on the luminescent ions and isolated Ce ions, respectively. It is interesting to observe that for the shorter decay process, a maximum occurred as the concentration of Ce³⁺ varied in the range of 1–10 mol%.     

Room temperature ferromagnetism in Fe-doped CeO2 thin films grown on LaAlO3 (001)

Ce_1 − xFe_xO_2 − δ/LaAlO₃(001) thin films (x = 0.01 and 0.03) have been prepared by pulsed laser deposition method and thoroughly characterized using X-ray diffraction (XRD), dc magnetization, near edge X-ray absorption fine structure (NEXAFS), and X-ray magnetic circular dichroism (XMCD). XRD data reveal a single-phase cubic structure with a strong crystallographic orientation along the (200) plane. Room temperature ferromagnetism is confirmed through isothermal hysteresis as well as temperature dependent magnetization measurements, which clearly show the ferromagnetic Curie temperature occurring at least above 350 K. The Fe L_3,2 edge NEXAFS spectra for both Fe-doped thin films exhibit mixed valent Fe²⁺/Fe³⁺ states, whereas Ce M_5,4 edge shows the 4+ state of Ce, throughout the doping. With the increase in Fe doping, Fe²⁺ state increases and a simultaneous decrease in magnetization value is also observed. The XMCD signal of both samples reveals the ferromagnetic ordering of substituted Fe ions in the ceria matrix. Our results indicate that ferromagnetism is intrinsic to the ceria system and is not due to any secondary magnetic impurity.