States Of Rare Earth Ions Research Articles

Reduction mechanism for Eu ions in Al2O3-containing glasses by heat treatment in H2 gas.

Superior functional glasses doped with rare-earth ions have been prepared by controlling the valence states of rare-earth ions. However, recent work has revealed unresolved questions about the controlling mechanism of rare-earth ions' valence states. To address these questions, oxide glasses with and without Al2O3 and doped with Eu(3+) ions were prepared by a melting process; then, the valence states of Eu(3+) ions were investigated during heating under a hydrogen environment. The Eu(3+) ions were reduced to Eu(2+) only in the glass containing Al(3+) ions; the reduction occurred in the center of the glass over a short heating period. It was discovered that the reduction of Eu(3+) ions concurrently occurred with the formation of OH bonds which were bound with Al(3+) ions. Considering this and the data for the H2 gas diffusion through the glass, we conclude that diffusing H2 gas molecules react with Al-O(-) bonds surrounding Eu(3+) ions to form AlOH bonds and reduce Eu(3+) ions to Eu(2+) via the extracted electrons. When H2 reacts with a glass structure, that hydrogen has transformed into -OH bonds and the hydrogen concentration in the glass decreases. In order to make up the lost hydrogen, more hydrogen molecules can enter into the glass, resulting in the fast reduction of Eu(3+) ions in the center of the glass.

Spectroscopic and visible luminescence properties of rare earth ions in lead fluoroborate glasses

The lanthanide doped lead lithium calcium zinc fluoroborate glasses (LLCZFB:Ln) of composition 20PbF2+10Li2O+5Cao+5ZnO+59B2O3+1Ln2O3 (where Ln=Sm, Eu and Dy in mol%) were prepared by conventional melt quench technique. The amorphous nature of these glasses was confirmed by X-ray diffraction studies. The glass transition temperatures (Tg) were studied by DSC analysis. The glass structure and spectroscopic properties were investigated using optical absorption, vibrational and fluorescence spectra. The FT-IR spectra and Raman spectra reveal the presence of BO3, BO4 and non-bridging oxygen׳s. The Judd–Ofelt intensity parameters Ωλ (λ=2, 4, 6) were determined from the spectral intensities of absorption bands. These parameters were used to calculate the radiative parameters such as radiative transition probability (AR), radiative life time (τR) and branching ratio (βr) for various excited luminescent states of rare earth ions. The visible emission spectra for different rare earth ions were recorded by exciting the samples at different wavelengths and the decay rates for the different rare earth ions were measured. Using the emission spectra, full width half maxima (FWFM), stimulated emission cross section (σEp) were evaluated. The nature of decay profiles of 4F9/2, 4G5/2 and 5D0 states of Dy, Sm and Eu ions respectively are analyzed. Comparison of luminescence features of these glasses and also with those reported for different glass hosts indicates that the LLCZFB:Dy glass has strong luminescence in the visible region.

Magnetic, thermal and transport behavior in single crystalline RCu2Ge2 (R=La, Ce, Pr and Sm) compounds

Magnetic behavior of single crystalline RCu 2 Ge 2 (R=La, Ce, Pr and Sm) compounds is reported. Single crystals were grown using Cu–Ge flux. The compounds crystallize in a tetragonal structure with space group I4/mmm. LaCu 2 Ge 2 shows a diamagnetic behavior. CeCu 2 Ge 2 , PrCu 2 Ge 2 and SmCu 2 Ge 2 orders antiferromagnetically at ≈ 4.2 , 15 and 14.5 K respectively with [001] as the easy axis of magnetization. CeCu 2 Ge 2 shows a Kondo behavior. The Kondo temperature T K , calculated using two different models is of order of 5 K. In PrCu 2 Ge 2 , an additional magnetic transition is observed below 4 K. The origin of the magnetic behavior is attributed to magnetoelastic effects. A possibility of change in easy axis of magnetization after field cycling (below 4 K) is discussed and such effect is rarely observed. The magnetic and specific heat behavior of CeCu 2 Ge 2 and PrCu 2 Ge 2 are analyzed on the basis of crystalline electric field (CEF) model. The CEF splited ground state of rare earth ions is calculated. The magnetic isotherm of SmCu 2 Ge 2 exhibits de Haas-van Alphen oscillations. It is inferred from the frequency spectrum analysis that, the Fermi surface cross-section along [001] is large compared to [100]. • RCu 2 Ge 2 single crystals are grown by self-flux • Anomalous magnetic behavior was observed in PrCu 2 Ge 2 , which was explained on the basis of possible change in the easy axis of magnetization after field cycling. • Crystal electric field calculations were done to explain the observed magnetic properties. • de Haas–van Alphen oscillations were observed in SmCu 2 Ge 2 .

Optical measurement of the effect of electric fields on the nuclear spin coherence of rare-earth ions in solids.

We show that the coherence properties of the nuclear spin states of rare-earth ions in solids can be manipulated by small applied electric fields. This was done by measuring the Stark effect on the nuclear quadrupole transitions of (151)Eu in Y(2)SiO(5) (YSO) using a combination of Raman heterodyne optical detection and Stark modulated quadrupole echoes to achieve high sensitivity. The measured Stark coefficients were 0.42 and 1.0 Hz cm/V for the two quadrupole transitions at 34.54 and 46.20 MHz, respectively. The long decoherence time of the nuclear spin states (25 ms) allowed us to make the measurements in very low electric fields of ∼ 10 V/cm, which produced 100% modulation of the nuclear spin echo, and to measure Stark shifts of ∼ 1 Hz or 20 ppm of the inhomogeneous linewidth.

Local Electronic and Crystal Structure of Magnetic RCo2As2 (R = La, Ce, Pr, Eu)

The experimental X-ray absorption data on the recently prepared magnetic arsenides RCo2As2 (R = rare earth) are provided. The oxidation state of rare-earth ions is explored by means of the X-ray absorption near edge structure (XANES) spectroscopy. Ce and Eu exhibit the intermediate valence state. The rare-earth valence instability sensitive to external perturbations opens the way for the influencing the magnetic state of RCo2As2 compounds.

Structural and optical aspects for Eu3+ and Dy3+ ions in heavy metal glasses based on PbO–Ga2O3–XO2 (X = Te, Ge, Si)

Structural and optical properties of Eu3+ and Dy3+ ions in PbO–Ga2O3–XO2 (X=Te, Ge, Si) glasses have been examined using X-ray diffraction, FT-IR and luminescence spectroscopy. Several crystalline phases such as TeO2, PbTe3O7, PbO and PbO2 were identified in lead tellurite system using X-ray diffraction analysis, in contrast to PbO–Ga2O3–XO2 (X=Ge, Si) glass samples. Characteristic emission bands due to the 5D0→7FJ (J=0–4) transitions of Eu3+ and the 4F9/2→6HJ/2 (J=15, 13, 11) transitions of Dy3+ were registered. Two spectroscopic parameters, such as luminescence intensity ratios associated to 5D0→7F2 and 5D0→7F1 transitions of Eu3+ as well as 4F9/2→6H13/2 and 4F9/2→6H15/2 transitions of Dy3+, and luminescence lifetimes for excited states of rare earth ions were determined. The luminescence intensity ratios for Eu3+ and Dy3+ ions decrease, whereas 5D0 (Eu3+) and 4F9/2 (Dy3+) luminescence lifetime increases, when covalence between Ln3+ and O2− ions is reduced and the phonon energy of the host glass is increased in PbO–TeO2→PbO–GeO2→PbO–SiO2 direction. It was confirmed by FT-IR measurements.

Influence of PbF2 concentration on spectroscopic properties of Eu3 + and Dy3 + ions in lead borate glasses

Effects of PbF2 concentration on spectroscopic properties of Eu3+ and Dy3+ ions in lead borate glasses have been studied. The samples where PbO was partially or totally substituted by PbF2 were examined using DSC, FT-IR, Raman, absorption and luminescence spectroscopies. Luminescence spectra and their decays were analyzed in detail. Especially, the luminescence intensity ratios R (Eu3+) and Y/B (Dy3+) as well as luminescence lifetimes of 5D0 (Eu3+) and 4F9/2 (Dy3+) excited states of rare earth ions have been determined.

Valence States of Rare-Earth Ions and Band Gaps in RBiBa2O6(R = La, Ce, Pr, Nd, Sm, Gd, Eu, and Dy) Photocatalysts

Valence States of Rare-Earth Ions and Band Gaps in RBiBa2O6(R = La, Ce, Pr, Nd, Sm, Gd, Eu, and Dy) Photocatalysts

Valence States of Rare-Earth Ions and Band Gaps in RBiBa2O6 (R = La, Ce, Pr, Nd, Sm, Gd, Eu, and Dy) Photocatalysts

We have measured magnetic susceptibility, powder X-ray diffraction (XRD), and diffuse reflectance spectra of RBiBa2O6 (R = La, Ce, Pr, Nd, Sm, Gd, Eu, and Dy). The valences of the rare earth elements estimated from magnetic susceptibility were trivalent except CeBiBa2O6 and PrBiBa2O6 in which Ce and Pr were tetravalent. In PrBiBa2O6, a significant effect of crystal fields on magnetic susceptibility was observed below 200 K. The (111) reflection in XRD, which is evidence for the R3+–O–Bi5+ ordering in the double-perovskite structure, was observed in all samples except CeBiBa2O6. In PrBiBa2O6, a weak (111) reflection was observed suggesting that a small amount of Pr3+ ions were present. The band gaps estimated from their optical spectra were approximately 0.7 eV for CeBiBa2O6, 1.1 eV for PrBiBa2O6, and 1.4–1.7 eV for R3+BiBa2O6. Thus the band-gaps were closely related to the valences of RBiBa2O6. We discuss the relationship between these physical properties and the photocatalytic activities previously reported.

Synthesis, Photoluminescence and Energy Transfer of Two Novel Tb(III) Ternary Complexes

Two novel Tb(III) ternary complexes with 2-aniline carbonyl benzoic acid (HAB) as the first ligand, imidazo [5,6-f] phenanthroline(IP) and 1,10-phenanthroline(phen)as the secondary ligands were synthesized. The structures of the ligands and Tb(III) complexes were confirmed by elemental analysis, IR spectra and UV spectra. The fluorescence spectra suggested that the luminescence intensity of Tb(HAB)3IP was obviously higher. Further investigation showed that HAB was the donor of energy, IP and phen which functioned as “energy ladder” could transfer the energy in the system of ternary complexes. If this “energy ladder” was located suitable, it could bridge the energy level difference between the triplet state of first ligand and excited states of rare earth ion .So, the intramolecular energy could transfer efficiently to central Tb3+ ion and competitive luminescence of HAB was also reduced.

Investigating material trends and lattice relaxation effects for understanding electron transfer phenomena in rare-earth-doped optical materials

Abstract Rare-earth-doped insulators and semiconductors play an important role in a wide range of modern optical technologies. Knowledge of the relative energies of rare-earth ions’ localized electronic states and the band states of the host crystal is important for understanding the properties of these materials and for determining the potential material performance in specific applications such as lasers, phosphors, and optical signal processing. Current understanding of the systematic variations of electron binding energies in these materials is reviewed with analysis of how lattice relaxation affects the results obtained from different experimental techniques. Detailed examples are presented for rare-earth-doped YAG and LaF3 material systems. A method for predicting the chemical shift of the 4f electrons of rare-earth impurities from the host crystal’s photoemission spectrum is also demonstrated. Furthermore, a simple model is presented that predicts host-dependent trends in the binding energies of the rare-earth ion states in materials ranging from the elemental metals to the ionic fluorides. By understanding the systematic changes in the relative energies for different states, different ions, and different host materials, insight is gained into electron transfer transitions, valence stability, and luminescence quenching that can accelerate the development of materials for optical applications.

Radiation effects on Yb- and Er/Yb-doped optical fibers: A micro-luminescence study

The integration of rare-earth doped optical fibers as part of fiber-based systems in space implies the development of waveguides tolerant to the radiation levels associated with the space missions. Erbium (Er)- or Ytterbium/Erbium (Yb/Er)-doped fibers have been shown to be very sensitive to ionizing radiations. Radiations lead to a strong increase of the fiber attenuation around the pump and amplified signal wavelengths. In this paper, we investigate by confocal luminescence microscopy the radiation-induced spectroscopic changes on prototype Yb- or Yb/Er-doped optical fibers. The set of tested fibers allows us to provide new insights into the relative influence of the P, Al doping on the radiation responses of their silica-based host matrix and on the transitions between the energy states of rare-earth ions.

Chemical bonding, electronic, and magnetic properties ofR3Co4Sn13intermetallics (R=La, Ce, Sm, Gd, and Tb): Density functional calculations

The structural, electronic, and magnetic properties in the intermetallic compounds, R3Co4Sn13 R = La, Ce, Sm, Gd, and Tb, have been investigated by performing density functional theory DFT and DFT+U calculations. The stability, bonding character, charge density, total and partial density of states, and band structure, as well as Fermi-surface feature, were analyzed. In particular, we clearly present the 4f electronic characters in these series. The antibonding states formed by Co 3d and Sn 5p play an important role in the superconductivity of La3Co4Sn13 .4 f electrons exhibit itinerancy and strongly couple with conduction electron, and these properties show strong dependence on the U parameter in the Ce compound. On the contrary, 4f electrons are fully localized at the deep energy level, and their energies do not change with the U parameter in the Gd compound. In Sm and Tb compounds, localization and itinerancy of 4f electrons coexist. In these compounds, magnetism mainly results from 4f electrons, while Co and Sn atoms hardly contribute a magnetic moment because of their sublattice structure and covalent interaction. Comparably, La compound is a nonmagnetic superconductor with the competition between superconductivity and magnetism. Ce3Co4Sn13 is a weak itinerant ferromagnetic metal with a semimetallic behavior. The antiferromagnetic ordering exists in Gd compound. Both Sm3Co4Sn13 and Tb3Co4Sn13 are ferromagnetic metals. The trivalent charge state of rare-earth ions is suggested in these compounds.

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li6Gd(BO3)3 doped with Eu3+ and Ce3+ and Li6Eu(BO3)3 crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce3+-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.

Laser cooling via excitation of localized electrons

Under appropriate conditions, absorption of light by a solid can initiate a process by which it is cooled. In particular, energy is extracted from a material when its absorption of a photon is followed by emission of a photon of higher energy. This up-conversion requires some of the solid's electrons to garner energy from atomic vibrations. Here, two schemes for laser cooling via localized electronic states are addressed. The first scheme utilizes the ground state and an excited state of a localized center. In this two-level scheme, the cooling process is initiated with photon absorption in the extreme low-energy tail of a localized state's vibrationally broadened absorption spectrum. The subsequent atomic relaxation transfers energy of especially large vibratory atomic strains into electrical energy that is then extracted via photon emission. The second scheme involves the ground state and two excited states of a localized center. Cooling is facilitated when (i) the photoexcitation of an electron from its ground state to the lower excited level is followed by (ii) electron-phonon-induced promotion to the uppermost level and the subsequent (iii) return of the electron to its ground state with emission of a photon of higher energy than that of the absorbed photon. However, competing relaxation processes contribute to heating. The net cooling power per unit volume is maximized for both schemes, thereby determining characteristics of localized electronic systems that foster optical cooling. The cooling power per unit volume is greatest at high temperatures and falls rapidly as the thermal energy is reduced below each system's luminescence Stokes shift. Moreover, cooling via the three-level scheme is most effective when (i) the energy separation between excited states is smaller than the thermal energy and (ii) the degeneracy of the highest-lying excited state is much larger than that of the center's middle level. These restrictive conditions appear to be satisfied for the three-level systems that to date exhibit laser cooling at or somewhat below room temperature: severely localized crystal-field-split $f$-electron states of rare-earth ions.

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li6Gd(BO3)3 doped by Eu3+ and Ce3+ ions, Li5.7Mg0.15Gd(BO3)3: Eu, and Li6Eu(BO3)3) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce3+ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.

Core-level in rare-earth metals and their trifluorides from exchange splitting photoemission and ground-state calculations

Photoemission from Eu, Gd, Tb, and their trifluorides has been investigated with respect to exchange splitting of core-levels. For ${\mathrm{EuF}}_{3}$, a splitting of the $4s$ and $5s$ lines has been observed despite the nonmagnetic Eu ion ground state. The major part of the observed splittings is reproduced by the density-functional theory based calculations. This indicates that the exchange splitting of the core-levels is present in the photoemission initial state of rare-earth ions with partially filled $4f$ shell.

Heat capacity of rare-earth ferroborates RFe 3(BO 3) 4

The heat capacity of ferroborates RFe 3(BO 3) 4 (R=Y, Y 0.5Gd 0.5, Gd, Er, Tb, Tm) were studied in the range 0.4–300 K. In TbFe 3(BO 3) 4 at 192 K and GdFe 3(BO 3) 4 at 156 K first-order structural phase transitions were observed. At 50% substitution of Gd for Y the transition temperature shifts to 234 K. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K in each member of this series, except TmFe 3(BO 3) 4 ( T N ∼ 2 4 K ). Besides, in GdFe 3(BO 3) 4 at 9 K a spin-reorientation first-order transition takes place. In the temperature range studied the rare-earth subsystem remains paramagnetic being fully polarized by the Fe subsystem. In a staggered field of the iron subsystem a Zeeman splitting of the ground state of rare-earth ions occurs, which leads to a Schottky-type anomaly and release of additional entropy.

4fn−15d→4fn emission of Ce3+, Pr3+, Nd3+, Er3+, and Tm3+ in LiYF4 and YPO4

High-resolution $4{f}^{n\ensuremath{-}1}5d\ensuremath{\rightarrow}4{f}^{n}$ emission spectra in the (vacuum) ultraviolet are reported for ${\mathrm{Ce}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Er}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ in $\mathrm{Li}\mathrm{Y}{\mathrm{F}}_{4}$ and $\mathrm{Y}\mathrm{P}{\mathrm{O}}_{4}$ host lattices. The positions and intensities of the zero-phonon lines are calculated and compared to the high-resolution emission spectra. For the thulium samples gated detection has been used to distinguish between the spin-forbidden and spin-allowed $4{f}^{n\ensuremath{-}1}5d\ensuremath{\rightarrow}4{f}^{n}$ emissions. Luminescence lifetimes for the spin-forbidden $4{f}^{n\ensuremath{-}1}5d\ensuremath{\rightarrow}4{f}^{n}$ emissions are calculated and compared with experimentally observed lifetimes. A good agreement between experiment and theory is found, demonstrating the validity of the model developed for the $4{f}^{n\ensuremath{-}1}5d$ and $4{f}^{n}$ states of lanthanide ions.

Exciplex emission and photoinduced energy transfer as a function of cavity dimension in naphthalene-linked aza-crown ethers

We report here the photophysical properties of two derivatives of N-( β-methylnaphthalene) aza-crown systems having different cavity dimensions. The aza-crown moiety is attached to β-position of naphthalene moiety by one >CH 2 unit in both the derivatives. The cavity size is found to have a pronounced effect on exciplex formation as well as energy transfer in the systems at room temperature and low temperature, respectively. Both the systems exhibit photoinduced electron transfer (PET) which is evident from their weaker fluorescence emission and their quenched singlet lifetimes as compared to that of free naphthalene. The systems also show a solvent sensitive red shifted broad structureless emission which is assigned to exciplex formation. The ratio of quantum yields of exciplex to monomer emission ( ϕ Exp/ ϕ M) is lower in the smaller aza-crown (L1) as compared to that in the larger aza-crown (L2) implying a different geometry of the two systems in the excited state. Semi-emperical calculations performed on the systems also corroborate the different geometry of the two systems. Complexation of alkali metals, rare earth ions and protons by the aza-crown moiety results in enhancement of fluorescence emission due to blocking of PET. In the presence of protons, L1 exhibits a new emission due to excimer formation which has not been observed in L2 under similar conditions. The rare earth ion complexes of L1 and L2 at low temperature exhibit energy transfer from the lowest triplet state of naphthalene to the rare earth ion states, the extent of energy transfer being greater in the larger aza-crown (L2) as compared to that in smaller aza-crown system (L1).