Divalent Lanthanide Ions Research Articles

Preparation of divalent rare earth ions in air by aliovalent substitution and spectroscopic properties of Ln 2+

When alkaline earth ions in borates, phosphates or borophosphates [SrB 4O 7, SrB 6O 10, BaB 8O 13, MBPO 5 (M=Ca,Sr)] are substituted partially and aliovalently by trivalent rare earth ions such as Sm 3+, Eu 3+, these rare earth ions can be reduced to divalent state by the produced negative charge vacancy V M″. The matrices must have appropriate structure containing a rigid three-dimensional network of tetragonal AO 4 groups (A=B,P). These groups can surround and isolate the produced divalent RE 2+ ions from the reaction with oxygen. Therefore, this reduction reaction can be carried out even in air at high temperature. The produced divalent rare earth ions can be detected by luminescence and XANES methods and their spectroscopic properties are discussed.

A theoretical calculation of vibronic coupling strength: the trend in the lanthanide ion series and the host-lattice dependence

This paper reports a comparison between theoretical and experimental aspects of the vibronic coupling strength in lanthanide ion compounds. The theoretical calculations also have the objective of applying and testing a model developed by one of us. Due to the amount of information and results no detailed comparison with calculations based on previous existing models is made. The emphasis is given to the 3 P 0→ 3 H 4, 5 D 0→ 7 F 2, 6 P 7/2→ 8 S 7/2 and 1 D 2→ 3 H 6 transitions of the Pr 3+, Eu 3+, Gd 3+ and Tm 3+ ions, respectively. The divalent lanthanide ions Eu 2+ and Sm 2+ in LiBaF 3 are also included in the analysis as model systems. In these systems the agreement between the theoretical and the experimental vibronic rates is quite satisfactory. The calculations reproduce the observation that the vibronic coupling is stronger in the beginning (Pr 3+) and at the end (Tm 3+) of the lanthanide ion series and weaker at the center (Eu 3+, Gd 3+), and also the experimentally observed host-lattice dependence. Furthermore, the model also predicts a stronger vibronic coupling for the Eu 2+ and Sm 2+ ions than for the isoelectronic Gd 3+ and Eu 3+ ions, as experimentally observed in fluoride host lattices.

Local structure and persistent spectral hole burning of Sm 2+ in silica-based fibers

Silica-based glasses doped with Sm 2+ have been prepared by using vapor-phase axial deposition method under a reducing atmosphere. The fluorescence spectra reveal that the reduction of Sm 3+ to Sm 2+ ions is accomplished in the silica glass codoped with Al 2O 3, while the Sm 2+ ions are not incorporated into the silica glass without Al 2O 3. The effects of Al 2O 3 codopant on the coordination change around divalent rare-earth ions are discussed according to our previous studies of 151Eu Mössbauer effects. It is assumed that the divalent rare-earth ions are preferentially coordinated by Al polyhedra, resulting in the stability of the lower valence state of the rare-earth ions. Persistent spectral hole burning is also demonstrated at room temperature for the 7F 0– 5D 0 transition of Sm 2+ in the aluminosilicate glass fiber. A hole width of 6 cm −1 is much narrower than the inhomogeneous width of 100 cm −1 , leading to the ratio of inhomogeneous-to-homogeneous width of ∼40.

Spectroscopic properties of SrZnCl 4:M 2+ and BaZnCl 4:M 2+ (M=Eu, Sm, Tm)

The emission spectra of the divalent rare-earth ions Eu 2+, Sm 2+ and Tm 2+, doped into the new compounds SrZnCl 4 and BaZnCl 4, are reported. Eu 2+ and Tm 2+ show broad 4 f n−1 5 d→4 f n emission. Due to the smaller crystal-field splitting, the emission bands for BaZnCl 4:M 2+ are shifted to higher energies when compared to SrZnCl 4:M 2+. For SrZnCl 4:Sm 2+ at 298 K broad 4 f 55 d→4 f 6 emission is observed, while at 20 K additional lines due to the 5D 0→ 7F J transition appear. BaZnCl 4:Sm 2+ shows only line spectra at low temperature. At 298 K a weak d→ f transition is observed. The results are discussed and compared with the corresponding measurements of SrCl 2 and BaCl 2 doped with the divalent rare-earth ions.

Luminescence of re 2+ (re = Sm, Yb) in barium octoborate

The reduction process from trivalent to divalent state for Sm 3+ and Yb 3+ ions in barium borates (BaB 8O 13) prepared in air was observed. The luminescence properties of these divalent rare-earth ions were studied. Yb 2+ shows an f-d broad emission band, due to the 4f 14 → 4f 135d transition, while the Sm 2+ ion shows an f → f transition. From the spectra of Yb 2+, and using the 5D 0 → 7F 0 transition of Sm 2+ as a structural probe, two crystallographic sites were found to be available for the cations in the host. Vibronic transitions of Sm 2+ were observed at low temperature. BaB 8O 13 was found to be a good host for reducing the trivalent rare-earth ions to divalent state and to exhibit interesting spectroscopic properties.

Ab initiocalculation on crystal fields ofSm2+in solids with Cl and F ligands

The linear combination of atomic orbitals (-molecular orbitals) crystal-field theory in the context of the superposition model was used to calculate the intrinsic crystal-field parameters for ${\mathrm{Sm}}^{2+}$ ions in solids with Cl and F ligands at different interionic distances. The five main contributions: point charge, charge penetration, exchange, overlap, and covalency considered in the present work can reasonably describe the experimental results. A direct comparison between divalent and trivalent lanthanide ions leads to the conclusion that the initial intrinsic crystal-field parameters and the power-law exponents corresponding to the distance dependence for divalent lanthanide ions are larger than those for trivalent lanthanide ions.

Redox Equilibrium of Rare-Earth Ions in Chlorine- and Bromine-Doped Fluoroaluminate Glasses

The effect of chloride and/or bromide ions on the redox equilibrium of rare-earth ions, Sm and Yb, was investigated in an AlF3-based YABC glass system. The large absorption bands of 4f-5d transition of Sm2+ and Yb2+ ions were found in the chlorine-doped samples in the visible and ultra-violet regions, respectively. Their intensities increased with the degree of incorporation. Change in the redox equilibrium of Yb ions was found to be a function of both the quantity and type of non-fluoride halide ions used as dopants. Br- was about 6 times more efficient than Cl- and co-doping resulted in a greater number of divalent Yb2+ ions. X-ray photoelectron spectra of Y3d and the thermodynamics data of various halide compounds and molecules reveal that the direct bond formation between rare-earth ions and non-fluoride ions and the lower free energies of mixed halide molecules would play key roles in the formation of divalent rare-earth ions in the glass melts.

Low Temperature EPR Spectra of the (Pr, RE)BaCuO Ceramics in the Orthorhombic Phase

AbstractThe EPR spectra of the members of the series Pr0.5RE0.5Ba2Cu3O6+x (RE = Pr, La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic phase are investigated at low temperature. The EPR spectra arise from two different paramagnetic centers, one from divalent copper ions in orthorhombic local crystal field symmetry and another from exchange coupled pairs of copper ions whose origin is discussed. The EPR spectra essentially depend on the time exposition. The EPR spectra of some divalent and trivalent rare earth ions are recorded. The crystal field analysis is done for the trivalent ytterbium ions. The semiconducting behaviour of the dc conductivity can be attributed to thermally activated small polaron hopping motion.

Charge-ordering in Sm 4Bi 3 and Yb 4As 3 with the anti-Th 3P 4 structure

Sm 4Bi 3 and Yb 4As 3 are mixed valence materials with the ratio of 1:3 of trivalent and divalent rare earth ions and are expected to show charge-ordering. In the specific heat of Sm 4Bi 3 and Yb 4As 3 we observed very sharp peaks at 260 K and at 293 K, respectively. Entropies estimated from these peaks are too small to be ascribed to full ordering, suggesting short range ordering at higher temperatures.

The luminescence of divalent and trivalent rare earth ions in SYAM-fluoride glass

The luminescence of several divalent and trivalent rare earth ions in SYAM fluoride glass (33 mole% SrF 2, 16% YF 3, 35% AlF 3, 16% MgF 2) is reported, and compared with the luminescence of these ions in SrB 4O 7 glass. This study concentrates on transitions of the broad-band type. The following dopant ions are studied: Sm 2+, Eu 2+, Yb 2+, Ce 3+, Pr 3+, Eu 3+, Gd 3+ and Tb 3+. Several efficiently luminescent glasses were found. The results lead to conclusions about the factors which determine the luminescence efficiency in glasses.

The luminescence of divalent and trivalent rare earth ions in the crystalline and glass modifications of SrB 4O 7

The luminescence of several rare earth ions in the crystalline and glass modification of SrB 4O 7 is investigated, with a special stress on the transitions of the broad-band type. The luminescence of all the three divalent rare earth ions is reported and discussed. Yb 2+ and Eu 2+ show mainly broad-band emissions while Sm 2+ shows line emissions of the 5 D 0 level to the 7 F j levels. The results lead to some conclusions about the factors which determine the luminescence efficiency in glasses.

Synthesis and optical properties of β″-alumina single crystals doped with divalent lanthanide ions

A two-step reduction ion-exchange technique is used to synthesize stable β″-alumina single crystals containing Yb2+ and Sm2+ ions. The luminescent properties of these materials are discussed. Yb2+ and Sm2+: β″-aluminas both exhibit parity-allowed optical absorptions in the UV and visible and produce strong single-band emission. Low-lying spin-forbidden states produce complex temperature-dependent luminescence decay in Yb2+:β″-alumina.

Doubly-valent rare-earth ions in halide crystals

The results of extensive research into the properties of divalent rare-earth ions in halide crystals have been reported, particularly in the past 20 years, and yet no review of these data is available. This paper concentrates on the spectroscopic properties of divalent rare-earth ions in alkaline earth halides, alkali halides, and other halide crystals. Special emphasis is given to Eu 2+, Sm 2+ and Yb 2+ which are the most-studied divalent rare-earth species. The optical properties of these ions in solution in halide crystals are summarized and aggregation and precipitation phenomena are described, using the results of EPR and ITC studies of aggregation kinetics.

The luminescence of ytterbium(II) in strontium tetraborate

The luminescence of SrB 4O 7:Yb 2+ is reported and discussed. The quantum efficiency is high, the Stokes shift small, and the emission maximum at short wavelength (≈ 360 nm). At low temperatures extended vibrational structure appears in all the luminescence spectra. A comparison with other divalent rare-earth ions in this host lattice is made.

Fluorescence, photoconductivity and polarization properties of divalent rare earth ions in fluoride crystals

The anomalous emission usually observed in CaF 2 and SrF 2 doped with Yb 2 and BaF 2 :Eu 2+ is due to an impurity trapped exciton which is the lowest excited state of these systems. The excited d electron of Re 2+ ions is delocalized in the s orbitals of next neighbor metal ions from where it can either autoionize or recombine with the RE 3+ ion giving rise to red shifted luminescence. When the electron is excited into the lower energy bands it relaxes into the excitation state and the emission is polarized but when it is excited into the higher energy bands it autoionizes without any prior relaxation and the fluorescence is unpolarized. The profile of the fluorescence decay curves shows that the exciton level is populated via relaxation and tunnel effects. The temperature dependence of the photoconductivity spectra enhances the difference between autoionization and thermal ionization

Ligand field calculation of the lower electronic energy levels of the lanthanide monoxides

Abstract The ligand field theory applied to the lanthanide monoxides predicts that the lower electronic states of these molecules are due to the configuration 4 f N −1 6 s (4 f N for EuO and YbO) of the divalent lanthanide ion Ln 2+ . We present here the results of a ligand field calculation of the energies of the lower electronic states for all the stable rare-earth monoxides.

Hydration energies of the divalent lanthanides and alkaline earth elements

The hydration energies of the divalent lanthanide ions and those of alkaline earth elements are proved to be dependent on their ionic radii. This dependence is described by one and the same equation.

Measurement of ionization rate of photoexcited divalent rare earth ions in fluorite-type crystals

The 4f→5d absorption spectra of Eu 2+ in BaF 2 and CdF 2 are broadened compared to the spectrum of the same ion in SrF 2 or CaF 2, as has long been realized. By convoluting a Lorentzian with the sharp spectrum of the latter, and adjusting the width good copies of the broadened spectra can be made. The lifetimes of the electron in the excited state are 25 and 70 femtoseconds for BaF 2 and CdF 2 respectively. The 4f→4f, 8S→ 6P 5 2 transitions of Eu 2+ in BaF 2 measured in two-photon spectrosco py are broadened into a Fano type antiresonance profile, from which a 70 fs lifetime is derived.

Some observations regarding the effects of background rare-earth impurities on the thermoluminescence and optical absorption of CaF2:Mn

At low Mn concentrations the optical absorption spectra of irradiated CaF2:Mn may be strongly influenced by the presence of trace rare-earth (RE) impurities. The RE absorption centres are predominantly due to divalent RE ions, either isolated or associated with NN F centres, known as photochromic (PC) centres, and ionised PC centres (i.e. an RE2+ ion with a NN anion vacancy). Some of these centres have a low thermal stability and anneal between room temperature and 200 degrees C. During the annealing of the centres noted above, it is observed that the Mn-related absorption peaks increase in intensity. After this increase, the spectrum is then characteristic of Mn-doped crystals. The temperatures over which the Mn-related absorption features disappear increase as the Mn content increases. During this anneal, intense thermoluminescence (TL) is observed. Accordingly, the temperature at which the TL is observed to be most intense shifts to higher values as the Mn content is raised. In samples intentionally doped with Ce (in addition to Mn) the authors find that the Ce 'quenches' very effectively the Mn-related absorption. If the Ce content is high enough, only Ce-related features can be seen in the absorption spectrum. The TL too is dominated by the Ce defects. They conclude that both the sensitivity and the thermal stability of the TL from CaF2:Mn are significantly affected by the presence of trace quantities (at the 10 p.p.m. level) of rare-earth ions. These data may be relevant to reported batch-to-batch irregularities in the TL properties of this material.

Effect of metal bound to the substrate site on calcium release from the phosphoenzyme intermediate of sarcoplasmic reticulum ATPase.

The ADP-sensitive phosphoenzyme intermediate (E1P) of sarcoplasmic reticulum ATPase was formed using CaATP as a substrate and release of its bound calcium was investigated. Our previous study (Shigekawa, M., Wakabayashi, S., and Nakamura, H. (1983) J. Biol. Chem. 258, 14157-14161) indicated that 1 mol of E1P has 3 mol of high affinity binding sites for Ca2+, of which two are transport sites for Ca2+, whereas the remainder is presumably the nucleotide binding site. The calcium bound to the substrate site was readily replaced by other divalent cations or lanthanide ions. These ions induced a pronounced change in the rate of Ca2+ release from the transport sites on the same phosphoenzyme. In E1P, whose substrate site had been deprived of a metal, Ca2+ release from the transport sites was fast. It was similar to the Ca2+ release from the normal E1P, whose substrate site was occupied by either Ca2+ or Mg2+, in that it was accelerated by increasing pH or high ATP and inhibited markedly by the treatment of the enzyme with phospholipase C. Release of Ca2+ from E1P, whose substrate site had been rendered metal-free, increased the fluorescence intensity of the enzyme-bound 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, which decayed rapidly upon addition of Mg2+ plus K+. This result suggests that the active site of the resultant divalent cation-free phosphoenzyme has a conformation similar to that for the normal ADP-insensitive phosphoenzyme, E2P. These data suggest that the basic mechanism for Ca2+ release from the phosphoenzyme may be similar whether or not the phosphoenzyme has bound metal at its substrate site, although this metal affects the rate of Ca2+ release extensively.