Rare Earth Impurities Research Articles

Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides LiRF4 (R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF4: Nd3+ single crystal, the splitting of optical spectral lines due to the interaction of Nd3+ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF4: U3+ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb3+, Ce3+, Nd3+, and U3+ ions in LiYF4 and LiLuF4 crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2s and 2p electrons of the nearest neighbor fluorine ions.

Crystal field parameters with Wannier functions: Application to rare-earth aluminates

A method to calculate the crystal field parameters {\it ab initio} is proposed and applied to trivalent rare earth impurities in yttrium aluminate and to Tb$^{3+}$ ion in TbAlO$_3$. To determine crystal field parameters local Hamiltonian expressed in basis of Wannier functions is expanded in a series of spherical tensor operators. Wannier functions are obtained by transforming the Bloch functions calculated using the density functional theory based program. The results show that the crystal field is continuously decreasing as the number of $4f$ electrons increases and that the hybridization of $4f$ states with the states of oxygen ligands is important. Theory is confronted with experiment for Nd$^{3+}$ and Er$^{3+}$ ions in YAlO$_3$ and for Tb$^{3+}$ ion in TbAlO$_3$ and a fair agreement is found.

The electronic level structure of lanthanide impurities in REPO4, REBO3, REAlO3, and RE2O3 (RE = La, Gd, Y, Lu, Sc) compounds

The vacuum referred binding energies of electrons in divalent and trivalent lanthanide impurity states and host band states in the rare earth (RE = La, Gd, Y, Lu, Sc) orthophosphates REPO4, orthoborates REBO3, aluminum perovskites REAlO3, and sesqui-oxides RE2O3 have been determined by combining the recently developed chemical shift model with spectroscopic data from the archival literature. The main trends in impurity and host band level locations with changing type of RE, which determines the site size, and with changing P, B, Al, or RE cation, which determines the strength of bonding with the oxygen ligands, are identified. Sc3+-based compounds are characterized by a relatively low energy for the conduction band bottom, or equivalently a high electron affinity, which is attributed to a relatively strong electron bonding in the 3d-shell of Sc2+.

Segregation of the Eu impurity as function of its concentration in the melt for growing of the lead telluride doped crystals by the Bridgman method

Behavior of a rare earth impurity of Eu in the PbTe single crystals grown by the Bridgman method from the melt with different initial concentrations of impurity NEuint(ml) of about 1×1020cm−3 and less is investigated with X-ray fluorescent element analysis, Secondary Neutral Mass Spectroscopy (SNMS), and magnetic measurements. The impurity distributions along and across the doped ingots are established. It is revealed that doping impurity enters into the bulk of doped crystals only if its initial concentration in the melt is high enough, approximately 1×1020cm−3. If this concentration is lower, about 1×1019cm−3 and less, the doping Eu impurity is pushed out onto the surface of doped ingot. The thickness of the doped surface layer is estimated to be in the order of several microns or somewhat more. The longitudinal distributions of Eu impurity along the axis of doped ingot–for NEuint(ml)=1×1020cm−3, as well as the transverse one in the surface layer where entire doping impurity is pushed out–for NEuint(ml)=1×1019cm−3, are strongly non-monotonic. Possible reasons for this unusual behavior of Eu doping impurity during the growth of PbTe:Eu crystals from the melt are analyzed.

Single-Stage Dual-Column HPLC Technique for Separation and Determination of Lanthanides in Uranium Matrix: Application to Burnup Measurement on Nuclear Reactor Fuel

A single-stage dual-column chromatographic technique has been developed in this study for separation and determination of lanthanides in a uranium matrix. A 5-cm-length reversed-phase column coated with tri-n-octylphosphine oxide (TOPO) was connected in series to a 10-cm-length reversed-phase monolithic column (dynamically modified into a cation exchange column) to accomplish individual isolation of lanthanides from the uranium matrix. The proposed technique eliminates the step of uranium matrix removal for the determination of lanthanides. Samples with a lanthanide-to-uranium ratio (1 part lanthanide to 105 parts uranium) were directly injected into the dual column for the quantitative determination of lanthanides without uranium matrix removal. In some studies, samples of lanthanides in the uranium matrix could be injected as much as 45 times consecutively into a high-performance liquid chromatography system for determination of lanthanides without any uranium elution. The retention behavior of Pu(IV), Pu(III), Am(III), and fission products was also investigated on the TOPO-coated support. The single-stage dual-column chromatographic technique was demonstrated for the determination of fission products such as La and Nd in the dissolver solution of pressurized heavy water reactor spent fuel for the measurement of atom percent fission burnup. The technique can also be employed to estimate lanthanide impurities in samples of UO2 (1 part lanthanide to 106 parts uranium) without removal of the uranium matrix.

Determination of Trace Rare Earth Impurities in Tantalum Pentaoxide by Electrothermal Vaporization ICP-MS Using in situ Volatilization for Matrix Removal

Determination of Trace Rare Earth Impurities in Tantalum Pentaoxide by Electrothermal Vaporization ICP-MS Using in situ Volatilization for Matrix Removal

Lanthanide impurities in wide bandgap semiconductors: A possible roadmap for spintronic devices

The electronic properties of lanthanide (from Eu to Tm) impurities in wurtzite gallium nitride and zinc oxide were investigated by first principles calculations, using an all electron methodology plus a Hubbard potential correction. The results indicated that the 4f-related energy levels remain outside the bandgap in both materials, in good agreement with a recent phenomenological model, based on experimental data. Additionally, zinc oxide doped with lanthanide impurities became an n-type material, showing a coupling between the 4f-related spin polarized states and the carriers. This coupling may generate spin polarized currents, which could lead to applications in spintronic devices.

A Review on How Lanthanide Impurity Levels Change with Chemistry and Structure of Inorganic Compounds

The energy of the 4f-5d transitions of divalent and trivalent lanthanide impurities in compounds depends strongly on the type of lanthanide, its valence, and the type of compound. Despite this large variability there is much systematic in 4f-5d transition energy. Once it is known for one lanthanide that for all others when in the same compound can be predicted. The same applies for the energy of electron transfer from the valence band to the 4f-shell of lanthanides which also behaves in a systematic fashion with type of lanthanide and type of compound. This work reviews my studies during the past fifteen years that are based on an analysis of data on all divalent and all trivalent lanthanides in more than 1000 different inorganic compounds collected from the archival literature. The established redshift and charge transfer models that form the basis to construct binding energy schemes showing all lanthanide levels with respect to the host bands are reviewed and the latest developments are addressed.

Si quantum dot structures and their applications

This paper presents briefly the history of emission study in Si quantum dots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented.The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.

Photoinduced Pockels effect in the Nd-doped ZnO oriented nanofilms

Photoinduced linear electrooptical (EO) effect was studied for Zinc oxide nanofilms doped by Nd. We have chosen Nd rare earth impurity with different contents to explore its influence on photoinduced EO. The kinetics of the photoinduced EO changes was studied. We have chosen the bicolor coherent 10 ns Nd:YAG laser pulses with power densities varying up to about 1 GW/cm2 as the photoinducing beam. The enhancement of the photoinduced liner EO and its relaxation was explored. Additionally, the study of the photoinduced trapping levels within the energy gap was performed and the explanation of the obtained results is given.

Lanthanide 4f-electron binding energies and the nephelauxetic effect in wide band gap compounds

Employing data from luminescence spectroscopy, the inter 4f-electron Coulomb repulsion energy U(6, A) in Eu 2+/3+ impurities together with the 5d-centroid energy shift ϵc(1,3+,A) in Ce3+ impurities in 40 different fluoride, chloride, bromide, iodide, oxide, sulfide, and nitride compounds has been determined. This work demonstrates that the chemical environment A affects the two energies in a similar fashion; a fashion that follows the anion nephelauxetic sequence F, O, Cl, Br, N, I, S, Se. One may then calculate U(6, A) from well established and accurate ϵc(1,3+,A) values which are then used as input to the chemical shift model proposed in Dorenbos (2012) [19]. As output it provides the chemical shift of 4f-electron binding energy and therewith the 4f-electron binding energy relative to the vacuum energy. In addition this method provides a tool to routinely establish the binding energy of electrons at the top of the valence band (work function) and the bottom of the conduction band (electron affinity) throughout the entire family of inorganic compounds. How the electronic structure of the compound and lanthanide impurities therein change with type of compound and type of lanthanide is demonstrated.

Ce3+ 5d-centroid shift and vacuum referred 4f-electron binding energies of all lanthanide impurities in 150 different compounds

A review on the wavelengths of all five 4f–5d transitions for Ce3+ in about 150 different inorganic compounds (fluorides, chlorides, bromides, iodides, oxides, sulfides, selenides, nitrides) is presented. It provides data on the centroid shift and the crystal field splitting of the 5d-configuration which are then used to estimate the Eu2+ inter 4f-electron Coulomb repulsion energy U(6,A) in compound A. The four semi-empirical models (the redshift model, the centroid shift model, the charge transfer model, and the chemical shift model) on lanthanide levels that were developed past 12 years are briefly reviewed. It will be demonstrated how those models together with the collected data of this work and elsewhere can be united to construct schemes that contain the binding energy of electrons in the 4f and 5d states for each divalent and each trivalent lanthanide ion relative to the vacuum energy. As example the vacuum referred binding energy schemes for LaF3 and La2O3 will be constructed.

Conductivity of Se95As5 chalcogenide glassy semiconductor layers containing the EuF3 rare-earth impurity in high electric fields

By investigating the I–V characteristics of an Al-Se95As5:EuF3-Te structure, it is established that, upon the application of a positive potential to Te, the current flows in it by the mechanism of space-charge-limited currents (SCLCs) of unipolar-injection, and the N-type I-V characteristic is observed for the opposite polarity. It is shown that these are processes of the thermal-field ionization of neutral and negatively charged U− centers, which play a dominant role in the current-flow mechanism in the investigated structures upon the application of an electric field with intensities exceeding 105 V/cm. These are also processes of the electron-hole recombination and capture of charge carriers at U− centers. The energy position and concentration of the local states, which correspond to the indicated centers and characterize the effect of the electric field—activation length are determined. It is established that it is the EuF3 impurities that predominantly affect the local-state concentration.

Rare-earth impurities in gallium nitride: The role of the Hubbard potential

We performed a first principles investigation on the electronic properties of 4f-rare earth substitutional impurities in zincblende gallium nitride (GaN:REGa, with RE=Eu, Gd, Tb, Dy, Ho, Er and Tm). The calculations were performed within the all electron methodology and the density functional theory. We investigated how the introduction of the on-site Hubbard U potential (GGA+U) corrects the electronic properties of those impurities. We showed that a self-consistent procedure to compute the Hubbard potential provides a reliable description on the position of the 4f-related energy levels with respect of the GaN valence band top. The results were compared to available data coming from a recent phenomenological model.

The spin effect in zinc-blende CdEuS and CdEuSe: GGA and GGA+U studies

We have investigated the structural, electronic and magnetic properties of substitutional europium rare earth impurity in cubic CdS and CdSe by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the framework of spin-polarized density functional theory (DFT). The electronic exchange-correlation energy is described by generalized gradient approximation GGA and GGA+U (U is the Hubbard correction). The GGA+U method is applied to the rare-earth 4f states. We have calculated the lattice parameters, bulk modulii, the first pressure derivatives of the bulk modulii and the cohesive energies. The calculated densities of states presented in this study identify the metallic behavior of CdEuS and CdEuSe when we use the GGA scheme, whereas when we use the GGA+U, we see that these compounds are half-metallic.

Testing a model-guided approach to the development of new thermoluminescent materials using YAG:Ln produced by solution combustion synthesis

The objective of this work is to test a model-guided approach, coupled with an efficient material synthesis method, for the development of new thermoluminescent (TL) material using yttrium aluminum garnet (YAG, Y3Al5O12) as a model material. We systematically investigated undoped and lanthanide-doped YAG using x-ray diffraction (XRD), TL, and radioluminescence (RL) to understand the role of the lanthanides in the TL process, i.e., whether they act as trapping centers or recombination centers. We also prepared samples with multiple dopants to test the possibility of creating materials with multiple TL peaks. The initial rise method of TL analysis was used to estimate the activation energies associated with the TL peaks.The role of the lanthanide impurities predicted using the lanthanide energy level diagram was largely confirmed, as evidenced by the TL curves, TL emission spectra and activation energy analysis. However, our data suggests that the exact role of the lanthanide dopants during the TL process depends on the thermal stability of the trapped charges, i.e. the same lanthanide can act as a trapping center in one system and as a recombination center in another system. These results demonstrate the possibility of introducing appropriate TL peaks and recombination centers in YAG produced by SCS by lanthanide doping, where the role of the lanthanide dopant is consistent with a model for the lanthanide energy levels. This allows for a more guided approach to the development of new TL materials with peaks in certain temperature range or multiple TL peaks, at least in conditions in which the model applies.

Ferromagnetic resonance studies of Fe thin films with dilute heavy rare-earth impurities

The structure and magnetic properties of Fe1−xGd(Tb)x films prepared by sputtering have been investigated using x-ray diffraction, vibrating sample magnetometer, and ferromagnetic resonance. Magnetic measurements show that the saturation magnetization decreases as the increase of Gd(Tb) content, which agrees with that Gd(Tb) atomic moments align antiparallel with Fe moments. Theoretical fittings of the angular dependence of ferromagnetic resonance field yield the second- and forth-order perpendicular anisotropy constants K⊥1 and K⊥2, which show a spin reorientation with increasing Gd(Tb) doping. The intrinsic contribution of ferromagnetic resonance linewidth as a result of Gilbert damping is separated from the extrinsic contribution because of the inhomogeneity of the films. It is found that the additions of Gd additives do not significantly improve the damping constant, whereas the Tb dopants increase the damping constant from 0.04 without Tb to 0.11 with 19% Tb, thus a remarkable enhancement of 175%.

Luminescence and micro-Raman investigations on inclusions of unusual habit in chrysoprase from Turkey

Chemical analyses performed on chrysoprase from Turkey have shown many trace elements as well as rare earth impurities. Quantitative chemical analyses of inclusions in minerals can improve our understanding of the chemistry of surface. The environmental scanning electron microscope (ESEM) with an attached X-ray energy dispersive system (EDS) is capable of producing rapid and accurate major element chemical analyses of individual inclusions in crystals larger than about 30μm in diameter. The samples were examined with lifetime-resolved and spatially-resolved cathodoluminescence (CL), and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Spatially resolved CL results at room temperature were recorded for two different areas. Bulk area displays with low CL emission and pores contain iron phases such as chromite, hematite and anatase which cause the green color. For the raw data in the lifetime resolved CL spectrum, at least three broad emission bands were detected in a yellow band of the highest intensity at about 550nm, a weaker orange band at about 650nm, and a red band at 720nm. It is assumed that there are links between the CL emissions and the presence of some transition metal and REE elements, but it is obvious that all trace elements do not play a direct role. Micro-Raman measurements were performed on chrysoprase and these showed a characteristic intensive Raman band peaked at 464cm−1 which can be inferred to ν2 doubly symmetric bending mode of [SiO4/M] centers. Raman spectrum of all inclusions found in the material are also given and discussed in detail.

Solid phase separation and ICP-OES/ICP-MS determination of rare earth impurities in nuclear grade uranium oxide

A simple, effective and low cost solid phase extraction procedure was standardized for the trace and ultra-trace level determination of rare earth impurities, such as, Ce, Dy, Sm, Gd, Eu, Er etc. which act as neutron poisons, in nuclear grade uranium oxide (U3O8 > 99.9% by weight). The method involves selective separation of these elements as their fluorides with the help of activated charcoal from major uranium matrix followed by determination by ICP-MS and high resolution ICP-OES. The residual uranium content of the solution was <10 μg/mL. The recovery of REEs ranges from 85 to 105%. The method was validated with nuclear grade uranium oxide standards CRM-I to CRM-V (BARC, Mumbai, India) in addition to some synthetic standards. The RSD of the method was ±12% (n = 3).

Photoluminescence of porous silicon saturated with tungsten-tellurite glass with rare-earth metal impurities

It has been shown that the presence of silicon nanoparticles in a layer of porous silicon saturated with tungsten-tellurite glass causes an increase in the photoluminescence quantum efficiency of erbium (1530 nm) by an order of magnitude in the case of long-wavelength excitation and an enhancement of the ytterbium photoluminescence (980 nm) by almost 50 times and erbium photoluminescence by 25 times in the case of short-wavelength pumping. This luminescence enhancement is explained by the formation of additional channels of transfer of external excitation by silicon nanocrystallites in porous silicon to impurity ytterbium and erbium ions in tungsten-tellurite glass.