Trivalent Impurities Research Articles

Physical Properties of Superbulky Lanthanide Metallocenes: Synthesis and Extraordinary Luminescence of [EuII(CpBIG)2] (CpBIG=(4‐nBu‐C6H4)5‐Cyclopentadienyl)

AbstractThe superbulky deca‐aryleuropocene [Eu(CpBIG)2], CpBIG=(4‐nBu‐C6H4)5‐cyclopentadienyl, was prepared by reaction of [Eu(dmat)2(thf)2], DMAT=2‐Me2N‐α‐Me3Si‐benzyl, with two equivalents of CpBIGH. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45 % quantum yield). The crystal structure is isomorphic to those of [M(CpBIG)2] (M=Sm, Yb, Ca, Ba) and shows the typical distortions that arise from CpBIG⋅⋅⋅CpBIG attraction as well as excessively large displacement parameter for the heavy Eu atom (Ueq=0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(CpBIG)2] (M=Sm, Eu, Yb), several physical analyses have been applied. Temperature‐dependent magnetic susceptibility data of [Yb(CpBIG)2] show diamagnetism, indicating stable divalent ytterbium. Temperature‐dependent 151Eu Mössbauer effect spectroscopic examination of [Eu(CpBIG)2] was examined over the temperature range 93–215 K and the hyperfine and dynamical properties of the EuII species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single‐crystal X‐ray data at 203 K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low‐frequency torsional and librational modes in [Eu(CpBIG)2]. X‐ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(CpBIG)2] (Ln=Sm, Eu, Yb) compounds are divalent. The XANES white‐line spectra are at 8.3, 7.3, and 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln2O3 standards. No XANES temperature dependence was found from room temperature to 100 K. XANES also showed that the [Ln(CpBIG)2] complexes had less trivalent impurity than a [EuI2(thf)x] standard. The complex [Eu(CpBIG)2] shows already at room temperature strong orange photoluminescence (quantum yield: 45 %): excitation at 412 nm (24270 cm−1) gives a symmetrical single band in the emission spectrum at 606 nm (νmax=16495 cm−1, FWHM: 2090 cm−1, Stokes‐shift: 2140 cm−1), which is assigned to a 4f65d1→4f7 transition of EuII. These remarkable values compare well to those for EuII‐doped ionic host lattices and are likely caused by the rigidity of the [Eu(CpBIG)2] complex. Sharp emission signals, typical for EuIII, are not visible.

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+.

Formation of microtubes in CdI2 crystals doped with BiI3

The formation of microtubes (MTs) with rectangular cross section in the volume of CdI2 layered crystals doped with Bi3+ trivalent impurity ions has been observed for the first time by scanning electron microscopy. The MTs grow predominantly in pores that appear in the volume of CdI2-BiI3 crystals as a result of a difference in the valence of host Cd2+ ions and guest Bi3+ ions substituted into regular crystal lattice sites. The formation of MT structures is related to the rolling of I-Cd-I trilayers containing impurity ions, which takes place as a result of the presence of uncompensated charges at the edges. Deviation of the MT cross section from circular is explained by nonuniformity of the elastic properties of I-Cd-I trilayers with Bi impurity, which is related to the appearance of vacancies in the cation subsystem upon replacement of Cd2+ ions by Bi3+ ions.

Effect of trivalent metal ion impurities (Al3+, Cr3+ and Fe3+) on the growth, structural and physical properties of potassium acid phthalate (KAP) crystals

Trivalent metal ion (Al, Cr, Fe) doped potassium hydrogen phthalate (KAP) crystals have been grown from aqueous solution at room temperature. Powder XRD and ICP-OES studies confirmed the metal ion doping into KAP crystals. The presence of functional groups in the crystal has been observed by FTIR analysis. Optical transmission studies were carried out by allowing the UV-NIR ray of wavelength between 190 and 1000nm to pass through the (010) face of the grown KAP crystals. Dielectric constant value of Fe-doped KAP at 100Hz was found to be significantly higher than that of undoped and Cr and Al-doped KAP. TG-DTA studies show the decomposition temperatures to be 255, 270, 258 and 287°C for pure, Al3+, Cr3+ and Fe3+ doped KAP crystals respectively. Microhardness studies reveal that the Cr3+ and Fe3+ doped crystals have higher hardness values than that of undoped and Al-doped KAP. The grown crystals were also subjected to second harmonic generation (SHG) efficiency tests.

EPR study of the effect of partial pressure of oxygen in the growth atmosphere on concentration of chromium centers in synthetic forsterite

Samples of synthetic forsterite doped with chromium and grown in an argon atmosphere with different oxygen concentrations (at a partial pressure of oxygen in the range from 0.03 to 0.78 kPa) have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been demonstrated how the oxidizing properties of the atmosphere during the growth of crystals affect the relative concentration of di-, tri-, and tetravalent chromium ions in the samples. The structure of trivalent chromium impurity centers has been discussed.

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.

Acoustic, elastic, and anharmonic properties of Sm1–x R x S solid solutions with trivalent impurities (R = Y, Lа, Tm)

Based on the known experimental data on the rigidity constants c ij (x) of single crystals of samarium monosulfide solid solutions (alloys) with yttrium, lanthanum, and thulium impurities, their anisotropic and isotropic acoustic (sound velocities), elastic (elasticity moduli and Poisson’s ratios), and anharmonic (Gruneisen parameters) properties are investigated. Anomalous behavior of these characteristics at isostructural electron phase transitions in the examined mixed systems in the intermediate valence state is discussed.

First Principle Calculation of $4{\rm f}^{\rm n}\rightarrow 4{\rm f}^{({\rm n}-1)}5{\rm d}$ Absorption Spectra of ${\rm Ce}^{3+}$ and ${\rm Pr}^{3+}$ Ions in Alkaline Earth Fluorides

In this paper we present the results of an ab initio calculations of df-absorption spectra for alkaline earth fluoride matrix doped with Pr3+ and Ce3+ ions. The density-functional theory (DFT) calculations have been performed in embedded-cluster approach. All our results were obtained with taking into account the lattice relaxation in the presence of trivalent impurity ion. The calculation have been made both for cubic and tetragonal centers. For the tetragonal centers we estimate the crystal field splitting of eg level.

Characterization of single crystal uranium-oxide thin films grown via reactive-gas magnetron sputtering on yttria-stabilized zirconia and sapphire

The microstructure and valence states of three single crystal thin film systems, UO2 on (11¯02) r-plane sapphire, UO2 on (001) yttria-stabilized zirconia, and U3O8 on (11¯02) r-plane sapphire, grown via reactive-gas magnetron sputtering are analyzed primarily with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD analysis indicates the growth of single crystal domains with varying degrees of mosaicity. XPS and UPS analyses yield U-4f, U-5f, O-1s, and O-2p electron binding energies consistent with reported bulk values. A change from p-type to n-type semiconductor behavior induced by preferential sputtering of oxygen during depth profile analysis was observed with both XPS and UPS. Trivalent cation impurities (Nd and Al) in UO2 lower the Fermi level, shifting the XPS spectral weight. This observation is consistent with hole-doping of a Mott–Hubbard insulator. The uranium oxide-(11¯02) sapphire system is unstable with respect to Al interdiffusion across the film–substrate interface at elevated temperature.

First-principles studies of the effects of impurities on the ionic and electronic conduction in LiFePO4

Olivine-type LiFePO4 is widely considered as a candidate for Li-ion battery electrodes, yet its applicability in the pristine state is limited due to poor ionic and electronic conduction. Doping can be employed to enhance the material's electrical conductivity. However, this should be understood as incorporating electrically active impurities to manipulate the concentration of native point defects such as lithium vacancies and small hole polarons which are responsible for ionic and electronic conduction, respectively, and not as generating band-like carriers. Possible effects of monovalent (Na, K, Cu, and Ag), divalent (Mg and Zn), trivalent (Al), tetravalent (Zr, C, and Si), and pentavalent (V and Nb) impurities on the ionic and electronic conductivities of LiFePO4 are analyzed based on results from first-principles density-functional theory calculations. We identify impurities that are effective (or ineffective) at enhancing the concentration of lithium vacancies or small hole polarons. Based on our studies, we discuss specific strategies for enhancing the electrical conductivity in LiFePO4 and provide suggestions for further experimental studies.

EVIDENCE OF MULTIPARTICLE OPTICAL CENTERS IN LiNbO3:Er3+-Yb3+ CRYSTAL

A characteristic feature of doped lithium niobate crystals is the liability of its structure to form complex multiparticle optical centers, which consist of two or more closely spaced trivalent impurity ions, even at low concentrations of dopants. This paper presents the results of studies aimed at identifying the characteristics of such centers in the fluorescent signal of the crystal LiNbO 3: Er 3+- Yb 3+ by combination of pulsed excitation at a wavelength of 808 nm with continuous excitation at 980 nm.

Texture and Surface Morphology Effects on Tetragonal Phase Stabilization in ZrO<SUB>2</SUB> Films Deposited by Metal-Organic Chemical Vapor Deposition

Tetragonal zirconia coatings (ZrO2) without doping any trivalent impurities have been deposited by metal-organic chemical vapor deposition (MOCVD) on (100) Si single crystals, using Zr(thd)4 precursor. The surface and cross-section morphologies were observed with Field-Emission-Gun Scanning Electron Microscopy (FEG-SEM). The crystalline structures were characterized by grazing incident X-ray diffraction (GIXRD). Crystallographic textures of these films were studied for both {011}t and {110}t planes by pole figure recording by X-ray diffraction under a 4-circle goniometer. The internal stresses were measured with the use of sin2 psi method. In order to study the relationship of microstructures and tetragonal phase stabilization in ZrO2 films, annealing experiments were taken at different temperature. The results show that the critical crystallite size for tetragonal to monoclinic phase transformation is different for samples with different initial microstructures. Besides the critical crystallite size and the residual stress, the texture and crystallite morphology of the ZrO2 films are responsible for the stabilization of the metastable tetragonal phase.

Ab initiostudy of point defects in the strongly correlated system CoO

Density functional theory is applied to study the effect of substitutional Fe, Al, and In impurities on the structural, electronic, and magnetic properties of stoichiometric and nonstoichiometric CoO. Ab initiocalculated hyperfine interactions at the Fe impurity are used to determine aliovalent states of iron dopant in CoO and Co${}_{1\ensuremath{-}x}$O systems as well as to characterize the local structure around the impurity. Various Hubbard potentials describing the on-site Coulomb interactions at the Fe impurity are considered, and their influence on the calculated hyperfine parameters and electronic structure of Fe-doped CoO is analyzed. Different vacancy-impurity configurations within the Co-deficient matrix are taken into account to find the site preferences for Fe, Al, and In dopants. The hyperfine parameters at the Fe impurity in Co${}_{1\ensuremath{-}x}$O are also investigated as a function of the distance between vacancy and impurity. This study shows that divalent Fe cations are created inside the almost perfect host lattice, while the trivalent Fe cations are stabilized in the system containing cobalt vacancies. Trivalent Co ions are induced in the cobalt sublattice defected by vacancies and impurities. The overall concentration of trivalent cations in Co${}_{1\ensuremath{-}x}$O is twice as large as the concentration of cation vacancies. Trivalent cations of different ionic radii prefer residing in the second-neighbor sites of a cationic sublattice with respect to the cobalt vacancy. The energy gap of CoO with Fe${}^{2+}$ ions is comparable to that calculated for the defect-free system. The band gap of Co${}_{1\ensuremath{-}x}$O with Fe${}^{3+}$ and Co${}^{3+}$ ions is reduced due to the acceptor states introduced by both trivalent cations. Band-gap reduction arising from acceptor levels created by Co${}^{3+}$ cations is also observed for Co${}_{1\ensuremath{-}x}$O defected by nonmagnetic trivalent impurities. The present first-principles studies support and supplement results of several M\ossbauer spectroscopy experiments.

Influence of Trivalent Metal Ions on the Metastable Zone Width of Ammonium Sulfate

AbstractThe metastable zone width (MZW) is a vital parameter for the design of industrial crystallization processes of inorganic salts and could be affected by the addition of impurities. Here, the effect of the trivalent ions Fe3+, Cr3+, and Al3+ on the MZW of ammonium sulfate was observed. All the experiments were performed with the help of ultrasonic speed measurements. Impurity concentrations up to 100 ppm at a solution pH of 4 increase the MZW depending on the metal ion. The pH value also showed a major influence on the effect of trivalent metal impurities. The results were compared to the distribution of metal‐aquo‐complexes existing in aqueous solutions contaminated by trivalent metal ions. The distribution was determined by using the computer‐based simulation software Hyss. The first hydrolysis product was identified to be active in widening the MZW.

Electron transfer processes in double lanthanide activated YPO 4

This work reviews electron transfer processes in double lanthanide doped YPO 4 studied using a diversity of optical techniques. The aim is to identify those transfer processes and to relate them with the location of lanthanide electron donor and lanthanide electron acceptor states within the bandgap of YPO 4. Electron transfer from the valence band to a trivalent lanthanide impurity or to the conduction band was studied by traditional single photon excitation on single lanthanide doped YPO 4. This work reports on YPO 4 doped with the electron donor Ce 3+ and co-doped with an electron acceptor Pr 3+, Er 3+, Nd 3+, Ho 3+, Dy 3+, Tm 3+, or Sm 3+ studied by means of thermoluminescence and optical stimulated luminescence techniques. The same samples were studied by pump and probe two photon spectroscopy utilizing synchrotron photons to transfer electrons from Ce 3+ to Ln 3+ defects and 445 nm laser diode to probe the concentration of created Ln 2+.

Impurity clustering and impurity-induced bands in PbTe-, SnTe-, and GeTe-based bulk thermoelectrics

Complex multicomponent systems based on PbTe, SnTe, and GeTe are of great interest for infrared devices and high-temperature thermoelectric applications. A deeper understanding of the atomic and electronic structure of these materials is crucial for explaining, predicting, and optimizing their properties, and to suggest materials for better performance. In this work, we present our first-principles studies of the energy bands associated with various monovalent (Na, K, and Ag) and trivalent (Sb and Bi) impurities and impurity clusters in PbTe, SnTe, and GeTe using supercell models. We find that monovalent and trivalent impurity atoms tend to come close to one another and form impurity-rich clusters and the electronic structure of the host materials is strongly perturbed by the impurities. There are impurity-induced bands associated with the trivalent impurities that split off from the conduction-band bottom with large shifts towards the valence-band top. This is due to the interaction between the $p$ states of the trivalent impurity cation and the divalent anion which tends to drive the systems towards metallicity. The introduction of monovalent impurities (in the presence of trivalent impurities) significantly reduces (in PbTe and GeTe) or slightly enhances (in SnTe) the effect of the trivalent impurities. One, therefore, can tailor the band gap and band structure near the band gap (hence transport properties) by choosing the type of impurity and its concentration or tuning the monovalent/trivalent ratio. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the whole class of PbTe-, SnTe-, and GeTe-based bulk thermoelectrics.

Spectroscopic and crystal field study of Sm3+ in different phases of TiO2

A systematic study of the photoluminescence spectra of samarium-doped crystalline TiO2 was conducted within the temperature range 10–300 K in order to reveal the energy levels of the trivalent impurity ion in different phases of titania. The sol–gel route was used to prepare Sm-doped TiO2 powders. The nanocrystalline anatase phase was obtained by annealing up to 800 °C, whereas the rutile phase was developed by annealing above 1000 °C. Crystal field calculations of Sm3+ energy levels in the two phases of TiO2 (anatase and rutile) were performed for the first time. The obtained set of energy levels assuming positioning of the Sm3+ ions in regular TiO2 lattice sites was consistent with the experimental results. The effect of the different phases of titania on the impurity is discussed.

Doping Effect in Nickel Oxide

Detailed investigations of nonstoichiometry as well as chemical and self-diffusion in nickel oxide have shown that doubly ionised cation vacancies and electron holes are the predominant defects in this material. The present work is an attempt to demonstrate that aliovalent impurities (Cr, Al, Na and Li) may considerably influence the concentration of these defects and, consequently, the oxidation rate of nickel at high temperatures. It has been shown that small amounts of tri-valent impurities (Cr, Al) bring about an increase of the oxidation rate, while mono-valent ones (Li, Na) decrease the rate of oxidation. These phenomena may satisfactorily be explained in terms of a doping effect. All experiments have been carried out as a function of temperature (1373-1673 K) and oxygen pressure (1-105 Pa) and consequently, it was possible to determine the influence of impurities not only on the oxidation rate but also on the activation energy of reaction and its pressure dependence. The results of these investigations could again be elucidated in terms of doping effect.

Extrinsic luminescence of BaF2: R 3+ crystals (R 3+ = La3+, Y3+, Yb3+)

This paper reports on a study of cross-luminescence in barium fluoride crystals doped by a variety of impurities (K+, Cd2+, Y3+, Yb3+, La3+). It is shown that doping of the crystal with a trivalent impurity gives rise to the formation of an additional cross-luminescence band peaking at 7.5 eV, the intensity of this band increasing with increasing impurity concentration.