Trivalent Impurities Research Articles

Role of dual doping in zinc oxide for optimizing thermoelectric performance

In this study, we have investigated the effect of dual doping on the thermoelectric performance of zinc oxide (ZnO). Pure ZnO is a wide band-gap semiconductor with a very low carrier concentration in its intrinsic form. Doping ZnO by trivalent impurity ions is considered a viable approach to improve its electrical conductivity by enhancing the concentration of charge carriers in the material. However, the above enhancement in the carrier concentration can also increase the electronic component of the system's thermal conductivity, thereby limiting its thermoelectric performance. It is expected that the dual doping of ZnO can decrease its thermal conductivity without adversely affecting its electrical conductivity. To test the above hypothesis, we doped ZnO with four different pairs of dopants (In/B, In/Al, In/Ga, and In/Bi) and compared the thermoelectric performance of the resulting materials over the temperature range of 350 K–773 K. For the above comparison, Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured, and corresponding power factors (PF) and figure of merits (ZT) were determined. Among all the samples investigated in the study, the In/Bi-doped ZnO exhibited the highest ZT value of 0.37 at 773 K.

Dissolution Energies of Impurities and Their Clusters in Powellite CaMoO4

The impurity defects in CaMoO4 are simulated by the method of interatomic potentials. The dissolution energies of monovalent, divalent, and trivalent impurities are calculated, their comparative analysis is performed, and the main patterns of change are presented. The most probable localization of defects is determined. In the case of heterovalent impurities, the most energetically favorable mechanism for their charge compensation has been found, both due to intrinsic crystal defects and under conjugate isomorphism. It is shown that the formation of impurity clusters with intrinsic crystal defects and (to a greater extent) the formation of clusters of different-valence impurities may significantly reduce the dissolution energy of impurities. The formation of neutral clusters of univalent impurities with oxygen vacancies not only makes it possible to increase the solubility of impurities but also reduces the probability of the formation of color centers.

Optical spectroscopy and unusual temperature shift of f – f zero-phonon luminescence lines: KTaO3:Er

We present a study of KTaO3:Er (≈0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO3. We provide information about energy level structures controlling f – f optical transitions in Er3+, on formation of minor cubic octahedral and major orthorhombic Er3+ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.

Defect structure and vibrational states in Eu-doped cubic gadolinium oxide.

We consider an effect of trivalent Eu impurities occupying two different crystallographic positions in cubic gadolinium oxide on its lattice structure and phonon spectrum. The numerical methods employed in the study take into account the shell model to describe interatomic interactions. Using cluster approach, the equilibrium lattice structures and phonon local symmetrized densities of states are calculated. The frequencies of resonant vibrations of various symmetry types induced by europium ions are determined using the Green function method. The participation of ions located around Eu impurities in the symmetrical resonant vibrations is discussed. The calculation results show that europium ions in two non-equivalent structural sites are responsible for the formation of the strongest Raman peak associated with the resonant vibrations at oxygen ions in doped Gd2O3.

Eu-induced lattice vibrations in Gd2O3 crystals

The effect of trivalent Eu impurity occupying a position with the S6 site symmetry in cubic Gd2O3 on the latter’s structure and phonon spectrum is investigated. This study was performed by means of computer modeling within a shell model. The equilibrium structures and phonon local symmetrized densities of states were calculated. In addition, frequencies of localized vibrations induced by Eu ion were determined. The calculated results were compared with the available experimental data in literature.

Optical behavior and TL kinetics of Eu3+ and Tb3+ doped zirconate thermoluminescent phosphors

The influence of Tb3+ and Eu3+ ions on the optical behavior and TL kinetics of the synthesized phosphors were analyzed here. The AZrO3 (A = Ca, Ba, Sr) phosphors activated with Eu3+ and Tb3+ ions were prepared using solid state reaction technique. The doping concentration of Eu3+ and Tb3+ ions is varied from 0 to 2 mol% for each. The PXRD analysis is done to study various crystallographic aspects of synthesized phosphors. It confirms the crystalline structure, phase purity and homogeneity of synthesized phosphors. The FESEM and TEM analyses show formation of particles with irregular shape and variable dimensions. The agglomeration of these particles is a resultant of prolonged heat treatments involved in solid state reaction method at elevated temperatures. The study of TL spectra of UV and gamma irradiated phosphors set forth their second order kinetics. The electrons were trapped in trap centers with high trap depth and hence high amount of energy (~ 3 eV) is required to vacate them. The doping of trivalent rare earth impurity ions in the perovskite host lattice leads to formation of these traps centers. With enhanced TL intensity and low fading characteristics, these phosphors are suitable alternatives for TL mapping and sensing applications.

Yb-doping effect on structure and lattice dynamics of Gd2O3

We report the results of a study that examines the effects of trivalent Yb impurities occupying different sites in cubic Gd2O3 on the latter’s structure and vibrational spectrum. The shell model is employed to compute relaxed structures and phonon symmetrized local densities of states. The lattice structure and dynamics of Yb-doped Gd2O3 are calculated using a cluster approach. The frequencies of localized vibrations caused by Yb ions are also determined. The results of lattice-dynamical calculations are compared with experimental data in literature.

First-principles study of impurity segregation in zirconia, hafnia, and yttria-stabilized-zirconia grain boundaries

The impurity segregation behavior in symmetric tilt Σ5 (310)/[001] grain boundaries (GBs) of cubic ZrO2, HfO2, and yttria-stabilized ZrO2 (YSZ) were studied using first-principles calculations. The substitutional impurities, including divalent (Mg, Ca), trivalent (Al, Y), and tetravalent (Si, Ti) elements, were considered. It is found that divalent and trivalent impurities tend to segregate to a narrow region within 4 Å of the GB plane, while the tetravalent impurities have a much more scattered segregation profile extending 8–15 Å from the GB plane. For ZrO2 GB, the calculated grain boundary impurity enrichment factor for yttrium dopant is about 1.7, which is in good agreement with the experimental value of 1.9. For YSZ GB, there exist a strong GB segregation tendency for Mg, Ca, and Si, and a very weak segregation tendency for Ti in the YSZ GB, which is consistent with experimental findings.

Atomistic Simulation of Impurities Segregation to Free Surfaces of α-Al2O3

Segregation of di- and trivalent impurities to the (00.1), (01.2) and (11.3) surfaces of α-Al2O3 has been investigated using molecular mechanics simulation. In this work, the focus has been on segregation energy calculations depending on impurity size, distance to the surface, and surface coverage. It has been shown the anisotropic segregation of dopants. The segregation energy to the most stable (00.1) surface has the lowest absolute value for all investigated impurities. For trivalent impurities at the (01.2) surface the dependence of the segregation energy on surface coverage has the minimum, which corresponds to the ordered arrangement of dopants. At the (11.3) surface the multilayer segregation has been observed, whereas at the (00.1) and (01.2) surfaces the model of monolayer segregation is acceptable in most studied cases.

Combustion derived Y doped CuO nanoparticle: its structural, morphological and optical properties

In the present paper, we report Yttrium doped CuO nanoparticles by varying weight ratios as 0.2, 0.4 and 0.6 as wt% of Y synthesized using an energy-efficient and solution combustion method with glycine in use as a fuel. Structural and optical characterization of Y doped CuO was investigated by annealing the samples to 400 °C (as-prepared), 600 °C and 800 °C. X-ray diffraction measurements indicate that the synthesized nano crystallite consists of monoclinic CuO phase with impurity phase (Y2O3) of Y at elevated temperature. Scanning Electron Microscopy observations show that the particles are more agglomerated with the addition of Y in CuO. The presence of Y is evidenced by the metal oxide peak shift in FTIR spectra. The effect of annealing and the impurity phase formation of the dopant were observed by the merging of the metal oxide peaks. The optical absorption results show that the optical bandgap energy of Y:CuO nanocrystals were much less as compared to that of the undoped CuO particles. Doping CuO with Y has shifted the absorption edge and narrowing down the Eg due to the existence of excess number of electrons by the trivalent impurity in the conduction band. Increasing the dopant concentration and the annealing has led to the partial curing of copper vacancies which has widened the bandgap. Photoluminescence (PL) spectra at the room temperature showed a strong band edge emission, and thereby confirm an increase in the concentration of defects upon doping with respect to the undoped CuO.

Investigation of microstructural, optical and dc electrical properties of spin coated Al:WO3 thin films for n-Al:WO3/p-Si heterojunction diodes

Tungsten trioxide (WO3) thin films have been prepared via sol-gel spin-coating technique for different concentrations of Al (0, 3, 6, 9 and 12wt.%) and investigated them by structural, optical, dc electrical and Al:WO3/p-Si heterojunction diodes. The XRD analysis reveals that the trivalent impurity of Al effectively influences the WO3. The UV–vis analysis shows an increase in the Al dopant that caused the band gap energy (Eg) to decrease. The SEM analysis shows that the size of the plate-like grains has reduced the Al:WO3 thin films. The composition ratio of W, O and Al elements has been confirmed by EDX spectrum. The dc electrical conductivity analysis has identified that Mott’s 3D variable hopping (VRH) mechanism influences the Al:WO3 thin films. The Al:WO3/p-Si p-n heterojunction diode parameters of ideality factor (n), barrier height (ΦB) and series resistance (Rs) have been calculated using the J-V method. From the current density-voltage-temperature (J-V-T) analysis, the inhomogeneity of barrier heights was extracted using the thermionic emission mechanism with Gaussian distribution (GD) function. As a result, the 9wt.% of Al:WO3/p-Si diode gives better results compared to the other Al:WO3/p-Si diodes.

Effect of point defects on Curie temperature of lithium niobate

AbstractThe effect of point defects on the Curie temperature (Tc) of LiNbO3 (LN) was investigated by combining Tc measurements with an analysis of the defect structures of LN doped with impurities having various valences. The data show that Tc of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These Tc variations were examined with respect to the defect structures of impurity‐doped LN, which are expressed in the form chemical formulae using Kröger‐Vink notation. The defect structures of divalent and trivalent impurity‐doped LN are and , respectively (NbLi: Nb at Li sites; VLi: vacancies at Li sites, MLi: impurities at Li sites, and Li/Nb = the congruent ratio). The defect structure in the case of tetravalent impurities is . Analyses of the defect structures indicated that the NbLi concentration decreases with divalent or trivalent impurity doping, which increases Tc. In contrast, the NbLi concentration increases with tetravalent impurity doping, which decreases Tc. In addition, the divalent or trivalent impurity concentrations at which the NbLi concentration becomes zero were found to correspond to the concentrations at which Tc is maximized, suggesting that Tc of LN depends on the NbLi concentration.

A rational approach to processing cerium-containing raw materials

A method for the online control of cerium extraction from rare-earth raw materials based on the oxidation of its trivalent ions with hydrogen peroxide, followed by the precipitation of hydroxide and additional acid treatment to remove trivalent impurities has been considered.

Diffusion of cation impurities by vacancy mechanism in α-Al2O3: Effect of cation size and valence

Molecular mechanics simulation was used to study the vacancy diffusion mechanism of impurities in corundum. Divalent and trivalent cation impurities of different sizes were investigated. The migration energies for different possible migration paths were obtained by the nudged elastic band method. The binding energies of defect clusters responsible for impurities diffusion were also estimated. It was shown that the two paths give the main contribution to the vacancy diffusion of impurities: migration along the C axis through an aluminum interstitial site, and jumps to the nearest aluminum sites at an angle to the C axis. The migration energies of isovalent impurities weakly depend on ion size. For divalent impurities the simulation revealed that the diffusion mechanism is changed from monovacancy to divacancy transport (mediated by defect clusters containing simultaneously the oxygen and aluminum vacancies) at some concentration of oxygen vacancies.

Ultraviolet-visible spectroscopic characterization of lanthanum beryllate crystals doped with Er, Nd, or Pr ions

Spectroscopic characterization of lanthanum beryllate La2Be2O5 (BLO) single crystals doped with trivalent ions of Eu, Nd or Pr, was carried out in the ultraviolet-visible spectral range using synchrotron radiation spectroscopy in combination with conventional optical absorption and luminescence spectroscopy techniques. On the basis of the obtained data, the energy level diagram for these trivalent impurity ions in BLO host lattice was developed; the optical and electronic properties of the crystals were determined; the possibility of the 4f-4f, 4f-5d and charge transfer transitions was analyzed; spectroscopic properties of the lattice defects formed during the introduction of trivalent impurity ions in the BLO host lattice, were investigated. We found that the lattice defects are responsible for a wide-band photoluminescence (PL) in the energy region of 400–600 nm. The most efficient excitation of the defect photoluminescence in the energy gap of BLO occurs in broad PL excitation-bands at 270 and 240 nm. The PL intensity of defects depends on the type of impurity ion and increases in the sequence: Pr-Nd-Er.

Investigation of defect structure of impurity-doped lithium niobate by combining thermodynamic constraints with lattice constant variations

The defect structures of impurity-doped congruent lithium niobates (c-LN) were determined for impurities with various valences, including divalent, trivalent, and tetravalent impurities, in a concentration range where antisite niobium (NbLi) exists. On the basis of the “Li site vacancy model,” six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The first structure is that in which impurities, vacancies and Nb exist as defects in the Li site and no defects exist in the Nb site (structure A), described as {[LiLi] 1-5x-jy[NbLi]x[MLi]y[VLi]4x+(j-1)y}[NbNb][OO] 3 (V: vacancy, M: impurity, j: valence of impurity, x, y: compositional variable (≠0), Li/Nb = congruent ratio). {[LiLi×] 1-5x-2y[NbLi••••]x[MLi•]y[VLi′]4x+y}[NbNb×][OO×] 3 is an example by the Kröger-Vink notation for divalent M. In the second structure, vacancies and Nb exist as defects in the Li site and impurities exist as defects in the Nb site (structure B), described as {[LiLi] 1-5x-(j-5)y[NbLi]x[VLi]4x+(j-5)y}{[NbNb] 1-y[MNb]y}[OO] 3. {[LiLi×] 1-5x+y[NbLi••••]x[VLi′]4x-y}{[NbNb×] 1-y[MNb′]y}[OO×] 3 is an example for tetravalent M. Since the relationship between impurity concentration and lattice constants for structures A and B differs, the defect structures can be differentiated by analyzing lattice constant variations as a function of impurity concentration. The results show that the defect structure of divalent and trivalent impurity-doped c-LN is structure A and that of tetravalent impurity-doped c-LN is structure B. The NbLi concentration increased with increasing tetravalent impurity concentration. In contrast, the NbLi concentration decreased with increasing divalent and trivalent impurities, leading to suppression of optical damage. The valence of an impurity determines whether the impurity is located in the Li site or Nb site in c-LN, consequently determining whether NbLi decreases or increases when the population of the impurity changes.

Spectroscopic, dielectric properties and local structure observation by EXAFS for Nd,Y:CaF2 crystal

Trivalent impurity ions are clustered easily in fluoride crystals. With the addition of Y3+ spectral properties are greatly enhanced. Low temperature absorption and emission spectra are performed to unfold the optical centers in Nd3+,Y3+:CaF2 crystals. The strongest absorption line at around 790 nm belongs to M luminescence quenched centers in 1%Nd3+:CaF2; and at 797 nm to L′ centers for a codoped one. For emission spectra 1049.3 nm and 1068.7 nm are attributed to M′ and N′ non quenched centers; 1054.6 nm to an L′ site in a 1%Nd3+,5%Y3+:CaF2 sample. EXAFS spectra, an intuitive approach, are applied to reveal that the first coordination number of Nd3+ increased from 9.2 to 10.7 with the addition of Y3+ ions, and the local lattice expanded.

Computational study of the energetics and defect clustering tendencies for Y- and La-doped UO2

The energetics and defect ordering tendencies in solid solutions of fluorite-structured UO2 with trivalent rare earth cations (M3+=Y, La) are investigated computationally using a combination of ionic pair potential and density functional theory based methods. Calculated enthalpies of formation with respect to constituent oxides show higher energetic stability for La solid solutions than for Y. Additionally, calculations performed for different atomic configurations show a preference for reduced (increased) oxygen vacancy coordination around La (Y) dopants. The current results are shown to be qualitatively consistent with related calculations and calorimetric measurements of heats of formation in other trivalent doped fluorite oxides, which show a tendency for increasing stability and increasing preference for higher oxygen coordination with increasing size of the trivalent impurity. The implications of these results are discussed in the context of the effect of trivalent impurities on oxygen ion mobilities in UO2, which are relevant to the understanding of experimental observations concerning the effect of trivalent fission products on the oxidative corrosion rates of spent nuclear fuel.

Simulation of forsterite protonation by the interatomic potential method

The interatomic potential method in an ionic approximation was used to model the protonation of forsterite crystals. The formation of isolated OH− groups in iron-free and iron-bearing crystals and neutral clusters of protonated cation vacancies was considered. It was shown that the presence of trivalent impurities may significantly facilitate protonation processes owing to their reduction to a divalent state or formation of clusters with cation vacancies. In most cases, charge balancing of hydrogen-bearing defects by magnesium vacancies is energetically favorable over that involving silicon vacancies.

The Electronic Structure of Lanthanide Impurities in TiO2, ZnO, SnO2, and Related Compounds

The vacuum referred binding energy of electrons in the 4fn levels for all divalent and trivalent lanthanide impurity states in TiO2, ZnO, SnO2, and related compounds MTiO3 and MSnO3 (M = Ca2 +, Sr2 +, Ba2 +) and Ca2SnO4 are presented. They are obtained by collecting data from the literature on the spectroscopy of lanthanide ions, and by combining that data with the chemical shift model. The model provides the energy at the top of the valence band and at the bottom of the conduction band, and it will be shown that those energies are in excellent agreement with what is known from techniques like photo-electron spectroscopy and electrochemical studies. Electronic level diagrams are presented that explain and predict aspects like absence or presence of lanthanide 4f-4f or 5d-4f emissions and the preferred lanthanide valence.