Energy levels and crystal-field calculations of trivalent ytterbium in yttrium aluminum garnet and yttrium gallium garnet

The splitting of the two ² F states of Yb³⁺:YAlG and Yb³⁺:YGaG, have been determined from fluorescence and absorption spectra at low temperatures. The levels of the ground states of Yb³⁺: YAlG are at: 0, 388, 613, 778 cm^–1, those of Yb³⁺: YGaG at 0, 308, 567, and 672 cm^–1. Crystal field calculations yield the following values for Yb³⁺:YAlG; A ₂ ⁰ =270 cm^–1, A ₄ ⁰ =–165 cm^–1, A ₄ ⁴ =–1155 cm^–1, A ₀ ⁶ =21 cm^–1, A ₄ ⁶ =–304 cm^–1 and for Yb³⁺:YGaG: A ₀ ² =110 cm^–1, A ₀ ⁴ =–125 cm^–1, A ₄ ⁴ =–1250 cm^–1, A ₀ ⁶ =10 cm^–1 and A ₄ ⁶ =–142 ^–1 A satisfactory agreement has been found for calculated and observed splitting patterns.

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Zusammenfassung Die Aufspaltung der ² F-Zustände von Yb³⁺:YAlG(I) und Yb³⁺:YGaG (II) wurde bei tiefen Temperaturen aus Fluoreszenz- und Absorptionsspektren bestimmt. Die Niveaus des Grundzustandes von (I) liegen bei 0, 388, 613, 778 ^–1, die von (II) bei 0, 308, 567 und 672 ^–1. Die Ligandenfeldtheorie ergibt folgende Werte für die Kristallfeldparameter: für (I) A ₂ ⁰ =270 cm^–1, A ₄ ⁰ =–165 cm^–1, A ₄ ⁴ =–1155 cm^–1, A ₀ ⁶ =21 cm^–1, A ₄ ⁶ =–304 cm^–1 unf für (II) A ₀ ² =110 cm^–1, A ₀ ⁴ =–125 cm^–1, A ₄ ⁴ =–1250 cm^–1, A=10 cm^–1 and A ₄ ⁶ =–142 ^–1. Die Übereinstimmung zwischen berechneter und beobachteter Aufspaltung war befriedigend.

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Résumé La division, par le champ de ligandes, des états ² F _7/2 et ² F _5/2 dans Yb³⁺:YAlG et Yb³⁺:YGaG a été déterminée des spectres de fluorescence et d'absorption. Les sous-niveaux de l'état fordamental se situent à 0, 388, 613, 778 cm^–1 dans Yb³⁺:YAlG, à 0, 308, 567, 672 cm^–1 Yb³⁺: YGaG. Le calcul fournit les valeurs suivantes pour les paramètres: Yb³⁺: YAlG A ₂ ⁰ =270 cm^–1, A ₄ ⁰ =–165 cm^–1, A ₄ ⁴ =–1155 cm^–1, A ₀ ⁶ =21 cm^–1, A ₄ ⁶ = -304 cm^–1 Yb³⁺:YGaG: A ₀ ² =110 cm^–1, A ₀ ⁴ =–125 cm^–1, A ₄ ⁴ =–1250 cm^–1, A ₀ ⁶ =10 cm^–1 et A ₄ ⁶ =–142 ^–1. L'accord entre les spectres calculé et observé est satisfaisant.

     

Nonstoichiometry and electronic defects in yttrium iron garnet

Results are presented of a thermogravimetrical analysis of yttrium iron garnet, Y₃Fe₅O_12?δ, in the temperature range 950–1270°C. From these measurements the oxygen vacancy concentration δ is obtained for partial oxygen pressures between 1 and 10^?5 atm. The data can be fitted with a relation $\text{δ = A}\text{exp}\text{(}\text{?E}\text{kT}\text{)}$. Values of A and E are given for different values of P_O2. The combined data from electrical conductivity measurements, measurements of Seebeck coefficients, and thermogravimetric analysis, are used to calculate the concentrations of point defects in the garnet lattice. The results are expressed in terms of equilibrium reaction constants. The model is also used to analyze diffusivity data.     

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y₃Al₅O₁₂ powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y₃Al₅O₁₂ crystallizes in a dynamic mode from the xerogel was determined to be 850?C950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230?C350 m²/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.     

Thermal decomposition behavior of precursors for yttrium aluminum garnet

Precursor powders for yttrium aluminum garnet (YAG) were synthesized by solution combustion reactions (nitrate–glycine reaction with stoichiometric and sub-stoichiometric amount of fuel) and simple decomposition of nitrate solution. The TG-DTA, FTIR and XRD analyses of the precursors and the typical heat-treated samples were carried out to understand the processes occurring at various stages during heating to obtain phase pure YAG. Precursors from all the reactions exhibited dehydration of adsorbed moisture in the temperature range of 30 to 300°C. The precursor from nitrate–glycine reaction with stoichiometric amount of fuel (precursor- A) contained entrapped oxides of carbon (CO and CO₂) and a carbonaceous contaminant. It exhibited burning away of the carbonaceous contaminant and crystallization to pure YAG accompanied by loss of oxides of carbon in the temperature ranges of 400 to 600 and 880 to 1050°C. The precursor from simple decomposition of nitrates (precursor-B) exhibited denitration cum dehydroxylation and crystallization in the temperature ranges of 300 to 600 and 850 to 1050°C. The precursor from nitrate–glycine reaction with sub-stoichiometric amount of fuel (precursor-C) contained entrapped carbon dioxide and exhibited its release during crystallization in the temperature range of 850 to 1050°C. This study established that, in case of metal nitrate–glycine combustion reactions, crystalline YAG formation occurs from an amorphous compound with entrapped oxides of carbon. In case of simple decomposition of metal nitrates, formation of crystalline YAG occurs from an amorphous oxide intermediate.     

Electron-phonon interactions in ce3+-doped yttrium aluminum garnet nanophosphors

Ce 3+-doped yttrium aluminum garnet nanophosphors with sizes near 30 and 250 nm have been synthesized by using chemical gelation and solvothermal methods, respectively. The size-dependent electron-longitudinal-optical-phonon coupling is investigated by fitting measured photoluminescence spectra within the framework of the Brownian oscillator model. Results show that the coupling strength is in a decreasing order from the bulk material to the nanophosphors of much smaller sizes.     

Characterization of yttrium aluminum garnet /YAG/ using 14 MeV neutron activation

A non-destructive method for the analysis of yttrium, aluminum and oxygen in Yttrium Aluminum Garnet /YAG/ by 14 MeV neutron activation analysis has been developed and evaluated. The fast neutron induces primarily /n, 2n/, /n, p/ or /n, / reactions with yttrium, aluminum, and oxygen to produce isotopes with measurable characteristic gamma-ray spectra. Concentrations of Y, Al and O were determined on the basis of calibrated nuclear decay emission spectra recorded on selected standards. After each analysis the crystal was returned to the furnace for further adjustments of the growth parameters, and any resultant elemental variations were observed in the next analysis. The accuracy of this method was determined by repeated analyses with various YAG crystals from a series of separate furnace runs.     

Intrinsic defects and dopants in LiNH2: a first-principles study

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ～0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.     

Influence of alkali metal impurities on properties of yttrium aluminum garnet doped with cerium

We consider the influence of Li, Na, and K metal dopants on the luminescent characteristics of Y_2.97Ce_0.03Al₅O₁₂ powders. It is shown that the photoluminescence intensity of yttrium aluminum garnet under excitation with blue light with a wavelength of 460 nm decreases with an increase of the content of these metals. The largest effect is observed for Li, the addition of which in amount of 0.05 wt.% leads to a decrease of intensity by 45%. The addition of the indicated metals does not influence the microstructure and the phase composition of the powders.     

Sol-gel synthesis of yttrium aluminum garnet (YAG): effects of the precursor nature and concentration on the crystallization

Journal of Sol-Gel Science and Technology - The sol–gel synthesis of yttrium aluminum garnet (YAG) was performed using different metal precursors (salts and alkoxides of aluminum and yttrium)...     

Pulse timing and optical interface between a neodymium: yttrium aluminum garnet laser and a Fourier transform ion cyclotron resonance mass spectrometer.

Laser desorption/ionization combined with pulsed (time-of-flight or Fourier transform ion cyclotron resonance) mass spectrometric detection is a powerful technique for analysis of involatile compounds and mixtures. Such experiments were originally conducted with pulsed CO2 lasers. Although a pulsed CO2 laser can be operated in single-shot mode, Nd: YAG lasers perform best with multiple flashes for warm-up before the final Q-switch output light pulse, thus creating the need to synchronize the desired final laser-output pulse with the event sequence for mass spectrometric analysis. In this paper, we describe a new and simple interface (both optical and electronic components) between a Continuum (formerly Quantel) Model YG 660A Nd:YAG laser and an Extrel FTMS-2000 mass spectrometer. The optics are modified from a prior pulsed CO2 laser interface from Extrel. Synchronization between the Nd:YAG laser and the mass spectrometer event sequence is achieved by means of a simple timing circuit that uses an inexpensive pulsing device and is triggered by pulses generated directly from the Extrel 1280 data system and cell controller, in contrast to the only prior published method that required an auxiliary microcomputer. The present interface method is highly flexible, and makes possible complex sequence events involving laser pulses for e.g.: desorption/ionization of solids; photoionization of gaseous neutrals; and photodissociation and photodetachment of gaseous ions.     

Synthesis and study of yttrium aluminum garnets doped with neodymium and ytterbium

Ytterbium- and neodymium-substituted yttrium aluminum garnets were obtained by hydroxide coprecipitation and by the cryochemical method. A continuous series of solid solutions in the Y₃Al₅O₁₂ — Yb₃Al₅O₁₂ system and limited solid solutions (Nd _x Y_{1 ? x} )₃Al₅O₁₂ (0 ≤ x ≤ 0.19) were obtained. The synthetic methods used made it possible to reduce the synthesis temperature from 1800 to 950°C and to obtain more homogeneous and nanosized material.     

Thermogravimetric study of yttrium isopropoxide and dispersant burnout in aluminum nitride ceramic processing

Yttrium isopropoxide, an yttrium oxide precursor, is sometimes used as a sintering aid for producing aluminum nitride ceramics. In the present work, this sintering aid was used with isopropanol as the solvent and polyethyleneimine as the dispersing agent. After initial ball milling and drying steps, the burnout behaviour of samples taken from isostatically pressed pellets was studied by thermal analysis in nitrogen and air. In addition to the milled and pressed pellets, each component was also analyzed separately. Complete conversion to yttrium oxide, with no residual carbon, would be a desirable property of this system. However, during the preparation of the aluminum nitride pellet, there was only partial yttrium isopropoxide decomposition. The nitrogen burnout residue contained carbon formed from the yttrium isopropoxide and dispersant overlapping thermal decomposition, mostly from an intermediary decomposition stage of the former that occurs between 300 and 550°C. The residual carbon content and the previous yttrium isopropoxide decomposition were estimated by thermogravimetry.     

Spectroscopy of multisites chromium (III) in yttrium aluminium garnet

Cr³⁺ multisites in an yttrium aluminium garnet host have been observed. The absorption, emission and excitation spectra and decay curves have been recorded. With the help of a high-resolution dye laser, the ²E and ⁴A₂ splittings were measured.     

Development and characterization of multifunctional yttrium iron garnet/epoxy nanodielectrics

Journal of Thermal Analysis and Calorimetry - In the present work, a series of yttrium iron garnet/epoxy nanocomposite systems were fabricated, and the morphology of nanocomposites were tested by...     

Effect of native defects and Co doping on ferromagnetism in HfO2: first-principles calculations

First-principles calculations of undoped HfO(2) and cobalt-doped HfO(2) have been carried out to study the magnetic properties of the dielectric material. In contrast to previous reports, it was found that the native defects in HfO(2) could not induce strong ferromagnetism. However, the cobalt substituting hafnium is the most stable defect under oxidation condition, and the ferromagnetic (FM) coupling between the cobalt substitutions is favorable in various configurations. We found that the FM coupling is mediated by the threefold-coordinated oxygen atoms in monoclinic HfO(2) and could be further enhanced in electron-rich condition.     

A first-principles study of the electronic structure and stability of a lithium aluminum hydride for hydrogen storage

LiAlH4 holds great promise for reversible hydrogen storage, where a fundamental understanding of hydrogen interaction with the metal elements is essential to further improve its properties. The present paper reports a first-principles study of its stability and electronic structure, using a full potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA) for high accuracy. The theoretically calculated heat of formation agrees well with experiment. The electronic structures show that the H atoms bond nonequivalently with the Al in the [AlH4]- ligand, which leads to complex dehydrogenation characteristics of LiAlH4.     

Four superhard carbon allotropes: a first-principles study

Using a generalized genetic algorithm, we propose four new sp(3) carbon allotropes with 5-6-7 (5-6-7-type Z-ACA and Z-CACB) or 4-6-8 (4-6-8-type Z4-A(3)B(1) and A4-A(2)B(2)) carbon rings. Their stability, mechanical and electronic properties are systematically studied using a first-principles method. We find that the four new carbon allotropes show amazing stability in comparison with the carbon phases proposed recently. Both 5-6-7-type Z-ACA and Z-CACB are direct band-gap semiconductors with band gaps of 2.261 eV and 4.196 eV, respectively. However, the 4-6-8-type Z4-A(3)B(1) and A4-A(2)B(2) are indirect band-gap semiconductors with band gaps of 3.105 eV and 3.271 eV, respectively. Their mechanical properties reveal that all the four carbon allotropes proposed in present work are superhard materials, which are comparable to diamond.