Ions In Garnets Research Articles

Promoted methane conversion to syngas over Fe-based garnets via chemical looping

Fe-based oxygen carries (OCs) have attracted wide attention due to the low cost and environmental compatibility for chemical looping methane partial oxidation. However, they usually suffer from low CH4 reactivity mainly due to insufficient synergy between catalytic CH4 activation and lattice oxygen mobility. In this work, Fe ions with different amount were incorporated into a novel garnet structure (Y3FexAl5-xO12), which exhibited drastically increased CH4 conversion with reduction time. This resulted from in-situ formed Fe0 sites at the early stage of reduction, which led to a more efficient reaction route involving methane catalytic decomposition on Fe0 and the resulted carbon oxidation easier by lattice oxygen, compared with methane directly oxidized by OCs. Moreover, the amount of Fe ions in garnet influenced surface exposed Fe0 sites and oxygen mobility, inducing Y3Fe2Al3O12 performed the best with almost 94 % methane conversion thanks to the combined advantages of larger surface exposed Fe0 and higher lattice oxygen mobility.

Distortion of the Ligand Environment of Gd3+ and Eu2+ Ions in Garnets: Y3Al5O12, Lu3Al5O12, and Y3Ga5O12

Using experimental second-rank parameters of the spin Hamiltonian of the rhombic centers Gd3+ and Eu2+ in three garnets and the superposition approximation formalism, relaxation of the nearest environment of impurity ions is estimated.

Temperature dependent Cr3+ photoluminescence in garnets of the type X3Sc2Ga3O12 (X = Lu, Y, Gd, La)

Broad band near-infrared (NIR) light sources have great potential, e.g., in optical communication and non-invasive medical imaging. Here we report the optical properties of X3Sc2Ga3O12 (X = Lu, Y, Gd, La) garnets doped with Cr3+ showing efficient 4T2 → 4A2 broad band NIR emission between 600 and 1000 nm. The chromium-doped garnets were investigated in view of their capability for blue to NIR conversion in high power phosphor converted LEDs (pcLEDs). Typical high power (In,Ga)N LEDs reach junction temperatures up to 450 K. Hence, the photoluminescence (PL) quenching temperature is a crucial parameter for on-chip phosphors. The Cr3+-doped garnets reveal very high quenching temperatures for the broad band NIR emission, ~ 700 K for Lu3Sc2Ga3O12, Y3Sc2Ga3O12, Gd3Sc2Ga3O12, and 450 K for La3Sc2Ga3O12. The high quenching temperatures are in line with a small Stokes shift (~ 2400 cm−1) and weak electron-phonon coupling. To gain further insight in the luminescence properties, low temperature photoluminescence measurements were done. The optical properties of Cr3+ ions in garnets were analyzed in terms of the crystal field strength, Racah parameters, and phonon coupling parameters. The suitability for application in pcLEDs for high power broad band NIR sources was confirmed by emission measurements of ceramic pellets upon blue LED excitation.

Pulsed electron‐nuclear double resonance diagnostics of Ce3+ emitters in scintillating garnets

Pulsed electron paramagnetic resonance (EPR) and pulsed electron‐nuclear double resonance (ENDOR) techniques have been applied to study the environment of luminescent Ce3+ ions in garnet based scintillator powders and ceramics. The presence of aluminum and gallium isotopes with large nuclear magnetic and quadrupole moments in the nearest neighborhood of the Ce3+ ion allows for the use of the hyperfine and quadrupole interactions with these ions for determination of the unpaired electron spatial distribution and the definition of the electric field gradient at the aluminum and gallium sites. Pulsed EPR and ENDOR techniques made it possible to study the coherent properties of the Ce3+ spin system in garnet powders and ceramics, which is important for spin manipulation on Ce3+ centers.

Magnetooptics of non-Kramers Eu3+ ions in garnets: analysis complemented by crystal-field splitting modeling calculations

Spectra of absorption, luminescence, magnetic circular dichroism (MCD), and magnetic circular polarization of luminescence (MCPL) in Gd3Ga5O12:Eu3+ and Eu3Ga5O12 garnets were studied within the visible spectral range at 300 K. Analysis of the spectral and temperature dependences of the magnetooptical and optical spectra made it possible to identify the magneto-dipole (MD) and electro-dipole (ED) 4f→4f transitions occurring between Stark sublevels of the 7FJ (J=1, 2) and 5D0 multiplets in Eu3+-containing garnet structures. Quantum mechanical “mixing” had significant influence on quasi-degenerate states of the non-Kramers rare-earth Eu3+ ion for Eu3Ga5O12 in MCD due to forbidden MD transition 7F1→5D0 and for Gd3Ga5O12:Eu3+ in MCPL due to MD 4f→4f transition 5D0→7F1 and forced ED-transition 5D0→7F2. A parameterized Hamiltonian defined to operate within the entire 4f(6) ground electronic configuration of Eu3+ ion was used to model the experimental Stark levels, including their irreducible representations and wavefunctions. The crystal-field parameters were determined through a Monte-Carlo method in which nine independent crystal-field parameters, Bkq, were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviation between 57 calculated-to-experimental levels was 0.73 meV.

Crystal field parameters for Yb 3+ ions at orthorhombic centers in garnets—Revisited

The major purpose of this paper is to clarify the deficiencies identified in the recent paper by Liu et al. (J. Lumin. 130 (2010) 103) as well as to reanalyze the available data and provide corrected results for the orthorhombic crystal field parameters (CFPs) for selected rare-earth ions in garnets. It appears that Liu et al., when utilizing the computer package for standardization of CFPs, have inadvertently confused the properties of CFPs expressed in the Wybourne notation with those in the extended Stevens operator notation. This confusion has led to misinterpretations concerning the orthorhombic standardization transformations and incorrect labeling of the CFP sets as supposedly ‘standardized’ for Yb 3+, Pr 3+, Nd 3+, and Er 3+ ions in various garnets. These deficiencies have prompted us to reconsider the CFP sets determined earlier by matching the experimental data, i.e. the orthorhombic spin Hamiltonian parameters ( g factors, g i , and hyperfine structure constants, A i ; i=x, y, z) and available optical spectral band positions, with the theoretical data calculated using the complete diagonalization method. To further verify the correctness of the present results the CFPs for orthorhombic Yb 3+ centers in Yb 3Al 5O 12 and Yb 3Ga 5O 12 garnets are calculated using the superposition model (SPM), which requires adoption of a well-defined symmetry-adapted axis system (SAAS). Hence, the SPM calculations enable reanalysis of available CFP sets based on the correct standardization procedure and establishing the correspondence between the SAAS and the ‘ nominal’ axis systems assigned to fitted CFP sets. Using the proper SAAS and general transformations of the axis systems, the relations between the calculated g i and/or A i values and the respective principal values determined by EPR experiments can be established. The consistent methodology utilized here may be helpful for proper reanalysis of spectroscopic data for rare-earth and transition-metal ions at orthorhombic symmetry sites in various crystals.

Diffusion in Minerals and Melts: Theoretical Background

Diffusion is due to thermally activated atomic-scale random motion of particles (atoms, ions and molecules) in minerals, glasses, melts, fluids, and gases (Fig. 1⇓). The random motion leads to a net flux when the concentration (more strictly speaking, the chemical potential) of a component is not uniform. Even though diffusion is a microscopic process, it can lead to macroscopic effects. For example, the initial phase of explosive volcanic eruptions (or more commonly encountered champagne eruptions) is powered by bubble growth, which in turn is controlled by diffusion that brings gas molecules into bubbles. This chapter provides a brief review of the theory of diffusion in minerals and melts (including glasses). More complete coverage of diffusion theory can be found in Crank (1975), Kirkaldy and Young (1987), Shewmon (1989), Cussler (1997), Lasaga (1998), Glicksman (2000), Balluffi et al. (2005), Mehrer (2007), and Zhang (2008). Figure 1. An example of random motion of particles. Initially (the left panel), all A particles (such as Fe2+ ions in garnet) represented by filled circles are in the lower side, and all B particles (such as Mg2+ ions in garnet) represented by open circles are in the upper side. Due to random motion, there is a net flux of A from the lower side to the upper side, and a net flux of B from the upper side to the lower side (the middle and right panels). As time increases, A and B will eventually become randomly and uniformly distributed in the whole system. This situation for diffusion is often encountered in diffusion experiments and is referred to as a diffusion couple. In minerals, diffusive transport is the only mechanism for particles to move from one location to another. For example, homogenization of a zoned crystal …

Effect of Mo3+ ions on Nd3+ spin-lattice relaxation in Y3Al5O12

The content of Mo3+ ions in YAG:Nd garnet samples prepared by different technologies has been studied, and the spin-lattice relaxation rate of these ions at temperatures of 4–5 K measured. It is concluded is drawn that Mo3+ ions can play the part of rapidly relaxing centers mediating the Nd3+ spin-lattice relaxation at liquid-helium temperatures. This may account for a number of features in the spin-lattice relaxation of rare-earth ions in garnets, observed earlier at low temperatures.

Local trigonal distortions for the substituting Cr 3+ ions in garnet crystals

The local trigonal distortions β − β 0 for the substituting Cr 3+ ions in garnet (YAlG, LuAlG and YGG) crystals are determined from a more reasonable method based on the studies of the zero-field splitting D, the first excited state splitting Δ( 2E) and the anisotropy of the g factor (characterized by Δg = g ∥ − g ⊥). The results show that the local octahedral environments of the substituting Cr 3+ ions are more regular than those of the host ions. This confirms the previous assumption for the local environment of Cr 3+ in YGG based on the studies of only part of these spectral parameters.

A theoretical investigation on the magneto-optical spectra of Ce-substituted yttrium aluminium garnet

Calculations of the Faraday rotation and ellipticity spectra at photon energies between 1.6 and 4.0 eV (wavelengths between 775 and 310 nm) resulting from Ce3+ ions in Ce-substituted yttrium aluminium garnets (YAG:Ce) based on the quantum theory are presented. The Faraday effect contributed by the Ce3+ ions is caused mainly by the intra-ionic electrical dipole transitions between the 4f and 5d configurations. The Faraday effect of the Ce3+ ions in both the diamagnetic YAG and the ferrimagnetic yttrium iron garnet (YIG) crystals has a paramagnetic spectral shape but the resonance frequencies of YAG:Ce are at 2.76 and 3.7 eV, which are obviously higher than those of YIG:Ce. This difference can be explained by the fact that the crystal field acting upon the rare-earth ions in garnets depends strongly on the nature of the next-nearest neighbours. The differences between the transition intensities for the right- and left-handed circularly polarized light from both the lowest and the second crystal-field (CF)-split 5d levels to the ground state are negative. Therefore the Faraday rotation spectrum contributed by the Ce3+ ions in YAG has a wave-like shape in the energy region considered. However, for YIG:Ce, the difference between the transition intensities for the right- and left-handed circularly polarized light from the second CF-split 5d level to the ground state is positive. So the corresponding spectrum for YIG:Ce has a different shape. The calculated results are in good agreement with experiment.

Near-infrared emission of Cr4+-doped garnets: Lifetimes, quantum efficiencies, and emission cross sections.

The lifetimes of the excited state of the tetrahedrally coordinated ${\mathrm{Cr}}^{4+}$ ion in several garnets were measured in the temperature range between 15 and 380 K. Both the nonradiative and radiative transition rates increase with temperature due to the coupling of phonons to the electronic states. This temperature dependence is described with the model of Struck and Fonger for the nonradiative decay rate and with the coth law for the radiative decay process. It is shown that even at low temperatures the nonradiative transition rate is larger than the radiative rate. With these data the quantum efficiencies are calculated to be between 8% and 33% at room temperature for the investigated crystals. The cross sections for the emission of the ${\mathrm{Cr}}^{4+}$ ion in the garnets are determined using the theory of McCumber. The ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{em}}$\ensuremath{\tau} product is highest for ${\mathrm{Cr}}^{4+}$:${\mathrm{Lu}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$, ${\mathrm{Cr}}^{4+}$:${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$, and for ${\mathrm{Cr}}^{4+}$:${\mathrm{Y}}_{3}$${\mathrm{Sc}}_{\mathit{x}}$${\mathrm{Al}}_{5\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{O}}_{12}$ crystals with low scandium content. Therefore, these crystals are most promising as tunable room-temperature lasers. A method for the calorimetric measurement of the quantum efficiency of the ${\mathrm{Cr}}^{4+}$ emission in yttrium aluminum garnet is also presented.

The influence of the admixing of different multiplets on the spin magnetic moment and Faraday effect of rare earth ions in Garnets

The influence of the admixing of the excited multiplets of the ground configuration with the ground multiplet on the spin magnetic moment of the crystal- field- and exchange-interaction-split ground state, the Faraday rotation and the magneto-optic coefficient induced by the rare earth ions in rare-earth-substituted garnets Y3-xRxFe5O12 (R=Ce and Pr) has been calculated in this paper. It is found that, in the case of Ce3+ ions, the admixing of the 4f1 2F7/2 multiplet with the 2F5/2 multiplet makes the spin magnetic moment of the crystal-field- and exchange-interaction-split ground state increase by 140% and the Faraday rotation increase by 180% at 633 nm wavelength, and 150% at 1150 nm at 294 K. In the case of Pr3+ ions, the admixing of the 4f2 3H5 multiplet with the 3H4 multiplet makes the spin magnetic moment of the crystal-field- and exchange-interaction-split ground state increase by 34% and the Faraday rotation increase by 59% at 294 K. The admixing of different multiplets of the Ce3+ ions makes the ratio between the magneto-optic coefficients at 50 and 294 K increase by 7% at 633 nm wavelength and 15% at 1150 nm.

Effective Hamiltonian and ground state properties of rare-earth ions in iron garnets

An effective Hamiltonian for ground Kramers doublet and quasidoublet of non-Kramers rare-earth (RE) ions in garnets in the presence of exchange interactions with ferric sublattices is shown to reflect the anisotropy of both crystalline field and exchange interaction with different consequences for Kramers and non-Kramers ions. For ten ions of the RE series all possible crystal-field ground states which are to be considered in RE garnets are examined. Main attention is paid to the search for RE moments with Ising-like character imprinted by the crystal field. The exchange interaction of such moments with iron sublattices can then be described by a single-parameter Hamiltonian. This greatly simplifies general many-parameter theories of the low-temperature magnetic phenomena. It is found that in principle the Ising-like ground states may occur in Sm 3+-, Tb 3+-, Dy 3+- and Tm 3+-iron garnets. The results are compared with available experimental data.

Valence control of Fe and CO ions in LPE garnet films by substitution of Ca or Ge

Results are presented for the valence control of Fe and Co ions in magnetic garnets for Ca- or Ge-substituted YIG. The measurement of spectral dependence of Faraday rotation was found to be a very sensitive method for observation of the valence state of Co and Fe ions in garnet films. It is observed that the values of the distribution coefficients for uncompensated Ca or Ge are three times lower than those obtained in the compensated state (simultaneous Ca and Ge substitution).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On standardization of the spin Hamiltonian and the ligand field Hamiltonian for orthorhombic symmetry

The idea that the ratio E/D ≡ λ can be confined by a proper choice of the axis system to the range (0,±1/3) serves as a basis for the standardization of the spin Hamiltonian and the ligand field Hamiltonian for orthorhombic symmetry. Due to recent results on the transformation of the Stevens operators Oqk, relations are given which allow for extension of the standardization to the spin Hamiltonian for paramagnetic centers with spin S≥2. Several EPR studies of 3dn and 4 f n ions in crystals are reconsidered. The results prove the usefulness of the clear choice of the parameters Bqk suggested by the standardization idea. Application of the standardization in the area of the ligand field theory, which seems rather novel, is also considered. Crystal-field parameters for several rare earth ions in garnets are recalculated. An arbitrary choice from among the equivalent sets of the crystal-field parameters used in the review by Morrison and Leavitt (1982) should be replaced by a transformation to the ‘‘standard’’ set.

Superposition model for zero-field splitting of Fe3+ and Mn2+ ions in garnets

In view of the results obtained the future of the SM for ZFS seems rather dim. Clearly, no universally valid ¯b2(R) dependence can be constructed. SM is capable to predict the signs and order of magnitudes of ZFS for a and d site S-state ions but here also a large quantitative differences may occur.

ESR study of the interstitial Co 2+ in YAG

ESR of Co 2+ ion in yttrium aluminum garnet is investigated and the interstitial Co 2+ spin resonance is observed in addition to the substitutional ones. Twenty-four magnetically inequivalent interstitials sites are identified. This is the first clear evidence of the interstitial magnetic ions in garnet.

上部マントル条件下におけるchrome-pyropeとferric iron-pyropeの安定領域

The stability fields of chrome-pyrope and ferric iron-pyrope have been determined in the pressure ranges 15 to 40 kilobars at 1000°C isothermal. The calcium-rich chrome-pyrope is more soluble at low pressure conditions than calcium-poor chrome-pyrope. The oxygen fugacity has affected stability of ferric iron-pyrope. The Fes+ ion can easily enter into the pyrope as Mg3Fe23+Si3O12 molecule in comparison with the andradite molecule at high pressure conditions.Behavior of Cr3+ and Fes3+ ions in garnets at high pressure conditions is very different. The solubility of these ions in garnets is controlled by presence of calcium ion. Under calcium-rich condition, the chrome-pyrope is more stable than the ferric iron-pyrope.

Growth-induced anisotropy arising from ordering of transition metal ions in garnets (invited)

The earlier study of the growth-induced anistropy (Ku) in epitaxial YIG produced by the preferential occupation of Ir4+ on octahedral sites has been extended to include most of the transition elements that have degenerate ground states and that can be incorporated in the garnet lattice with the appropriate valence states. Among the 3d elements only Co2+ meets these requirements. For Co2+ charge compensated by Zr4+, Ku increases linearly with the Co/Fe ratio in the melt, reaching 2×105 ergs/cm3 in (111) films for a Co/Fe atomic ratio of 0.17. Among the possible 4d and 5d ions only Ru, Mo, and W are expected to be stable in the desired valence state. Ruthenium yields a large in-plane growth-induced anisotropy (Ku = −1.2×105). Lattice constant, SEM and optical studies indicate that Mo and W have negligible solubility in YIG. For as-grown bubble samples the coercive force (Hc) increase linearly with Ir content, as does Ku. For Ku = 2.3×105, Hc = 70 Oe. The increase of Hc with Ru concentration is substantially smaller. For Ku = −1.2×105, Hc = 2.5 Oe. For YIG (Ir, Ca) samples annealed to reduce Ku (initially 3.5×105) to zero, Hc is 3 Oe, indicating that Ir content alone does not cause the large coercive force. However to data we have not been able to produce an Ir containing material with an easy axis normal and a low value of Hc.

Radiationless decay processes of Nd^3 + ions in solids

Laser photoacoustic spectroscopy measurements were made on Nd3 + ions in garnet, vanadate, and pentaphosphate host crystals. The variations of signal intensities with chopping frequency of the incident light are not in agreement with the predictions of standard photoacoustic signal generation theory. The results are distinctly different for concentrated and dilute Nd-doped crystals, indicating that the mechanisms for generating heat have different characteristics in these two types of samples. The determination of radiative quantum efficiencies of these materials by photoacoustic spectroscopy techniques is also described. These results are compared with those obtained by other methods.