Synthesis of garnet structured Li7+x La3Y x Zr2-x O12 (x = 0–0.4) by modified sol–gel method

Preparation of lithium garnet Li₇La₃Zr₂O₁₂ (LLZ) in cubic phase by solid state method requires high temperature sintering around 1,200 °C for 36 h in Al₂O₃ crucible with intermittent grinding. Synthesis of LLZ in cubic phase at lower temperatures by wet chemical methods was reported earlier, however that decompose at high temperature around 850 °C. In this work we report the systematic studies on synthesis of garnet structured electrolytes by modified sol–gel method by the simultaneous substitution of Li⁺ and Y³⁺ for Zr⁴⁺ according to the formulae Li_7+x La₃Y _x Zr_2-x O₁₂ (x = 0, 0.1, 0.2, 0.3 and 0.4). The present investigation revealed that the cubic garnet phase is obtained at much lower temperature for Li₇La₃Zr₂O₁₂ and the simultaneous increase of both Li⁺ and Y³⁺ in Li_7+x La₃Y _x Zr_2-x O₁₂ requires slightly higher sintering temperatures for the formation of cubic garnet phase. SEM micrographs of the Li_7+x La₃Y _x Zr_2-x O₁₂ (x = 0, 0.1, 0.2, 0.3 and 0.4) annealed at minimum sintering temperature required for the formation of cubic garnet phase revealed the increase in grain size and relatively dense structure with increase of x in Li_7+x La₃Y _x Zr_2-x O₁₂.     

Thermodynamic stability of calcium vanadium garnet ferrites upon formation of substitutional solid solutions

The thermodynamic stability of calcium vanadium garnet ferrites Ca₃Fe_{3.5 ? x} Ti_2x V_{1.5 ? x} O₁₂ upon isomorphic substitution of titanium ions for iron and vanadium ions was studied by the EMF method using ZrO₂(Y₂O₃) ceramic solid electrolyte. Temperature-dependent ΔG ⁰ was determined. Isomorphic substitutions of titanium ions for iron and vanadium ions in the inequivalent sublattices of the garnet structure in a temperature range of 1100–1483 K cause changes in ΔG ⁰ due to the entropy and enthalpy components and has a minimal value when x = 0.15.     

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.     

Electric field gradients in EuScFe garnets

The electric quadrupole interactions at the octahedral, tetrahedral, and dodecahedral sites in the Eu_3?ySc_2+yFe₃O₁₂ (0 ≤ y ≤ 0.5) garnet system were studied with ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy. The electric field gradient tensor at the three sites was calculated using a monopole-point-dipole model. It is shown that the dipole contribution to the electric field gradient tensor is not negligible even for very small values of the oxygen dipole polarizability. The importance of the overlap and second-order 4f contributions is discussed.     

Kathodolumineszenz der neuen Selten Erd-aktivierten Granate A2A′Sb2Zn3O12 (A = Gd,Y; A′ = Sr,Ca)

By activation of the new garnet host lattices A₂A′Sb₂Zn₃O₁₂ (A = Gd, Y; A′ = Sr, Ca) with the trivalent rare earth ions (Ln³⁺ = Pr, Eu, Tb, Tm) a cathodoluminescence in the visible region is observed. The influence of the electronic structure and concentration of the activator on the relative intensity as well as the host lattice participation in the energy transfer processes are discussed.     

Absorption spectra and energy levels of Sm3+:Y3Al5O12

Absorption spectra between 0.2 and 6.7 μm are reported for trivalent samarium in single-crystal yttrium aluminum garnet, Y₃Al₅O₁₂, at liquid-helium, liquid-nitrogen, and room temperatures. Energy levels (4f⁵[SL]Jμ) associated with sextet, quartet, and doublet states of Sm³⁺ are established from data between 1500 and 46000 cm⁻¹. Most samarium ions occupy yttrium ion sites, which have D₂ point-group symmetry in the lattice. A free-ion wavefunction calculation predicts the assignment of 35 isolated [SL]J manifolds with an rms deviation of 32 cm⁻¹ between calculated and observed centers of gravity. A hamiltonian consisting of coulombic, spin—orbit, and crystalline electric field (D₂ symmetry) terms is diagonalized for the two lowest sextet states [6H]J and [6F]J. The calculated Stark levels based on a single set of B_km crystal field parameters are compared to the observed levels (54) yielding an rms deviation of 4.9 cm⁻¹.     

Preparation of nanostructured thin films of yttrium iron garnet (Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>) by sol–gel technology

The synthesis of hydrolytically active heteroligand coordination compounds [M(C₅H₇O₂)_3?x(C₅H₁₁Oⁱ)_x] (M = Fe³⁺ and Y³⁺) using iron and yttrium acetylacetonates has been studied. The gel formation kinetics in solutions of these compounds upon hydrolysis and polycondensation has also been studied. Thin films of a solution of these precursors have been applied to polished sapphire substrates by dip coating technology. The crystallization of nanostructured yttrium iron garnet (Y₃Fe₅O₁₂) films during heat treatment of xerogel coatings under various conditions has been studied. How the phase composition, microstructure, and particle size depend on the synthesis parameters has been recognized.     

Blue-light-excitable broadband yellow-emitting CaGd2HfSc(AlO4)3:Ce3+ garnet phosphors for white light-emitting diode devices with improved color rendering index

The current commercial white light-emitting diodes (LEDs) are generally based on the combination of blue LED chips and Y₃Al₅O₁₂:Ce³⁺ yellow phosphors. However, because of the lack of red component, such white LED devices exhibit cool white-light emissions with low color rendering index (Ra < 75, R9 < 0). Therefore, it is urgent to discover new blue-light-excitable yellow-emitting phosphors with enhanced red emissions for fabricating high color-quality white LEDs. In the present work, we demonstrate a novel broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor for blue-light-excited white LEDs with improved color rendering index. The as-prepared CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor possesses a cubic structure with Ia$\overline{3}$d space group, and the unit cell parameters of the representative CaGd₂HfScAl₃O₁₂:2%Ce³⁺ phosphor are a = b = c = 12.450 Å, α = β = γ = 90°, and V = 1,929.59(4) ų. Impressively, we find that the CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor shows an intense absorption band in the 300–500 nm wavelength range with a maximum at 452 nm owing to the 4f→5d transition of Ce³⁺ ions. On 452 nm excitation, the optimal CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample exhibits a broad asymmetric yellow emission band in the wavelength range of 470–750 nm with peak at 564 nm and full width at half maximum of 151 nm. The Commission Internationale de l’Eclairage chromaticity coordinates and internal quantum efficiency of the CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample are (0.4485, 0.5157) and 30.4%, respectively. Finally, a white LED device is fabricated by combing a 450 nm blue LED chip with commercial Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphor, which generates white light with low color rendering index (CRI; Ra = 74.7, R9 = ?12.7) and high correlated color temperature (CCT = 6,554 K) under the 60 mA driving current. In sharp contrast, another white LED device, which is made by coating our as-prepared CaGd₂HfScAl₃O₁₂:2%Ce³⁺ yellow-emitting phosphors onto the surface of a 450 nm blue LED chip, produces white-light emission with high CRI value (Ra = 84.5, R9 = 26.3) and relatively low CCT of 5,649 K. This work reveals that the newly discovered broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ phosphors can serve as a potential color converter in high-color-quality phosphor-converted white LEDs.     

Preparation of nanostructured thin films of yttrium aluminum garnet (Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>) by Sol—Gel technology

The synthesis of hydrolytically active heteroligand coordination compounds [M(C₅H₇O₂)_3–x(C₅H₁₁Oⁱ)_x] (where M = Al³⁺ and Y³⁺) using aluminum and yttrium acetylacetonates has been studied. The gel formation kinetics in their solutions upon hydrolysis and polycondensation has also been studied. Thin films of a solution of these precursors have been applied to polished sapphire substrates by dip coating. The crystallization of nanostructured yttrium aluminum garnet (Y₃Al₅O₁₂) films during heat treatment of xerogel coatings under various conditions has been studied. How the phase composition, microstructure, and particle size depend on the synthesis parameters has been recognized.     

Synthesis of fluoride garnets {Na3}[M3+2](Li3)F12 (M = Al,Cr, and Fe) from aqueous solution and their properties

The garnet-type fluorides, Na₃M₂Li₃F₁₂ (M = Al, Cr, and Fe) were synthesized as the coprecipitates of ～1 to 10 μm powders from HF solutions. The garnet structures were always obtained under acidic conditions. The incorporation of water molecules into the structure of the Cr and Fe garnets was observed. After heat treatment at 300°C, very small unknown peaks were observed in the X-ray powder pattern in addition to the garnet phase which had a slightly smaller value of the lattice constant than that of hydrous garnets. From the measurement of magnetic properties and Mössbauer effects, the Fe and Cr garnets were found to be paramagnetic with both ions in the trivalent state. Presentation of infrared spectra of the garnets is also included.     

Conduction mechanism in Y3−2xCa2xFe5−xVxO12 garnet ferrites

A series of polycrystalline garnet ferrites with composition Y_3−2xCa_2xFe_5−xV_xO₁₂ (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were prepared by the standard ceramic technique to study the effect of Ca²⁺ and V⁵⁺ ions substitution on their DC electrical conductivity, thermoelectric power, charge carrier concentration and charge carrier mobility at different temperatures. It was found that the DC electrical conductivity increases linearly with increasing temperature ensuring the semiconducting nature of samples. The lines representing the temperature dependence of σ_dc are broken at two-phase transition temperature (T_σ1, T_σ2=T_C) giving three distinct regions (I, II and III). The activation energy for electrical conduction increases going from ferrimagnetic state (regions I and II) to paramagnetic state (region III) through the transition temperature T_σ2 (Curie temperature). It also increases going from region I to region II thorough the temperature T_σ1. The dc electrical conductivity does not vary uniformly with Ca²⁺ and V⁵⁺ ion substitution. The values of the thermoelectric power were positive for samples of 0.0⩽x⩽0.6 indicating that the majority of the carrier are holes in these samples while it were negative for samples of x⩾0.8 indicating that the majority of charge carriers are electrons in this samples. Using the values of the DC electrical conductivity and thermoelectric power, the values of the charge carrier concentration and the charge carrier mobility were calculated.     

Time-resolved electronic Raman spectrum of terbium aluminum garnet excited with visible and UV laser sources

Excitation profiles for the intensities of electronic Raman transitions between crystal field components of the ⁷F₆ and ⁷F₅ manifolds of terbium aluminum garnet are recorded for excitation in the spectral region where absorption bands due to levels of the ⁵D₄ manifold occur. The intensities of the electronic transitions are not enhanced which is thought to be caused by the small values of electric dipole matrix elements of the resonating electronic states in comparison to the values of such elements to other intermediate states which occur in the expression for the scattering tensor. Fluorescence from the ⁵D₄ levels is induced and resonance fluorescence are time resolved with respect to the Raman transitions. We report electronic Raman transitions excited with the 308.0 nm line of an XeCl excimer laser. As opposed to excitation with visible laser sources, transitions are recorded which terminate on all the crystal field levels of the ⁷F_5…0 levels. In addition, fluorescence from ⁵D₃ to the ground state of terbium aluminum garnet is also observed.     

Effects of NiO addition on the structure and electric properties of Dy3−xNixFe5O12 garnet ferrite

Polycrystalline garnet ferrites Dy_3?xNi_xFe₅O₁₂ with varying Ni substitutions (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) have been prepared by the standard ceramic technique and their crystalline structures were investigated by using X-ray diffraction and IR spectroscopy. The X-ray diffraction analysis showed that all samples have a single cubic garnet phase. The materials prepared are identified by infrared rays which indicate the presence of three absorption bands ν₂, ν₃ and ν₄ which represent the tetrahedral, octahedral and dodecahedral sites respectively which characterize the garnet ferrite.The dielectric constant (?), and dielectric loss (tan δ) of the prepared samples were measured at 1 kHz in the temperature range 300–700 K. The dielectric constant (?), and dielectric loss (tan δ) are functions of temperature.The initial magnetic permeability has been studied at different temperatures. The initial magnetic permeability (μ_i) increases gradually with increasing temperature and then drops suddenly at a certain temperature T_c.     

Crystallographic Analysis of the Garnet Structure Type: Symmetry in the Structural Anatomy

The crystallographic analysis of the garnet structure type (Al₂Ca₃(SiO₄)₃, Ia3d symmetry) establishes the determining role of the [AlCa₆Si₆] cation group with the local symmetry \(\bar 3\)in the structure formation. The glide planes of three directions coherently assemble these groups. High structural stability (three not symmetry-fixed coordinates per 80 atoms, the fundamental volume V*~17 ų) promotes the occurrence of various atomic compositions in it. The relation with the β-W structure type and the structure of menezesite mineral is shown.     

Lithium garnet based free-standing solid polymer composite membrane for rechargeable lithium battery

Electrolytes with high lithium-ion conductivity, better mechanical strength and large electrochemical window are essential for the realization of high-energy density lithium batteries. Polymer electrolytes are gaining interest due to their inherent flexibility and nonflammability over conventional liquid electrolytes. In this work, lithium garnet composite polymer electrolyte membrane (GCPEM) consisting of large molecular weight (W_avg ~?5?×?10⁶) polyethylene oxide (PEO) complexed with lithium perchlorate (LiClO₄) and lithium garnet oxide Li_6.28Al_0.24La₃Zr₂O₁₂ (Al-LLZO) is prepared by solution-casting method. Significant improvement in Li⁺ conductivity for Al-LLZO containing GCPEM is observed compared with the Al-LLZO free polymer membrane. Maximized room temperature (30 °C) Li⁺ conductivity of 4.40?×?10^?4 S cm^?1 and wide electrochemical window (4.5 V) is observed for PEO₈/LiClO₄?+?20 wt% Al-LLZO (GCPEM-20) membrane. The fabricated cell with LiCoO₂ as cathode, metallic lithium as anode and GCPEM-20 as electrolyte membrane delivers an initial charge/discharge capacity of 146 mAh g^?1/142 mAh g^?1 at 25 °C with 0.06 C-rate.     

Structural and thermal investigation of gadolinium gallium mixed oxides obtained by coprecipitation: Observation of a new metastable phase

Polycrystalline gadolinium gallium mixed oxides were prepared by coprecipitation and annealing at various temperatures below 1000 °C. The oxide materials appear to be X-ray amorphous after a heat treatment at 500 °C for 30 h, but after 30 h at 800 and 900 °C a major, unreported, hexagonal phase, isostructural with TAlO₃ compounds (where T=Y, Eu, Gd, Tb, Dy, Ho, Er) appears to crystallize. On the other hand, a highly energetic mechanical treatment of the amorphous powder previously annealed at 500 °C changes considerably the shape and position of exothermal events occurring in the range from 700 up to 900 °C. Subsequent annealing at 900 °C of the mechanically treated powder gives rise to the complete formation of the Gd₃Ga₅O₁₂ garnet structure at the expense of the hexagonal phase and of the minor Gd₄Ga₂O₉ oxide phase. However, a 7.0 wt% contamination is found to be due to tetragonal zirconia coming from vials and balls colliding media. The garnet phase may have strong deviations from the nominal stoichiometry of the garnet, as suggested by the refined lattice parameter obtained from the powder diffraction patterns and by the remarkable absence of intensity relative to the (220) Bragg peak position.     

Determination of Rare Earth Elements in Garnet Minerals,Geological Materials by Inductively Coupled Plasma–Atomic Emission Spectral and Mass Spectral Analysis

Abstract

The performance of inductively coupled plasma–atomic emission spectrometry (ICP‐AES) for the determination of 14 lanthanides and Yttrium was evaluated by comparison with inductively coupled plasma–mass spectral (ICP‐MS) analysis. The geochemical reference samples (GRS), DNC‐1(diabase), AGV‐1(andesite), Sy‐2(syenite), MRG‐1(gabbro), AN‐G(anorthosite), AC‐E(granite), and MAG‐1(marine mud) were chosen as test materials and analyzed for checking the precision and reproducibility of the methods. The mineral garnet is separated from the black sands of the southwest coast of India, and the combined cation exchange–ICP method of AES analysis and MS analysis were carried out for the determination of rare earth elements. Both techniques are within the requirements needed for garnet minerals. The determination of rare‐earth elements in these minerals, which contain other elements as major contribution and trace distribution of rare‐earth elements, shows that ICP applied under the proper working condition lives up to the expectations. Major element analysis gives the formula of garnet of Manavalakurichi (MK) as (FeCaMg)_2.79Al_2.07Si_3.05O₁₂ approximated to Fe₃Al₂Si₃O₁₂, hence of almandine-type garnet. The enrichment of heavy lanthanides compared to the light lanthanides indicates that these lanthanides occupy the coordinaton site of Fe²⁺ by replacement. Both techniques are excellent in determining the very low concentration of lanthanides in geological materials, specifically garnet.     

A neutron diffraction study of the d and d lithium garnets Li3Nd3W2O12 and Li5La3Sb2O12

The garnets Li₃Nd₃W₂O₁₂ and Li₅La₃Sb₂O₁₂ have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950 °C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group with lattice parameters a=12.46869(9) Å (Li₃Nd₃W₂O₁₂) and a=12.8518(3) Å (Li₅La₃Sb₂O₁₂). Li₃Nd₃W₂O₁₂ contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li₅La₃Sb₂O₁₂ contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d⁰ and d¹⁰ phases shows that replacement of a d⁰ cation with d¹⁰ cation of the same charge leads to an increase in the lattice parameter due to polarisation effects.     

Zirconium-Containing Compositions with a Component Ratio Characteristic of the Garnet Structure: Physicochemical and Catalytic Properties

The possibility for the formation of garnet structures in the Mn–Fe–Zr–O and Ca–Sm–Zr–O systems obtained by the precipitation of the corresponding salts is studied. It is shown that, in the Mn–Fe–Zr–O system, garnet is crystallized at 860–920°C, for which probable cation distribution is estimated to be {Zr_2.5 ⁴⁺Mn_0.5 ²⁺}[Mn₂ ²⁺](Fe_2.5 ³⁺Mn_0.5 ³⁺)O₁₂. In the Ca–Sm–Zr–O system, the perovskite CaZrO₃, pyrochlore Sm₂Zr₂O₇, and CaO are formed at 900–1200°C, but compounds with garnet structures are not found. The reported systems are characterized by surface areas of 300–450 m²/g at 450°C, and they have the polydisperse distribution of pores over sizes. The introduction of surfactants at the stage of component mixing enables an increase in the overall pore volume and mechanical strength of these systems. The Mn–Fe–Zr and Ca–Sm–Zr compositions are active catalysts for the complete oxidation of hydrocarbons.