Garnet Research Articles

Synthesis, structural and conductive properties of Nd doped garnet-type Li7La3Zr2O12 Li-ion conductor

Garnet type solid electrolyte Li7La3Zr2O12 (LLZO) is the most promising candidate among solid electrolytes for all solid state Li batteries. In this work, small amount of Nd doping (5%–20%) to the garnet structure is proposed to improve its ionic conductivity. Nd doped garnet type solid electrolytes for Li-ion batteries were synthesized through a conventional solid state reaction method. The effect of Nd doping on the microstructure, morphology and ionic conductivity of the LLZO was studied by powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and Electrochemical impedance spectroscopy (EIS) methods. Instead of La whose valence is similar to that of Nd, XRD and Raman analyzes revealed that Nd takes the place of the higher valence element Zr. In order to compensate the valence difference, the ratio of Li increases in the structure. On the other side, results showed that Nd doped LLZO samples formed as a mixture of both tetragonal and cubic phases. According to EIS measurements, among the prepared samples, 5% Nd doped LLZO exhibits the highest ionic conductivity of 2.47 × 10−6 S cm−1 at room temperature.

Phase composition, crystal structure, and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y3−xAl5NbxO12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO4 secondary phase at the grain boundary of Y3Al5O12. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb5+ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of εr ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τf ~ − 34.7 ppm/°C can be obtained in x = 0.1 (Y2.9Al5Nb0.1O12.1) sintered at 1700 °C for 6 h.

Modulation of structure and magnetic properties of terbium iron garnet by the introduction of Bi3+ and Ce3+

Modulation of structure and magnetic properties of terbium iron garnet by the introduction of Bi3+ and Ce3+

New aluminate-germanate hosts with garnet structure for solid state lighting applications

In this paper, two newly developed garnet phosphors, BaY2Al4GeO12:Ce3+ and SrY2Al4GeO12:Ce3+ are reported. The materials were prepared by simple, facile combustion synthesis. The emission for these phosphors is in yellow region when excited by blue light. Decay for BaY2Al4GeO12:Ce3+ is nearly exponential with average lifetime of 70 ns. In case of SrY2Al4GeO12:Ce3+, decay curve could be fitted to second order exponential with fast component τ1 = 24.5 ns and slow component τ2 = 57 ns. Capability of excitation by blue LED, yellow emission, simple preparation make these phosphors suitable for white LED lamp fabrication. These could be suitable replacements for YAG:Ce.

Deposition temperature dependence of thermo-spin and magneto-thermoelectric conversion in Co2MnGa films on Y3Fe5O12 and Gd3Ga5O12

We have characterized Co2MnGa (CMG) Heusler alloy films grown on Y3Fe5O12 (YIG) and Gd3Ga5O12 (GGG) substrates at different deposition temperatures and investigated thermo-spin and magneto-thermoelectric conversion properties by means of a lock-in thermography technique. X-ray diffraction, magnetization, and electrical transport measurements show that the deposition at high substrate temperatures induces the crystallized structures of CMG, while the resistivity of the CMG films on YIG (GGG) prepared at and above 500 °C (550 °C) becomes too high to measure the thermo-spin and magneto-thermoelectric effects due to large roughness, highlighting the difficulty of fabricating highly ordered continuous CMG films on garnet structures. Our lock-in thermography measurements show that the deposition at high substrate temperatures results in an increase in the current-induced temperature change for CMG/GGG and a decrease in that for CMG/YIG. The former indicates the enhancement of the anomalous Ettingshausen effect in CMG through crystallization. The latter can be explained by the superposition of the anomalous Ettingshausen effect and the spin Peltier effect induced by the positive (negative) charge-to-spin conversion for the amorphous (crystallized) CMG films. These results provide a hint to construct spin-caloritronic devices based on Heusler alloys.

Synthesis, Structure, Transport Properties, Electrochemical Stability Window, and Lithium Plating/Stripping of Mg and Nb Codoped Li7La3Zr2O12 Garnet-Type Solid Electrolytes

In this work, the Mg and Nb codoped garnet-type solid electrolytes Li7+x–yLa3–xMgxZr2–yNbyO12 (0 ≤ x ≤ 0.4; 0.4 ≤ y ≤ 0.8) for all-solid-state Li-ion batteries were prepared by conventional solid-state method. The formation of a single-phase garnet structure (space group Ia3̅d) with lattice constant decreased from 12.94452(19) Å (x = 0, y = 0.4) to 12.92286(14) Å (x = 0.4, y = 0.8) have been observed. The La and Zr were substituted by Mg (24c) and Nb (16a), respectively, which greatly altered the conductivity of the sample. Additionally, a high-density structure was obtained through sintering at 1150 °C. The Mg and Nb codoped Li7La3Zr2O12 (LLZO) achieved Li-ion conductivity, ranging from 7.49 × 10–4 S·cm–1 (x = 0, y = 0.4) to 1.95 × 10–4 S·cm–1 (x = 0.4, y = 0.8) at 25 °C. The Li-ion 3D diffusion pathway was constructed by the bond valence sum mapping of Mg and Nb codoped LLZO. The Mg and Nb codoped LLZO retains remarkable electrochemical stability against lithium after 600 cycles at 0.1 mA·cm–2. This work may inspire the rational design of garnet-based electrolyte materials to attain high performance in the energy storage field.

Cation ordering, twinning, and pseudo-symmetry in silicate garnet: The study of a birefringent garnet with orthorhombic structure

AbstractThe crystal structure of a birefringent garnet (~Adr53Grs47) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/a 1 2/d (unconventional setting for Fddd) space group symmetry, with unit-cell parameters of a = b = 11.966(3) Å, c = 11.964(3) Å, α = β = 90°, γ = 90.29(2)°, V = 1713.0(7) Å3, Z = 8. The orthorhombic garnet displays very high birefringence (δ ~0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y1 and Y2 sites, respectively. Diffraction peaks (such as 101 and 103) violating the Ia3d symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between ~Adr53Grs47 and ~Adr100) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.

Phase stabilization and luminescence properties of Gd3Al5O12: Ce nanopowders prepared by Pechini method: Effect of pH and annealing temperature

Gd3Al5O12: 1 at. % Ce3+ (GAG: Ce) nanopowders have been synthesized by the Pechini sol-gel method. The effect of pH of the solution and the annealing temperature on phase stabilization, secondary phase evolution, particle morphology and luminescence properties of GAG: Ce3+ were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and photoluminescence spectroscopy. XRD analysis indicated that the growth of cubic structure of garnet is strongly influenced by the pH of the solution and the annealing temperature. Mixed phases of Gd3Al5O12, GdAlO3 in orthorhombic and hexagonal phases have been observed. The powder prepared with pH = 7 was suitable for obtaining more stable GAG phase. Rietveld refinement revealed that the percentage of the cubic phase increased from 38% to 79% when increasing the annealing temperature from 1000 °C to 1500 °C. FE-SEM/EDS observations showed that increasing annealing temperatures from 1000 °C to 1500 °C resulted in an increase of average particle size from 700 nm to 1.5 μm and a decrease of Gd/Al from 0.79 to 0.59. The characteristic emission band of Ce3+ ions from the 5d → 4f transition localized at 530 nm was observed for all samples. The higher emission intensity was obtained for the sample prepared with pH = 7 and annealed at 1500 °C showing that the annealing at higher temperatures enhances the phase stability to form a purer garnet structure and increases the emission performance of GAG: Ce3+ nanopowders prepared at neutral pH.

Magnetic and structural characteristic of Gd-YIG

The substitution's effect of the of Y3+ by Gd3+ on the structural and magnetic properties of gadolinium-doped yttrium iron garnet, GdxY3-xFe5O12, with 0.0 ≤ x ≤ 1.3 has been presented. The crystalline structures and magnetic properties were investigated using X-ray Diffraction (XRD), Transmission electron microscope (TEM), Transform Infrared Spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM). Furthermore, Curie temperature (Tc) helped us to explain the magnetic behavior of the samples by determining the strength of super-exchange interaction. The XRD patterns of GdxY3-xFe5O12 indicated the pure garnet structure without any extra phase and the sizes of particles were in the range 33–42 nm. The powder morphology was analyzed by transmission electron microscopy (TEM), evidencing agglomerated particles with regular shapes. FT-IR spectra confirmed the garnet phase formation and there is a shift for lower wave numbers as the Gd3+ concentration increases. It was found that the a-d super-exchange interactions play an important role in the saturation magnetization of the sample. In addition, the variation of Gd3+ content affects the Curie temperature (Tc) by determining the strength of super-exchange interaction.

Ultrasonic Spray Pyrolysis Synthesis and Photoluminescence of LuAG:Ce Thin Films.

LuAG:Ce (Lu3Al5O12:Ce) is one of the most important color converters in white lighting industry. Especially, LuAG:Ce film attracts more attention due to the outstanding advantages, such as the efficient heat dissipation, the saving of rare earth, and so on. Here, LuAG:Ce film on sapphire was successfully prepared by the ultrasonic spray pyrolysis process. The phase, microstructure and photoluminescence of LuAG:Ce films were investigated. LuAG:Ce films had a thickness of around 5 μm, which were well crystallized at 1000 °C in air atmosphere to form the typical garnet structure. Under the protection of CO atmosphere, increasing the annealing temperature greatly enhanced the photoluminescence performance. After annealing at 1500 °C for 5 h in CO atmosphere, 3.0 mol.% Ce3+ doped LuAG:Ce film exhibited the highest emission and excitation intensity. The emission intensity of 3.0 mol.% Ce3+ doped LuAG:Ce film annealed at 1500 °C in CO atmosphere increased up to five times, when compared with the best LuAG:Ce film annealed at 1000 °C in air atmosphere. The effects of Ce3+ doping concentration on the photoluminescence were also examined. As the Ce3+ doping concentration increased from 0.2 mol.% to 7.0 mol.%, the color of LuAG:Ce films changed from yellowish green to greenish yellow. When coupling the 3.0 mol.% Ce3+ doped LuAG:Ce film with a 0.5 W 450 nm blue laser, the formed device successfully emitted white light.

Luminescence investigation of Eu2+/Ce3+/Mn2+ doped Y2Mg2Al2Si2O12 phosphors

To investigate the energy transfers and tunable luminescence, Eu2+/Ce3+/Mn2+-doped Y2Mg2Al2Si2O12 phosphors were synthesized by sintering at a temperature of 1400 °C. The phase and luminescence characteristics were investigated subsequently. The XRD results indicate that the synthesized phosphors have the pure phase and garnet structure. Eu2+/Ce3+-, Eu2+/Mn2+- and Eu2+/Ce3+/Mn2+-doped phosphors show tunable luminescence because of the energy transfer processes from Eu2+ to Ce3+ and Mn2+. White lights with different correlated color temperatures are generated by the synthesized phosphors. The dependence of the emission intensity on temperature shows that the thermal quenching temperature of the Y1.955Eu0.030Ce0·015Mg1·985Mn0·015Al2Si2O12 phosphor is higher than 175 °C. Our results demonstrate that the synthesized phosphors can be applied to near-ultraviolet LED chip-excited white-light-emitting diodes.

Multiobjective Machine Learning-Assisted Discovery of a Novel Cyan-Green Garnet: Ce Phosphors with Excellent Thermal Stability.

Ce-doped garnet phosphors play an important role in the white light-emitting diode (LED) family. In the past years, a lot of trial-and-error experiments guided by experience to discover phosphors suitable for white LEDs have been presented. The working temperature of phosphors may reach 200 °C in white LEDs, and so, the exploration of phosphors with excellent thermal stability at the desired wavelength continues to be a challenge. In the present study, to discover novel cyan-green garnet:Ce phosphors, wavelength and thermal stability machine learning models were built by constructing reasonable features. Among the 171,636 compounds with garnet structures predicted by our models, 25 samples were selected for preparation and characterization by multiobjective optimization based on active learning. Lu1.5Sr1.5Al3.5Si1.5O12:Ce performed the best with excellent thermal stability (≥60% emission intensity was retained at 640 K) and exhibited emission peaks of about 505 nm, and it is a very promising phosphor for future applications, especially in high-temperature operating environments.

Phase evolution and chemical stability of Nd-doped natural garnet waste form by microwave sintering

Advanced materials and processes are needed to immobilize fission products in the form of chemically durable waste. Nd-doped Ca3Zr2Fe2SiO12 (GZG) ceramics with different sintering temperatures (1100–1300 °C), sintering times (0.5–6 h) and solid solutions (0 ≤ x ≤ 2.5) were prepared by microwave sintering. The results show that by adjusting a series of key parameters, natural garnet structures with excellent chemical stability are obtained. XRD shows that a single phase of solid solution can be obtained by increasing the sintering temperature and controlling the actinide solid solution below the solid solution limit. The sintering temperature only affects the compactness of the solid solution. Thermodynamic calculations show that high temperature is favorable for the synthesis of GZG, and it is positive and irreversible. The normalized leaching rate of Nd3+ in the GZG structure is approximately ∼ 10−6 g m−1 d−1. The release form of Nd in aqueous solution is inner diffusion. The experimental results show that the naturally occurring garnet structure is an excellent waste form.

Quantitative definition of strength of chromophores in gemstones and the impact on color change in pyralspite garnets

AbstractThe color of most garnets is derived from multiple color‐producing elements (chromophores). In this study we selectively isolate the ultraviolet–visible–near infrared (UV–Vis–NIR) spectra of four major chromophores (Fe2+, Mn2+, Cr3+ and V3+) by collecting and extrapolating the spectra of four representative pyrope‐almandine‐spessartine (pyralspite) garnets. In order to study the color strength of these isolated chromophores and their impact on color change in garnet, we have proposed a new quantitative definition of strength of a chromophore. With the isolated garnet chromophore spectra, hypothetical garnet spectra with various chemical compositions can be simulated. By using these hypothetical garnet spectra, the impact of each chromophore on the color change behavior in pyralspite garnet was also quantitatively studied. Most pyralspite garnets can be successfully modeled using these four chromophores, however, this model cannot adequately describe those garnets containing the Fe3+/Fe2+ intervalence charge‐transfer transitions. Finally, a comprehensive excel data processing template (Appendix 3) written with VBA macro code was generated to help other researchers calculate colorimetric parameters and color panels from UV–Vis–NIR spectrum. In the template, we provided multiple types of conversion matrices and power distribution curves of different illuminants, so that different colorimetric parameter conversion and color panel calculation from UV–Vis–NIR spectrum can be selected with different combinations based on people's preference. The user guide of the excel template can be found in Appendix 4. The detailed calculation procedures and formulae used for the excel template can be found in Appendix 5.

An energy transfer strategy for highly luminescent green-emitting Ce3+/Tb3+ codoped Ca2LaHf2Al3O12 garnet phosphors in white light-emitting diodes

Bright, energy-efficient, and stable white lighting sources, such as light-emitting diodes, are desirable to mitigate climate change. A near-ultraviolet-excitable, green-emitting Ca2LaHf2Al3O12:Ce3+,Tb3+ garnet phosphor with high photoluminescence efficiency, prepared by solid-state synthesis, is described. X-ray diffraction Rietveld refinement reveals its garnet structure with space group Ia3¯d and crystallographic parameters a = b = c = 12.528 Å, α = β = γ = 90°, and V = 1966.470 Å3. On 408 nm excitation, it produces bright green light peaking at 543 nm owing to the 5D4→7F5 transition of Tb3+ ions and demonstrates an ultrahigh internal quantum efficiency (80%) and high external quantum efficiency (59.2%). The luminescence intensity of Ca2LaHf2Al3O12:0.05Ce3+,0.6Tb3+ is double that of a Ca2LaHf2Al3O12:0.05Ce3+ green phosphor owing to Ce3+→Tb3+ energy transfer. This Ca2LaHf2Al3O12:0.05Ce3+,0.6Tb3+ green phosphor was used to fabricate a near-UV-pumped white LED device; the resulting luminous efficacy and color rendering index at 3397 K were 17.56 lm/W and 93.1, respectively.

Full-Spectrum White Light-Emitting Diodes Enabled by an Efficient Broadband Green-Emitting CaY2ZrScAl3O12:Ce3+ Garnet Phosphor.

Phosphor-containing white light-emitting diodes (LEDs) with low color-correlated temperatures (CCTs) and high color rendering indexes (CRIs) are highly desirable for energy-efficient and environmentally friendly solid-state light sources. Here, we report a new and efficient blue light-excited, green-emitting Ce3+-activated CaY2ZrScAl3O12 phosphor, which underpins the fabrication of high-color quality and full-visible-spectrum warm-white LED devices with ultrahigh CRI values (Ra > 96 and R9 > 96). A family of CaY2ZrScAl3O12:Ce3+ phosphors with different Ce3+ dopant concentrations were prepared by high-temperature solid-state synthesis. X-ray diffraction and corresponding Rietveld refinement reveal a garnet structure with an Ia3̅d space group and crystallographic parameters a = b = c = 12.39645(8) Å, α = β = γ = 90°, and V = 1904.99(4) Å3. Luminescence properties were studied in detail as a function of Ce3+ with the optimal concentration 1% mol. Impressively, CaY2ZrScAl3O12:1%Ce3+ exhibits a broad excitation band from 370 to 500 nm, peaking at ∼421 nm, which is well matched with emission from commercial blue LED chips. Under 421 nm excitation, the CaY2ZrScAl3O12:1%Ce3+ phosphor produces dazzling green light in a wide emission band from 435 to 750 nm (emission peak: 514 nm; full width at half-maximum: 113 nm), with a high internal quantum efficiency of 63.1% and good resistance to thermal quenching (activation energy of 0.28 eV). A white LED device combining a 450 nm blue LED chip with CaY2ZrScAl3O12:1%Ce3+ green phosphor and commercial CaAlSiN3:Eu2+ red phosphor as color converters demonstrates bright warm-white light with excellent CIE color coordinates of (0.3938, 0.3819), low CCT of 3696 K, high CRI (Ra = 96.9, R9 = 98.2), and high luminous efficacy of 45.04 lm W-1 under a 20 mA driving current. New green phosphors enable the design and implementation of efficient luminescent materials for healthy solid-state lighting.

Determination of different garnet films characteristics prepared via sol-gel spin or dip-coatings techniques

In order to convert high-energy radiation, such as gamma or X-rays, into a visible light, a certain type of material is needed. Such compounds are usually referred to as scintillators. Over the years many different candidates to fit the requirements were examined. However, compounds with garnet structure have attracted a particularly large amount of attention. Cerium doped yttrium and lutetium aluminum garnets (YAG:Ce, LuAG:Ce), have high density, high thermal stability, a rather intensive emission/excitation and high quantum efficiency which are needed for a good scintillator. However, further optimization and improvement is still needed especially on the shortening of the decay time. One way to approach this problem is to alloy the aforementioned compounds with different elements, such as boron or magnesium.

Microwave dielectric properties of Li3A3Te2O12 (A = Y, Yb) garnets for low temperature cofired ceramic technologies

Te6+-containing microwave dielectric ceramics Li3A3Te2O12 (A = Y, Yb) with low firing temperatures were prepared using the solid-state reaction method. Li3Y3Te2O12 and Li3Yb3Te2O12 can be obtained as garnet single-phase ceramics with low sintering temperatures of 940 ℃ and 950 ℃, respectively. The cations, such as Li+, Te6+, and Y3+/Yb3+, fully occupied the tetrahedral, octahedral, and dodecahedral sites of garnet structure, respectively. Li3A3Te2O12 (A = Y, Yb) ceramics exhibit εr ∼ 7.83 ± 0.2 and 5.94 ± 0.2, Q × f ∼ 47,800 ± 500 GHz and 41,800 ± 500 GHz, and τf ∼ –47 ± 3.0 ppm/°C and –76 ± 3.0 ppm/°C, respectively. The full width at half-maximum (FWHM) values of the A1g Raman mode correlate negatively with the Q × f values. Moreover, Li3A3Te2O12(A = Y, Yb) ceramics possess good chemical compatibility with the Ag electrode, making them promising candidates for low-temperature cofired ceramics (LTCC) technologies.

Energy-saving chromium-activated garnet-structured phosphor-converted near-infrared light-emitting diodes

Phosphor-converted near-infrared light-emitting diodes based on garnet structure as the host for chromium incorporation are promising light sources for modern compact devices and are also an excellent alternative to traditional tungsten lamps.

Electronic structure, optical dispersion and luminescence properties of terbium gallium garnet crystal

Electronic structure, optical dispersion and luminescence properties of terbium gallium garnet (Tb3Ga5O12, TGG) were investigated.