Rare-earth Garnets Research Articles

Specific Heat and Phonon Transport in Er-Containing Rare-Earth–Aluminum Garnets at Liquid-Helium Temperatures

The temperature dependences of the specific heat and the transport characteristics of thermal-frequency phonons in single crystals of the solid solutions of rare-earth aluminum garnets are studied at liquid-helium temperatures in the presence of Schottky-type low-energy excitations. The kinetic characteristics of phonons as functions of the solid solution composition are measured. The relation between the kinetic and thermophysical characteristics of the material in the solid solutions of rare-earth aluminum garnets is analyzed under conditions of a nonstationary process and the spatial inhomogeneity caused by the coordinate dependence of the state of low-energy excitations. The thermalization conditions in the nonequilibrium phonon–low-energy excitation system are estimated.

Magnetic properties of lithium-containing rare earth garnets Li3RE3Te2O12 (RE = Y, Pr, Nd, Sm-Lu)

The synthesis, structural description, temperature dependent magnetic susceptibilities and field dependent magnetizations of a series of rare earth garnets of the form Li3RE3Te2O12 (RE = Y, La, Pr, Nd, Sm-Lu) are reported. The structure of Li3Dy3Te2O12 is refined from powder x-ray diffraction data. The field dependent magnetizations saturate for some of the members by 9 Tesla at 2 K. Of particular interest for further study in this family is the behavior of the Pr, Tb, Dy and Yb garnets.

Structural, magnetic, dielectric and high frequency response of synthesized rare earth doped bismuth nano garnets (BIG)

Abstract Rare earth doped bismuth (Bi) iron garnets (BIG) having composition Bi3−xRxFe5O12 for x = 1.5 (here R stands for rare earth elements like Y, Gd, Pr, Ho and Yb) have been studied in this work. The samples were prepared using sol-gel method. X-Ray diffraction (XRD), Scanning electron microscope (SEM), Fourier transform infrared spectroscope (FTIR), Vibrating sample magnetometer (VSM) and Vector network analyzer (VNA) were employed on the samples to evaluate the properties of the doped Bi nanogarnets. Single-phase garnet structure was confirmed through XRD and the crystallite sizes were found to be 66–73 nm, depending upon the type of rare earth ions. SEM images helped to observe the morphology and measurement of grain size of the doped Bi nanogarnets. Vibrational bands showed the presence of garnet phase and were confirmed through FTIR analysis. Detailed dynamic magnetic and dielectric properties were measured using VNA from 1 to 6 GHz. Real and imaginary part of the permittivity, permeability, dielectric and magnetic losses, ac conductivity, electric modulus and Q factor were calculated for the rare earth doped Bi nanoferrites. Gd doped Bi nanoferrites showed higher permittivity values whereas Pr doped show higher ac conductivity as compared to other samples. Yb doped Bi nanoferrites have Q factor values at 4.8 GHz. Static magnetic properties of the doped Bi nanogarnets were investigated using magnetic loops. Magnetic remanence (Mr) Saturation magnetization (Ms) and coercivity (Hc), Bohr magneton, anisotropy constant and initial permeability were also calculated. The dielectric and magnetic studies of the rare earth doped Bi nanogarnets suggested the applications of these rare earths doped nanogarnets for microwave devices and high frequency applications in GHz regime.

Structural, morphology and luminescence study of Er+3-doped garnet-type Li5La3Nb2O12 electrolytes as a potential new phosphor

Erbium doped garnet-type Li5La3Nb2O12 electrolytes were synthesized by conventional solid state reaction in air at 900 °C. Formation, microstructure and luminescence properties were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–VIS absorption and luminescence techniques. The limits of the Li5La3-xErxNb2O12 solid solution (0 ≤ x ≤ 0.05) were established by XRD and SEM. For x > 0.05, garnet structure coexists with segregated Er2O3. Rietveld refinement of room temperature powder XRD patterns shows that this solid solution exhibits a cubic garnet structure (a ~ 12.79 Å and S.G. Ia-3d) and trivalent Er ions replace La cations in cubic coordination (located in 24c site). The typical green, red and infrared luminescence of Er3+ ions can be efficiently excited under several wavelength lights in the UV–VIS–NIR region. Optical measurements have been improved dispersing Li5La3-xEr-xNb2O12 powder in KBr pellets. Our results indicate the potentiality of rare earth doped Li5La3Nb2O12 garnets as a new family of phosphors.

Thermal radiation properties of porous rare-earth garnet ceramics

Thermal radiation properties of porous rare-earth garnet ceramics

Estimation of Thermal Expansion Coefficients of Rare-Earth Garnets in a Wide Temperature Range

Simple expressions were obtained for the thermal expansion coefficients of the rare-earth garnets, which allow to estimate their values in a wide temperature range in cases, when either the lattice unit cell parameters or the density of the material are known at 300 K. The expression giving the relationship between the thermal expansion and thermal conductivity coefficients for the rare-earth garnets has also been obtained. It is shown that the calculated values obtained with the use of these expressions are in a good agreement with the available experimental data for the garnets of different composition.

Failure Analysis of Multilayered Suspension Plasma-Sprayed Thermal Barrier Coatings for Gas Turbine Applications

Improvement in the performance of thermal barrier coatings (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria-stabilized zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multilayered TBCs consisting of advanced topcoat materials fabricated by suspension plasma spraying (SPS). The investigated topcoat materials were YSZ, dysprosia-stabilized zirconia, gadolinium zirconate, and ceria–yttria-stabilized zirconia. All topcoats were deposited by TriplexPro-210TM plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high-temperature applications.

Magnetic and optical property studies on cubic Gd3Fe5−xCoxO12 nanogarnets for spintronics

Gadolinium-based rare earth garnets were processed in the form of nanostructures and studied for their optical and magnetic properties at room-temperature.

Ferroelectrics of homogeneously deformed rare-earth garnet crystals, excited by elastic wave propagation

The effect of deformation on the electric properties in rare-earth garnet compounds is elucidated. It is shown that inhomogeneous deformation causes the emergence of electric polarization in garnet crystals on account of nonequivalent low-symmetry sites of rare-earth ions in the cubic structure of these crystals, whose symmetry of the environment has no inversion center. The polarization of a rare-earth ion subsystem in garnet crystals is studied upon the elastic wave propagation therein.

Luminescence of Eu:Y3Al5O12, Eu:Lu3Al5O12, and Eu:GdAlO3 Nanocrystals Synthesized by Solution Combustion

Nanocrystals of rare-earth garnets Y3Al5O12 and Lu3Al5O12 and perovskite GdAlO3 highly doped (10–20 at%) with Eu3+ are synthesized by the solution combustion technique and subsequent annealing in air at 800 and 1300oC. Their structure, morphology, and phase composition are studied. These materials exhibit intense red luminescence under UV excitation. Eu:GdAlO3 luminescence has CIE 1931 color coordinates (0.632, 0.368); dominant wavelength, 599.6 nm; and color purity, >99%. Judd–Ofelt parameters, luminescence branching ratios, and lifetimes of the Eu3+ 5D0 state are determined. The luminescence quantum yield for Eu:GdAlO3 (10 at%) reaches 74% with a lifetime of 1.4 ms for the 5D0 state. The synthesized materials are promising for red ceramic phosphors.

Heat capacity of rare-earth aluminum garnets

Heat capacity of the single crystalline ErxY3-xAl5O12 (x = 0, 0.6, 1.1, 3), Er2HoAl5O12, and Ho1.5Y1.5Al5O12 garnets was measured in the temperature range from 1.9 to 220 K at zero magnetic field and at fields up to 9 T. The Schottky anomalies were found for the garnets with magnetic ions. The heat capacity was fitted by a sum of the Debye, Einstein, and Schottky contributions. The entropy and magnetic entropy were evaluated. The magnetic entropy magnitude in the garnets with erbium ions suggests the application of these garnets in adiabatic demagnetization refrigerators.

Field-induced magnetic transition in a mixed rare-earth aluminum garnet Er2HoAl5O12

The temperature dependence of the ac magnetic susceptibility of a single-crystal mixed rare-earth garnet Er2HoAl5O12 has been investigated within the range from 1.8 to 300 K in a zero constant field and in applied bias fields of up to 9 T. In the absence of a constant magnetic field the magnetic susceptibility followed the Curie–Weiss law. The application of a constant magnetic field caused a magnetic phase transition, the temperature of which increased with increasing magnetic field. The temperature of the maximum of the ac magnetic susceptibility, which is a characteristic of the phase transition, did not show a noticeable dependence on the frequency of the alternating magnetic field.

Evaluation of stability region for scandium-containing rare-earth garnet single crystals and their congruent-melting compositions

Single crystals of scandium-containing rare-earth garnets in system R-Sc-C-O (R3+=Y, Gd; C3+=Al, Ga) have been grown by the Czochralski technique. X-ray diffraction analysis has been used to refine crystal compositions. The fundamental difference between the melt compositions and compositions of grown crystals has been found (except for compositions of congruent-melting compounds, CMC). The specific features of garnet solid solution formation have been established and the ternary diagrams with real or hypothetical phases have been built. The dinamics of coordination polyhedra changes with the formation of substitutional solid solutions have been proposed based on the mathematical modeling and experimental data. Possible existence of CMC with garnet structure in different systems as well as limit content of Sc ions in dodecahedral and octahedral sites prior to their partial substitution of ions, located in other sites, have been evaluated. It was established that the redistribution of cations over crystallographic sites (antistructural point defects) due to system self-organization to maintain its stability may be accompanied by cation ordering and the symmetry change of individual polyhedrons and/or the whole crystal.

Magnetic Hysteresis of Molecular Faraday Effects of Phthalocyanine-Based Thin Films on Bi, Al-Substituted DyIG Substrates at Room Temperature and Demagnetization of the Ferrimagnetic Substrates by Photothermal Effects of Phthalocyanines

We report the switching of magneto-optical signals of a ferrimagnetic substrate by a selective laser irradiation of a phthalocyanine-based thin film on the substrate followed by application of an external magnetic field. We prepared thin films based on μ-oxo-bis[hydroxyl{2,9(or 10),16(or 17),23(or 24)-tetra-tert-buthylphthalocyanato}silicon], (SiPc)2, onto a ferrimagnetic rare earth garnet Bi0.8Dy2.2Fe4.3Al0.7O12 substrate, whose Curie temperature is appropriate for demagnetization by laser irradiation. The magnetic hysteresis of the magnetic circular dichroism (MCD) signals of (SiPc)2 was observed, which reflects the ferrimagnetic properties of the substrate. To the best of our knowledge, this is the first observation of the magnetic hysteresis of molecular Faraday effects at room temperature. After the selective pulsed laser excitation of (SiPc)2, the MCD intensity of the substrate decreased, which reflects the demagnetization of the substrate, and indicates that the photothermal energy produced in the ...

Ferroelectricity of domain walls in rare earth iron garnet films

In this paper, we report on electric polarization arising in a vicinity of Bloch-like domain walls in rare-earth iron garnet films. The domain walls generate an intrinsic magnetic field that breaks an antiferroelectric structure formed in the garnets due to an exchange interaction between rare earth and iron sublattices. We explore 180° domain walls whose formation is energetically preferable in the films with perpendicular magnetic anisotropy. Magnetic and electric structures of the 180° quasi-Bloch domain walls have been simulated at various relations between system parameters. Singlet, doublet ground states of rare earth ions and strongly anisotropic rare earth Ising ions have been considered. Our results show that electric polarization appears in rare earth garnet films at Bloch domain walls, and the maximum of magnetic inhomogeneity is not always linked to the maximum of electric polarization. A number of factors including the temperature, the state of the rare earth ion and the type of a wall influence magnetically induced electric polarization. We show that the value of polarization can be enhanced by the shrinking of the Bloch domain wall width, decreasing the temperature, and increasing the deviations of magnetization from the Bloch rotation that are regulated by impacts given by magnetic anisotropies of the films.

A method for solid phase synthesis of phosphors under increased pressure; creation of remote phosphors

A method for phosphors solid-phase synthesis under high pressure is proposed. Using the method, phosphors based on rare earth garnets were synthesized at a temperature considerably lower (1300 °C) than the temperature of the synthesis by conventional method (above 1650 °C). Using organic, inorganic ligaments and synthesized phosphors, remote phosphors were created. The chromaticity coordinates of created LEDs on the base of blue LEDs and remote phosphors fall into the bin of white and green colours. Luminous efficiency reaches 140 lm/W.

Athermal distributed Brillouin sensors utilizing all-glass optical fibers fabricated from rare earth garnets: LuAG

An all-glass optical fiber derived from single-crystal LuAG is investigated for its potential use in athermal Brillouin distributed strain sensors. Such sensor systems are comprised of fiber whose Brillouin frequency shift is independent of temperature, but not independent of strain. Bulk Brillouin spectroscopy measurements on the precursor LuAG crystal are performed to gain insight into the crystal-to-glass transition. Results suggest that both the mass density and acoustic velocity are reduced relative to the crystal phase, in common with the other rare earth aluminosilicates. Advantages of the LuAG derived fiber over other rare earth garnet-derived fibers for the sensing application are a stronger strain response and larger Brilloun gain with narrower Brillouin spectral width.

Crystal field and magnetism with Wannier functions: rare-earth doped aluminum garnets

Using the recently developed method we calculated the crystal field parameters in yttrium and lutetium aluminum garnets doped with seven trivalent Kramers rare-earth ions. We then inserted calculated parameters into the atomic-like Hamiltonian taking into account the electron-electron, spin-orbit and Zeeman interactions and determined the multiplet splitting by the crystal field as well as magnetic ĝ tensors. We compared calculated results with available experimental data. Very good agreement with the spectroscopic data and qualitative agreement with experimental ĝ tensors was found.

Photoluminescence study of rare earth doped Yttrium aluminum garnet—YAG:RE (RE: Eu3+, Pr3+ and Tb3+)

Yttrium aluminum garnet (YAG) phosphor powders doped with rare earth ions (Eu3+, Pr3+and Tb3+) were synthesized by mixed fuel combustion method at low temperature. The XRD analysis of the powders confirms the formation of pure phase of YAG even at low temperature. A systematic study of the fluorescence properties of YAG:Eu3+, YAG:Pr3+ and YAG:Tb3+ shows respective characteristics excitation and emission spectra confirms the incorporation of ions in YAG.

High-pressure phase transition in Y3Fe5O12

Yttrium iron garnet (YIG, Y3Fe5O12) was examined up to 74 GPa and 1800 K using synchrotron x-ray diffraction in a diamond anvil cell. At room temperature, YIG remained in the garnet phase until abrupt amorphization occurred at 51 GPa, consistent with earlier studies. Upon laser heating up to 1800 K, the material transformed to a single-phase orthorhombic GdFeO3-type perovskite of composition (Y0.75Fe0.25)FeO3. No evidence of decomposition of the sample was observed. Both the room-temperature amorphization and high-temperature transformation to the perovskite structure are consistent with the behaviour of other rare earth oxide garnets. The perovskite sample was compressed between 28–74 GPa with annealing to 1450–1650 K every 3–5 GPa. Between 46 and 50 GPa, a 6.8% volume discontinuity was observed without any accompanying change in the number or intensity of diffraction peaks. This is indicative of a high-spin to low-spin electronic transition in Fe3+, likely in the octahedrally coordinated B-site of the perovskite. The volume change of the inferred spin transition is consistent with those observed in other rare earth ferric iron perovskites at high pressures.