Rare-earth Aluminum Garnets Research Articles

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Engineering atomic size mismatch in Pr3+, La3+ codoped Lu3Al5O12 garnet single crystals for tailored structure and functional properties

This study provided the first in-depth investigation of the effects of large dopant incompatibility (Pr3+ and La3+ ions) on the small host lattice element (Lu3+) in Lu3Al5O12 (LuAG) single crystal. The growth of such complex crystals from the melt presented many challenges. By engineering the ionic radius ratio of RE- and M-site cations, a single-crystal phase stabilized by configurational entropy was achieved. This investigation elucidated the crystallization behavior of configurationally disordered rare-earth aluminum garnet oxide (Lu1−x−yPrxLay)3Al5O12 from the melt and characterized its functional properties, including microstructural, optical, photoluminescence, and scintillation properties, between 5 and 300 K. Relaxation of the imposed strain energy led to local perturbations and destabilization of the garnet structure. Multielemental EDS mapping, micro-Raman spectroscopy, and thermoluminescence revealed the mechanism by which atomic size mismatch drove a smooth transition from the garnet to the perovskite phase in high entropy garnets. The optical, photoluminescence, and scintillation measurements provided fundamental insights into property changes driven by incompatibility doping. Standard and modified Judd-Ofelt theory analysis of absorption spectra determined the phenomenological Judd-Ofelt parameters Ωλ and radiative lifetimes. Atomic size mismatch engineering offers a promising approach to overcoming the limitations of conventional eutectic synthesis methods.

Compositional screening of Ce-doped (Gd,Lu,Y)3(Al,Ga)5O12 ceramics prepared by quenching from melt and their luminescence properties

Quenching from the melt using an Optical Floating Zone furnace was investigated as a possible high-throughput preparation method in order to screen novel scintillating materials. To validate this method, polycrystalline rare-earth aluminum garnets and yttrium gallium aluminum garnets were synthesized and characterized by X-ray diffraction, photoluminescence and radioluminescence emission spectra as well as radioluminescence imaging and compared to the previously reported properties of those materials. In order to achieve rapid, but quantitative comparison of the materials, pellets fabricated from the synthesized powders were sintered; two different sintering conditions were investigated and compared. A simplistic energy deposition and light absorption model for brightness of opaque samples under X-ray irradiation was developed to estimate the relative scintillation efficiency of the materials. Based on the results, Y3Al5O12, LuY2Al5O12 and GdY2Al5O12 seem to have the highest scintillation efficiency among the prepared samples.

Epitaxial Growth of Single Crystal YAG for Optical Devices

We report the latest progress on fabrication of rare earth doped single crystal yttrium aluminum garnet (YAG) core/undoped YAG cladded fibers. Rare-earth doped single crystal core fibers were grown with laser heated pedestal growth methods. In a second step, epitaxial methods were used to grow a single crystalline undoped YAG cladding onto the core fiber. Hydrothermal and liquid phase epitaxy methods utilize the core doped fiber as the seed. X-ray diffraction of cladding reveals an equilibrium (110) morphology. Energy-dispersive X-ray spectroscopy analysis shows there is minimal diffusion of rare-earth dopants into the cladding structure. The use of scandium doping is shown to substitute at the Al3+ site, thereby allowing an additional tunability of refractive index of core structure material besides conventional Y3+ site dopants. The use of these epitaxial growth methods enables material compatibility, tuning of refractive index, and conformal growth of cladding structures onto core fibers for optical devices.

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.

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.

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.

Heat capacity of rare-earth cuprates, orthovanadates, and aluminum garnets, gallium garnets, and iron garnets

The correlation between the heat capacities of rare-earth cuprates, orthovanadates, and garnets with ionic radius R3+ has been analyzed. It has been shown that the values of Cp0 change consistently depending on the radius R3+ within the corresponding tetrads (La-Nd, Pm-Gd, Gd-Ho, Eu-Lu).

Positron Lifetime Measurements of Vacancy Defects in Complex Oxides

Native defects in complex oxides play a crucial role in determining their optical, electrical, and magnetic properties and it is di cult to identify and characterize them. Positron lifetime spectroscopy is a powerful technique to study vacancy defects; however its application to complex oxides has been limited. In this work we apply positron lifetime spectroscopy to study open volume defects in rare earth doped yttrium aluminum garnet (YAG) complex oxides grown in argon atmosphere. In YAG single crystals, positron lifetime measurements identi ed isolated aluminum vacancies and complexes of aluminum vacancy and neighbor oxygen vacancies. Thermoluminescence measurements were also performed to elucidate the interaction between trapping defects and luminescence centers. By combining positron lifetime and thermoluminescence, both the defect type and its e ect on the optical properties of YAG crystals were revealed.

Scintillating Properties of Rare Earth Aluminum Garnets

Rare earth aluminum garnets, RE3Al5O12, where RE are Lu, Y or their mixture in asked ratio, doped with Ce3+ or Pr3+ are efficient scintillators characterized by high or medium light yield (L.Y.) up to 27000 ph/MeV [1,2]. These scintillators are tested and used in various applications such as medical imaging (PET, PEM, radiography etc. [3]) or in 2D X-ray microradiography [1]. Crystalline samples of above mentioned aluminum garnets can be prepared as the single crystals grown by the Czochralski or Bridgman methods [4,5], as epitaxial layers grown by the liquid phase epitaxy (LPE) and in the form of ceramics [7]. In particular the epitaxial layers seem to be very promising medium materials regarding their good scintillating properties and cost [5,6]. Scintillation properties of garnet structure materials were investigated by pulse height spectroscopy using a HPMT photomultiplier [2]. Scintillating properties of garnet structure materials can be influenced by (i) garnet sample composition, by (ii) annealing at different temperatures and atmospheres and by (iii) effective doping if energy transfer is present. The main goal of this report is to present short summary of scintillating properties (L.Y. and energy resolution) of various aluminum garnet structure materials depending on the above described modifications (i)-(iii).

Dopant segregation in rare earth doped lutetium aluminum garnet single crystals grown by the micro-pulling down method

Nd, Ho, Er, and Tm doped lutetium aluminum garnet (LuAG) single crystals were grown using the micro-pulling down (μ-PD) method. The crystals were produced from the melts containing 3mol% of the dopants. The axial and radial dopant distribution were measured by electron probe micro analysis. Nd3+ and Ho3+ ions concentrated in the rim (k0<1) and the radial concentration showed concave curvature. k0 is the segregation coefficient of the dopant with respect to the given host phase. In the case of Er3+ and Tm3+ ions, the k0 value is considered to be nearly 1 and the radial dopant distribution profiles were almost flat.

Formation of ultrafine eutectic-like microstructures of various rare earth oxide-Al2O3 systems by use of amorphous phases

Ultrafine eutectic-like microstructures of various rare earth (RE) oxide-Al2O3 systems were formed by use of amorphous phases. This new method uses a low migration rate in the amorphous phases. Mixtures of RE oxide (RE: Yb, Dy, Er, Ho, Gd, Sm, Eu) and Al2O3 powders with the eutectic compositions were melted and quenched rapidly to form the amorphous phases. A heat treatment of the amorphous phases of various eutectic systems at 1000 and 1300 °C, for 30 min, formed RE aluminum garnet (RE3Al5O12)/Al2O3 phases or RE aluminum perovskite (REAlO3)/Al2O3 phases. Scanning electron microscopy observation of these materials heat-treated at 1300 °C showed eutectic-like microstructures, in which crystals of eutectic component were entangled with each other. Furthermore, the microstructures were much finer than those of materials generally prepared from eutectic melts. In this study, it was confirmed that this method is useful for the formation of ultrafine eutectic-like microstructures for many eutectic systems.

Defect structure of rare earth aluminium garnets obtained by the glycothermal method

Rare earth aluminium garnets (REAG), Gd3Al5O12, Y3Al5O12 and Yb3Al5O12, having the particle size of 50–120, 30–50 and 30–50 nm, respectively, were prepared by glycothermal reaction. The unit cell parameters of REAGs were larger than the values reported in JCPDS cards. Rietveld analysis indicated the presence of Al vacancies in the 24d sites and oxygen vacancies in the 96h sites, and partial substitution of Al ions in the 16a sites with RE ions was also suggested. The occupancy of RE ions in 16a sites increased with the decrease in ionic size of the RE element.

Cover picture: Extrinsic defect structure of RE3Al5O12 garnets

AbstractThe cover picture of Rapid Research Letters refers to the article [1] by Chris Stanek et al. The image visually describes the results of calculations pertaining to the defect structure in rare earth aluminum garnet corresponding to Me2+ and Me4+ dopants. In particular, this result aids the development of garnet scintillators.

Vibrational properties of the rare earth aluminum garnets

In this work we present single crystal infrared (IR) reflectance spectra for the Dy3Al5O12 and Er3Al5O12 compounds, their corresponding optical constants, as well as a discussion concerning the vibrational, dielectric, and elastic properties of various members of the rare earth aluminum garnet family. In the framework of the rigid ion model, the frequencies of the experimentally unresolved infrared active modes, the IR eigenvectors and the partial phonon density of states have been calculated. The role of the different atomic species in the various regions of the IR vibrational spectrum has been elucidated. In particular, it is found that the rare earth ionic mass plays a crucial role in the low frequency IR modes (&amp;lt;200 cm−1), while the high frequency modes are related mainly to the molecular modes of the AlO4 tetrahedral subunit. From the volume dependence of the IR active mode frequencies, their mode Grüneisen parameters are estimated and discussed. Finally, by using a modification of the semiempirical Brout’s formula, the bulk moduli have been determined and are compared to those derived from interatomic potential calculations.

Lattice Dynamical Properties of the Rare Earth Aluminum Garnets (RE3Al5O12)

This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE3Al5O12). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al–O and the dodecahedral RE–O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.

Nonlocal effects in diffusive propagation of heat pulses in systems with trapping centers of nonequilibrium phonons

The propagation of heat pulses in systems with defects as trapping centers of nonequilibrium phonons is investigated theoretically. Among these defects are point defects involving two-level systems (TLSs) of different nature. It is demonstrated that, in addition to the principal signal, one more signal can be detected by the bolometer due to reemission of the nonequilibrium TLS energy at a certain ratio of relaxation times in the phonon and TLS subsystems. The temperature and concentration dependences of the time of signal arrival at the bolometer are analyzed. The results of theoretical investigations are compared with experimental data on the propagation of weakly nonequilibrium thermal phonons in solid solutions of the Y3−xErxAl5O12 rare-earth yttrium aluminum garnets.

Rare earth doped yttrium aluminum garnet (YAG) selective emitters

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study presents a spectral emittance model for films and cylinders of rare earth doped yttrium aluminum garnets. Good agreement between experimental and theoretical film spectral emittances was found for erbium and holmium aluminum garnets. Spectral emittances of films are sensitive to temperature differences across the film. For operating conditions of interest, the film emitter experiences a linear temperature variation whereas the cylinder emitter has a more advantageous uniform temperature. Emitter efficiency is also a sensitive function of temperature. For the holminum aluminum garnet film the efficiency is 0.35 at 1446 K, but only 0.27 at 1270 K.

Rare earth doped high temperature ceramic selective emitters

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study develops a spectral emittance model for films of rare earth containing materials. Although there are several possible rare earth doped high temperature materials, this study was confined to rare earth aluminum garnets. Good agreement between experimental and theoretical spectral emittances was found for erbium, thulium and erbium–holmium aluminum garnets. Spectral emittances of these films are sensitive to temperature differences across the film. Emitter efficiency is also a sensitive function of temperature. For thulium aluminum garnet the efficiency is 0·38 at 1700 K but only 0·19 at 1262 K.