Rare Earth Sesquioxides Research Articles

X-ray diffraction and spectroscopy study of nano-Eu2O3 structural transformation under high pressure

Nanoscale materials exhibit properties that are quite distinct from those of bulk materials because of their size restricted nature. Here, we investigated the high-pressure structural stability of cubic (C−type) nano-Eu2O3 using in situ synchrotron X−ray diffraction (XRD), Raman and luminescence spectroscopy, and impedance spectra techniques. Our high-pressure XRD experimental results revealed a pressure-induced structural phase transition in nano−Eu2O3 from the C−type phase (space group: Ia-3) to a hexagonal phase (A−type, space group: P-3m1). Our reported transition pressure (9.3 GPa) in nano-Eu2O3 is higher than that of the corresponding bulk-Eu2O3 (5.0 GPa), which is contrary to the preceding reported experimental result. After pressure release, the A−type phase of Eu2O3 transforms into a new monoclinic phase (B−type, space group: C2/m). Compared with bulk−Eu2O3, C-type and A-type nano−Eu2O3 exhibits a larger bulk modulus. Our Raman and luminescence findings and XRD data provide consistent evidence of a pressure-induced structural phase transition in nano-Eu2O3. To our knowledge, we have performed the first high-pressure impedance spectra investigation on nano-Eu2O3 to examine the effect of the structural phase transition on its transport properties. We propose that the resistance inflection exhibited at ∼12 GPa results from the phase boundary between the C−type and A−type phases. Besides, we summarized and discussed the structural evolution process by the phase diagram of lanthanide sesquioxides (Ln2O3) under high pressure.

X-ray diffraction and Raman studies on Ho: Eu2O3

In this paper the structural parameters of mixed rare earth sesquioxides (Eu1-xHox)2O3 (0 ≤ x ≤ 1) is reported for the first time. The oxide samples are characterized by angle dispersive X-ray diffraction (ADXRD) and Raman scattering techniques. ADXRD measurements confirmed the formation of single phase in the cubic bixbyite structure with space group Ia-3. Complete miscibility of the two components is confirmed by the Vegard's law. A random distribution of the two cations in the two rare earth (RE) ion sites is observed. Symmetry modes are assigned for the observed 13 out of 22 expected Raman bands and the dependency of average cationic radii, RRE, is discussed. A low frequency band shows an anomalous mode softening with decrease in Eu content. This mode disappears above x = 0.8, which coincides with a distinct slope change observed in the RE(24d) positional coordinate estimated from refined x-ray diffraction data.

Structural and microstructural tailoring of rare earth sesquioxides by swift heavy ion irradiation

The structural transformations induced in Gd2O3 and Eu2O3 irradiated with high‐energy ions (94‐MeV Pb and 1042‐MeV Xe), where ionization processes (electronic stopping) dominate the ion slowing‐down, have been studied via the combination of XRD and Raman experiments. Irradiation induces a cubic‐to‐monoclinic phase transformation; the radiation‐induced monoclinic phase shows strong preferential orientation. This structural modification is accompanied by changes in the photoluminescence properties of sesquioxides. The results show that swift heavy ion irradiation is a powerful tool to control the properties of oxides at the nanoscale.

Metal-Oxygen Hybridization and Core-Level Spectra in Actinide and Rare-Earth Oxides

ABSTRACT We employ a combination of the density-functional theory and the dynamical mean-field theory to study the electronic structure of selected rare-earth sesquioxides and dioxides. We concentrate on the core-level photoemission spectra, in particular, we illustrate how these spectra reflect the integer or fractional filling of the 4f orbitals. We compare the results to our earlier calculations of actinide dioxides and analyze why the core-level spectra of actinide compounds display a substantially reduced sensitivity to the filling of the 5f orbitals.

Pressure-Induced Structural Transition Trends in Nanocrystalline Rare-Earth Sesquioxides: A Raman Investigation

The article presents a concise review of our investigations on the preferred transition paths followed as well as high-pressure-induced structural changes in nanocrystalline rare-earth sesquioxides that include Y2O3, Sm2O3, Gd2O3, Eu2O3, Dy2O3, Ho2O3, and Yb2O3. The starting phase in all samples was predominantly cubic, as characterized using X-ray diffraction and Raman spectroscopy. The pressure-induced structural changes were primarily tracked via changes in phonon modes in the Raman spectra. The structural transition sequences demonstrated behaviors differing from the polycrystalline trends usually observed; however, it was interesting to note that the onset pressures and subsequent shifts in the phonon modes followed a trend similar to the unpressurized samples when observed with respect to the f-electron number. The mode Gruneisen parameters were estimated from the high-pressure data, which indicated a swifter response to external stimuli as the particle size reduced below an average of 50 nm. It was...

Comparison of the high-pressure behavior of the cerium oxidesCe2O3andCeO2

The high-pressure behavior of $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ was studied using angle-dispersive x-ray diffraction to 70 GPa and compared with that of $\mathrm{Ce}{\mathrm{O}}_{2}$. Up to the highest pressure $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ remains in the hexagonal phase (space group 164, $P\overline{3}2/m1)$ typical for the lanthanide sesquioxides. A theoretically predicted phase instability for 30 GPa is not observed. The isothermal bulk modulus and its pressure derivative for the quasihydrostatic case are ${B}_{0}=111\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=4.7\ifmmode\pm\else\textpm\fi{}0.3,$ and for the case without pressure-transmitting medium ${B}_{0}=104\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=6.5\ifmmode\pm\else\textpm\fi{}0.4$. Starting from ambient-pressure magnetic susceptibility measurements for both oxides in highly purified form, we find that the Ce atom in $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ behaves like a trivalent $\mathrm{C}{\mathrm{e}}^{3+}$ ion ($2.57{\ensuremath{\mu}}_{\mathrm{B}}$ per Ce atom) in contrast to previously published data. Since x-ray emission spectroscopy of the $L\ensuremath{\gamma}$ $(4{d}_{3/2}\ensuremath{\rightarrow}2{p}_{1/2})$ transition is sensitive to the $4f$ -electron occupancy, we also followed the high-pressure dependence of this line for both oxides up to 50 GPa. No change of the respective line shape was observed, indicating that the $4f$-electron configuration is stable for both materials. We posit from this data that the $4f$ electrons do not drive the volume collapse of $\mathrm{Ce}{\mathrm{O}}_{2}$ from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase.

Evidence for 5d-σ and 5d-π covalency in lanthanide sesquioxides from oxygen K-edge X-ray absorption spectroscopy.

The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy (XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.

Tuning the composition of rare earth sesquioxides Gd2−xLaxO3:Eu3+ to control phase transitions at a high temperature to design new highly sensitive luminescence-based thermal sensors

Coupling Rietveld XRD data analysis with the optical emission spectroscopy to probe the local environment, gives a perfect view of the C → B phase transition and allows designing new highly sensitive thermal sensors at high temperatures.

Structural properties and the pressure-induced C → A phase transition of lanthanide sesquioxides from DFT and DFT + U calculations

Density functional theory (DFT) calculations were performed to analyze the structural properties of rare-earth lanthanide sesquioxides (Ln2O3) with the hexagonal A and the cubic C phases. The calculations were performed with the DFT-based Augmented Plane Wave plus local orbital (APW + lo) method, using both the local density approximation (LDA) and the Wu and Cohen parametrization of the generalized gradient approximation (WC-GGA) for the exchange and correlation energy. We also considered the addition of the Hubbard U term in the WC-GGA approximation (GGA + U approach), to take into account the strongly correlated Ln-4f electrons involved. The predicted equilibrium properties for both phases of each Ln2O3 system, i.e. crystal equilibrium volume, bulk modulus and its first pressure derivative are in good agreement with the corresponding experimental results. The obtained predictions with LDA and WC-GGA reveal that the A phase is stable for the lighter systems of the Ln2O3 sesquioxide series, and the C phase is favored for the heavier Ln2O3, in a qualitatively good agreement with the experimental data. However, the predicted C to A phase transition pressure PC→A, as well as the sesquioxide equilibrium volumes corresponding to the lanthanides of the middle of the series, underestimates the experimental values. The implementation of the U term gives a correct description of the insulating ground state of these systems, and also improves the predictions for the studied structural properties with respect to those predicted by LDA and WC-GGA. However, PC→A is extremely sensitive to the U parameter, giving the GGA + U approach method very poor predictions for this quantity. The reasons of these effects are analyzed in detail.

Flux growth and physical properties characterizations of Y_1866Eu_0134O_3 and Lu_156Gd_041Eu_003O_3 single crystals

Y1.866Eu0.134O3 and Lu1.56Gd0.41Eu0.03O3 single crystals of the cubic rare-earth sesquioxide phase were grown for the first time by a new flux method, in air and at temperatures between 1250 and 1100 degrees C. Magnetic susceptibility measurements performed in the former crystals suggest that a preferential dissolution of Eu3+ cations occurs on the C-2-site. Transmission measurements established the promising optical quality of the samples, especially for Lu1.56Gd0.41Eu0.03O3 in the red spectral range where most of the scintillation light is emitted. The Lu1.56Gd0.41Eu0.03O3 single crystal, grown and tested for the first time, is an efficient X-ray scintillator and its characteristics seem promising for X-ray imaging with medium spatial resolution, in terms of light output and low afterglow. Emission and transmission spectra were measured and Eu3+ Judd-Ofelt analysis was performed in both crystals. (C)2015 Optical Society of America

4f fine-structure levels as the dominant error in the electronic structures of binary lanthanide oxides.

The ground-state 4f fine-structure levels in the intrinsic optical transition gaps between the 2p and 5d orbitals of lanthanide sesquioxides (Ln2 O3 , Ln = La…Lu) were calculated by a two-way crossover search for the U parameters for DFT + U calculations. The original 4f-shell potential perturbation in the linear response method were reformulated within the constraint volume of the given solids. The band structures were also calculated. This method yields nearly constant optical transition gaps between Ln-5d and O-2p orbitals, with magnitudes of 5.3 to 5.5 eV. This result verifies that the error in the band structure calculations for Ln2 O3 is dominated by the inaccuracies in the predicted 4f levels in the 2p-5d transition gaps, which strongly and non-linearly depend on the on-site Hubbard U. The relationship between the 4f occupancies and Hubbard U is non-monotonic and is entirely different from that for materials with 3d or 4d orbitals, such as transition metal oxides. This new linear response DFT + U method can provide a simpler understanding of the electronic structure of Ln2 O3 and enables a quick examination of the electronic structures of lanthanide solids before hybrid functional or GW calculations.

Photoluminescence of europium(III)-doped (YxSc1−x)2O3 nanoparticles: Linear relationship between structural and emission properties

Europium (III)-doped (YxSc1−x)2O3 nanocrystalline powders of different compositions (x=0, 0.25, 0.5, 0.75 and 1) are synthesized using a polymer complex solution method. Comparative structural and optical characterization is carried out with powder X-ray diffraction and photoluminescence excitation, emission and emission decay measurements. Detailed information about unit cell parameter and energy levels of europium (III) ions are presented for all powder compositions. The second rank crystal field parameters (B02) and (B22) and crystal field strength (Nv) are calculated from the energies of Stark components of the 7F1 manifold of the europium (III) emissions. It has been shown that the crystal field parameter changes with the powder composition and shows a linear relationship with the average ionic radii of cations in the solid solutions. A similar dependence is also found for the magnitude of splitting of 7F1 manifold vs. unit cell parameter values. Moreover, this linear relationship is demonstrated for the whole family of europium (III)-doped mixed rare-earth sesquioxide hosts (RR’O3; R, R′=rare earth ions) by taking into consideration results published in the literature so far. A simple, semi-empirical equation which relates structural properties (unit cell parameters, ionic radii) of the sesquioxide host material and the emission properties of europium (III) ions (energy splitting of the 7F1 manifold, crystal field parameters and crystal field strength) are presented.

Phase transformations inLn2O3materials irradiated with swift heavy ions

Phase transformations induced in the cubic C-type lanthanide sesquioxides, ${\mathrm{Ln}}_{2}{\mathrm{O}}_{3}$ (Ln = Sm, Gd, Ho, Tm, and Lu), by dense electronic excitation are investigated. The structural modifications resulting from exposure to beams of 185 MeV Xe and 2246 MeV Au ions are characterized using synchrotron x-ray diffraction and Raman spectroscopy. The formation of a B-type polymorph, an X-type nonequilibrium phase, and an amorphous phase are observed. The specific phase formed and the transformation rate show dependence on the material composition, as well as the ion beam mass and energy. Atomistic mechanisms for these transformations are determined, indicating that formation of the B-type phase results from the production of anti-Frenkel defects and the aggregation of anion vacancies into planar clusters, whereas formation of the X-type and amorphous phases requires extensive displacement of both anions and cations. The observed variations in phase behavior with changing lanthanide ionic radius and deposited electronic energy density are related to the energetics of these transformation mechanisms.

Overview of transparent optical ceramics for high-energy lasers at NRL.

In this review, we present our recent research progress at the Naval Research Laboratory in the development of highly transparent and rugged ceramic window materials such as MgAl2O4 spinel and β-SiC; high-power solid-state laser gain materials based on sesquioxide such as Yb(3+):Y2O3, Yb(3+):Lu2O3, and Ho(3+):Lu2O3; and composite ceramics in the application for high-energy lasers. Various powder synthesis/purification methods and powder post-process techniques necessary to create high-purity powders are described. Ceramic fabrication processes and chemical, morphological, and optical properties of the ceramics developed at the Naval Research Laboratory (NRL) are highlighted. We also report high-efficiency lasing from a hot-pressed rare-earth sesquioxide single layer and composite ceramics made from coprecipitated powder.

Stabilization of cubic phase in dense Eu2O3 ceramics

Eu2O3, a technologically relevant lanthanide sesquioxide, crystallographically morphs from cubic to monoclinic symmetry at 1170°C. A combined x-ray diffraction/confocal Raman microscopy study is performed on sintered Eu2O3 pellets. After sintering at 1500°C, cubic grains are experimentally observed to form at specific locations, mainly monoclinic grain boundaries, of the surface of Eu2O3 dense ceramics. They cover roughly 10% of the surface and present low crystallinity. The large internal stresses that arise during densification are proposed to be the cause for the observed phenomenology. This surprising phase coexistence advances the appealing prospect of room temperature stabilization of the cubic phase in dense lanthanide sesquioxides sintered at high temperatures.

Rare-Earth-Doped Sesquioxides for Diode-Pumped High-Power Lasers in the 1-, 2-, and 3-μm Spectral Range

Sesquioxides combine excellent thermomechanical properties, comparably low phonon energies and a strong crystal field, making them ideally suited for broad band emitting high power laser materials in the infrared and mid-infrared spectral range. In this paper, the latest progress on diode pumped highly efficient high-power solid-state lasers based on rare-earth doped sesquioxides, Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , as well as different mixed sesquioxides (Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with a + b + c = 1 is reviewed. While Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped sesquioxides allow for record high efficiencies at several 100 W of output power in continuous wave operation and record high average powers in pulsed mode in the 1-μm spectral range, Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> - and Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped sesquioxides enable efficient broad band laser operation and ultrashort pulse duration around 2 μm. Finally, Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped sesquioxides enabled the highest efficiencies and output powers demonstrated so far at room temperature in the mid-infrared range at 3 μm.

Electronic and Optical Properties of Rare Earth Oxides: &amp;lt;i&amp;gt;Ab Initio&amp;lt;/i&amp;gt; Calculation

In this work, we have investigated the electronic and optical properties of the technologically important rare earth oxide compounds—X2O3 (X: Gd, Tb) using the density functional theory within the GGA. The band structure of X2O3 have been calculated along high symmetry directions in the first brillouin zone. The real and imaginary parts of dilectric functions and the other optical responses such as energy-loss function, the effective number of valence electrons and the effective optical dielectric constants of the rare earth sesquioxides (Gd2O3 and Tb2O3) were calculated.

Spectroscopic properties of newly flux grown and highly Yb3+-doped cubic RE2O3 (RE = Y, Gd, Lu) laser crystals

We have developed laser grade and highly Yb3+-doped cubic rare-earth sesquioxides (RE2O3, RE=Y, Gd, Lu) single crystals by means of a newly designed high-temperature solution growth method which gives them spectroscopic properties different from those obtained by high-temperature solidification methods. Absorption and emission cross sections, visible anti-Stokes emission, fluorescence decays of the new cubic Gd2O3:Yb3+ single crystals are presented and compared to those of Y2O3:Yb3+ and Lu2O3:Yb3+ crystals. The case of cubic Gd2O3:Yb3+ is particularly interesting since its basic spectroscopic properties have never been detailed and we were capable of achieving Yb-doping level as high as 20%.

Raman scattering of rare earth sesquioxide Ho2O3: A pressure and temperature dependent study

Pressure and temperature dependent Raman scattering studies on Ho2O3 have been carried out to investigate the structural transition and the anharmonic behavior of the phonons. Ho2O3 undergoes a transition from cubic to monoclinic phase above 15.5 GPa, which is partially reversible on decompression. The anharmonic behavior of the phonon modes of Ho2O3 from 80 K to 440 K has been investigated. We find an anomalous line-width change with temperature. The mode Grüneisen parameter of bulk Ho2O3 was estimated from high pressure Raman investigation up to 29 GPa. Furthermore, the anharmonic components were calculated from the temperature dependent Raman scattering.

Electronic structure of lanthanum sesquioxide: A Compton scattering study

We present the first-ever experimental and theoretical momentum densities of La2O3. The Compton line shape is measured using a 20 Ci 137Cs Compton spectrometer at an intermediate resolution with full width at half maximum of 0.34a.u. The experimental Compton profile is compared with the theoretical electron momentum densities computed using linear combination of atomic orbitals (LCAO) method with density functional theory (DFT). It is seen that the generalized gradient approximation (GGA) within DFT reconciles better with the experiment than other DFT based approximations, validating the GGA approximation for rare-earth sesquioxides. The energy bands and density of states computed using LCAO calculations show its wide band gap nature which is in tune with the available reflectivity and photo-absorption data. In addition, Mulliken׳s population and charge density are also computed and discussed.