Ab initio study on structure and phase transition of A- and B-type rare-earth sesquioxides Ln2O3 (Ln=La–Lu, Y, and Sc) based on density function theory

Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001)

We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm₂O₃ deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm₂O₃ phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm₂O₃ hexagonal phase and its gradual transition to a mixture with cubic C-Sm₂O₃ showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.

Assessment of PBE+U and HSE06 methods and determination of optimal parameter U for the structural and energetic properties of rare earth oxides

Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO₂(111) and Nd₂O₃(0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV-3 eV is also found to be the optimal range of U for the reaction energies of bulk La₂O₃, Ce₂O₃, Nd₂O₃, Er₂O₃, and Ho₂O₃. U = 1 eV gives the best results for Pr₂O₃, Pm₂O₃, Eu₂O₃, Tm₂O₃, and Lu₂O₃, whereas Gd₂O₃ could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd₂O₃(0001) and CeO₂(111), although larger U values are required to obtain sufficient localization of 4f electrons.

Structural and Lattice-Dynamical Properties of Tb2O3 under Compression: A Comparative Study with Rare Earth and Related Sesquioxides

We report a joint experimental and theoretical investigation of the high pressure structural and vibrational properties of terbium sesquioxide (Tb₂O₃). Powder X-ray diffraction and Raman scattering measurements show that cubic Ia3̅ (C-type) Tb₂O₃ undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic C2/m (B-type) phase at ∼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3̅m1 (A-type) phase at ∼1₂ GPa. Thus, Tb₂O₃ is found to follow the well-known C → B → A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb. Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results. Moreover, Raman-active modes of the three phases have been monitored as a function of pressure, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase. Finally, we extract the bulk moduli and the Raman-active mode frequencies together with their pressure coefficients for the three phases of Tb₂O₃. These results are thoroughly compared and discussed in relation to those reported for rare earth and other related sesquioxides as well as with new calculations for selected sesquioxides. It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals.

Efficient and tunable one-dimensional charge transport in layered lanthanide metal-organic frameworks

The emergence of electrically conductive metal-organic frameworks (MOFs) has led to applications in chemical sensing and electrical energy storage, among others. The most conductive MOFs are made from organic ligands and square-planar transition metal ions connected into two-dimensional (2D) sheets stacked on top of each other. Their electrical properties are thought to depend critically on the covalency of the metal-ligand bond, and less importance is given to out-of-plane charge transport. Here, we report a series of lanthanide-based MOFs that allow fine tuning of the sheet stacking. In these materials, the Ln3+ ions lie between the planes of the ligands, thus connecting organic layers into a 3D framework through lanthanide-oxygen chains. Here, efficient charge transport is found to occur primarily perpendicular to the 2D sheets. These results demonstrate that high conductivity in layered MOFs does not necessarily require a metal-ligand bond with highly covalent character, and that interactions between organic ligands alone can produce efficient charge transport pathways.

Pressure-Induced Phase Transitions in Sesquioxides

Pressure is an important thermodynamic parameter, allowing the increase of matter density by reducing interatomic distances that result in a change of interatomic interactions. In this context, the long range in which pressure can be changed (over six orders of magnitude with respect to room pressure) may induce structural changes at a much larger extent than those found by changing temperature or chemical composition. In this article, we review the pressure-induced phase transitions of most sesquioxides, i.e., A2O3 compounds. Sesquioxides constitute a big subfamily of ABO3 compounds, due to their large diversity of chemical compositions. They are very important for Earth and Materials Sciences, thanks to their presence in our planet’s crust and mantle, and their wide variety of technological applications. Recent discoveries, hot spots, controversial questions, and future directions of research are highlighted.

Microstrain-assisted polymorphic phase transitions in (Eu1−x La x )2O3

Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.

Influence of nanostructure formation on the crystal structure and morphology of epitaxially grown Gd2O3 on Si(001)

The influence of growth conditions on the layer orientation, domain structure and crystal structure of gadolinium oxide (Gd₂O₃) on silicon (001) has been investigated. Gd₂O₃ was grown at low (250°C) and high (850°C) temperatures with different oxygen partial pressure as well as a temperature ramp up during growth. At low temperature, the cubic bixbyite type of crystal structure with space group Ia{\bar 3} was grown at low oxygen partial pressure. The layers consist of two domains oriented orthogonal to each other. The epitaxial relationships for the two domains were found to be Gd₂O₃(110)[001]||Si(001)[110] and Gd₂O₃(110)[001]||Si(001)[{\bar 1}10], respectively. Applying additional oxygen during growth results in a change in crystal and domain structures of the grown layer into the monoclinic Sm₂O₃-type of structure with space group C2/m with (20\bar 1) orientation and mainly two orthogonal domains with the epitaxial relationship Gd₂O₃(20\bar 1)[010]||Si(100)〈110〉 and a smooth surface morphology. Some smaller areas have two intermediate azimuthal orientations between these variants, which results in a six-domain structure. The change in crystal structure can be understood based on the Gibbs-Thomson effect caused by the initial nucleation of nanometre-sized islands and its variation in diameter with a change in growth conditions. The crystal structure remains stable even against a temperature ramp up during growth. The layers grown at high temperature exhibit a nanowire-like surface morphology, where the nanowires have a cubic crystal structure and are aligned orthogonal to each other along the 〈110〉 in-plane directions. An increase in oxygen supply results in a reduced length and increased number of nanowires due to lower adatom mobility. The results clearly indicate that both kinetic and thermodynamic factors have a strong impact on the crystal structure, epitaxial relationship and morphology of the grown layers.

Cooperative upconversion luminescence of Er3+ in Gd2O3-xSx phosphor

Gd₂O_3-xS_x:Er crystals were prepared through high-temperature solid-state reaction method in vacuum, with the vacuum synthesis mechanism determined by thermal analysis. The crystal structure and upconversion luminescence properties were investigated respectively by XRD, TEM and spectrophotometer. Well crystallized Gd₂O₂S:Er phosphors were prepared under 1000°C in vacuum with a certain excessive amount of sulfur content than stoichiometric. It is confirmed that with the increasing sulfur content the green emission was enhanced and red emission was weakened. The cooperative upconversion luminescence of Er³⁺ in non-stoichiometric Gd₂O_3-xS_x crystals was interpreted as a result of two photon absorption and the photon avalanche process.

Investigation of NdxY0.25–xZr0.75O1.88 inert matrix fuel materials made by a co-precipitation synthetic route

Yttria-stabilized zirconia (YSZ) is a material that is being considered for use as an inert matrix fuel in nuclear reactors, but a complete characterization of these materials is required for them to be licensed for use. A series of NdxY0.25–xZr0.75O1.88 materials have been synthesized using a co-precipitation method, and the thermal stability of these materials has been studied by annealing them at 1400 and 1500 °C. (Nd was used as surrogate for Am.) The long-range and local structures of the materials were characterized via powder X-ray diffraction, scanning electron microscopy, wavelength dispersive spectroscopy, and X-ray absorption spectroscopy at the Zr K- and Y K-edges. These results were compared with the previous characterization of Nd-YSZ materials synthesized using a ceramic method. The results indicated that the ordering in the local metal–oxygen polyhedral remains relatively unaffected by the synthetic method, but there was increased long-range disorder in the materials prepared by the co-precipitation method. Further, it was found that the materials produced by the co-precipitation method were unexpectedly unstable when annealed at high temperature. This study highlights the importance of determining the effect of synthetic method on material properties and demonstrates how the co-precipitation route could be used to produce inert matrix fuels.

Unraveling energy conversion modeling in the intrinsic persistent upconverted luminescence of solids: a study of native point defects in antiferromagnetic Er2O3

Lattice defects like anion Frenkel pair (a-Fr) works on transporting the O defect. For bio-window, it increases the electron transitions between high-coordinated O-defect and metastable a-Fr, a core–shell works to quench the low-coordinated O defect to reduce green band luminescences.

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 with a scanning transmission X-ray microscope (STXM).

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 features of Sm- and Gd-doped ceria studied by synchrotron X-ray diffraction and μ-Raman spectroscopy

A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce1-xREx)O2-x/2 samples (RE ≡ Sm, Gd; x = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and μ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO2 structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO2 and the RE2O3 structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at ∼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the hybrid region was confirmed, and hints of their composition were obtained by μ-Raman spectroscopy. The obtained results indicate a possible explanation for the non-Vegard behavioral trend of the cell parameters.

Charge distribution as a tool to investigate structural details. III. Extension to description in terms of anion-centred polyhedra

The charge distribution (CHARDI) method is a self-consistent generalization of Pauling's concept of bond strength which does not make use of empirical parameters but exploits the experimental geometry of the coordination polyhedra building a crystal structure. In the two previous articles of this series [Nespoloet al.(1999).Acta Cryst.B55, 902–916; Nespoloet al.(2001).Acta Cryst.B57, 652–664], we have presented the features and advantages of this approach and its extension to distorted and heterovalent polyhedra and to hydrogen bonds. In this third article we generalize CHARDI to structures based on anion-centred polyhedra, which have drawn attention in recent years, and we show that computations based on both descriptions can be useful to obtain a deeper insight into the structural details, in particular for mixed-valence compounds where CHARDI is able to give precise indications on the statistical distribution of atoms with different oxidation number. A graph-theoretical description of the structures rationalizes and gives further support to the conclusions obtainedviathe CHARDI approach.

Phase transformation of molecular beam epitaxy-grown nanometer-thick Gd₂O₃ and Y₂O₃ on GaN

High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Biomimetic synthesis of chiral erbium-doped silver/peptide/silica core-shell nanoparticles (ESPN)

Peptide-modified silver nanoparticles have been coated with an erbium-doped silica layer using a method inspired by silica biomineralization. Electron microscopy and small-angle X-ray scattering confirm the presence of an Ag/peptide core and silica shell. The erbium is present as small Er(2)O(3) particles in and on the silica shell. Raman, IR, UV-Vis, and circular dichroism spectroscopies show that the peptide is still present after shell formation and the nanoparticles conserve a chiral plasmon resonance. Magnetic measurements find a paramagnetic behavior. In vitro tests using a macrophage cell line model show that the resulting multicomponent nanoparticles have a low toxicity for macrophages, even on partial dissolution of the silica shell.

Nanometer-Thick Single-Crystal Hexagonal Gd2 O3 on GaN for Advanced Complementary Metal-Oxide-Semiconductor Technology

Hexagonal-phase single-crystal Gd2 O3 is deposited on GaN in a molecular beam epitaxy system. The dielectric constant is about twice that of its cubic counterpart when deposited on InGaAs or Si. The capacitive effective thickness of 0.5 nm in hexagonal Gd2 O3 is perhaps the lowest on GaN-metal-oxide-semiconductor devices. The heterostructure is thermo dynamically stable at high temperatures and exhibits low interfacial densities of states after high-temperature annealing.

Structural and electronic properties of lutecia from first principles

The structural and electronic properties of lutecia (C-type Lu(2)O(3)) have been investigated using the projected augmented wave (PAW) method based on the periodic density functional theory (DFT). Two models for the localized Lu 4f electrons have been employed, in which the f electrons are treated as a part of the inner core and as valence electrons, respectively. With the former model, the Perdew-Burke-Ernzerhof (PBE) calculations yield a crystal structure in good agreement with experiments, while with the latter model, the PBE+U results show that the optimum U(eff) parameter value is dependent on the property investigated. The variation of structural properties with respect to the model used and the U(eff) value chosen has been rationalized in terms of the repulsive interaction between Lu 4f and O 2p electrons owing to the insufficient self-interaction cancelation associated with the localized f electrons in the DFT functional. Based on the calculated results, a practical scheme is proposed for the calculation of Lu(2)O(3) and the related materials in cases where the f electronic properties are relevant.