Pr Segregation Research Articles

Pyrochlore-like ZrO2-PrOx compounds: The role of the processing atmosphere in the stoichiometry, microstructure and oxidation state

The object of this work is to study the relation between composition, microstructure and oxidation state of Pr2±xZr2∓xO7±y materials produced by the laser–floating zone (LFZ) technique. Three compositions are studied, nominally Pr1.7Zr2.3O7+y, Pr2Zr2O7+y and Pr2.24Zr1.76O7±y, all within the pyrochlore field in the ZrO2–PrOx phase diagram. Samples have been processed under four different atmospheres (O2, air, N2 and 5%H2(Ar)), so as to vary the environmental conditions from oxidising to reducing. Sample colouration ranged from dark brown to bright green, owing to varying Pr4+ content. A close correlation is found between the phase homogeneity, the microstructure and the Pr content. Pr–deficient samples present a homogeneous microstructural aspect and composition, whereas Pr–rich compositions always break into 5–25 µm–sized grains with pyrochlore phases at the grain centre and ill–crystallised, Pr–rich oxidised phases at the grain–boundaries. Raman spectroscopy shows that different types of oxygen disorder occur depending on composition and processing atmosphere: in Pr–poor samples oxygen interstitials are created to compensate for Zr4+ excess charge, whereas in Pr–rich samples oxygen disorder occurs around the Pr3+ or Pr4+ ions substituting for Zr4+, because of size–mismatch. Magnetic measurements showed a high Pr4+ content, which has been attributed to several factors: the highly oxidised state of the feedstock material, the segregation of Pr and O–rich grain boundaries in compositions with praseodymium molar rate> 0.5, and the lower oxide–ion conductivity for PZO compositions, compared to either Pr–poor or Pr–rich compositions. Post–processing thermal annealing in a vacuum at 1000 °C enabled total Pr reduction, with the exception of the Pr–rich P2.24 samples, where some Pr4+ ions remained in the oxidised state.

Formation of Ruddlesden-Popper Faults and Their Effect on the Magnetic Properties in Pr0.5Sr0.5CoO3 Thin Films.

Epitaxial Pr0.5Sr0.5CoO3 thin films have been grown on single-crystalline (La0.289Sr0.712)(Al0.633Ta0.356)O3(001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden-Popper faults (RP faults) can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the RP faults is characterized by atomic-resolution chemical mapping. The formation of the RP faults not only contributes to the epitaxial strain relaxation but also significantly decreases the ferromagnetic long-range order of the films, resulting in lower magnetizations than those of the fault-free films. Our results provide a strategy for tuning the magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process.

Thermal, vibrational and optical properties of PrLuO3 interlanthanides from hydrothermally-derived precursors.

PrLuO3 interlanthanides were prepared at temperatures ranging from 800 °C to 1600 °C using hydrothermally-derived precursors. The chemical reactions observed include the conversion and segregation of Pr(OH)3 and LuO(OH), respectively, into PrO2 and Lu2O3 cubic oxides below 1200 °C, followed by the production of a mixture of hexagonal P63/mmc and orthorhombic Pnma PrLuO3 phases at 1400 °C. Phase-pure orthorhombic PrLuO3 was obtained at 1600 °C, which was corroborated by Raman and micro far-infrared spectroscopic analyses. Photoluminescence, colorimetric and lifetime measurements were carried out in PrLuO3 samples. Dominant emission verified in samples calcined at 1400 °C corresponds to the hypersensitive 3P0 → 3F2 transition with a color purity of 97% (decay times of 12 μs and 3 μs), while emissions for the phase-pure samples correspond to the 3P0 → 3H6 and 3P0 → 3H5 transitions with a color purity of 94% (a single luminescence lifetime of 12 μs). The optical properties of PrLuO3 interlanthanides thereby suggest that they can be used as luminescent materials in both structural arrangements.

Effect of Pr Addition on Properties of Sn-0.5Ag-0.7Cu-0.5Ga Lead-Free Solder

In this paper, the effect of Pr addition on the microstructure and properties of Sn-0.5Ag-0.7Cu-0.5Ga lead-free solder was investigated. It was found that the properties of Sn-0.5Ag-0.7Cu-0.5Ga-xPr solder, such as wettability and mechanical properties, could be obviously improved, and the optimal content of Pr was about 0.06 wt.%. The microstructure of Sn-0.5Ag-0.7Cu-0.5Ga-0.06Pr solder showed that the β-Sn matrix and intermetallic compound (IMC) grains were significantly refined, and refinement and homogenization of the microstructure achieved maximum efficiency, which played the role of fine grain strengthening and second phase strengthening. However, as the content of Pr exceeded 0.06 wt.%, some uneven distributed black phases of PrSn3 were found in the β-Sn matrix, which seriously worsened the microstructure and properties of the solders. As a surface-active element, the segregation of Pr at the molten solder interface could give rise to decreasing the interface tension. Consequently, adding a suitable amount of Pr could play a positive role in improving the properties of the solders.

Grain Boundary Plane Effect on Pr Segregation Site in ZnO Σ13 [0001] Symmetric Tilt Grain Boundaries

As grain boundary (GB) and GB segregation often have significant impact on various properties of polycrystalline materials, their atomic structures as well as the location of segregated dopant should be intensively investigated. We have thus reported several papers about segregation of Pr (praseodymium) in ZnO (zinc oxide) GBs for case study. In this study, we study the atomic structure of Pr‐doped ZnO [0001]/(130) Σ13 symmetric tilt GB, and the results are compared with that for the (250) GB [Sato et al., Phys. Rev. B, 87,140101 (2013)]. Although atomic arrangements of these GBs can be characterized using the same kinds of structural units (SUs), Pr segregation sites relative to the SU vary with GBs. It is suggested that change in strain distribution for different GBs would cause the variation in the segregation sites and Pr would prefer the Zn site of locally highest tension.

Electrical conductivity and grain boundary composition of Gd-doped and Gd/Pr co-doped ceria

We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150°C to 700°C in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300°C. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd0.62Ce0.38O2-δ, and Gd0.29Pr0.16Ce0.55O2-δ in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic–electronic conductivity in the near-boundary region.

(Invited) Increased Oxide Ion Diffusivity and Surface Exchange on Pr2NiO4 Base Oxide by Au Dispersion

Mass transport property in nano-size structure controlled materials are attracting much interest because such effects have a possibility to improve the performance of the electrochemical solid state devices such as Solid Oxide Fuel Cells (SOFCs). There are many approaches and ideas for the improvement of oxide ion conductivity in electrolyte by nano size effects. In this study, we demonstrated the 3 dimension (3D) tensile strain in Pr2NiO4, mixed conductor, was successfully introduced by dispersing Au particles in grain. The maximum lattice distortion estimated from X-ray diffraction measurement was 0.23 % at 2 mol% Au dispersion. Au dispersed Pr2NiO4 shows a large positive effect on oxide ion diffusivity and surface exchange coefficient. The doubled oxygen permeation flux was achieved with dispersing 2 mol% Au in PNCG at 873 K, indicating the tensile strain in PNCG lattice positively works for the oxygen permeation property. Since oxygen permeation is controlled by interstitial oxygen concentration and its mobility, further details study on effects of tensile strain on oxygen diffusivity were studied by using 18O tracer diffusion. The depth profiles of x mol% Au/PNCG (x=0, 1, 2, 3) are studied with SIMS analysis, and the estimated oxygen tracer diffusivity (D*) and the surface exchange coefficient (k*) are much increased with dispersing Au, and the enhancement in D value was well agreed with that in oxygen permeation rate. Redox titration measurement suggests that 3D tensile strain increased amount of excess oxygen. Interstitial oxygen in Pr2NiO4 is introduced in Rock salt layer of K2NiF4 structure, and this is origin for the fast oxygen diffusivity and high surface activity for oxygen dissociation. Recently, there is much attention of surface composition in oxide ionic conductors using the low-energy ion scattering (LEIS) technique to understand the surface reaction and degradation. LEIS analysis was also performed on Au dispersed Pr2NiO4 and it was found that surface of Pr2NiO4 was enriched with Pr. However, Pr segregation was prevented by introduction of Au and so tensile strain also positively works for preventing Pr segregation. On the other hand, surface oxygen content is also decreased by Au dispersion and so surface oxygen vacancy seems to be much enriched. Therefore, increased surface exchange coefficient by Au dispersion could be assigned to the high concentration of oxygen vacancy on surface of Pr2NiO4 dispersed with Au. In summary, although Au dispersion was not uniform, effect of tensile strain on oxide ion mobility could be observed in Pr2NiO4 by Au dispersion. Furthermore, tensile strain affected the surface composition, which may enhance the k* values. If the better dispersion state with fine particles could be achieved, there is a possibility to develop the higher oxide ionic conductor with uniform strained material.

Understanding Structural Variability Induced by Pr segregation in SrTiO3 Grain Boundaries

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Crystal growth and scintillation properties of Pr-doped oxyorthosilicate for different concentration

0.05, 0.1 and 0.25 mol% Pr (with respect to Lu) doped Lu 2SiO 5 (LSO) single crystals were grown by the micro-pulling down (μ-PD) method. The grown crystals were transparent, and a slight segregation of Pr 3+ was observed both in the crystal cross-section and growth direction. Transparency in the visible wavelength range was about 80% in all the crystals. Intense absorptions related with the Pr 3+ 4f–5d transitions were observed around 230 and 255 nm, and weak absorptions due to the 4f–4f transitions were detected around 450 nm. In radioluminescence spectra, the Pr 3+ 5d–4f transitions were observed around 275and 310 nm, and emissions due to the 4f–4f transition were observed around 500 nm. In the pulse height analysis using 137Cs gamma-ray excitation, Pr 0.1% doped sample showed the highest light yield of 2,800 ph/MeV. In the decay time measurements using different excitation sources (photoluminescence, X- and gamma-ray), two different processes related to the 5d–4f emission peaks were found. Fast decay component corresponds to direct excitation of Pr 3+ (4–6 ns) and slower component (25 ns) reflects the energy migration process from the host lattice to the emission center.

Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y 2 SiO 5 :Pr 3+ and Lu 2 SiO 5 :Ce 3+ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y 2 SiO 5 :Pr 3+ nanocrystal demonstrates the intense fluorescence from the high crystal field components of split 1 D 2 manifold of Pr 3+ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison ofthe data obtained for nano-and bulk Y 2 SiO 5 :Pr 3+ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr 3+ and Y 3+ . Lu 2 SiO 5 :Ce 3+ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.

Atomic Structures and Electrical Properties of ZnO Grain Boundaries

Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain‐boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well‐defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain‐boundary orientation relationship and grain‐boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage (I–V) characteristics across ZnO grain boundaries. High‐resolution transmission electron microscopy (HRTEM) observations and lattice‐statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of Pr‐doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three‐plus state and would not produce acceptor states in the boundary. First‐principles calculations revealed that Pr doping instead promotes the formations of acceptor‐like native defects, such as Zn vacancies. We believe that such acceptor‐like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co‐codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain‐boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co‐codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single‐grain‐boundary varistors can be designed by controlling grain‐boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single‐grain‐boundary varistors.

Role of Pr Segregation in Acceptor-State Formation at ZnO Grain Boundaries

The role of Pr doping on double Schottky barrier formations at ZnO single grain boundaries was investigated by the combination of current-voltage measurements, atomic-resolution Z-contrast scanning transmission electron microscopy, and first-principles calculations. Although Pr segregated to the specific atomic site along the boundaries, it was found not to be the direct cause of nonlinear current-voltage properties. Instead, under appropriate annealing conditions, Pr enhances formations of acceptor-type native defects that are essential for the creation of double Schottky barriers in ZnO.

Grain boundary dependency of nonlinear current–voltage characteristics in Pr and Co Doped ZnO Bicrystals

Pr and Co doped ZnO bicrystals with various types of grain boundaries were fabricated to investigate their current–voltage characteristics. In addition, their grain boundary structures and chemical compositions were investigated using high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy. It was found that all the boundaries are atomically joined, but have some precipitates of Pr6O11. Co dopants are distributed over the grain interiors and boundaries, while Pr dopants segregate at the fabricated grain boundaries except for a highly coherent Σ1 boundary. The Σ1 boundary with no Pr segregation shows an almost Ohmic characteristic, and coincidence-site lattice and random boundaries with clear Pr segregations show nonlinear characteristics. The change in the nonlinearity is attributed to the difference of the Pr concentration that is associated with the grain boundary structure.

Incorporation kinetics of rare earth impurities in Si during molecular beam epitaxy

Rare earth doping (Er, Pr) was performed by Si during molecular beam epitaxy (MBE) and incorporation characteristics were studied using ex-situ secondary ion mass spectrometry. We observed considerable surface segregation for the growth temperature, 300–800°C, and kinetic aspect of Er and Pr segregation in the low temperature regime is demonstrated. An attempt is made to make a sharp doping profile using Sb-based heteroepitaxy.

Compositional Analysis and High Resolution Imaging of Grain Boundaries in Pr-DOPED ZnO Ceramics

AbstractAn electron microscopy investigation has been conducted in ZnO:Pr:Co varistor structures prepared by a new in-diffusion method. It has been shown that some boundaries host intergranular precipitates of a Prx Oy-based phase. All observations lead to the conclusion that grain boundaries exhibit Pr segregation, while Co is homogeneously distributed in the ZnO matrix. No continuous intermediate layer is present at the ZnO/ZnO boundary nor at the Pr2O3/ZnO interfaces. This is consistent with a barrier model based solely on a space charge depletion region(s).