Probing Complex Disorder in Ce_1‑<i>x</i>Gd_<i>x</i>O_{2‑<i>x</i>/2} Using the Pair Distribution Function Analysis

In this work the first Pair Distribution Function (PDF) study on Ce_1‑<i>x</i>Gd_<i>x</i>O_{2‑<i>x</i>/2} (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration <i>x</i>_Gd (0 ≤ <i>x</i>_Gd ≤ 1) of the CGO solid solutions was investigated through high resolution synchrotron radiation powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions the presence of positional disorder, which has been explicitly mapped into the real space. The <i>average</i> structural models, as obtained by the Rietveld method, fit well the experimental PDF data only for a spatial range <i>r</i> > ∼10 Å. The same models applied at lower <i>r</i> values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < <i>r</i> < ∼6 Å range was obtained for all the CGO samples applying a <i>biphasic</i> model encompassing both a fluorite CeO₂-like and a C-type Gd₂O₃-like phases. This provides evidence that extended defects at local scale exist in the CGO system. Gd-rich and Ce-rich droplets coexist in the subnanometric range

Defect Structure of Y‑Doped Ceria on Different Length Scales

An exhaustive structural investigation of a Y-doped ceria (Ce_1–<i>x</i>Y_<i>x</i>O_{2–<i>x</i>/2}) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO₂- and Y₂O₃-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y₂O₃. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level

Assessing Phase Stability in High-Entropy Materials by Design of Experiments: The Case of the (Mg,Ni,Co,Cu,Zn)O System

In this study, we aimed to explore the phase stability of high-entropy oxides (HEOs) beyond their conventional equimolar composition, which presents the maximum configurational entropy. This task is challenging due to the large number of compositional parameters involved. We used the design of experiments as a strategy to investigate the compositional range of stability of the rock salt (RS) structure in the (Mg,Ni,Co,Zn,Cu)O quinary system, featuring the prototypical HEO Mg0.2Ni0.2Co0.2Zn0.2Cu0.2O. Our study revealed that the chemical nature of the RS-native oxides (NiO, MgO, and CoO) significantly affects the phase stability of the RS-HEO, suggesting that the HEO stability is not solely governed by the balance of configurational entropy and enthalpy of mixing. In addition, a single high-entropy phase can be achieved on a wide out-of-equimolar set of compositions, thereby broadening the compositional range that should be explored in the search for innovative materials with unique properties and applications

Phase Transformations in the CeO₂–Sm₂O₃ System: A Multiscale Powder Diffraction Investigation

The structure evolution in the CeO₂–Sm₂O₃ system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO₂ fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated

Easy Accommodation of Different Oxidation States in Iridium Oxide Nanoparticles with Different Hydration Degree as Water Oxidation Electrocatalysts

In this paper, we present a comprehensive study on low hydration Ir/IrO₂ electrodes, made of an Ir core and an IrO₂ shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir–O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality X-ray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state <i>I</i>/<i>E</i> curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrO_<i>x</i>. The main conclusion of this study is that the Ir core is noninteracting with the IrO_<i>x</i> shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in operando XAS experiments assessed that the catalytic cycle involves Ir­(III) and (V), as previously established for the highly hydrated IrO_<i>x</i> material

Synthesis of a Cu-Filled Rh₁₇S₁₅ Framework: Microwave Polyol Process Versus High-Temperature Route

Metal-rich, mixed copper–rhodium sulfide Cu_3−δRh₃₄S₃₀ that represents a new Cu-filled variant of the Rh₁₇S₁₅ structure has been synthesized and structurally characterized. Copper content in the [CuRh₈] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu₃Rh₃₄S₃₀ obtained by a rapid, microwave-assisted reaction of CuCl, Rh₂(CH₃CO₂)₄ and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu_3−δRh₃₄S₃₀ (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu_3−δRh₃₄S₃₀ is metallic just like Rh₁₇S₁₅, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh₁₇S₁₅ host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh–Rh interactions in favor of Cu–Rh interactions as revealed by quantum-chemical calculations

Intermediate-Valence Ytterbium Compound Yb₄Ga₂₄Pt₉: Synthesis, Crystal Structure, and Physical Properties

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb₄Ga₂₄Pt₉ was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group <i>C</i>2<i>/m</i>, Pearson symbol <i>mS</i>74, <i>a</i> = 7.4809(1) Å, <i>b</i> = 12.9546(2) Å, <i>c</i> = 13.2479(2) Å, β = 100.879(1)°, <i>V</i> = 1260.82(6) ų, <i>R</i>_<i>F</i> = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an <i>ABABB</i> stacking of two slabs with trigonal symmetry and compositions Yb₄Ga₆ (<i>A</i>) and Ga₁₂Pt₆ (<i>B</i>). The hard X-ray photoelectron spectrum (HAXPES) of Yb₄Ga₂₄Pt₉ shows both Yb²⁺ and Yb³⁺ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor