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1. Phase purity and evolution in sol–gel derived single component and multicomponent rare‐earth disilicates

   https://doi.org/10.1111/jace.19672  

   Salanova, Alejandro; Opila, Elizabeth J.; Ihlefeld, Jon F. (January 2024, Journal of the American Ceramic Society)

   Abstract Rare‐earth disilicates are a focus of study for use as environmental barrier coatings in gas‐turbine engines. These coatings require thermomechanical and thermochemical stability at elevated temperatures and properties can be tailored through the use of multicomponent rare‐earth disilicates. Producing rare‐earth disilicates via sol–gel is documented in literature, but there are differing procedures with varying phase purities. This work establishes trends that dictate the effects of water content, pH, and heat treatment conditions that determine the final phase purity of Yb, Er, Lu, Sc, and Y disilicate powders made via sol–gel. The phase(s) of the powders were identified and quantified using X‐ray diffraction (XRD) to extract weight fractions. In situ XRD during heating from room temperature to 1200°C was used to observe the crystallization and phase evolution of the sol–gel‐based powders, allowing for the identification of a rarely reported low temperature triclinic phase in ytterbium‐, erbium‐, and lutetium‐based disilicate sol–gels that forms prior to transformation into a monoclinic phase. Ex situ XRD allowed for the phase identification of sol–gels processed at 1400°C. These experiments demonstrated that phase‐pure disilicates could be formed under conditions with no intentional water additions, a target pH of 2, and long heat treatment times at high temperatures (e.g., 1400°C). These conditions remain valid for not only single‐cation rare‐earth disilicates of Yb, Er, Lu, Sc, and Y but also a multicomponent disilicate containing equimolar concentrations of all of these cations. 

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2. Thermal Analysis of High Entropy Rare Earth Oxides

   https://doi.org/10.3390/ma13143141  

   Ushakov, Sergey V.; Hayun, Shmuel; Gong, Weiping; Navrotsky, Alexandra (July 2020, Materials)

   null (Ed.)

   Phase transformations in multicomponent rare earth sesquioxides were studied by splat quenching from the melt, high temperature differential thermal analysis and synchrotron X-ray diffraction on laser-heated samples. Three compositions were prepared by the solution combustion method: (La,Sm,Dy,Er,RE)2O3, where all oxides are in equimolar ratios and RE is Nd or Gd or Y. After annealing at 800 °C, all powders contained mainly a phase of C-type bixbyite structure. After laser melting, all samples were quenched in a single-phase monoclinic B-type structure. Thermal analysis indicated three reversible phase transitions in the range 1900–2400 °C, assigned as transformations into A, H, and X rare earth sesquioxides structure types. Unit cell volumes and volume changes on C-B, B-A, and H-X transformations were measured by X-ray diffraction and consistent with the trend in pure rare earth sesquioxides. The formation of single-phase solid solutions was predicted by Calphad calculations. The melting point was determined for the (La,Sm,Dy,Er,Nd)2O3 sample as 2456 ± 12 °C, which is higher than for any of constituent oxides. An increase in melting temperature is probably related to nonideal mixing in the solid and/or the melt and prompts future investigation of the liquidus surface in Sm2O3-Dy2O3, Sm2O3-Er2O3, and Dy2O3-Er2O3 systems. 

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3. Crystal growth and phase formation of high‐entropy rare‐earth monoclinic aluminates

   https://doi.org/10.1111/jace.19320  

   Pianassola, Matheus; Loveday, Madeline; Lalk, Rebecca; Pestovich, Kimberly; Melcher, Charles L.; Zhuravleva, Mariya (July 2023, Journal of the American Ceramic Society)

   Abstract For the first time, high‐entropy rare‐earth monoclinic aluminate crystals were grown via directional solidification using the micro‐pulling‐down method. Five high‐entropy compositions were formulated with a general formula RE₄Al₂O₉, where RE is an equiatomic mixture of five rare‐earth elements. The rare‐earth elements included were Lu, Yb, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, and La. High‐temperature powder X‐ray diffraction and Rietveld structure refinement indicated that all crystals were a single monoclinic phase and that rare‐earth average ionic radius did not affect phase purity. At room temperature, the refined lattice parameters increased consistently with increasing average ionic radii of the five compositions. One of the crystals had a typical high‐temperature phase transition of single‐RE RE₄Al₂O₉in the range of 1100–1150°C, which consisted of a lattice contraction upon heating. Differential scanning calorimetry indicated a thermal event corresponding to that phase transition. Electron probe microanalysis revealed Al‐rich inclusions on the surface of the crystals. Crystals containing Tb had dark surface features that became lighter after annealing in a reducing atmosphere, which indicated that Tb⁴⁺may be responsible for the dark features. 

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4. 2.2.2-Cryptand as a bidentate ligand in rare-earth metal chemistry

   https://doi.org/10.1039/d0qi00746c  

   Chung, Amanda B.; Huh, Daniel N.; Ziller, Joseph W.; Evans, William J. (November 2020, Inorganic Chemistry Frontiers)

   null (Ed.)

   The 2.2.2-cryptand ligand (crypt) that is heavily used in reductions of rare-earth metal complexes to encapsulate alkali metals has been found to function as a bidentate ligand to rare-earth metal ions in some cases. The X-ray crystal structures of the reduced dinitrogen metal complex, [{(R 2 N) 2 Ce(crypt-κ 2 -O,O′)} 2 (μ–η 2 :η 2 -N 2 )] (R = SiMe 3 ), and the ytterbium metallocene, (C 5 Me 5 ) 2 Yb(crypt-κ 2 -O,O′), are presented to demonstrate this binding mode. The implications of this available binding mode in rare-earth metal cryptand chemistry are discussed. 

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5. Computational thermodynamics-guided alloy design and phase stability in CoCrFeMnNi-based medium-and high-entropy alloys: An experimental-theoretical study

   https://doi.org/10.1016/j.scriptamat.2024.116264  

   Silva, David DS; Bertoli, Gustavo; Neto, Nelson_D Campos; Schell, Norbert; Clarke, Kester D; Kaufman, Michael J; Clarke, Amy J; Coury, Francisco G; Bolfarini, Claudemiro (November 2024, Scripta Materialia)

   A computational thermodynamics approach has been employed to design CoCrFeMnNi-based medium- and high- entropy alloys (M/HEAs) with systematically varied compositions (Co((80-X)/2)Cr((80-X)/2)FeXMn10Ni10 with x = 30, 40, and 50 at.%) and phase stability. Since the formation of sigma phase, usually brittle and undesirable, is a common concern, when this class of alloys is subjected to elevated temperatures (600–1000 ◦C), predicting its formation becomes essential. Thus, its formation and the phase equilibria were studied using the CALPHAD method, and two empirical methods, namely, valence electron concentration (VEC) and paired sigma-forming element (PSFE). Isothermal aging treatments at 900–1100 ◦C for 20 h were performed, since CALPHAD and VEC/PSFE predictions diverged. Both prediction methods were compared with experimental characterization by a combination of scanning electron microscopy and high-energy synchrotron X-ray diffraction. The predictions from the VEC/PSFE and CALPHAD calculations (depending on the database used) were shown to be quite accurate. 

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