Abstract The challenge of growing rare-earth (RE) sesquioxide crystals can be overcome by tailoring their structural stability and melting point via composition engineering. This work contributes to the advancement of the field of crystal growth of high-entropy oxides. A compound with only small REs (Lu,Y,Ho,Yb,Er) 2 O 3 maintains a cubic C-type structure upon cooling from the melt, as observed via in-situ high-temperature neutron diffraction on aerodynamically levitated samples. On the other hand, a compound with a mixture of small and large REs (Lu,Y,Ho,Nd,La) 2 O 3 crystallizes as a mixture of a primary C-type phase with an unstable secondary phase. Crystals of compositions (Lu,Y,Ho,Nd,La) 2 O 3 and (Lu,Y,Gd,Nd,La) 2 O 3 were grown by the micro-pulling-down (mPD) method with a single monoclinic B-type phase, while a powder of (Lu,Y,Ho,Yb,Er) 2 O 3 did not melt at the maximum operating temperature of an iridium-rhenium crucible. The minimization of the melting point of the two grown crystals is attributed to the mismatch in cation sizes. The electron probe microanalysis reveals that the general element segregation behavior in the crystals depends on the composition.
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Thermal expansion and phase transformation in the rare earth di-titanate ( R 2 Ti 2 O 7 ) system
Characterization of the thermal expansion in the rare earth di-titanates is important for their use in high-temperature structural and dielectric applications. Powder samples of the rare earth di-titanates R 2 Ti 2 O 7 (or R 2 O 3 ·2TiO 2 ), where R = La, Pr, Nd, Sm, Gd, Dy, Er, Yb, Y, which crystallize in either the monoclinic or cubic phases, were synthesized for the first time by the solution-based steric entrapment method. The three-dimensional thermal expansions of these polycrystalline powder samples were measured by in situ synchrotron powder diffraction from 25°C to 1600°C in air, nearly 600°C higher than other in situ thermal expansion studies. The high temperatures in synchrotron experiments were achieved with a quadrupole lamp furnace. Neutron powder diffraction measured the monoclinic phases from 25°C to 1150°C. The La 2 Ti 2 O 7 member of the rare earth di-titanates undergoes a monoclinic to orthorhombic displacive transition on heating, as shown by synchrotron diffraction in air at 885°C (864°C–904°C) and neutron diffraction at 874°C (841°C–894°C).
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- Award ID(s):
- 1838595
- NSF-PAR ID:
- 10320215
- Date Published:
- Journal Name:
- Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials
- Volume:
- 77
- Issue:
- 3
- ISSN:
- 2052-5206
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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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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Tatsuki Ohji, Ph. D. Editor (Ed.)ZrW2O8 (ZrO2•2WO3) and HfW2O8 (HfO2•2WO3) have been the focus of thermal expansion studies due to their isotropic negative thermal expansion (NTE) measured previously at temperatures below 775◦C. This work presents measurements of these materials at their thermodynamically stable temperature ranges of 1105 and 1257◦C for ZrW2O8 and 1105–1276◦C for HfW2O8, where they were characterized with in situ, powder X-ray diffraction. The linear coefficients of thermal expansion were measured to be −5.52 × 10−6 and −4.87 × 10−6◦C−1 for ZrW2O8 and HfW2O8, respectively. The mechanism leading to this NTE is discussed. Powder samples were synthesized by a solution-based process called the organic–inorganic steric entrapment method. In situ characterization in air was carried out at the National Synchrotron Light Source II using a hexapole lamp, optical furnace and theAdvanced Photon Source using a quadrupole lamp, optical furnace to achieve elevated temperatures.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1107/S2052520621004479
