Tuning the melting point and phase stability of rare-earth oxides to facilitate their crystal growth from the melt

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)2O3 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)2O3 crystallizes as a mixture of a primary C-type phase with an unstable secondary phase. Crystals of compositions (Lu,Y,Ho,Nd,La)2O3 and (Lu,Y,Gd,Nd,La)2O3 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)2O3 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.

In Situ High-Temperature Structural Analysis of High-Entropy Rare-Earth Sesquioxides

“…The melting point of Gd 2 O 3 is reported in the range of 2392–2445 °C − while composition 1 solidifies ∼2350 °C. This is in agreement with previous work that demonstrated that the melting point of RE 2 O 3 can be suppressed in high-entropy compositions with similar AIR . The B- to H-type transition temperature of Gd 2 O 3 is reported in the range of 2170–2195 °C, , while composition 1 had this transition at ∼2150 °C, which is close to the lower end of this range.…”

“…This is in agreement with previous work that demonstrated that the melting point of RE 2 O 3 can be suppressed in high-entropy compositions with similar AIR. 41 The B-to H-type transition temperature of Gd 2 O 3 is reported in the range of 2170−2195 °C, 36,39 while composition 1 had this transition at ∼2150 °C, which is close to the lower end of this range. Therefore, the polymorphic behavior of single-RE 2 O 3 can be used to predict the type of phase transformations that may occur in high-entropy RE 2 O 3 .…”

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

“…Measurements up to temperatures close to the melting point of these compounds would be advisable to capture the full dependence of the structural behavior on temperature. Maximum experimental temperatures around 2100°C would be required since the melting points of single‐RE RE 4 Al 2 O 9 are in the range of 1950–2040°C, 4 and the melting points of high‐entropy oxides is usually lower or similar to their single‐cation analogous oxides 24,41,42 . Such extreme temperature measurements require careful treatment to avoid reactions between the sample and sample holder.…”

“…In this work, we demonstrate crystal growth of high‐entropy RE 4 Al 2 O 9 for the first time, contributing to the emerging field of crystal growth of high‐entropy oxides 24–34 . The phase formation and optical quality of crystals containing five REs, including Lu, Yb, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, and La were evaluated.…”

“…A (Nd,Sm,Eu,Y,Yb) 4 Al 2 O 9 polycrystalline ceramic was evaluated for environmental barrier applications, and no phase transitions were observed up to 1200°C, 22 which is an indication that the high‐entropy design could contribute to the phase stability of RE monoclinic aluminates. Additionally, suppression of polymorphic transitions in complex RE sesquioxides (RE 2 O 3 ) has been previously attributed to the high‐entropy compositional approach 23,24 . This inspires the investigation of the crystal growth of high‐entropy RE 4 Al 2 O 9 since high optical quality and attractive luminescence properties could be achieved if the lattice expansion upon cooling is suppressed.…”