Synthesis and Single Crystals of Refractory Oxides of Lanthanides and Thorium

Technical Report · Fri Feb 19 00:00:00 EST 2021

DOI:https://doi.org/10.2172/1755124· OSTI ID:1755124

Kolis, Joseph William ^[1]

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1. Clemson Univ., SC (United States)

At the completion of this program, we can report that we developed a considerable degree of technical improvements in our ability to perform hydrothermal reactions at high temperatures and pressures. We can now routinely perform reactions at 700-750°C and 200 MPa. Currently we are in the process of exploiting this new technology synthesizing a range of exotic new materials investigating relatively poorly understood materials. Our initial efforts focused on the chemistry of rare earth oxides with tetravalent and pentavalent oxides. We recently published a study of the lanthanides with Nb⁵⁺ and Ta⁵⁺ ions, where we grew oxides such as RENdO₄ and RETaO₄ as high quality single crystals. These compounds were targeted as potential hosts for luminescent and scintillation materials, particularly given that they are among the densest oxide hosts and hence have good potential as absorbers for high energy radiation like X-rays and gamma rays. We also isolated a range of unusual new rare earth tantalates with very complex structures. indicating that the chemistry is very sensitive to conditions. We performed some fairly comprehensive examinations of the solid-state chemistry of rare earth ions with various tetravalent metal ions especially Si⁴⁺, Ge⁴⁺, Sn⁴⁺ and Ti⁴⁺. Given the potential role of rare earth silicates in immobilizing radioactive waste elements in long-term storage, and the similarity of our hydrothermal fluids with known geological conditions, this chemistry continues to be relevant. We prepared an extensive series of new lanthanide germanates (e.g. RE₁₃Ge₆O₃₁(OH), BaRE₁₀(GeO₄)₄O₈). and found that there is there is almost no overlap between the chemistry of the rare earth silicates. Stannic oxide (SnO₂) is much more refractory and requires higher temperatures and of mineralizer concentrations. One significant result is the growth of RE₂Sn₂O₇ pyrochlore single crystals. These are of interest because the rare earth stannate pyrochlores are known to display a wide range of magnetic frustration such as spin ice behavior. We grew high quality single crystals of rare earth germanate and stannate pyrochlores and this led to a collaboration with Professor Kate Ross at Colorado State. Preliminary measurements, indicate that the crystals contain no detectable defects or site disorder. Initial neutron diffraction on single crystals was performed at Oak Ridge, and more detailed experiments involving the Ross group are underway at both NIST and ORNL. This particular chemistry has turned out to be the most potentially significant work on this project and the collaborative effort with Prof. Ross is the topic of a DoE renewal project on quantum materials. Our initial foray into the hydrothermal chemistry of rare earth titanates has also been very promising and a range of cubic and polar ferroic phases of the light rare earths RE₂Ti₂O₇ (RE = La - Pr) in the P2₁ phase. We also discovered an interesting new phase Ce₂Ti₄O₁₁ that can have implications in heavy metal immobilization and storage. along with a series of new rare earth titanates (La₅Ti₄O₁₅(OH) Sm₃TiO₅(OH)₃ and Lu₅Ti₂O₁₁(OH) with exceptionally complex structures. One interesting sidelight has been high temperature hydrothermal chemistry terbium, including the growth of large crystals of TbO(OH). This is not a new compound but it is the first time it has been grown as large single crystals. The Tb atom density is almost as high as that in Tb₂O₃ and has a very high Verdet constant (ca. 70), making it a very attractive candidate as a Faraday rotator. Unfortunately it is not in a cubic structure but he material is hard, stable, pure and inexpensive, so should still be an attractive Faraday oscillator. We recently received a patent on this material. We also synthesized K₂Tb(Ge₂O₇) containing stable octahedral Tb⁴⁺ ions, which appears to be the first example of a well-characterized Tb⁴⁺ complex. Given that Tb⁴⁺ has been proposed as a benign surrogate for more treacherous tetravalent ions such as Cf⁴⁺ and Bk⁴⁺, we think that Tb⁴⁺ silicates can be a particularly useful study for actinide immobilization and related work. We also began reaction studies with rare earths and both ReO₂ and RuO₂. These resulted in large single crystals of species like RE₅Ru₂O₁₂, RE₄Re₂O₁₁, REReO₄ and RE₂ReO₅. Several of these samples have already been sent to ORNL for magnetic and neutron diffraction studies.

• Research Organization: Clemson Univ., SC (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division (MSE)
• DOE Contract Number: SC0014271
• OSTI ID: 1755124
• Report Number(s): DOE-Clemson-14271
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
hydrothermal
single crystals
refractory oxides
magnetic systems
rare earths