Introduction to rare earth materials

Ashlee J. Howarth

Ashlee J. Howarth is an Associate Professor and Concordia University Research Chair at Concordia University in Montréal. She was born and raised in London, Ontario. She obtained her undergraduate degree from the University of Western Ontario in 2009, and then went on to do her PhD in inorganic materials chemistry at the University of British Columbia under the supervision of Michael O. Wolf. Before joining the faculty at Concordia, she completed an NSERC Postdoctoral Fellowship at Northwestern University with Joseph T. Hupp and Omar K. Farha. At Concordia, the Howarth group is focused on the design and synthesis of rare-earth cluster-based metal–organic frameworks targeting applications in pollution remediation, catalysis, drug delivery, X-ray detection, and chemical sensing.

Takao Mori

Takao Mori is Deputy Director and Field Director at the Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Japan, and also Professor at the University of Tsukuba. He received his PhD at the University of Tokyo, Department of Physics in 1996. He is also a Program Manager at the Japan Science and Technology Agency (JST) Mirai Large-scale Program. He is now involved in the development of high performance thermoelectric materials and devices, and also thermal management technology.

Zhiguo Xia

Zhiguo Xia is a professor at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China. He obtained his PhD (chemistry) in 2008 from the Department of Chemistry, Tsinghua University, Beijing, China. His current research interests focus on inorganic luminescent materials, including rare earth doped phosphors and luminescent metal halides, focusing on developing their structural design, synthesis, structure–property correlation and the versatile photonics applications.

The rare-earth elements include Sc, Y and the 15 lanthanoids from La to Lu. Owing, in part, to their diverse coordination numbers and geometries and relatively localized orbitals, the rare-earth materials find application in lighting, displays, catalysis, hydrogen storage, photovoltaics, magnetism, magnetocalorics, thermoelectrics, biomedical science and sensing, amongst others.¹ In addition to their various applications, there are diverse classes of materials that can be synthesized using rare-earth elements including coordination complexes, polymers, metal–organic frameworks (MOFs),² solid-state inorganic materials, and nanoparticles, etc.¹ Therefore, in addition to the structural diversity of rare-earth materials, the unique electronic properties of the rare-earth elements, particularly the lanthanoids, engenders the resulting materials with tunable luminescence, and magnetic behaviour, etc.^1,2 This themed collection showcases both the structural diversity and versatile physical properties of rare-earth materials. Here we have chosen several papers from this themed collection to feature, but there are other papers in the collection that advance the materials chemistry of rare-earth elements too, and we believe that all papers will be of high interest to the community.

Rare-earth coordination polymers and MOFs are of interest due to the interplay between organic linker molecules and rare-earth metal nodes, which can lead to unique polymeric structures, sometimes having porosity.² In one example, de Andrade et al. (https://doi.org/10.1039/D4TC03823A) demonstrated the importance of short bridging linkers, and thus reduced metal–metal distances (8.3–8.5 Å compared to >9.2 Å), to generate upconversion luminescence in 1D rare-earth coordination polymers. Firmino et al. (https://doi.org/10.1039/D4TC02589J) highlighted the utility of phosphonate linkers for the construction of robust rare-earth MOFs, leading to an extremely thermally stable MOF that maintains structural features up to 800 °C. In another example that underscores the diversity that rare-earth elements bring to MOF chemistry, Loukopoulos et al. (https://doi.org/10.1039/D4TC03317E) reported the first hexagonal 6-connected RE₆-cluster building block to be observed in a MOF, which also gives rise to a novel MOF topology. This themed collection also features research from Chen et al. (https://doi.org/10.1039/D4TC03221G) on the encapsulation of dyes in the pores of rare-earth MOFs as a strategy for tuning the photoluminescence properties of the resulting composite materials. Psalti et al. (https://doi.org/10.1039/D4TC02806F) reported a series of near-infrared (NIR) emitting MOFs where the organic linker used to sensitize the NIR emission allows for excitation in the visible region of the electromagnetic spectrum (450 nm) – a rare feature compared to most NIR emitting rare-earth MOFs reported to date. While Djanffar et al. (https://doi.org/10.1039/D4TC00781F) showed that mixed-metal rare-earth cluster-based MOFs have potential as luminescent thermometers, with high thermal sensitivity near room temperature.

Magnetism of rare-earth materials is mainly derived from the f-electrons, which as a signature mostly possess relatively large magnetic moments that manifest in diverse and interesting behavior.^3,4 In a study on lanthanide calcium oxyborates LnCa₄O(BO₃)₃, with a series of f-electron rare-earth elements, Azrour and coworkers discovered strong magnetic anisotropy originating from the polar crystal structure of this system (https://doi.org/10.1039/D4TC03249G). This results in excellent rotating magnetocaloric effect (RMCE) values in the He cryogenic region, particularly for the Er compound, rivalling those of well-known Gd₃Ga₅O₁₂.⁵ Direct solid-state cooling via materials is attracting increasing interest for potential expanded applications for magnetocaloric cooling and Peltier cooling.^6–8 Moving away from oxides, Li et al., focused on rare-earth–chalcogen coordination and synthesized a novel series of rare-earth chalcogenidotetrachloride clusters (https://doi.org/10.1039/D4TC02778G). While a super-exchange antiferromagnetic interaction is observed for the Gd phase, with the magnitude being relatively small because of the weak bridging effect of the chloro ligand, interestingly in contrast, a ferromagnetic interaction is indicated for only the Dy sulfide phase. The smallest Dy–Dy separation of the sulfide phase resulted in the strongest dipole–dipole ferromagnetic coupling and therefore, enabled it not to be engulfed by the antiferromagnetic super-exchange interactions. Calculations are carried out to confirm this, and optical properties of the novel compounds are also characterized.

In addition to magnetism, luminescence properties of the rare-earth materials have also been an intensively investigated topic.⁹ There are some reports on traditional rare-earth ion doped phosphor materials in this themed collection, such as BaY₂Sc₂Al₂SiO₁₂:Ce³⁺ (https://doi.org/10.1039/D4TC02906B) and Ca₃Sc₂Si₃O₁₂:Ce³⁺,Cr³⁺,Li⁺ (https://doi.org/10.1039/D4TC03017F), however, their applications have been expanded from white LEDs, to emerging near-infrared laser-driven lighting. Moreover, some interesting luminescence properties and mechanisms have been investigated, like the concentration quenching behavior of Stokes and upconversion luminescence for Pr³⁺-doped Y₃Al₅O₁₂ (https://doi.org/10.1039/D4TC03386H). Balhara et al. also took an interesting approach when they utilized negative thermal expansion (NTE) in the host Sc₂Mo₃O₁₂ and synthesized the Sm³⁺ doped material (https://doi.org/10.1039/D4TC01817F). Thermal quenching of the photoluminescence of rare-earth phosphors has been an issue for applications, and utilizing NTE host matrixes to counter that is an effective emerging strategy. Their Sm³⁺ phosphors are reddish orange emitters and display robust anti-thermal quenching behavior. Another hot topic in the field of luminescent materials is rare-earth halide perovskites or their derivates, which can be easily prepared in the form of nanocrystals, and their applications have been expanded. Li et. al. reported rare-earth-based Cs₂NaRECl₆ (RE = Tb, Eu) halide double perovskite nanocrystals with multicolor emissions for anticounterfeiting and LED applications (https://doi.org/10.1039/D4TC01697A). Ding et. al. contributed a very nice review on the lead-free lanthanide-based Cs₃LnCl₆ metal halides (https://doi.org/10.1039/D4TC03748K). In this leading review, the authors summarized several synthesis approaches towards both Cs₃LnCl₆ polycrystals and nanocrystals. The association of the crystal/electronic structure, optical properties and applications is discussed.

As a traditional research topic with new emerging directions, rare-earth materials will continually serve traditional applications like lighting, catalysis, magnetism, etc., while bringing about some newer research in photonic quantum technologies, perovskites based optoelectronic devices, etc. As guest editors of this themed collection, we extend our gratitude to the colleagues who have contributed to and reviewed the articles included in this collection. We also hope this collection serves as both inspiration and as a valuable resource for researchers across all disciplines in rare-earth materials.

Acknowledgements

TM acknowledges support from JST Mirai Program JPMJMI19A1.

References

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2. F. Saraci, V. Quezada-Novoa, P. R. Donnarumma and A. J. Howarth, Rare-earth metal–organic frameworks: from structure to applications, Chem. Soc. Rev., 2020, 49, 7949–7977.
3. D. Gignoux and D. Schmitt, in Handbook of Magnetic Materials, ed. K. H. J. Buschow, North-Holland, Amsterdam, 1997, ch. 2, p. 239.
4. T. Mori, Rare earth higher borides, in Handbook on the Physics and Chemistry of Rare-earths, ed. J. -C. Bunzli and V. Pecharsky, Elsevier, 2020, vol. 58, pp. 39–154.
5. P. Mukherjee, A. C. Sackville Hamilton, H. F. J. Glass and S. E. Dutton, Sensitivity of magnetic properties to chemical pressure in lanthanide garnets Ln₃A₂X₃O₁₂, Ln = Gd, Tb, Dy, Ho, A = Ga, Sc, In, Te, X = Ga, Al, Li, J. Phys.: Condens. Matter, 2017, 29, 405808.
6. S. Fähler and V. K. Pecharsky, Caloric effects in ferroic materials, MRS Bull., 2018, 43, 264–268.
7. V. Franco, J. S. Blazquez, J. J. Ipus, J. Y. Law, L. M. Moreno-Ramirez and A. Conde, Magnetocaloric effect: From materials research to refrigeration devices, Prog. Mater. Sci., 2018, 93, 112–232.
8. N. S. Chauhan and T. Mori, Cooler, stronger, smaller: improving thermoelectric cooling, Natl. Sci. Rev., 2024, 12, nwae445.
9. X. Q. Zhou, L. X. Ning, J. W. Qiao, Y. F. Zhao, P. X. Xiong and Z. G. Xia, Interplay of defect levels and rare earth emission centers in multimode luminescent phosphors, Nat. Commun., 2022, 13, 7589.

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