Emergent Transitions: Discord between Electronic and Chemical Pressure Effects in the RE Al3 (RE = Sc, Y, Lanthanides) Series

Amber Lim; Katerina P Hilleke; Daniel C Fredrickson

doi:10.1021/acs.inorgchem.2c03393

Emergent Transitions: Discord between Electronic and Chemical Pressure Effects in the RE Al₃ (RE = Sc, Y, Lanthanides) Series

Inorg Chem. 2023 Mar 20;62(11):4405-4416. doi: 10.1021/acs.inorgchem.2c03393. Epub 2023 Jan 3.

Authors

Amber Lim¹, Katerina P Hilleke¹, Daniel C Fredrickson¹

Affiliation

¹ Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin53706, United States.

PMID: 36595300
DOI: 10.1021/acs.inorgchem.2c03393

Abstract

Atomic packing and electronic structure are key factors underlying the crystal structures adopted by solid-state compounds. In cases where these factors conflict, structural complexity often arises. Such is born in the series of REAl₃ (RE = Sc, Y, lanthanides), which adopt structures with varied stacking patterns of face-centered cubic close packed (FCC, AuCu₃ type) and hexagonal close packed (HCP, Ni₃Sn type) layers. The percentage of the hexagonal stacking in the structures is correlated with the size of the rare earth atom, but the mechanism by which changes in atomic size drive these large-scale shifts is unclear. In this Article, we reveal this mechanism through DFT-Chemical Pressure (CP) and reversed approximation Molecular Orbital (raMO) analyses. CP analysis illustrates that the Ni₃Sn structure type is preferable from the viewpoint of atomic packing as it offers relief to packing issues in the AuCu₃ type by consolidating Al octahedra into columns, which shortens Al-Al contacts while simultaneously expanding the RE atom's coordination environment. On the other hand, the AuCu₃ type offers more electronic stability with an 18-n closed-shell configuration that is not available in the Ni₃Sn type (due to electron transfer from the RE d_z² atomic orbitals into Al-based states). Based on these results, we then turn to a schematic analysis of how the energetic contributions from atomic packing and the electronic structure vary as a function of the ratio of FCC and HCP stacking configurations within the structure and the RE atomic radius. The minima on the atomic packing and electronic surfaces are non-overlapping, creating frustration. However, when their contributions are added, new minima can emerge from their combination for specific RE radii representing intergrowth structures in the REAl₃ series. Based on this picture, we propose the concept of emergent transitions, within the framework of the Frustrated and Allowed Structural Transitions principle, for tracing the connection between competing energetic factors and complexity in intermetallic structures.