Andrew Boothroyd

Abstract:

Tin is known for its asymmetric crystal structure and numerous solid phase transitions, with molecular dynamics studies suggesting the beta to gamma phase transition exhibits a strong orientation dependence. In this study, shock compression experiments are conducted on tin single crystals and polycrystals to probe the effects of the crystal orientation on this phase transition through Hugoniot measurements, with peak pressures between 9 and 13 GPa. A strong order-of-magnitude orientation dependence of the elastic limit is found; however, the transition and post-transition behavior show at best only qualitative differences to the velocimetry profiles, with no quantitative variation. A dependence of the transition on the peak pressure is also observed. Explanations of these results based on potential transformation pathways identified through prior static high pressure work are discussed.

Abstract:

Geometrical frustration in the face-centered-cubic (fcc) lattice presents a fundamental challenge in determining antiferromagnetic order, as the ground state is highly sensitive to subtle differences in competing magnetic interactions and structural symmetry. Here, we explore the magnetostructural interplay in two halide double perovskites, Cs2NaFeCl6 and Cs2AgFeCl6. Although both materials have a cubic structure at room temperature, neutron diffraction shows that they adopt different antiferromagnetic structures upon cooling. Cs2NaFeCl6 experiences a transition to an AFM-III order below 2.6 K, governed by J 1 and J 2 (first and second nearest-neighbor) magnetic exchange interactions. Cs2AgFeCl6, however, adopts an AFM-I order below 17 K, accompanied by a significant tetragonal distortion confirmed from both neutron diffraction and polarized Raman spectroscopy. Thermal expansion measurements reveal anomalous lattice expansion at the magnetic transitions in both compounds but are substantially stronger in Cs2AgFeCl6. Combining these findings with density functional theory (DFT) studies, we conclude that the strength of magnetoelastic coupling dictates the magnetic ground state. A strong J 1 in Cs2AgFeCl6 induces a large tetragonal lattice distortion, relieving magnetic frustration and stabilizing the AFM-I phase. In contrast, weaker magnetoelastic coupling in Cs2NaFeCl6 causes minimal distortion, favoring the AFM-III phase via the J 1–J 2 mechanism. Our findings show that magnetic interactions can be a primary driving force for structural phase transitions in these materials, while the strong structural distortion could determine the selection of magnetic ground-state ordering.

Abstract:

MAX phases are a family of atomically laminated materials with various potential applications. Mn2GaC is a prototype magnetic MAX phase, where complex magnetic behaviour arises due to competing interactions. We have resolved the room temperature magnetic structure of Mn2GaC by neutron diffraction from single-crystal thin films and we propose a magnetic model for the low temperature phase. It orders in a helical structure, with a rotation angle that changes gradually between 120° and 90° depending on temperature.