Structural characterization of the candidate Weyl semimetal CeGaGe

Weyl semimetals have a variety of intriguing physical properties, including topologically protected electronic states that coexist with conducting states. Possible exploitation of topologically protected states in a conducting material is promising for technological applications. Weyl semimetals that form in a non-centrosymmetric structure that also contain magnetic moments may host a variety of emergent phenomena that cannot be seen in magnetic, centrosymmetric Weyl materials. It can be difficult to distinguish definitively between a centrosymmetric structure and one of its non-centrosymmetric subgroups with standard powder X-ray diffractometers in cases where two atoms in the compound have nearly the same atomic number, as is the case for the candidate Weyl semimetal CeGaGe. In these cases, a careful single-crystal neutron diffraction experiment with high-angle reflections provides complimentary information to X-ray diffraction and definitively resolves any ambiguity between centrosymmetric and non-centrosymmetric crystal structures. Single-crystal neutron diffraction measurements on the candidate Weyl semimetal CeGaGe confirms that its structure is non-centrosymmetric, described by space group 109 \(\left(I4_1md\right)\) rather than the centrosymmetric space group 141 \(\left(I4_1/amd\right)\). There are many high-angle reflections in the data set that give clear, physically intuitive evidence that CeGaGe forms with \(I4_1md\) symmetry since Bragg planes of these reflections can contain Ga with no Ge or vice versa whereas the Bragg planes for a structure with \(I4_1/amd\) symmetry would have a mix of Ga and Ge. Further, in some crystals we have studied, there is clear evidence for a structural transition from body-centered \(I4_1md\) symmetry to primitive \(P4_3\) and/or \(P4_1\) symmetry.