Control of Polarity in Kagome-NiAs Bismuthides.

A 2×2×1 superstructure of the P6/mmc NiAs structure is reported in which kagome nets are stabilized in the octahedral transition metal layers of the compounds NiPdBi, NiPtBi, and MnPdBi. The ordered vacancies that yield the true hexagonal kagome motif lead to filling of trigonal bipyramidal interstitial sites with the transition metal in this family of "kagome-NiAs" type materials. Further ordering of vacancies within these interstitial layers can be compositionally driven to simultaneously yield kagome-connected layers and a net polarization along the c axes in NiBi and NiPdBi, which adopt Fmm2 symmetry. The polar and non-polar materials exhibit different electronic transport behaviour, reflecting the tuneability of both structure and properties within the NiAs-type bismuthide materials family.