Control of Polarity in Kagome-NiAs Bismuthides*

Kagome materials have attracted considerable attention due to the intrinsic properties such as flat bands, Dirac points and quantum magnetic frustration. Here, we report several new NiAs-type bismuthide materials in which the coupled formation of vacancies and interstitials generates kagome transition metal layers, which can further compositionally tuned towards breaking of inversion symmetry to give polar materials. A superstructure of the P6₃/mmc NiAs structure is reported in which kagome nets are stabilized in the octahedral transition metal layers of the compounds Ni_0.7Pd_0.2Bi, Ni_0.6Pt_0.4Bi, and Mn_0.99Pd_0.01Bi. 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 axis in Ni_0.9Bi and Ni_0.79Pd_0.08Bi, which adopt polar Fmm2 symmetry. Interestingly, the polar Ni_0.9Bi exhibits an unconventional electronic transport, in contrast with the conventional metallic behavior of the non-polar members of this materials family, pointing to potentially extensive compositional control of structure and properties in a chemistry that affords the intensely studied kagome nets. This research demonstrates the ability to combine the important structural motifs and selectively break or maintain important crystallographic symmetries in the kagome-NiAs bismuthide family, which will stimulate extensive searches for new or polar kagome materials and associate studies of their physical properties, with ramifications for the control and understanding of electronic structure in the solid state.