Twisting of 2D kagom\'e sheets in layered intermetallics

Abstract

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5{\deg}) in the metallic kagom\'e MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ~ 300 K to 100 K, no evidence for a phase transition was found in physical properties measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagom\'e network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modelling of the displacive disorder in the crystal structure shows the presence of second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits inter-layer twisting, but with a different pattern, consistent with the change in electron count due to removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.