Honeycomb layered frameworks with metallophilic bilayers

Honeycomb layered frameworks have garnered traction in a wide range of disciplines owing not only to their unique honeycomb configuration, but also to the plenitude of physicochemical and topological properties such as fast ionic conduction, diverse coordination chemistry and structural defects amongst others typically exploited for energy storage applications. In turn, honeycomb layered frameworks manifesting metallophilic bilayer arrangements of cations sandwiched between the transition metal cation slabs have recently garnered attention due to the presence of anomalous fractional valency state of the cations always accompanied by metallophilic interactions constituting the cationic bonds within the bilayered structure. The concepts needed to characterise the aforementioned peculiarities and other phenomena such as conductor-semiconductor-insulator phase transition and magnetoresistance in these materials cut across multi-disciplines ranging from materials science and solid-state chemistry to condensed matter physics, suggesting applications that fall beyond energy storage. This Review highlights the exciting advancements in the science of honeycomb layered frameworks with metallophilic bilayers. First, the latest tactics and techniques including but not limited to X-ray absorption spectroscopy (XAS) and high-resolution transmission electron microscopy (HRTEM) particularly necessary for characterising recent honeycomb layered frameworks with metallophilic bilayers are described, with emphasis on silver-based oxides. Second, new strategies and concepts related to topochemically- or temperature-induced cationic-deficient phases expanding the compositional space of honeycomb layered frameworks focused on cationic bilayer architectures are also accentuated. Third, the latest condensed matter theoretic advances towards a full, atomistic description of the bilayered structure in such frameworks are detailed, especially related to critical phenomena at the cusp of the monolayer-bilayer phase transition. This entails, in part, describing honeycomb layered frameworks as optimised lattices within the congruent sphere packing problem, equivalent to a particular two-dimensional (2D) conformal field theory. Within this picture, the monolayer-bilayer phase transition represents the bifurcation of the honeycomb lattice into its bipartite constituents, related to a 2D-to-3D crossover. Altogether, it is hoped that this Review will give the reader a panoramic view of the honeycomb layered frameworks with important applications within the emerging field of quantum matter, potentially redefining their frontier. Thus, the scope of this Review is expected to be worthwhile for recent graduates and emerging experts alike not only in the materials science and chemistry community but also in other diverse fields of interest.