  This Review highlights the exciting advancements in the science of honeycomb
layered frameworks with metallophilic bilayers that have recently garnered
attention particularly due to reports of anomalous fractional valency states of
silver cations sandwiched between transition metal slabs of other cations.
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. 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.
