  Prussian blue analogues (PBAs) are a broad and important family of
microporous inorganic solids, famous for their gas storage, metal-ion
immobilisation, proton conduction, and stimuli-dependent magnetic, electronic
and optical properties. The family also includes the widely-used double-metal
cyanide (DMC) catalysts and the topical hexacyanoferrate/hexacyanomanganate
(HCF/HCM) battery materials. Central to the various physical properties of PBAs
is the ability to transport mass reversibly, a process made possible by
structural vacancies. Normally presumed random, vacancy arrangements are
actually crucially important because they control the connectivity of the
micropore network, and hence diffusivity and adsorption profiles. The
long-standing obstacle to characterising PBA vacancy networks has always been
the relative inaccessibility of single-crystal samples. Here we report the
growth of single crystals of a range of PBAs. By measuring and interpreting
their X-ray diffuse scattering patterns, we identify for the first time a
striking diversity of non-random vacancy arrangements that is hidden from
conventional crystallographic analysis of powder samples. Moreover, we show
that this unexpected phase complexity can be understood in terms of a
remarkably simple microscopic model based on local rules of electroneutrality
and centrosymmetry. The hidden phase boundaries that emerge demarcate
vacancy-network polymorphs with profoundly different micropore characteristics.
Our results establish a clear foundation for correlated defect engineering in
PBAs as a means of controlling storage capacity, anisotropy, and transport
efficiency.
