High-Pressure Behaviour of Prussian Blue Analogues: Interplay of Hydration, Jahn-Teller Distortions and Vacancies

We report a high-pressure crystallographic study of four hydrated Prussian blue analogues: M[Pt(CN)6] and M[Co(CN)6]0:67 (M = Mn2+, Cu2+) in the range 0–3GPa. Mn[Co(CN)6]0:67 was
studied by single-crystal X-ray diffraction, whereas the other systems were only available in polycrystalline form. The Mn-containing compounds undergo pressure-induced phase transitions from Fm¯3m to R¯3 at 1.0–1.5GPa driven by cooperative tilting of the octahedral units. No phase transition was found for the orbitally disordered Cu[Co(CN)6]0:67 up to 3GPa. Mn[Co(CN)6]0:67 is significantly softer than the other samples, with a bulk modulus of 14GPa compared to 35GPa of the powdered samples. The discrepant pressure responses are discussed in terms of the presence of structural defects, Jahn-Teller distortions, and hydration. The implications for the development
of polar systems are reviewed based upon our high-pressure study.