Structure of porous copper prussian blue analogues: Nature of their high H₂ storage capacity

Within porous Prussian blue to copper the highest H₂ storage capacity is observed. Such behavior finds explanation in the crystal structure for Cu₃[M(CN)₆]₂ with M) Fe, Co, Ir. The crystal structure of Prussian blue analogues is usually solved and refined with a cubic unit cell in the Fm3 jm space group, which corresponds to a random vacancy distribution. However, a careful evaluation of X-ray diffraction powder patterns of copper-containing compositions reveals a deviation from that structural model. The crystal structure for the considered series of copper hexacyanometallates(III) was found to be also cubic but in the Pm3 jm space group related to a nonrandom vacancy distribution. To this model 50% of vacancies for the building block, [M(CN)₆], corresponds, which is quite different from the value of 33.3% in the Fm3 j m structural model. Mössbauer spectra and high pressure H₂ adsorption isotherms support the assignment of the Pm3 jm space group for the studied series of copper Prussian blue analogues. The implications of a nonrandom vacancy distribution on the physical properties of these materials are discussed.