Phase separation driven by mobile cations in(Na1−xKx)0.35

Cation substitution effects on structural and optical properties are investigated for films of Prussian-blue-type cyanides, (Na1−xKx)0.35Co[Fe(CN)6]0.74zH2O(0.07≤x≤0.85). Substitution of K+ for Na+ induces a phase transition from high-spin (HS) phase with the electronic configuration of Co2+(t2g5eg2)-Fe3+(t2g5) to low-spin (LS) phase with the electronic configuration of Co3+(t2g6)-Fe2+(t2g6). At around room temperature, the HS phase (LS phase) is observed in the region of x≤0.2(x≥0.8) while two phases coexist in the intermediate-x region (0.2≤x≤0.8). We interpreted the two-phase region in terms of intrinsic phase separation into small-x (Na+-rich) and large-x (K+-rich) regions because the cations can migrate within the crystal to minimize the total free energy. We further confirmed that absorption spectra in the infrared, visible and Co/Fe K edge regions are quantitatively reproduced by the phase-separation model