Cation Adaptive Structures Based on Manganese Cyanide Prussian Blue Analogues: Application of Powder Diffraction Data to Solve Complex, Unprecedented Stoichiometries and New Structure Types

Abstract

A Mn(II) salt and A⁺ CN^- under anaerobic conditions react to form 2-D and 3-D extended structured compounds of A_m Mn^II _n (CN)_m+2n stoichiometry. Here, the creation and characterization of this large family of compounds, for example AMn^II ₃ (CN)₇ , A₂ Mn^II ₃ (CN)₈ , A₂ Mn^II ₅ (CN)₁₂ , A₃ Mn^II ₅ (CN)₁₃ , and A₂ Mn^II [Mn^II (CN)₆ ], where A represents alkali and tetraalkylammonium cations, is reviewed. Cs₂ Mn^II [Mn^II (CN)₆ ] has the typical Prussian blue face centered cubic unit cell. However, the other alkali salts are monoclinic or rhombohedral. This is in accord with smaller alkali cation radii creating void space that is minimized by increasing the van der Waals stabilization energy by reducing ∠Mn-N≡C, which, strengthens the magnetic coupling and increases the magnetic ordering temperatures. This is attributed to the non-rigidity of the framework structure due the significant ionic character associated with the high-spin Mn^II sites. For larger tetraalkylammonium cations, the high-spin Mn sites lack sufficient electrostatic A⁺ ⋅⋅⋅NC stabilization and form unexpected 4- and 5-coordinated Mn sites within a flexible, extended framework around the cation; hence, the size, shape, and charge of the cation dictate the unprecedented stoichio-metry and unpredictable cation adaptive structures. Antiferromagnetic coupling between adjacent Mn^II sites leads to ferrimagnetic ordering, but in some cases antiferromagnetic coupling of ferrimagnetic layers are compensated and synthetic antiferromagnets are observed. The magnetic ordering temperatures for ferrimagnetic A₂ Mn^II [Mn^II (CN)₆ ] with both octahedral high- and low-spin Mn^II sites increase with decreasing ∠Mn-N≡C. The crystal structures for all of the extended structured materials were obtained by powder diffraction.

Keywords: Mn; Prussian blue analogue; Rietveld analysis; cation adaptive structure; cyanometallate; extended 3-D structure; ferrimagnet.