Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3 Me)2 MnII 5 (CN)12 and (NEt2 Me2 )2 MnII 5 (CN)12 : A Cation-Adaptive Structure

Abstract

The reactions of Mn^II (O₂ CCH₃ )₂ with NEt₃ Me⁺ CN^- and NEt₂ Me₂ ⁺ CN^- form (NEt₃ Me)₂ Mn^II ₅ (CN)₁₂ (1) and (NEt₂ Me₂ )₂ Mn^II ₅ (CN)₁₂ (2), respectively. Structure model-building and Rietveld refinement of high-resolution synchrotron powder diffraction data revealed a cubic [a=24.0093 Å (1), 23.8804 Å (2)] 3D extended structural motif with adjacent tetrahedral and octahedral Mn^II sites in a 3:2 ratio. Each tetrahedral Mn^II site is surrounded by four low-spin octahedral Mn^II sites, and each octahedral Mn^II site is surrounded by six high-spin tetrahedral Mn^II sites; adjacent sites are antiferromagnetically coupled in 3D. Compensation does not occur, and magnetic ordering as a ferrimagnet is observed at T_c =13 K for 2 based on the temperature at which remnant magnetization, M_r (T)→0. The hysteresis has an unusual constricted shape with inflection points around 50 and 1.2 kOe with a 5 K coercivity of 16 Oe and remnant magnetization, M_r , of 2050 emuOe mol^-1 . The unusual structure and stoichiometry are attributed to the very ionic nature of the high-spin N-bonded Mn^II ion, which enables the maximization of the attractive van der Waals interactions through minimization of void space via a reduced ∠ MnNC. This results in an additional example of the A_x Mn^II _y (CN)_x+2y (x=0, y=1; x=1, y=3; x=2, y=1; x=2, y=2; x=2, y=3; x=3, y=5; and x=4, y=1) family of compounds possessing an unprecedented stoichiometry and lattice motif that are cation adaptive structured materials.

Keywords: Rietveld analysis; cyanometallates; extended 3D structures; ferrimagnet; manganese.