Crystal Structure of (NH4)2MnF5

Abstract

(NH₄)₂MnF₅ crystallizes from acid solution as orthorhombic crystals of space group Pnma with $\text{a\hspace{0.17em}=\hspace{0.17em}6.20\hspace{0.17em}±\hspace{0.17em}0.03, b\hspace{0.17em}=\hspace{0.17em}7.94\hspace{0.17em}±\hspace{0.17em}0.01,}\mathrm{and}\text{c\hspace{0.17em}=\hspace{0.17em}10.72\hspace{0.17em}±\hspace{0.17em}0.01 Å}$ , and four formula units per cell. The stoichiometry is achieved by the growth parallel to $b$ of infinite kinked anionic chains of composition (MnF₅)_n⁻²ⁿ, formed through sharing of opposite vertices of MnF₆ octahedra. The shared fluorine atoms lie on mirror planes $\mathrm{(y\hspace{0.17em}=\hspace{0.17em}}\frac{1}{4},\frac{3}{4})$ , as do the hexagonal networks of ammonium ions that bind the anionic chains together. Mn–F bond distances, corrected for the markedly anisotropic thermal motions, are 1.85 and 2.10 Å. The observed elongation of the MnF₆ octahedra in the chain direction is largely, but not wholly, interpretable as a Jahn–Teller distortion.