Relating highly distorted Jahn–Teller MnO6 to colouration in manganese violet pigments

Abstract

‘Manganese violet’ pigments have been known for over 150 years, but are receiving renewed interest due to their non-toxicity and earth-abundant components. For the first time we report here a detailed study into the structural aspects that define the sought-after pigment properties of these materials. This work identified two polymorphs, designated as α- and β-NH₄MnP₂O₇ that provide the strong colouration and compared them to a commercially available sample. Rietveld analysis of neutron powder diffraction data indicated that the α-polymorph crystallised in space group P2₁/c (a = 7.4252(3) Å, b = 9.6990(4) Å, c = 8.6552(4) Å and β = 105.627(3)°) and exhibited a highly distorted MnO₆ coordination sphere. The apparent [2 + 2 + 2] distortion gives rise to the optical properties and appears to be driven, in part, by a “plasticity effect” in the Mn coordination induced by the pyrophosphate ligand. A second polymorph β-NH₄MnP₂O₇ was found to crystallise in space group P (a = 8.4034(6) Å, b = 6.1498(4) Å, c = 6.1071(4) Å, α = 104.618(5)°, β = 100.748(5)° and γ = 96.802(6)°) and possessed similarly distorted MnO₆ octahedra, but was found to differ from α-NH₄MnP₂O₇ in the relative dimensions of the intersecting framework tunnels that contained the ammonium cations. UV-visible spectroscopy was used to characterise optical behaviour and a combination of TGA-MS and in situ high temperature X-ray powder diffraction were used to determine thermal decomposition pathways.