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

'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 alpha- and beta-NH4MnP2O7 that provide the strong colouration and compared them to a commercially available sample. Rietveld analysis of neutron powder diffraction data indicated that the a-polymorph crystallised in space group P2(1)/c (a = 7.4252(3) angstrom, b = 9.6990(4) angstrom, c = 8.6552(4) angstrom and beta = 105.627(3)degrees) 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 beta-NH4MnP2O7 was found to crystallise in space group P (1) over bar (a = 8.4034(6) angstrom, b = 6.1498(4) angstrom, c = 6.1071(4) angstrom, alpha = 104.618(5)degrees, beta = 100.748(5)degrees and gamma = 96.802(6)degrees) and possessed similarly distorted MnO6 octahedra, but was found to differ from alpha-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.