‘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.

There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P2O74−); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca2P2O7.nH2O and Sr2P2O7.nH2O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond ∼8 Å in both phases, with this local order found to resemble crystalline analogues. Further studies, including 1H and 31P solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P–O–P bond angles within the P2O7 units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to ∼450 °C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P2O7 anions, leading to the hydrolysis of some P–O–P linkages and the formation of HPO42− anions within the amorphous matrix. The latter anions then recombined into P2O7 ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials.

Two previously uncharacterized members of the Rb−Mn−P−O system, RbMnP2O7 and β-RbMnHP3O10, have been synthesized using a phosphoric acid flux synthetic route and their crystal and magnetic structures determined using neutron powder diffraction. The crystal structure of RbMnP2O7 (space group P21/c, a = 7.3673(2) Å, b = 9.6783(2) Å, c = 8.6467(2) Å, and β = 105.487(1)°) was found to be isostructural with RbFeP2O7. The polymorph β-RbMnHP3O10 was also isolated as a single phase and found to crystallize in the space group C2 (a = 12.2066(5) Å, b = 8.5243(3) Å, c = 8.8530(4) Å, β = 107.233(2)°). Both structures consist of frameworks of corner-sharing MnO6 octahedra linked together by condensed phosphate anions, with Rb+ cations located in the intersecting channels. In both cases the Mn3+ octahedra exhibit unusual Jahn−Teller distortions indicative of a plasticity effect driven by the steric requirements of the condensed phosphate anions, and this causes a strong violet coloration similar to that observed in the manganese violet pigment; the structure of this has yet to be determined. Magnetic susceptibility measurements show that both RbMnP2O7 (TN = 20 K) and β-RbMnHP3O10 (TN = 10 K) undergo a phase transition at low temperatures to an antiferromagnetically ordered state. Low-temperature neutron powder diffraction studies show that the magnetic ground states of each of these materials involve both ferromagnetic and antiferromagnetic super-superexchange interactions between orbitally ordered Mn3+, which are mediated by PO4 tetrahedra. These interactions are compared and discussed.

The synthesis of La1+xBa2−xMn2O6 (0.2 ≤ x ≤ 0.4) under reducing conditions is reported. Neutron powder diffraction studies indicate an A-site ordered, oxygen deficient bilayer Ruddlesden–Popper structure. La3+ ions preferentially occupy oxygen vacant layers within the perovskite bilayers, leading to a structure analogous to the cuprate superconductor La1.85Ba0.15CaCu2O6+γ, with “MnO2” layers constructed from corner-linked square pyramidal MnO5 units. Magnetic measurements suggest that no long range magnetic order exists in this mixed Mn2+/Mn3+ system. Significantly, the low temperature oxidation of this phase leads to the synthesis of La1+xBa2−xMn2O6.95, which constitutes the first isolation of a complete barium analogue of La1+xSr2−xMn2O7, the important low field colossal magnetoresistive material. Neutron diffraction data confirm the retention of A-site order and a change in Jahn–Teller distortion. Low temperature neutron data and magnetic susceptibility suggest canted A-type antiferromagnetic order.

The crystal structure of the layered acid phosphate, AlH2P3O10·2H2O, has been determined and provides a new structure-type for a series of metal phosphates with interlamellar regions likely to be highly suited to intercalation behaviour.

The synthesis of La1+xBa2−xMn2O6 (0.2 ≤ x ≤ 0.4) under reducing conditions is reported. Neutron powder diffraction studies indicate an A-site ordered, oxygen deficient bilayer Ruddlesden–Popper structure. La3+ ions preferentially occupy oxygen vacant layers within the perovskite bilayers, leading to a structure analogous to the cuprate superconductor La1.85Ba0.15CaCu2O6+γ, with “MnO2” layers constructed from corner-linked square pyramidal MnO5 units. Magnetic measurements suggest that no long range magnetic order exists in this mixed Mn2+/Mn3+ system. Significantly, the low temperature oxidation of this phase leads to the synthesis of La1+xBa2−xMn2O6.95, which constitutes the first isolation of a complete barium analogue of La1+xSr2−xMn2O7, the important low field colossal magnetoresistive material. Neutron diffraction data confirm the retention of A-site order and a change in Jahn–Teller distortion. Low temperature neutron data and magnetic susceptibility suggest canted A-type antiferromagnetic order.

There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P2O74−); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca2P2O7.nH2O and Sr2P2O7.nH2O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond ∼8 Å in both phases, with this local order found to resemble crystalline analogues. Further studies, including 1H and 31P solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P–O–P bond angles within the P2O7 units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to ∼450 °C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P2O7 anions, leading to the hydrolysis of some P–O–P linkages and the formation of HPO42− anions within the amorphous matrix. The latter anions then recombined into P2O7 ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials.

‘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.