Manganese(III) phosphate
{{Chembox
| OtherNames = {{Unbulleted list
| Manganic phosphate
| Manganese monophosphate
}}
| Section1 = {{Chembox Identifiers
| CASNo = 10236-39-2
| InChI = 1S/Mn.H3O4P/c;1-5(2,3)4/h;(H3,1,2,3,4)/q+3;/p-3
| PubChem = 14389607
| SMILES = [O-]P(=O)([O-])[O-].[Mn+3]
}}
| Section2 = {{Chembox Properties
| AtmosphericOHRateConstant =
| Appearance = Purple (anhydrous)
Pale-green (monohydrate)
| Density = 3.4 g/cm3 (anhydrous)
3.16 g/cm3 (monohydrate)
| Formula = MnPO4
| MolarMass = 149.91 g/mol (anhydrous)
167.92 g/mol (monohydrate)
| MeltingPtC = 400
| MeltingPt_notes= (decomposes, anhydrous)
| SolubleOther = Insoluble in acetonitrile, ethanol, and acetone
}}
| Section3 = {{Chembox Structure
| CrystalStruct = Orthorhombic
| SpaceGroup = Pmna
| LattConst_a = 9.65 Å
| LattConst_b = 5.91 Å
| LattConst_c = 4.78 Å
| UnitCellVolume = 272 Å3
}}
}}
Manganese(III) phosphate is an inorganic chemical compound of manganese with the formula MnPO4. It is a hygroscopic purple solid that absorbs moisture to form the pale-green monohydrate,{{cite journal |author1=Yiqing Huang |author2=Jin Fang |author3=Fredrick Omenya |author4=Martin O'Shea |author5=Natasha A. Chernova |author6=Ruibo Zhang |author7=Qi Wang |author8=Nicholas F. Quackenbush |author9=Louis F. J. Piper |author10=David O. Scanlon |author11=M. Stanley Whittingham |title=Understanding the stability of MnPO4 |journal=Journal of Materials Chemistry A |date=2014 |volume=2 |issue=32 |pages=12827–12834 |doi=10.1039/C4TA00434E |language=en}} though the anhydrous and monohydrate forms are typically each synthesized by separate methods.
Production and properties
Manganese phosphate monohydrate is produced by the reaction of an Mn(II) salt, such as manganese(II) sulfate, and phosphoric acid, followed by oxidation by nitric acid. Another method of producing the monohydrate is by the comproportionation of permanganate and Mn(II) in phosphoric acid:{{cite journal |author1=Eiichi Narita |author2=Taijiro Okabe |title=Inhibition of catalytic decomposition of acid permanganate solutions |journal=Industrial & Engineering Chemistry Product Research and Development |date=1982 |volume=21 |issue=4 |pages=662–666 |doi=10.1021/i300008a030 |language=en}}{{cite journal |author1=Narita Eiichi |author2=Okabe Taijiro |title=The Thermal Decomposition of Manganese(III) Phosphate Monohydrate |journal=Bulletin of the Chemical Society of Japan |date=1983 |volume=56 |issue=9 |pages=2841–2842 |doi=10.1246/bcsj.56.2841 |language=en}}{{cite journal |author1=Slobotka Aleksovska |author2=Vladimir M. Petruševski |author3=Bojan Šoptrajanov |title=Infrared spectra of the monohydrates of manganese(III) phosphate and manganese(III) arsenate: relation to the compounds of the kieserite family |journal=Journal of Molecular Structure |date=1997 |volume=408-409 |pages=413–416 |doi=10.1016/S0022-2860(96)09720-7 |bibcode=1997JMoSt.408..413A |language=en}}
:MnO4– + 4 Mn2+ + 10 PO43– + 8 H+ → 5 [Mn(PO4)2]3– + 4 H2O
The diphosphomanganate(III) ion slowly converts to the monohydrate. Heating of the monohydrate does not yield the anhydrous form, instead, it decomposes to manganese(II) pyrophosphate (Mn2P2O7) at 420 °C:
:4 MnPO4·H2O → 2 Mn2P2O7 + 4 H2O + O2
To produce the anhydrous form, lithium manganese(II) phosphate is oxidized with nitronium tetrafluoroborate under inert conditions.
The anhydrous form is sensitive to moisture. In the absence of moisture, it decomposes at 400 °C, but when moisture is present, it slowly transitions to an amorphous phase and decomposes at 250 °C.
Structure and natural occurrence
File:Purpurite - Sandamab Pegmatite, Erongo Region, Namibia 2.jpg
The anhydrous form has an olivine structure and naturally occurs as the mineral purpurite. The monohydrate has a monoclinic structure, similar to that of magnesium sulfate monohydrate, but has distortions at the octahederal manganese center due to the Jahn-Teller effect. It naturally occurs as the mineral serrabrancaite.{{cite web |title=Purpurite |url=https://www.mindat.org/min-3311.html |website=mindat.org |access-date=5 September 2023}}{{cite web |title=Serrabrancaite|url=https://www.mindat.org/min-7288.html|website=mindat.org |access-date=5 September 2023}}
The monohydrate form has cell parameters of a = 6.912 Å, b = 7.470 Å, β = 112.3°, and Z = 4. It consists of interconnected distorted trans-[Mn(PO4)4(H2O)2] octahederons.{{cite journal |author1=Philip Lightfoot |author2=Anthony K. Cheetham |author3=Arthur W. Sleight |title=Structure of manganese(3+) phosphate monohydrate by synchrotron x-ray powder diffraction |journal=Inorganic Chemistry |date=1987 |volume=26 |issue=21 |pages=3544–3547 |doi=10.1021/ic00268a025 |language=en}}