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Manganese nitrides

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Manganese nitrides are salts of manganese and the nitride ion. Four of these compounds are stable at atmospheric pressure. The most important is Mn3N2, which catalyzes nitrogen fixation and is a high-temperature antiferromagnet. The others are Mn6N5–6, Mn4N,[1][2] and Mn2N.[3] The compounds generally form as surface layer during combustion of manganese metal in nitrogen or ammonia gas, and homogenous samples can be difficult to obtain.

Formation

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The compounds generally form from combustion of manganese metal in nitrogen or ammonia gas, but as a passivating surface layer; consequently homogenous samples are difficult to obtain. A sufficiently activated manganese sponge results from distillation of manganese amalgam.[4] As described in 1894, a sponge is essential to Mn2N synthesis:[5] manganese powder, if used instead, instead absorbs excess nitrogen, although the resulting nitrogen-rich salt anneals with manganese metal to the correct stoichiometry.[3]

Alternatively, manganese(II) chloride undergoes a non–self-propagating solid state metathesis with magnesium nitride at 550 °C to form Mn3N2; higher temperatures or differently-sized cations give Mn2N instead.[6] Excess molten sodamide at 240 °C reduces manganese oxides to nitrides, with the final product dependent on stoichiometry, through the following reaction.

3 Mn2O3 + 9 NaNH2 → 2 Mn3N2 + 9 NaOH + N2 + 3 NH3

The waste sodium hydroxide selectively dissolves in an aqueous ethanol wash.[7] Manganocene ammonolyzes at 700 °C to give Mn3N2.[8] Manganese azides decompose when heated to give Mn3N2 or Mn6N5–6 and nitrogen gas.[2][9]

Structures and properties

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Several manganese nitrides are stable at atmospheric pressure. The most important is Mn3N2, which catalyzes nitrogen fixation and is a high-temperature antiferromagnet.[6] Other salts include Mn6N5–6, Mn4N,[1][2] and Mn2N.[3][10] Splat quenching may also form a metastable Mn8N that decomposes without diffusion.[1] In general, no binary compounds of manganese and nitrogen are known in which manganese exhibits a formal oxidation state higher than Mn2+, but manganese does form a wide variety of homoleptic nitrido complexes and ternary salts, with oxidation states ranging from Mn+ to Mn+6.5; indeed, the stablest homoleptic complex is Li7MnN4.[11]

Except for Mn6N5–6,[9] manganese nitrides are generally stable against hydrolysis, but act as Brønsted bases in concentrated acid.[12]

Nitrogen-doped manganese experiences a slight freezing point depression, with a eutectic point estimated near 1213 °C and 4 at% nitrogen. Further addition of nitrogen increases the melting point to roughly 1270 °C.[1]

Mn4N is an antiperovskite[11] superlattice with Pearson symbol cP5 and space group Fm3m, the same structure as an iron nitride homologue.[1][3] The cell period is 0.3864 nm, and each nitrogen atom is very nearly at the center of the cell.[1] Any defects present are believed to be of Frenkel type.[11] It is the only truly ferromagnetic phase in the manganese-nitride system, with a Curie temperature around 470 °C. However, dissolved hydrogen in the compound is believed to slightly increase the Curie point. The compound metamorphosizes at 890 °C to the hexagonal close-packed Mn2N phase.[1]

Despite the name, Mn2N in fact exhibits substantial variation in its stable composition with temperature, and the formula Mn2N is only accurate near room temperature. It becomes nitrogen-deficient when heated, with composition Mn5–6N above 890 °C. These shifts in composition also correspond to a gradual change in the unit cell, such that the compound has various hexagonal unit cells at high temperatures.[1][11] At its most nitrogen-rich (and coldest), though, the compound has an 0.5668×0.4909×4.537 nm3 orthorhombic unit cell with space group D14
2h. Pbna. It is isostructural with Mo2C.[3][11]

Mn3N2 is face-centered tetragonal, with Pearson symbol tI6 and space group I4/mmm, analogous to thorium hydride.[1] The unit cell has periods a = 0.42046 nm and c = 1.2131 nm, corresponding to three nearly-cubic unit cells stacked atop each other, but with substantial disorder corresponding to nitrogen vacancies.[13] It is a metallic conductor[1] and a Pauli antiferromagnet, with Néel temperature roughly 645 °C.[2] Around 710 °C, it reversibly decomposes to Mn2N, releasing the excess nitrogen as gas[1] and consuming 25 kJ/mol enthalpy.[2] Likewise, it decomposes in a 400 °C hydrogen atmosphere to a body-centered cubic alloy.[14]

Mn6N5–6 resembles CrN.[2] It is face-centered tetragonal like Mn3N2, but lacks the vacancies that cause such a large fineness ratio in the latter. Instead Mn6N5–6 is nearly cubic.[1][2] When nitrogen-poor, it has lattice parameters a = 0.42 nm and c = 0.41 nm,[1] but the unit cell dimensions vary substantially with nitrogen content and temperature, and the material becomes truly cubic at 400 °C. Consequently it exhibits substantial crystal twinning.[2] Around 580 °C, it decomposes to Mn3N2 and nitrogen gas,[1] but requires a very high nitrogen vapor pressure (even at lower temperatures) to stabilize the phase.[1][2] The decomposition is somewhat reversible, recovering Mn6N5.18 upon cooling. Between 150 °C and 325 °C, it undergoes a magnetic phase transition, aligning moments along one symmetry axis; the Néel temperature is then 387 °C.[2]

In principle, a hexagonal MnN monolayer should exhibit very strong spin polarization, thus behaving as a very strong ferromagnet.[15]

See also

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References

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  1. ^ a b c d e f g h i j k l m n o Gokcen, N. A. (1990). "The Mn-N (manganese-nitrogen) system". Bulletin of Alloy Phase Diagrams. 11 (1): 33–42. doi:10.1007/bf02841582.
  2. ^ a b c d e f g h i j Leineweber, Andreas; Niewa, Rainer; Jacobs, Herbert; Kockelmann, Winfried (26 Oct 2000) [29 Aug 2000]. "The manganese nitrides η-Mn3N2 and θ-Mn6N5+x". Journal of Materials Chemistry (10). Royal Society of Chemistry: 2827–2834. doi:10.1039/B006969H.
  3. ^ a b c d e Mekata Mamoru; Haruna Junsuke; Takaki Hideo (July 1968) [29 Jan 1968]. "Neutron diffraction study of antiferromagnetic Mn2N". Journal of the Physical Society of Japan. 23 (1): 234–238. Bibcode:1968JPSJ...25..234M. doi:10.1143/JPSJ.25.234.
  4. ^ Lihl, Franz; Ettmayer, Peter; Kutzelnigg, Alfred (1962). "Beitrag zum System Mangan-Stickstoff" [Report on the manganese-nitrogen system]. Zeitschrift für Metallkunde (in German). 53 (11). DeGruyter: 715. Bibcode:1962IJMR...53..715L. doi:10.1515/ijmr-1962-531104.
  5. ^ Prelinger, Otto (10 May 1894). "Über Stickstoffverbindungen des Mangans" [On the nitrogen compounds of manganese] (PDF). Monatshefte für Chemie und verwandte Teile anderer Wissenschaften (in German): 391–401.
  6. ^ a b Rognerud, E. G.; Rom, C. L.; Todd, P. K.; Singstock, N. R.; Bartel, C. J.; Holder, A. M.; Neilson, J. R. (2019). "Kinetically-controlled low-temperature solid-state metathesis of manganese nitride Mn3N2". Chemistry of Materials. 31 (18): 7248–7254. doi:10.1021/acs.chemmater.9b01565. OSTI 1569452.
  7. ^ Miura Akira; Takei Takahiro; Kumada Nobuhiro (2013) [30 April 2013]. "Low-temperature nitridation of manganese and iron oxides using NaNH2 molten salt". Inorganic Chemistry. 52 (20). American Chemical Society: 11787–11791. doi:10.1021/ic401951u. PMID 24074443.
  8. ^ Walter, Carsten; Menezes, Prashanth W.; Orthmann, Steven; Schuch, Jona; Connor, Paula; Kaiser, Bernhard; Lerch, Martin; Driess, Matthias (2018). "A molecular approach to manganese nitride acting as a high performance electrocatalyst in the oxygen evolution reaction". Angewandte Chemie. 130 (3): 706–710. Bibcode:2018AngCh.130..706W. doi:10.1002/ange.201710460.
  9. ^ a b Choi Jonglak; Gillan, Edward G. (2009) [6 Feb 2009]. "Solvothermal metal azide decomposition routes to nanocrystalline metastable nickel, iron, and manganese nitrides". Inorganic Chemistry. 48 (10). American Chemical Society: 4470–4477. doi:10.1021/ic900260u. PMID 19341302.
  10. ^ Gokcen 1990 labels it as the ζ and ζ′ phases, but does not give a stoichiometry, as discussed below.
  11. ^ a b c d e Niewa, R. (2002) [21 Aug 2001]. "Nitridocompounds of manganese". Z. Kristallogr. 217. Munich: Oldenbourg Wissenschaftsverlag: 8–23. doi:10.1524/zkri.217.1.8.20801.
  12. ^ Lyutaya, M. D.; Goncharuk, A. B. (March 1977) [July 10, 1974]. "Manganese nitrides". Poroshkovaya Metallurgiya. 171 (3). New York: Plenum: 208–212 [65–70]. doi:10.1007/BF00794089. UDC 621.762.
  13. ^ Leineweber et al. 2000. Gokcen 1990 gives a much smaller value for a ~0.3 nm).
  14. ^ Laassiri, Said; Zeinalipour-Yazdi, Constantinos D.; Catlow, C. Richard A.; Hargreaves, Justin S J. (2018) [21 April 2017]. "The potential of manganese nitride based materials as nitrogen transfer reagents for nitrogen chemical looping". Applied Catalysis B: Environmental. 223. Elsevier. §3.2.1. Bibcode:2018AppCB.223...60L. doi:10.1016/j.apcatb.2017.04.073.
  15. ^ Xu Zhenming; Zhu Hong (2018). "Two-dimensional manganese nitride monolayer with room temperature rigid ferromagnetism under strain" (PDF). Journal of Physical Chemistry C. 122 (26): 14918–14927. doi:10.1021/acs.jpcc.8b02323.
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