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Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high pressure annealing of a pre-structured nanocrystalline precursor

Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high pressure annealing of a pre-structured nanocrystalline precursor
Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high pressure annealing of a pre-structured nanocrystalline precursor
Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies have been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of pre-structuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The or-thorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.
0002-7863
9503-9511
Salamat, Ashkan
5af74416-eb78-4ee4-844c-2499a79951a8
Hector, Andrew Lee
f19a8f31-b37f-4474-b32a-b7cf05b9f0e5
Gray, Benjamin M.
768c4268-32e4-436f-83d9-630f53cc464e
Kimber, Simon A.J.
568a23f5-7a81-44b3-9afd-42762854722c
Bouvier, Pierre
6b2e78fa-e40c-4271-addd-3f25488b10f3
McMillan, Paul F.
8bca9219-06e0-4621-bdae-ad059709a147
Salamat, Ashkan
5af74416-eb78-4ee4-844c-2499a79951a8
Hector, Andrew Lee
f19a8f31-b37f-4474-b32a-b7cf05b9f0e5
Gray, Benjamin M.
768c4268-32e4-436f-83d9-630f53cc464e
Kimber, Simon A.J.
568a23f5-7a81-44b3-9afd-42762854722c
Bouvier, Pierre
6b2e78fa-e40c-4271-addd-3f25488b10f3
McMillan, Paul F.
8bca9219-06e0-4621-bdae-ad059709a147

Salamat, Ashkan, Hector, Andrew Lee, Gray, Benjamin M., Kimber, Simon A.J., Bouvier, Pierre and McMillan, Paul F. (2013) Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high pressure annealing of a pre-structured nanocrystalline precursor. Journal of the American Chemical Society, 135 (25), 9503-9511. (doi:10.1021/ja403368b).

Record type: Article

Abstract

Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies have been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of pre-structuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The or-thorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

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Published date: 30 May 2013
Organisations: Electrochemistry

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Local EPrints ID: 353272
URI: http://eprints.soton.ac.uk/id/eprint/353272
ISSN: 0002-7863
PURE UUID: 0a65963e-8b87-4241-9d4a-3c937705d9ed
ORCID for Andrew Lee Hector: ORCID iD orcid.org/0000-0002-9964-2163

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Date deposited: 04 Jun 2013 10:24
Last modified: 15 Mar 2024 02:52

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Contributors

Author: Ashkan Salamat
Author: Benjamin M. Gray
Author: Simon A.J. Kimber
Author: Pierre Bouvier
Author: Paul F. McMillan

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