Synthesis of Tetragonal and Orthorhombic Polymorphs of Hf₃N₄ by High-Pressure Annealing of a Prestructured Nanocrystalline Precursor

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 has 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 prestructuring 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 orthorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

Redox supercapacitor performance of nanocrystalline molybdenum nitrides obtained by ammonolysis of chloride- and amide-derived precursors

“…5 shows that peak positions do not maintain this cubic geometry at lower temperature, specifically the angle between the 111 and 200 reflections becomes larger. This type of distortion has been previously observed in Hf 3 N 4 , where it was initially described in terms of a rhombohedrally distorted defect rocksalt lattice [46] and later found to be due to a tetragonally distorted defect fluorite lattice [40]. No specific distortion was found that 8 modelled these molybdenum nitride samples well.…”

“…The surprising finding is that the two precursors result in very different molybdenum nitride phase behaviours. Our general interest in such reactions stems from the potential to access higher metal oxidation states [38,39,40] and to make controlled material morphologies [41]. Herein we targeted small particle molybdenum nitride samples that may act as good charge storage materials and tested their performance as electrodes in aqueous electrochemical capacitors.…”

Pressure-stabilized hafnium nitrides and their properties

“…Among them, hafnium nitrides (HfN x ), display high melting points (HfN: 3330 ℃ [1]), high incompressibility (HfN: B 0 =260-306 GPa [2]; Hf 3 N 4 : B 0 = 227-260 GPa [3,4]), oxidation resistance at high temperature [5] and superconductivity (HfN: T c =8.8 K) [6], and have been widely exploited by experiments [7][8][9][10] and calculations [8][9][10][11] in recent years.…”

High-Pressure Annealing of a Prestructured Nanocrystalline Precursor to Obtain Tetragonal and Orthorhombic Polymorphs of Hf3N4

“…1). 7 Increasing the pressure to 20 GPa and carrying out extended heating to 2000 K resulted in a new crystalline diffraction pattern that we identified as an orthorhombic cotunnite-type phase, containing 25% vacancies distributed along with N 3-ions on the anion sites (Fig. 2).…”

“…7 Its diffraction pattern showed only a broad, diffuse signal indicative of an amorphous or nanocrystalline material. Based on our structural solution for the phase that crystallized at moderate P and T (see later) we analysed the local structure of the nanocrystalline starting compound that was thought previously to be based on a rocksalt (NaCl) arrangement of ions.…”