High pressure synthesis of high oxidation state metal nitrides using a pre-structured nanocrystalline chemical precursor

16-07-2013

Transition metal nitrides containing metal ions in high oxidation states are a promising arena for the discovery of new semiconducting materials. Most metal nitride compounds prepared at high temperature and high pressure from the elements show metallic bonding. However, amorphous or nanocrystalline compounds can be prepared via metal-organic chemistry routes giving rise to precursors with a high nitrogen to metal ratio. Using state of the art X-ray diffraction techniques together with advancements in high pressure laser heating, this work highlights that the composition and structure of a metastable nanocrstyalline precursor can be retained under high pressure-temperature (high-P,T) conditions. In particular, nanocrystalline Hf3N4 with a tetragonal defect-fluorite structure can be crystallised under high-P,T conditions. Increasing the pressure and temperature of crystallisation leads to the formation of a fully recoverable orthorhombic (defect cottunite) polymorph. This approach identifies a novel pathway to the synthesis of new nitrogen rich transition metal nitrides.

  • Share

Solid state compounds with early transition metals in high oxidation states often provide insulators or wide bandgap semiconductors, e.g. TiO2 and ZrO2. The corresponding nitrides do not typically achieve maximum oxidation states and are dominated by metallic bonding. This is due to the highly exothermic formation enthalpy of the lower metal nitrides allied with the thermodynamic stability of gaseous nitrogen. Higher nitrides such as Ti3N4 should be stable [1] and could be accessed if a suitable synthesis route were found. Lower band gaps are expected relative to the oxides, giving rise to semiconducting and photocatalytic properties, as well as highly coloured compounds that can be used as pigments e.g. Ta3N5 [2]. Recent elemental combination reactions under high pressure and temperature in laser heated diamond anvil cells (LH-DAC) have led to new phases such as PtN2, OsN2 and Hf3N4 [1]. However, most of these reactions at high pressure have only led to surface conversion or mixed phases and consequently low yields. Alternatively, synthesis routes involving metal-organic chemistry such as ammonolysis of metal amide complexes can lead to amorphous or nanocrystalline solids with a very high N:metal ratio [3]. Here we have shown that one of these materials can provide an ideal precursor for high-P,T synthesis experiments and that the precursor structure plays a key role in directing the polymorphism of the products.

Diffuse scattering pattern from nanocrystalline Hf3N4 and the Bragg rings resulting from crystallisation of the tetragonal and orthorhombic phases

Figure 1. Diffuse scattering pattern from nanocrystalline Hf3N4 (centre) and the Bragg rings resulting from crystallisation of the tetragonal (left) and orthorhombic (right) phases of Hf3N4.  Superimposed are a schematic of a diamond anvil cell and the atom connectivities.

For the synthesis, we used in situ X-ray diffraction (Figure 1) and the LH-DAC facilities at beamline ID27 to follow the progress of transformations of the precursor and to identify the products. The precursor was nanocrystalline Hf3N4 material prepared by ammonolysis of the tetra-alkylamide complex Hf(NEtMe)4 (Et = C2H5; Me = CH3). The samples were loaded into DACs inside a glove box and charged with a nitrogen pressure transmitting medium under rigorously inert conditions. The precursor showed only a broad, diffuse signal indicating an amorphous or nanocrystalline material. Upon heating to 1500 K at 11 GPa a series of sharp crystalline peaks was observed to appear. We determined that a tetragonally distorted version of an anion-deficient fluorite structure had formed (space group I4/m), with all the metal sites filled with Hf4+ cations, and the anion sites occupied by N3- and vacancies (Figure 2a). 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 along with N3- ions on the anion sites (Figure 2b). Neither of these polymorphs have been described in previous high-P,T studies and we did not observe the formation of the cubic Th3N4-structured Hf3N4 phase which is obtained by elemental combination under similar conditions, indicating that the metal nitride exhibits a rich structural polymorphism among stable and metastable phases under high P and high T conditions. We used our refined cell parameters to obtain compressibility (V(P)) data for the two new polymorphs. The defective fluorite structure was less resistant to compression (bulk modulus Ko = 200 GPa) than the corresponding tetragonal oxide (HfO2) or Th3P4-structured Hf3N4, as expected due to the presence of vacancies on the anion sites. However the new dense cotunnite phase had a higher Ko value, indicating a greater resistance to mechanical stress.

Rietveld refinement fits to the XRD patterns of Hf3N4

Figure 2. Rietveld refinement fits to the XRD patterns of Hf3N4 at different conditions with the corresponding structural model (ID27, λ = 0.37380 Å). The data points are shown as black dots and the Rietveld fit as a red line. The refined background is shown in green and the difference plot in blue. (a) Diffraction pattern of Hf3N4 obtained at 12 GPa and 1500 K using a tetragonally-distorted fluorite model. (b) Hf3N4 at 19 GPa after laser heating at 2000 K using a defect cottunite structure model.

Based on our structural solution for the tetragonal phase that crystallised at moderate P and T, we then undertook a re-analysis of the local structure of the nanocrystalline starting compound that was thought previously to be based on a rocksalt (NaCl) arrangement of ions. Here we used pair distribution function (PDF) analysis techniques with data collected at ID15B and diffraction data collected at BM01 (SNBL) to suggest a new model based on the same defective fluorite structure found for the crystalline polymorph (Figure 3). This result changes our understanding of local structural arrangements in nanocrystalline or amorphous materials produced from chemical synthesis experiments for heavy metal cations such as Hf3+ or Zr3+, compared with lighter elements such as Ti4+ that do exhibit a rocksalt like arrangement [3].

Print

Figure 3. a) Rietveld (SNBL, λ = 0.69775 Å) and b) PDF (ID15B, λ = 0.13788 Å) fits to the diffraction data collected with nanocrystalline Hf3N4. Both data sets identify a tetragonally-distorted fluorite cell confirming that the structure of the precursor is retained at elevated pressure and temperatures (12 GPa and 1500 K).

We have demonstrated that the high density orthorhombic form of Hf3N4 can be recovered to ambient conditions, as is likely also the case for the tetragonal form. These compounds can now be produced in larger quantities for the examination of their properties. The approach of combining chemical precursor synthesis with high-P,T techniques and synchrotron X-ray diffraction could be applied to other transition metal containing systems to lead to a new family of high oxidation state nitrides.

 

Principal publication and authors
Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high pressure annealing of a pre-structured nano-crystalline precursor, A. Salamat (a,b), A.L. Hector (c), B.M. Gray (c), S.A.J. Kimber (b), P. Bouvier (d), P.F. McMillan (e), J. Am. Chem. Soc. 135, 9503–9511 (2013).
(a) Harvard University, Cambridge (USA)
(b) ESRF
(c) University of Southampton, Southampton (UK)
(d) CNRS, Université Grenoble-Alpes, Grenoble (France)
(e) University College London, London (UK)

 

References
[1] A. Salamat, A.L. Hector, P. Kroll and P.F. McMillan, Coord. Chem. Rev. 257, 2063 (2013).
[2] M. Jansen, E. Guenther, H.P. Letschert, German Patent 199 07 618.9, 1999; Y. Moriya, T. Takata and K. Domen, Coord. Chem. Rev. 257, 1957 (2013).
[3] D.V. Baxter, M.H. Chisholm, G.J. Gama, V.F. DiStasi, A.L. Hector and I.P. Parkin, Chem. Mater. 8, 1222 (1996); A.W. Jackson, O. Shebanova, A.L. Hector and P.F. McMillan, J. Solid State Chem. 179, 1383 (2006).

 

Top image: Novel pathways to new materials using a pre-structured nanocrystalline precursor under extreme synthesis conditions.