High-pressure polymorphism of a photocatalytically-active titanium oxynitride Phase, Ti2.85O4N
We have made use of the I15 beamline at Diamond Light Source to investigate the high pressure behaviour of Ti2.85O4N, a novel oxynitride synthesized by the unusual route of atmospheric pressure chemical vapour deposition, at pressures up to 68 GPa. At ambient pressure this phase adopts the pseudo-brookite Cmcm structure, and anisotropic compression was observed up 18 GPa, at which pressure a first order phase transition was observed to new orthorhombic (Pmc21) structure. Further compression led to the observation of a second phase transition to a high-pressure monoclinic structure at 42 GPa.
Figure 1: The positions recorded for diffraction peaks observed in the XRD data as a function of increasing pressure (GPa). The data indicate the appearance of a new phase (assigned by Le Bail refinement to Pmc21 symmetry) above 15-20 GPa. The low pressure Cmcm phase persists to at least 23-27 GPa indicating that the transformation is first order. The dhkl vs pressure data clearly reveal peaks due to e and d phases of N2 used as a pressure-transmitting medium as well as TiC and weak features due to an unidentified impurity phase.
New metal nitride and oxynitride compounds and their crystalline polymorphs are currently being explored for photocatalysis applications in fields ranging from water-splitting for energy generation to water decontamination and disinfection procedures and production of fine chemicals. Titanium oxides are well known to possess photocatalytic properties but these are mainly active under UV illumination. However, doping the materials with nitrogen causes the bandgap to shift into the visible range and it becomes possible to envisage processes driven by harvesting solar energy at the Earth’s surface. Recently, part of our group based at UCL used atmospheric pressure chemical vapour deposition (APCVD) and combinatorial methods to discover a new titanium oxynitride phase, Ti2.85O4N that was shown to be an active photocatalyst under UV and visible light illumination. It is well known that high pressure treatment causes solid state materials to undergo polymorphic phase changes into more densely packed crystalline structures. In many cases these high density polymorphs can be recovered metastably to ambient pressure conditions and the technique can be used to adjust and control the electronic properties including the bandgap. In this way we can explore a much wider range of new materials based at a given composition by incorporating high pressure studies in our materials discovery process. The best way to do this is to study the phase changes that occur during compression and decompression in the diamond anvil cell, using synchrotron X-ray diffraction combined with laboratory spectroscopy to determine the structural changes that occur, and if the new phases can be recovered to atmospheric pressure. In the present study we used the intense monochromatic beam of high energy X-rays (? = 0.4441 Å) available from the 3.5 T wiggler device at the Extreme Conditions beamline I15 at Diamond to probe the structural changes occurring in Ti2.85O4N at pressures ranging up to 68 GPa (680,000 atm).
Figure 2: Non-hydrostatic compression data obtained with no pressure-transmitting medium at 24, 30, 42 and 68 GPa. The Le Bail refined X-ray diffraction patterns are of the Pmc21 high pressure phase of Ti2.85O4N. Asterisks (*) mark peaks resulting from the remaining contributions of the ambient pressure Cmcm phase. At 68 GPa additional contributions indicating a second transformation into a monoclinc (P21/c) phase are observed.
The new compound Ti2.85O4N has a structure based on the archetype Ti3O5 that is one of a Magneli series of TinO2n-1 phases with n = 3. At room temperature it is an insulator with the ß-Ti3O5 monoclinic structure. During heating it undergoes a phase transition at 450 K to a metallic monoclinic structure (ß’) with a 6% increase in unit cell volume. Further heating to 500 K leads to a second phase transition to the orthorhombic a-Ti3O5 structure.1 In the low T ß phase the Ti d-electrons are localised within a metal-metal bond, and become delocalised above the transition temperature: breaking the metal-metal bond results in the unit cell expansion and metallic properties. Early work found that lowering the Fermi level by substituting Ti4+ by cations of lower charge results in a lowered transition temperature.2-4 Hyett et al.5 demonstrated that a similar effect is obtained by anionic substitution in the new compound Ti2.85O4N prepared by CVD, comparable with that achieved in a-Ti3O5 at T > 500 K. We decided to explore the high pressure polymorphism and electronic properties of this phase. A thin film of Ti2.85O4N prepared by APCVD from reaction between gaseous titanium(IV) chloride, ethyl acetate and ammonia deposited onto a glass surface was loaded into a cylindrical diamond anvil cell with 200-300 µm diamond culets. Runs were carried out either with no pressure-transmitting medium or using nitrogen cryogenically loaded into the DAC with the sample. Pressures were determined using the ruby fluorescence method.6
Figure 3: Representation of the unit cell of Ti2.85O4N, showing the titanium ions (grey) and the disordered anion sites (red). View of the (a) 100 face and (b) 001 face. Fros (a) can be seen the three co-ordinated anion site between the rigid titania layers that allows the relatively higher compressibility in the b and c directions.
At ambient pressure, the Cmcm structure had lattice parameters, a = 3.80(1) Å, b = 9.62(1) Å and c = 9.88(1) Å. In the first series of runs using N2 as a pressure transmitting medium we obtained high quality data that could be analysed using Rietveld structure refinement methods. However, above 16-18 GPa the data quality deteriorated, partly due to the d-e phase transition occurring in the N2 pressure medium, accompanied by a phase transition in the sample (Fig. 1). We carried out our second series of runs with no pressure-transmitting medium, and observed the sample phase transition at around 18 GPa. The diffraction data showed a new orthorhombic phase (Pmc21symmetry) emerging but the low pressure Cmcm phase was still present up to ~42 GPa, indicating a first-order polymorphic phase transition had taken place. That result indicates that the high pressure phase might be recoverable to ambient conditions. However, all our synchrotron experiments in the diamond anvil cell at Diamond showed that the crystalline phase did not survive the decompression stage. During further compression to 68 GPa, at least two of the peaks of the orthorhombic structure had developed shoulders indicating a further phase change to a monoclinic (P21/c) structure (Fig. 2). Figure 3 shows the structure of the compound and the relatively higher compressibility in the b and c directions.
High-Pressure Behavior and Polymorphism of Titanium Oxynitride Phase Ti2.85O4N Salamat A, Hyett G, Quesada Cabrera R, McMillan P, Parkin I The Journal of Physical Chemistry C 2010 114 (18), 8546-8551
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This work was supported by a Royal Society/Wolfson Research Merit award to IPP and an EPSRC Senior Research Fellowship to PFM. GH thanks the Ramsay Memorial Trust for a research fellowship. Nicholas A. Spencer, an undergraduate student from the University of Glasgow, assisted with the experiments.