A labile hydride strategy for the synthesis of heavily nitridized BaTiO3

Journal name:
Nature Chemistry
Year published:
Published online


Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800–1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N3− species in the lattice place significant constraints on the composition and structure—and hence the properties—of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (<500 °C), using a perovskite oxyhydride as a host. The lability of H in BaTiO3−xHx (x ≤ 0.6) allows H/N3− exchange to occur, and yields a room-temperature ferroelectric BaTiO3−xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O–H–N intermediates. These findings suggest that this ‘labile hydride’ strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds.

At a glance


  1. Two-step synthesis of oxynitride BaTiO3−xN2x/3.
    Figure 1: Two-step synthesis of oxynitride BaTiO3−xN2x/3.

    Perovskite BaTiO3 (oxide) converted to BaTiO3–xHx (oxyhydride) by CaH2 reduction16. BaTiO3–xHx is converted into BaTiO3–xN2x/3 (oxynitride) by the low-temperature NH3 treatment (this work), via the oxyhydride–nitride BaTiO3–xHzNy. Ba, dark green; Ti, light grey; O, red; H, blue; N, light green. Here the lability of H in the oxyhydride allows H/N3– exchange to occur by low-temperature ammonolysis (375–550 °C) to yield BaTiO3−xN2x/3 via mixed O–H–N intermediates. The photos displayed below each structure show the colour change of the specimens for x = 0.6. Compositions are approximated (left to right) as BaTiO3 (Ti4+), BaTi3.4+O2.4H0.6 (Ti3.4+), BaTiO2.4H0.14N0.22 (Ti3.6+) and BaTiO2.4N0.4 (Ti4+). The colour change is related to the amount of Ti 3d t2g electrons (see the text for details).

  2. Anion compositions, cell parameters and magnetism of the NH3-treated oxyhydrides.
    Figure 2: Anion compositions, cell parameters and magnetism of the NH3-treated oxyhydrides.

    a, Temperature dependence of magnetic susceptibilities χ of BaTiO2.4H0.6 before (red) and after (blue) the ammonia treatment at TR = 500 °C. The non-magnetic nature of the sample after NH3 treatment indicates the absence of electrons in the Ti 3d t2g band. b, Nitrogen content (y) dependence of cell parameters (a, c, V) for BaTiO2.4H0.6 and its samples before the reaction (y = 0) and ammonolysed at 375 °C, 400 °C, 425 °C and 500 °C. The cell volume decreases with increasing TR, which implies a gradual oxidation of Ti. A tetragonal distortion is observed for samples with TR ≥ 425 °C along the a and c axes, as labelled. c, Temperature dependence of the lattice parameters for BaTiO2.4N0.4 shows a tetragonal-to-cubic transition at around 100–150 °C, consistent with the SHG results (see Fig. 5a). d, The hydrogen content x versus the nitrogen content y, where the x and y values were determined, respectively, by TDS and elemental analysis. The solid lines in bd are guides to the eye.

  3. Structural refinement for BaTiO2.4N0.4.
    Figure 3: Structural refinement for BaTiO2.4N0.4.

    a,b, Refined PXRD (a) and PND (b) patterns of BaTiO2.4N0.4 that show the observed (red), calculated (green) and difference (blue) profiles. The upper band and lower band of vertical ticks represent the positions of the calculated Bragg reflections of BaTiO2.4N0.4 and TiN (minor impurity), respectively. The PXRD refinement converged successfully to Rwp = 5.12%, Rp = 3.78% and χ2 = 1.20. The PND refinement also converged to Rwp = 3.86%, Rp = 3.05% and χ2 = 2.71 (see Supplementary Note 1 for the definition of Rwp, Rp and χ2). The refined parameters are listed in Table 1.

  4. Systematic evolution of transport properties from oxyhydride SrTiO2.75H0.25 to oxyhydride–nitride SrTiO2.75HzNy (y + z < 0.25) to oxynitride SrTiO2.75Ny (y ≈ 0.16).
    Figure 4: Systematic evolution of transport properties from oxyhydride SrTiO2.75H0.25 to oxyhydride–nitride SrTiO2.75HzNy (y + z < 0.25) to oxynitride SrTiO2.75Ny (y ≈ 0.16).

    a, Temperature dependence of resistivity. From bottom to top, the SrTiO2.75H0.25 film and its ammonia-treated films at TR = 375, 400, 450, 475 and 500 °C. With increasing TR, the electrical resistivity increases and a metal-to-insulator crossover can be seen. b, The carrier density ne of the films estimated from Hall measurements as a function of TR demonstrates a systematic reduction of ne with TR.

  5. Ferroelectric properties of BaTiO2.4N0.4.
    Figure 5: Ferroelectric properties of BaTiO2.4N0.4.

    a, The temperature dependence of SHG intensity for the BaTiO2.4N0.4 bulk sample reveals a centrosymmetric-to-non-centrosymmetric phase transition around 150 °C. b, Variation of p- and s-polarized SHG intensities, I2ωp and I2ωs, for the BaTiO2.4N0.4 thin film as a function of rotation angle of the electric field of the fundamental light, θ, measured in the transmission set-up (Supplementary Fig. 8). The polar plots fitted well using a 4mm model with a four-fold axis perpendicular to the film. cf, In-phase VPFM (c), out-of-phase VPFM (d), in-phase LPFM (e) and out-of-phase LPFM (f) images taken after the electric-field poling (Supplementary Fig. 9), which clearly demonstrate polarization switching. g,h, Corrected VPFM (g) and LPFM (h) images obtained by poling in a different way (Supplementary Fig. 10). i,j, Averaged line profiles of the dashed rectangle areas in g and h, respectively. Also shown is the in-plane piezoelectric displacement simulated by FEM. Here the in-plane displacement is the piezoelectric displacement in the scanning direction when a tip is scanned across 180° up and down domain walls (Supplementary Fig. 11). The VPFM response is contributed not by out-of-plane piezoelectric displacements, but by a buckling effect of the PFM tip. a.u., arbitrary units.


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Author information


  1. Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

    • Takeshi Yajima,
    • Fumitaka Takeiri,
    • Kohei Aidzu,
    • Wataru Yoshimune,
    • Masatoshi Ohkura,
    • Takafumi Yamamoto,
    • Yoji Kobayashi &
    • Hiroshi Kageyama
  2. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan

    • Takeshi Yajima
  3. Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA

    • Hirofumi Akamatsu,
    • Shiming Lei &
    • Venkatraman Gopalan
  4. Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

    • Koji Fujita &
    • Katsuhisa Tanaka
  5. NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, MS 6100, Gaithersburg, Maryland 20899-6100, USA

    • Craig M. Brown
  6. School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK

    • Mark A. Green
  7. CREST, Japan Science and Technology Agency, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

    • Hiroshi Kageyama


T. Yajima and F.T. contributed equally. T. Yajima, F.T. and H.K. conceived and designed the study. F.T., K.A., M.O., W.Y. and T. Yajima performed the synthesis, laboratory PXRD, synchrotron PXRD, XPS and elemental analysis. T.Yam., C.M.B., M.A.G. and H.K. obtained the neutron data. The structural refinement was performed by K.A., T. Yamamoto and T. Yajima. W.Y. and T. Yajima fabricated the thin films. H.A., K.F., S.L., V.G. and K.T. conducted the SHG and PFM measurements and FEM simulations. All authors discussed the results. F.T. and H.K. wrote the manuscript, with contributions and feedback from all the authors, mainly T. Yajima, Y.K., H.A. and K.F.

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