SSC RESEARCH LINES

Nitride-based materials

Led by Prof. Amparo Fuertes

SSC

The similarities in electronegativity, polarizability, ionic radii and coordination numbers of oxygen and nitrogen allow the formation of the same structural types when combined with cations as well as the mutual substitution of both anions at the same crystallographic sites. This can result in the formation of solid solutions where the formal valence of one or more cations may change according to the O/N ratio. The design of new nitrides based on phases and crystal structures already known for oxides is  a useful tool for exploring and/or modifying a large variety of physical properties, either through the formation of completely new compounds or of solid solutions.

We have investigated hafnium nitride halide superconductors that show high critical temperatures within non oxidic compounds. We investigate the effect of nitrogen on the properties of important materials such as CeO2 and TiO2, and we design new oxynitrides with magnetic, electrical, photocatalytic and luminescent properties. Oxynitrides of transition metals, alkaline earth metals and rare earth metals are an important  group of materials to expand and tune the properties of oxides. The differences in polarizability, electronegativity and anion charge of nitrogen and oxygen induces changes in the physical and chemical properties by nitrogen introduction. The effects on properties arise from the higher covalency of metal-nitrogen bond and the changes in the energies of electronic levels, and are important in slightly doped nitrogen metal oxides as in stoichiometric oxynitrides. The stabilization of new perovskite oxynitrides, with the oxidation states of cations tuned by O/N stoichiometry, has lead to new photocatalysts, magnetic and dielectric materials. The lower electronegativity of nitrogen and larger crystal field splitting induced by N3- shifts the emission wavelenghts of phosphors to the red, and oxynitridosilicates have been investigated as components of white LED’s.

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Research Highlights

Mechanism of intercalation of Na in beta-HfNCl

We have investigated the influence of the nature of the transition metal, the co-intercalated molecules, the staging and the doping level on the superconducting properties of intercalated zirconium and hafnium nitride halides. We have discovered an unprecedent re-entrant mechanism during synthesis of superconducting Na0.5HfNCl by ultra slow electrochemical intercalation of sodium. HfNCl host layers transform to an alternative geometry and then revert to their original structure, revealing that intercalation reactions may proceed by very different mechanisms to those expected in the conventional slab-gliding picture.

N-doping of metal oxides

Nitrogen doping is an important method for modifying the properties of oxides, for example, tuning the band gap of the photocatalyst TiO2 from the UV to the visible region. We have prepared mesoporous films of N-TiO2 showing photocatalytic activity under visible light. We have shown for the first time that ceria (CeO2), a relevant material with important applications as oxide ion-conducting and oxygen-storage component in catalysts, can be doped with nitrogen that would be a significant defect when this material is exposed to reducing, nitrogen-rich atmospheres. Films of CeO2-x-yNx show photocatalytic activity under visible light in the decomposition of acetaldehyde.

Perovskite oxynitrides: electrical, magnetic and photocatalytic properties

Polar hexagonal perovskite BaWON2

Our research in perovskite oxynitrides has lead to the discovery of large/colossal magnetoresistance in EuNbO2N, EuWO2-xN1+x and in the double perovskite Sr2FeMoO4.9N1.1. We have discovered new vanadium and chromium perovskites RVO3-xNx and RCrO3-xNx (R= rare earth), showing the effect of nitride introduction on the magnetic interactions between the spins of the transition metal or the rare earth and on the electrical properties. New antiferromagnetic double perovskites Sr2FeWO5N have been prepared by topochemical ammonolysis of cation ordered Sr2FeWO6.  

We have discovered new hafnium oxynitride perovskites RHfO2N that show photocatalytic activity under visible light in water oxidation or reduction. Anion order in pseudocubic perovskite oxynitrides has been investigated by neutron diffraction indicating a cis distribution of nitrides confined in planes, which is induced by the higher covalency of B-N bonds.

The new compound BaWON2  is the first example of a hexagonal perovskite oxynitride. It crystallizes in the non centrosymmetric space group P63mc and shows complete order of N and O in hexagonal and cubic layers. Synergetic second order Jahn- Teller effect, anion order and electrostatic repulsion between hexavalent W cations induce large distortions at two inequivalent face sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity. 

Luminescent oxynitrides

Phosphors based on rare earth-doped silicon oxynitrides attract attention because of their good luminescent properties, low toxicity, thermal stability and colour tunability. We have discovered new oxynitridosilicates with apatite and beta-K2SO4 structure that show orange and red emission wavelengths under excitation with blue or UV light. The new oxynitride orthosilicates LaMSiO3N (M=Sr, Ba) are isotypic to alpha’-M2SiO4 (M= Sr, Ba) with beta-K2SO4 structure and can be activated either with Ce3+ or Eu2+.  Under excitation with UV-blue light the emission wavelengths are red shifted with respect to the high efficient phosphors Sr2SiO4:Eu or Ba2SiO4:Eu, from c.a. 550 nm to 650-700 nm.

Publications

  • Selected recent publications

    1. A.Fuertes. Prediction of Anion Distributions Using Pauling’s Second Rule, Inorganic Chemistry, 45, 9640-9642 (2006).
    2. E.Martínez-Ferrero, Y.Sakatani, C.Boissière, D.Grosso, A.Fuertes, J.Fraxedas and C.Sanchez. Nanostructured Titanium Oxynitride Porous Thin Films as Efficient Visible-Active Photocatalysts. Advanced Functional Materials, 17, 3348-3354 (2007).
    3. A.B. Jorge, J.Fraxedas, A.Cantarero, A.J.Williams, J.Rodgers, J.P.Attfield and A.Fuertes. Nitrogen Doping of Ceria. Chemistry of Materials, 20, 1682-1684 (2008).
    4. A.B.Jorge, J.Oró-Solé, A.M.Bea, N.Mufti, T.T.Palstra, J.A.Rodgers, J.P.Attfield and A.Fuertes. Large Coupled Magnetoresponses in EuNbO2N. Journal of the American Chemical Society, 130, 12572-12573 (2008).
    5. M.Yang, J.A.Rodgers, L.C.Middler, J.Oró-Solé, A.Belén-Jorge, A.Fuertes and J.P.Attfield. Direct Solid State Synthesis at High Pressures of New Mixed-Metal Oxynitrides: RZrO2N (R=Pr, Nd and Sm). Inorganic Chemistry, 48, 11498-11500 (2009).
    6. A.Fuertes. Synthesis and properties of functional oxynitrides – from photocatalysts to CMR materials. Dalton Transactions, 39, 5942-5948 (2010).
    7. M. Yang, J.Oró-Solé, A. Kusmartseva, A.Fuertes, and J. P.Attfield. Electronic Tuning of Two Metals and Colossal Magnetoresistances in EuWO1+xN2-x Perovskites. Journal of the American Chemical Society, 132, 4822-4829 (2010).
    8. M.Yang, J. Oró-Solé, A.Fuertes, and J. P. Attfield. Topochemical Synthesis of Europium Molybdenum Oxynitride Pyrochlores. Chemistry of Materials, 22, 4132-4134 (2010).
    9. M.Yang, J.Oró-Solé, J.A. Rodgers, A. B. Jorge, A.Fuertes, and J. P.Attfield. Anion Order in Perovskite Oxynitrides. Nature Chemistry, 3, 47-52 (2011).
    10. A.Fuertes. Chemistry and Applications of Oxynitride Perovskites”. Journal of Materials Chemistry, 22, 3293-3299 (2012).
    11. A. B. Jorge, Y.Sakatani, C.Boissière, C.Laberty-Roberts, G.Sauthier, J.Fraxedas, C.Sanchez, and A.Fuertes. “Nanocrystalline N-doped Ceria Porous Thin Films as Efficient Visible-Active Photocatalysts”. Journal of Materials Chemistry, 22, 3220-3226 (2012).
    12. P.Camp, A.Fuertes and J.P.Attfield. Sub-extensive Entropies and Open Order in Perovskite Oxynitrides. Journal of the American Chemical Society, 134, 6762-6766 (2012).
    13. S.Thomas, J.Oró-Solé, B.Glorieux, V.Jubera, V.Buissette, T.Lemercier, A.Garcia, and A. Fuertes. New Luminescent Rare Earth Activated Oxynitridosilicates and Oxynitridogermanates with the Apatite Structure. Journal of Materials Chemistry, 22, 23913-23920 (2012).
    14. J.Oró-Solé, L.Clark, W.Bonin, J.P.Attfield, A.Fuertes. Anion-ordered Chains in a d1 Perovskite Oxynitride; NdVO2N. Chemical Communications 4, 2430 – 2432 (2013).
    15. L.Clark, J. Oró Solé, A. Fuertes; J.P. Attfield. Thermally Robust Anion-chain Order in Oxynitride Perovskites. Chemistry of Materials 25, 5004−5011 (2013).
    16. J. Oró Solé, L. Clark, N. Kumar, W Bonin, A. Sundaresan, J.P. Attfield, CNR Rao, A. Fuertes. Synthesis, Anion Order and Magnetic Properties of RVO3-XNx Perovskites (R = La, Pr, Nd; 0 <= x <= 1)’. Journal of Materials Chemistry C 2, 2212–2220 (2014).
    17. V.Meignen, J.Oró-Solé, W.Bonin, M.Morcrette, M.R.Palacín, J.P.Attfield, A.Fuertes. Re-entrant Layer Reconstruction during Intercalation in Hafnium Nitride Chloride. Chemical Science 5, 2974-2978, (2014).
    18. A.Fuertes. Metal Oxynitrides as Emerging Materials with Photocatalytic and Electronic properties. Materials Horizons, 2, 453-461 (2015).
    19. A.P. Black, K.A. Denault, J. Oró-Solé, A.R. Goñi, A.Fuertes. Red luminescence and ferromagnetism in europium oxynitridosilicates with a beta-K2SO4 structure. Chemical Communications 51, 2166-2169 (2015).
    20. A.P. Black, K.A. Denault, C.Frontera, R.Seshadri, A.R. Goñi, A.Fuertes. Emission colour tuning through coupled N/La introduction in Sr2SiO4:Eu2+. Journal of Materials Chemistry C 3, 11471–11477 (2015).
    21. A.P.Black, H.E.Johnston, J.Oró-Solé, B.Bozzo, C.Ritter, C.Frontera, J.P.Attfield, A.Fuertes. Nitride Tuning of Lanthanide Chromites. Chemical Communications, 52, 4317-4320 (2016).
    22. R.Ceravola, J.Oró-Solé, A.P. Black, C.Ritter, I.Puente Orench, I. Mata, E.Molins, C.Frontera, A.Fuertes. Topochemical Synthesis of Cation Ordered Double Perovskite Oxynitrides. Dalton Transactions, 46, 5128-52131 (2017).
    23. A.Fuertes. Synthetic Approaches in Oxynitride Chemistry. Progress in Solid State Chemistry, 51, 63-70 (2018).
    24. A.P.Black, H.Suzuki, M.Higashi, C.Frontera, C.Ritter, CD. De, A.Sundaresan, R.Abe, A.Fuertes. New Rare Earth Hafnium Oxynitride Perovskites with Photocatalytic Activity in Water Oxidation and Reduction. Chemical Communications, 54, 1525-1528 (2018).
    25. H. Johnston, A.P.Black, P.Kayser, J.Oró-Solé, D.A.Keen, A.Fuertes, J.P.Attfield. Dimensional Crossover of Correlated Anion Disorder in Oxynitride Perovskites. Chemical Communications, 54, 5245-5247 (2018).
    26. R.Ceravola, C.Frontera, J.Oró-Solé, A.P.Black, C.Ritter, I.Mata, E.Molins, J.Fontcuberta and A. Fuertes. Topochemical nitridation of Sr2FeMoO6. Chemical Communications, 55, 3105-3108 (2019).
    27. R.Verreli, A.P.Black, C.Frontera, J.Oró-Solé, M.E. Arroyo de Dompablo, A.Fuertes, M.R.Palacín. On the Study of Ca and Mg Deintercalation from Ternary Tantalum Nitrides. ACS Omega, 4, 8943-8952 (2019).
    28. A.Fuertes. Nitride tuning of transition metal perovskites. APL Materials 8, 020903 (2020).            
    29. J. Oró-Solé, C.Frontera, A.P.Black, A.Castets, K.L. Velásquez-Méndez, J.Fontcuberta, A.Fuertes. Structural, Magnetic and Electronic Properties of EuTi0.5W0.5O3-xNx perovskite oxynitrides. Journal of Solid State Chemistry 286, 121274 (2020).
    30. J.Oró-Solé, I.Fina, C.Frontera, J.Gàzquez, C. Ritter, M.Cunquero, P. Loza- Alvarez, S. Conejeros, P.Alemany, E. Canadell, J. Fontcuberta,  A. Fuertes. Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2. Angewandte Chemie International Edition, 59, 18395 (2020).

     

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