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.
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.
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.
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.
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.