Metal oxynitrides as emerging materials with photocatalytic and electronic properties

Abstract

Oxynitrides of transition metals, alkaline earth metals and rare earth metals are intensively investigated as a group of materials to expand and tune the properties of oxides. The differences in polarizability, electronegativity and anion charge of nitrogen and oxygen induce changes in the physical and chemical properties of oxides by nitrogen introduction. The effects on properties arise from the higher covalency of the 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. More intense recent progress in oxynitride research has been made in some specific fields such as photocatalysis in water splitting and other processes as the observed small band gaps lead to activity in the visible light range. The stabilization of new perovskite oxynitrides, with the oxidation states of cations tuned by N/O stoichiometry, has led to new magnetic and dielectric materials. The lower electronegativity of nitrogen and larger crystal field splitting induced by N³⁻ shifts the emission wavelengths of phosphors to the red, and oxynitridosilicates have been investigated as components of white LEDs.

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Amparo Fuertes

Amparo Fuertes is a research professor of the Spanish National Research Council (CSIC), at the Institute of Materials Science of Barcelona. She started her career at the University of Valencia where she obtained the PhD in 1986, in the field of transition metal carboxylates with magnetic properties of low dimensionality. In 1988 she joined the Institute of Materials Science of Barcelona and initiated the Laboratory of Solid State Chemistry. Her research interests focus on the synthesis and characterization of oxides and nitride-based materials with diverse applications including superconductivity, colossal magnetoresistance, luminescence and catalysis.

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1. Introduction

Research on metal oxynitrides has increased significantly in the last decade because of their emerging applications as photocatalysts, pigments, phosphors, dielectrics and magnetic materials.^1–6 The introduction of nitrogen in an oxidic compound expands the possibilities of tuning properties with respect to the most used strategy of cation substitution. Nitrogen is less electronegative and more polarizable than oxygen and this increases the covalency of bonding. The interelectronic repulsion decreases and the nephelauxetic effect (expansion of the electron cloud) increases. The crystal field splitting is larger as a consequence of the higher charge of nitride. The −3 charge of the nitride anion may allow the formation of new compounds where cations show higher oxidation states than in oxides. These effects induce important changes in properties. For instance in an insulating oxide the band gap decreases with nitride introduction and the optical properties may change drastically. As a result of the band gap reduction the photocatalytic activity of semiconductors may be shifted from the UV to the visible light range. The emission wavelengths of rare earths are shifted to the red because of lowering of the energy of 5d orbitals, which is a consequence of the larger nephelauxetic effect and crystal field splitting (Fig. 1a). Additional effects such as symmetry distortions or anion order may further modify the luminescence properties. The Fermi level in a late transition metal nitride in the charge transfer regime⁷ is placed at higher energies than in the corresponding oxide (Fig. 1b). This decreases the operating voltage in lithium or sodium batteries making the transition metal (oxy)nitrides attractive as electrode materials.^8,9 The higher covalency and larger polarization of bonds with metals affect the electronic properties leading to new magnetic, conducting and dielectric materials. This review focuses on recent developments in significant applications of metal oxynitrides as electronic and photocatalytic materials. These include nitrogen doped binary oxides (N-TiO₂ and N-CeO₂) and ternary oxynitrides (TaON) to complex quaternary and higher oxynitrides of transition metals, alkaline earth metals and rare earth metals (perovskites and oxynitridosilicates), showing a variety of optical, magneto/electric (dielectric, thermoelectric, ferromagnetic, spin glass or magnetoresistance) and photocatalytic (in water splitting and organic reactions) properties.

Fig. 1 (a) Schematic diagram of energies of 5d levels and emissions of rare earth cations as Eu²⁺ or Ce³⁺ affected by bonding (nephelauxetic effect) and crystal field splitting in different environments. (b) Schematic diagram of band energies for a semiconducting, late transition metal nitride and oxide (charge transfer type).

2. Synthesis: reactions in NH₃ and N₂ at high temperature

The two general approaches for the synthesis of oxynitrides are the treatment at high temperatures of reactants in NH₃ and N₂. Ammonolysis reactions are used starting with precursor oxides or mixtures of binary oxides, nitrides and carbonates. Ammonia dissociates appreciably into N₂ and H₂ at temperatures higher than 500 °C (Fig. 2a). The kinetics of the dissociation is slow and its extent can be controlled by the flow rate. For instance, at 700 °C the % of decomposition varies from ca. 10% for a flow rate of 150 cm³ min⁻¹ to ca. 45% for 50 cm³ min^−1 10 (Fig. 2b). Under high flow rate conditions and at high temperature, NH₃ is very reactive and highly nitriding. Typical flow rates are between 180 and 1000 cm³ min⁻¹. The key parameters of temperature, flow rate, time and sample placement in the reaction tube among other factors determine the gas composition (NH₃, N₂ and H₂) around the sample and should be considered to optimize the purity of the oxynitride phase.¹¹ The reducing power of NH₃ together with the presence of H₂ makes more difficult the stabilization of ionocovalent oxynitrides of the metals more electronegative (e.g. late transition metals) which form interstitial nitrides with the cations in low oxidation states.^12,13 On the other hand in ammonolysis reactions some cations may be oxidized under the same conditions. For instance the rare earth vanadium oxynitrides RVO_3−xN_x (R = La, Pr, Nd) are prepared in NH₃ between 650 and 800 °C starting with the precursors RVO₄ (monazite or zircon).¹⁴ The ammonolysis reaction proceeds through an initial reduction of RVO₄ precursors to the perovskites RVO₃ that are progressively nitrided and reoxidised to RVO_3−xN_x materials with varying proportions of V⁴⁺/V³⁺. A similar oxidation reaction takes place for rare earth molybdenum pyrochlores R₂Mo₂O₇ which under ammonia lead topotactically to oxynitrides R₂Mo₂O_7−xN_x with molybdenum in oxidation states between +4 and +6.¹⁵

Fig. 2 (a) Equilibrium constant of dissociation of NH₃ as a function of temperature using the equation²⁶ log K_p = 2095/T − 2.7884 log T + 0.0003986T + 2.6253 where K_p is the equilibrium constant of the formation of NH₃, and (b) % of decomposition of NH₃ as a function of flow rate at different temperatures (produced with data of ref. 10).

Because of the moderate temperatures during the synthesis of oxynitrides under NH₃ the sintering of samples is difficult. Dense ceramics are needed for different applications and for the measurement of electrical properties. High pressure with and without sample heating has been used for producing densities above 85%.¹⁶ The homogeneity of the samples in terms of N stoichiometry (and then of the oxidation states of cations) is important and in most cases several treatments are needed to accomplish it.

More homogeneous and sintered samples are obtained in the synthesis under N₂, which is performed at higher temperatures, typically between 1300 and 1500 °C. This requires a strict control of H₂O and O₂ impurities around the sample to avoid the formation of oxides. The use of carbonates as reactants is common but they constitute an additional source of oxygen. CO₂ decomposes into CO and O₂ at temperatures above ca. 1300 °C and also it may react with nitrides at lower temperatures oxidizing N³⁻ to N₂ and giving rise to oxides.¹⁷ Amides,¹⁸ imides,¹⁹ urea²⁰ and azides²¹ have also been used as nitrogen sources.

High pressure has been used in few examples to stabilize quaternary oxynitrides containing small cations. The perovskites LnZrO₂N can be prepared at room pressure only for Ln=La and are synthesized at 2–3 GPa and 1200–1500 °C for smaller rare earth metals such as Pr, Sm or Nd.²² More recently MnTaO₂N with LiNbO₃ type structure has been synthesized at 6 GPa and 1400 °C.²³

3. N-doped TiO₂ and CeO₂

The introduction of nitrogen into TiO₂ and CeO₂ takes place with concomitant creation of anion vacancies as the charge compensation mechanism.^24,25 The oxynitrides TiO_2−x−yN_x and CeO_2−x−yN_x with anatase and fluorite structures, respectively, are prepared under NH₃ from the corresponding oxides. Under these conditions both systems behave similarly chemically. Using a constant reaction time of several hours the nitrogen content of the samples increases to a maximum of ca. 0.1 atoms per formula unit at a temperature between 550 and 700 °C. At higher temperatures the reduction of Ti⁴⁺ to Ti³⁺ or that of Ce⁴⁺ to Ce³⁺ competes with the formation of the oxynitride and the nitrogen content decreases. For powder samples of CeO₂ the solid solution Ce_1–y^IVCe_y^IIIO_{2−3x/2−y/2}N_x (x ≤ 0.09, y ≤ 0.06) is formed between 600 and 680 °C, with the cell parameter of the cubic fluorite structure increasing with N content.²⁵ At temperatures higher than 800 °C the reduced oxides CeO_1.78 and Ce₇O₁₂ are formed. In the ammonolysis of anatase TiN is produced at temperatures higher than 800 °C, and N doped rutile is also observed at high temperature.²⁴

N-doped TiO₂ (anatase) has been extensively investigated since in 2001 Asahi et al. reported the visible light induced photooxidation of acetaldehyde (CH₃CHO) to CO₂ on this material.²⁷ Nitrogen doping shifts the photocatalytic activity of TiO₂ from UV to the visible range. DFT calculations show that there is no shift of both the top and bottom positions of the O 2p valence band, as well as of the conduction band, with respect to the undoped material. The red shift of the absorption edge is caused by N 2p localized states just above the top of the O 2p valence band.²⁸ Visible light photocatalytic activity has also been reported in oxidation of other compounds (carbon monoxide, formaldehyde, acetic acid among many others) and for water splitting.²⁹ Antibacterial, antiviral and antiallergen properties under visible or indoor lighting have also been reported.

N-doped CeO₂ has shown photocatalytic activity under visible light in the decomposition of acetaldehyde and other organic compounds^30–32 (Fig. 3). The shift of the band gap in powder samples has been estimated by diffuse reflectance spectroscopy from 2.96 eV for CeO₂ to 2.65 eV for CeO_2−x−yN_x with x = 0.084.³⁰ DFT calculations suggest that in contrast to N doped TiO₂, the band edges in N-CeO₂ are lowered with respect to CeO₂ on both sides of the energy gap.³¹ Nitrogen introduction into CeO₂ has also been reported to remarkably improve scavenging of free radical oxygen species in polymer electrolyte membranes of fuel cells.³³

Fig. 3 Dependence of the CO₂ evolution rate for the decomposition of acetaldehyde over a film of CeO_2−x−yN_x with x = 0.05 on the cut-off wavelength of the irradiated light. (Reproduced from ref. 30 with permission of The Royal Society of Chemistry.)

4. TaON

The stable polymorph of tantalum oxynitride at room pressure is monoclinic, isostructural to baddeleyite ZrO₂ (β-TaON). Tantalum is heptacoordinated and there are two crystallographically independent sites for anions where N and O order.³⁴ As in many oxynitrides and other mixed anion compounds this order can be rationalised and predicted by using Pauling's second crystal rule.^35,36 This oxynitride is an important visible active photocatalyst for water splitting.^37–39 As Ta₃N₅, TaON is a semiconductor with a band gap of 2.4 eV allowing visible light absorption, and has band edge potentials suitable for H₂ and O₂ production (from the reduction and oxidation of H₂O respectively). This requires that the minimum of the conduction band and the maximum of the valence band of the photocatalyst would be lower and higher than the oxidation–reduction potential of H⁺/H₂ and O₂/H₂O respectively.⁶ If no co-catalyst is used the oxidation of water on TaON is efficient but the activity in the reduction is not high.³⁷ Efficient reduction and oxidation of water has been achieved in two steps,^40,41 and more recently in one step by using ZrO₂ as a co-catalyst.⁴² Both Ta₃N₅ and β-TaON have also been reported as photocatalysts in oxidation of organic molecules.⁴³

The synthesis of β-TaON is usually done by ammonolysis of Ta₂O₅ at 800–850 °C, using either low flow rates of NH₃ (typically 20–50 cm³ min⁻¹) or some PH₂O (bubbling NH₃ in water) to prevent the formation of the nitride Ta₃N₅.⁴⁴ Nanoparticles of β-TaON are prepared by a Ca-assisted urea route⁴⁵ and efficient photoanodes have been prepared by electrophoretic deposition.^46,47

γ-TaON is a metastable polymorph that can be prepared starting with Ta₂O₅ using lower ammonia flow rates than for TaON and shorter reaction times.⁴⁸ It shows a crystal structure related to VO₂(B)⁴⁹ consisting of sharing edge octahedra of tantalum in layers that are connected through the Ta(O,N)₆ octahedra corners. δ-TaON with anatase structure is another metastable polymorph that has been recently obtained in powder and thin film forms.^50,51 As powder it forms by ammonolysis of amorphous citrate precursors at 760 °C, and transforms into the more stable β-TaON between 800 and 850 °C. A cotunnite polymorph of TaON was predicted at high pressure⁵² and it has been recently stabilized at 33 GPa at room temperature.⁵³ The cotunnite phase shows a very high bulk modulus K_o ≈ 370 GPa. γ-TaON has been reported to show efficient visible-light photocatalytic activity in water splitting⁵⁴ whereas δ-TaON films show high mobility of electron carriers and a band gap of 2.37 eV.⁵¹

5. Perovskite oxynitrides

Oxynitrides with a perovskite structure have been reported for alkaline earth or rare earth cations in the A site and the transition metals Ti, Zr, V, Nb, Ta, Mo, W and Fe in the B sites and nitrogen contents up to a maximum of 2.16 atoms per formula shown for A = La and B = W.^2,3,55–64 TaThN₃ is one of the few reported examples of fully nitrided perovskites⁶⁵ whereas many antiperovskite nitrides (with one nitrogen per formula) have been reported.⁶⁶ Ruddlesden–Popper oxynitrides are known for the series (SrO)(SrNbO₂N)_n (n = 1, 2),⁶⁷ A₂TaO₃N (A = alkaline earth metal)⁶⁸ and the rare earth aluminates R₂AlO₃N.^69,70 The tolerance factors of existing perovskite oxynitrides range from 0.878 to 1.038⁷¹ as calculated from ionic radii⁷² (0.827 to 1.042 from observed interatomic distances), in contrast to those of oxides, from 0.74 to 1.00 (0.822 to 1.139 from bond valence parameters).⁷¹

The synthesis of perovskite oxynitrides can be done under NH₃ starting with binary metal oxides and carbonates or with oxide precursor phases. The use of precursors is needed for rare earth perovskites, as the rare earth oxides are poorly reactive at the usual synthesis temperatures of ammonolysis reactions (up to 1100 °C). The synthesis in N₂ has been performed at temperatures between 1100 and 1500 °C starting with binary nitrides, oxynitrides, oxides or carbonates.^17,73 The oxide and nitride anions order in many reported perovskite oxynitrides.^73–77 In layered Sr₂TaO₃N and Sr₂NbO₃N with K₂NiF₄-type structures the nitrogen atoms occupy the equatorial sites of the octahedra,^75,78 which show larger bond strength sums, in agreement with Pauling's second crystal rule.³⁶ In pseudo cubic perovskites all the anion sites show the same bond strength sums and the anion order cannot be rationalized based on this rule. In the perovskites SrTaO₂N, SrNbO₂N and more recently in RVO₂N (R = La, Pr, Nd), a local cis arrangement of nitrides has been suggested from electron diffraction and high resolution neutron diffraction studies.^14,79,80 Long-range disorder of this arrangement results in 4 sites with N/O mixed occupancies close to 50/50. The existence of zig-zag N–M–N chains confined in planes that are prone to disorder has been suggested to explain the observed results (Fig. 4a). The local cis order has also been suggested for the cubic perovskite BaTaO₂N from neutron pair distribution function analysis⁸¹ and has been found energetically favoured from DFT calculations.^82–84 It can be explained in terms of maximum utilization of d orbitals in π bonding with the lone pairs of the anions (Fig. 4b).⁸⁵ In temperature dependent neutron diffraction studies of ATaO₂N and ATaON₂ perovskites the cis order has been observed up to 1100 °C (the maximum temperature investigated).⁸⁶ The combination of the four most common types of octahedral tilting and anion order (considering the cis and trans configurations of N in AMO₂N or O in AMON₂) results in 14 possible space groups.^77,87

Fig. 4 (a) N/O order in ABO₂N perovskite oxynitrides showing N–M–N zig-zag chains (blue line) confined in planes.⁷⁹ (b) π bonding of d orbitals with ON lone pairs showing three and two interactions in the cis and trans configurations respectively.⁸⁵

5.1 Photocatalytic properties

Tantalum(V), niobium(V) and titanium(IV) perovskite oxynitrides have been reported as photocatalysts in water splitting. The band gaps and band edge positions of CaTaO₂N, SrTaO₂N, BaTaO₂N, LaTaON₂, LaTiO₂N and BaNbO₂N^88–90 are suitable for both water oxidation and reduction under visible light. For efficient conversion of solar energy, band gaps smaller than 2 eV (equivalent to an absorption edge of λ > 600 nm) are convenient, and the band gaps of this group of compounds range from 1.8 eV (for LaTaO₂N) to 2.5 eV (for CaTaO₂N). Co-catalysts like transition metal oxides and sacrificial agents are used to enhance the photocatalytic activity as they provide active centres for the oxidation/reduction reactions and may improve charge carrier separation and lifetime. The tantalum perovskites evolve H₂ from water (λ > 420 nm) but they are not active in O₂ production and show efficiencies in water reduction up to 20% (for LaTaON₂).^91,92 Overall water splitting with visible light up to 600 nm has been recently reported for the double tantalum/magnesium perovskites with formula LaMg_xTa_1−xO_1+3xN_2−3x (x ≥ 1/3).^93,94 Among the niobium perovskite oxynitrides ANbO₂N (A = Ca, Sr, Ba) and LaNbON₂, the compound CaNbO₂N showed the highest activity for both H₂ and O₂ production (λ > 420 nm), SrNbO₂N showed activity only for O₂ evolution and the remaining perovskites were not active or showed poor activity.^95,96 The lower activities with respect to the tantalum materials are likely due to the presence of reduced oxidation states of niobium. LaTiO₂N has a band gap of 2.1 eV and is active for O₂ and H₂ production in the presence of sacrificial agents.⁹⁷ The co-catalyst CoO_x improves the efficiency of LaTiO₂N to 27.1% at 440 nm which is the highest reported for oxynitrides with absorption edges near to 600 nm.⁹⁸

5.2. Electrical and magnetic properties

Tantalum perovskite oxynitrides have been investigated as dielectric materials since relative permittivities at room temperature of 4900 and 2900 were reported for SrTaO₂N and BaTaO₂N, respectively, with a relaxor-type ferroelectric behaviour.⁹⁹ As both compounds crystallize in centrosymmetric space groups the local Ta-O,N dipoles counterbalance by each other and different possibilities for the origin of the dielectric properties have been discussed. Local cis ordering of nitrides in the TaO₄N₂^79,81 octahedra together with polar cooperative disorder¹⁰⁰ of N–M–N chains¹⁰¹ have been suggested. Classical ferroelectricity has been recently observed in compressively strained SrTaO₂N epitaxial thin films grown on SrTiO₃ substrates.¹⁰² DFT calculations under epitaxial strain of this material suggest that small domains of the trans N ordered ferroelectric phase are responsible for this behaviour, and microstructural analysis suggests the coexistence of ferroelectric (trans type) and relaxor ferroelectric (cis type) domains.¹⁰³ Recently the new structurally related oxynitride MnTaO₂N with a polar LiNbO₃-type structure has been reported. (Fig. 5)²³ It is insulating, and dielectric measurements have not been reported, but the calculated spontaneous polarization is similar to multiferroic Mn oxides with a LiNbO₃ structure. It shows a long range, helical order of Mn²⁺ spins and strong frustration interpreted as a consequence of octahedral tilting and N/O disorder.

Fig. 5 Polar LiNbO₃ type structure of MnTaO₂N.²³

The dielectric properties of thin films of LaTiO₂N¹⁰⁴ and N-doped ATiO₃ (A = Sr, Ba) have also been investigated.¹⁰⁵ The dielectric constant of LaTiO₂N films was 325 whereas in N doped Ba_xSr_1−xTiO₃ films it improved by a factor of 1.44 with respect to the undoped material.

Molybdenum perovskite oxynitrides with composition AMoO_3−xN_x (A = alkaline earth) have been reported as thermoelectric materials. They show low electrical conductivities and Seebeck coefficients from 15 to 30 μV K⁻¹.¹⁰⁶

Tantalum, niobium or tungsten perovskite oxynitrides with europium at A sites and formula EuMO_3−xN_x are ferromagnetic because of the ordering of Eu(II) S = 7/2 spins, with Curie temperatures of 5.2, 5.1 and 12 K for M = Nb, Ta and W respectively.^59,107,108 When mixed oxidation states of the transition metals near to stochiometric Eu²⁺Nb⁺⁵O₂N and Eu²⁺W⁺⁶ON₂ are present (Nb⁴⁺ or W⁵⁺ doping) these perovskites show colossal magnetoresistance below T_c because of coupling between the Eu²⁺ spins and d-band carriers of Nb or W. In EuWO_1+xN_2−x the electrical conductivity is tuned by the nitrogen content and the doping (p or n) mechanism.

Lanthanum or praseodymium vanadium oxynitride perovskites with formula RVO₂N show spin freezing transitions at low temperatures¹⁴ and the oxynitride perovskite Sr₂FeMoO₅N shows a broad ferromagnetic transition with T_c in the temperature range of 150–200 K.⁶⁴

6. Rare earth activated (oxy)nitridosilicates with luminescence properties

Oxynitridosilicates of alkaline earth metals and rare earth metals have been widely investigated as hosts for Eu²⁺ or Ce³⁺ to produce luminescent materials with long wavelengths and broad emission bands.^4,5 Within pure nitrides the compounds A₂Si₅N₈ (A = Sr, Ba) doped with Eu²⁺ have found applications as phosphors in white LEDs as they show high quantum efficiencies, strong absorptions in the blue light region and a range of emission wavelengths from 580 to 680 nm.^109,110 These nitridosilicates show a condensed and rigid structure formed by corrugated layers of corner sharing [SiN₄] tetrahedra which are three dimensionally connected.⁵ Remarkable long emission wavelengths (up to 650 nm) and high efficiency are shown also by Eu²⁺ doped CaAlSiN₃¹¹¹ and the nitridoaluminate SrLiAl₃N₄, structurally related to nitridosilicates.¹¹² In cerium doped CaSiN₂ wavelength emissions up to 630 nm under excitation at around 460 nm and quantum efficiencies up to 40% have been reported.¹¹³

The synthesis of oxynitridosilicates is frequently performed by treatment of mixtures of Si₃N₄, SiO₂, alkaline earth carbonates and rare earth oxides in N₂ or N₂/H₂ at high temperatures (typically 1400–1600 °C). Silicon diimide has also been used as a source of silicon and nitrogen.¹⁹ Ammonolysis of precursor silicates may produce oxynitridosilicates at lower temperatures (below 1050 °C).¹¹⁴

The oxynitridosilicates (Sr,Ca,Ba)Si₂O₂N₂ doped with Eu²⁺ are highly efficient phosphors showing yellow-green broad emissions (with λ_emiss from 492 to 563 nm) under activation in the UV-blue range.¹¹⁵ They show layered structures where sheets of the alkaline earth cations alternate with double layers of [SiO₂N₂]²⁻ corner shared tetrahedra and composition [Si₂N₂O]. More recently the layered oxynitrides Ba₃Si₆O₁₂N₂ doped with Eu²⁺ have been reported to show green emission with a maximum at 527 nm.¹¹⁶

The structure of recently discovered compounds A₃Si₂O₄N₂ (A = Sr, Ca) are formed by rings of 12 corner-sharing [SiO₂N₂]²⁻ tetrahedra where the alkaline earth cations are placed.^117,118 When activated with Eu²⁺ they show emission wavelengths up to 600 nm under excitation in the UV-blue light range.

The new oxynitride orthosilicates LaMSiO₃N (M = Sr, Ba)¹¹⁹ are isotypic to α′-M₂SiO₄ (M = Sr, Ba) (Fig. 6a) with a β-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 highly efficient phosphors Sr₂SiO₄:Eu or Ba₂SiO₄:Eu,¹²⁰ from ca. 550 nm to 650–700 nm (Fig. 6b).

Fig. 6 (a) Crystal structure of LaMSiO₃N compounds (M = Sr, Ba) showing polyhedra of the two cation sites of the β-K₂SO₄ structure and the isolated [SiO₃N]⁵⁻ tetrahedra. Anions are represented by red (oxygen) and light blue (mixed oxygen–nitrogen) spheres. (b) Excitation (λ_em = 550 nm, dotted line) and emission (λ_ex = 405 nm, solid line) spectra of LaSr_0.95Eu_0.05SiO₃N and Sr_1.95Eu_0.05SiO₄.¹¹⁹

7. Conclusions and future prospects

The precedent sections have shown that recent research on metal oxynitrides has led to important new materials. The consequences of introducing nitride into an oxidic network are relevant on physical and chemical properties because of differences in charge, electronegativity and polarizability of both anions. The higher charge of nitride allows the stabilization of phases with higher oxidation states of cations with respect to the analogous oxides. On the other hand the synthesis conditions of oxynitrides under NH₃ or N₂ at high temperatures are reducing and may allow the stabilization of low oxidation states. The diversity of new materials increases with the possibility to tune the degree of anion order which may affect the properties. For electric/magnetic applications the sinterability and chemical homogeneity of ceramics are still issues because of the possibility of domains with a different N/O ratio and the limited thermal stability of some transition metal oxynitrides. High pressure methods have been used to solve this problem and have been useful to isolate new quaternary or multinary phases of late transition metals.²³ These are still scarce because the higher electronegativity of metals like Fe, Mn or Co promotes the formation of interstitial binary nitrides under the reducing conditions of N₂ or under NH₃ at high temperatures. The development of new methods of preparation of thin films with defined compositions and nano/microstructures is also a challenge.

The developments in the last few years have been more active in the reviewed applications in photocatalysis, dielectrics, magnetic materials and solid-state lighting, but the results in other important fields are promising. For instance the introduction of nitrogen into the cathode material Li₂FeSiO₄ has been predicted to decrease the lithium de-insertion voltage associated with the Fe³⁺/Fe⁴⁺ redox couple,⁹ and the oxynitrido phosphates Na₂Fe₂P₃O₉N and Li₂Fe₂P₃O₉N have been recently suggested as cathode materials for sodium and lithium batteries.¹²¹ The growth of new groups in the field and the improvement of synthetic approaches are expected to fuel new applications and materials in the near future.

Acknowledgements

This work was supported by the Spanish Ministerio de Economía y Competitividad, Spain (Project MAT2011-24757 and MAT2014-53500-R).

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