Nitride tuning of transition metal perovskites Cite as: APL Mater. 8, 020903 (2020); https://doi.org/10.1063/1.5140056 Submitted: 25 November 2019 . Accepted: 21 January 2020 . Published Online: 10 February 2020 Amparo Fuertes COLLECTIONS Paper published as part of the special topic on New Perspectives on Emerging Advanced Materials for Sustainability Note: This paper is part of the Special Issue on New Perspectives on Emerging Advanced Materials for Sustainability. 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APL Materials PERSPECTIVE scitation.org/journal/apm Nitride tuning of transition metal perovskites Cite as: APL Mater. 8, 020903 (2020); doi: 10.1063/1.5140056 Submitted: 25 November 2019 • Accepted: 21 January 2020 • Published Online: 10 February 2020 Amparo Fuertesa) AFFILIATIONS Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain Note: This paper is part of the Special Issue on New Perspectives on Emerging Advanced Materials for Sustainability. a)Author to whom correspondence should be addressed: [email protected] ABSTRACT Transition metal perovskite oxynitrides are emergent materials for applications as visible light-active photocatalysts for water splitting and CO2 reduction and as thermoelectric, dielectric, and magnetic materials. They have been reported for early transition metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W in the B sites and alkaline earth or rare earth metals in the A sites. Nitrogen is less electronegative and more polarizable than oxygen, and nitride is more charged than oxide. As a consequence, the introduction of nitride in an oxidic perovskite has important effects on the covalency of bonds, the energy of electronic levels, and the valence states of the cations. This work discusses fundamental and recent developments of perovskite oxynitrides of transition metals of groups 4, 5, and 6 as photocatalytic and electronic materials, focusing on the important aspects of synthetic methodologies, crystal structures, and anion ordering, in connection with the observed physical and chemical properties. Some examples of compounds with late transition metals and complex structures such as layered and double perovskites are also discussed. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5140056., s I. INTRODUCTION to the visible light.9 The higher charge of N3− increases the ionic polarizability and the dielectric constant. The possibility of for- Transition metal perovskite oxides are of permanent inter- mation of permanent electric dipoles8 is affected by the order of est because they show innumerable important properties and the two anions,10–13 which also has an impact on the bandgap14 applications. Their chemical diversity may be expanded by mod- and other relevant properties. To compensate the −3 charge ifying the anion composition with the introduction of nitride of nitride, the oxidation states of the transition metals have that shows electronic and crystal chemical characteristics close to be high. They should be stable in the reducing conditions 1 to oxide. Nitrogen is less electronegative and more polarizable under NH3 or at high temperatures under N2 flow that are than oxygen, and nitride is more charged than oxide. These used for the synthesis of oxynitrides. These two requirements crucial differences between the two anions are the important have directed the majority of explored compositions of perovskite factors affecting the chemical and physical properties of per- oxynitrides to early transition metals from groups 4, 5, and 6, ovskite oxynitrides ABO3−xNx (A = alkaline earth or rare earth which have stable high valence states and show relative low metal; B = transition metal). They have been investigated for electronegativities. several important applications, more intensively as water split- The most common method for the synthesis of perovskite ting photocatalysts active under visible light for hydrogen gen- oxynitrides is ammonolysis of ternary oxide precursors.15,16 2 3 eration and as materials with high dielectric constants. Per- The direct solid state reaction under NH3 between individual ovskite oxynitrides also show remarkable applications as non-toxic metal oxides or salts has been used for perovskites contain- pigments4 and as thermoelectric,5 magnetic,6 and ferroelectric ing alkaline earth cations, and also high temperature reactions 7,8 materials. between nitrides, oxynitrides, and oxides under N2 have been When substituting oxide in a semiconductor, nitride introduces performed.17 2p states at the top of the valence band, decreasing the bandgap Research in perovskite oxynitrides was initiated in the 1970s and affecting, for instance, the photocatalytic activity that shifts by Marchand and co-workers, who reported the first example of APL Mater. 8, 020903 (2020); doi: 10.1063/1.5140056 8, 020903-1 © Author(s) 2020 APL Materials PERSPECTIVE scitation.org/journal/apm 18 layered Nd2AlO3N with K2NiF4-type structure and, later, the alka- line earth tantalum and niobium perovskites BaTaO2N, BaNbO2N, 15,19 SrTaO2N, and CaTaO2N. The interest in the field was stim- ulated in 2000 with the publication in Nature by Jansen and Letschert4 of new non-toxic red-yellow pigments in the solid solution Ca1−xLaxTaO2−xN1+x and, later, with other early reports from different groups of new compounds with additional appli- cations.3,20–22 Research during the first decade after these ini- tial reports was reviewed by Ebbinghaus et al. in 201023 and Fuertes in 2012.24 This perspective aims at giving a present overview of the reported compounds in the three groups of tran- sition metals, identifying new challenges, and focusing on rele- vant aspects and progress in the synthesis pathways, important structural features such as anion order, and chemical and physical properties. II. TITANIUM, ZIRCONIUM, AND HAFNIUM COMPOUNDS A. Titanium Within the group 4, the titanium compounds are the most FIG. 1. Crystal structure of the perovskite SrTaO N29 projected along (a) [001] and investigated because of their photocatalytic activity and dielec- 2 (b) [010] directions of the superstructure 2ap × 2ap × 2ap of the perovskite sub- tric properties. LaTiO2N and NdTiO2N were first prepared by cell (of parameter a ) showing N/O occupancies determined by neutron diffraction. ○ p Marchand et al. by ammonolysis at 900 C of Ln2Ti2O7 (Ln = La, Oxygen, nitrogen, strontium, and tantalum atoms are represented as white, black, 25 Nd). For Ln = La, another method treating this oxide and urea golden, and blue spheres, respectively. SrTaO2N has an apparent I4/mcm space 26 group at room temperature because of tilting, but N/O order lowers the tetrago- under N2 has also been suggested. The crystal structure of the lanthanum compound was initially reported to be triclinic I-117 nal symmetry to Fmmm. (c) N/O order in ABO2N perovskite oxynitrides showing cis 11 from the refinement of neutron powder diffraction data. Further disordered N-M-N zig-zag chains (blue line) confined in planes. investigation on a highly crystalline sample prepared by a flux method indicated the orthorhombic symmetry Imma, using elec- tron diffraction and refinement from synchrotron and neutron pow- Highly sintered ceramics of this perovskite cannot be obtained 27 ○ der diffraction data. The distribution of nitride and oxide was because it decomposes completely at 1100 C under N2 into a mix- 41 found totally disordered in the two studies, and more recently as ture of LaTiO3, La2O3, and TiN. Using DFT calculations, spon- partially ordered28 with occupancies in agreement with a cis con- taneous polarization has been predicted in thin films, induced by 42 figuration of nitrides as suggested for2 SrTaO N, SrNbO2N, and epitaxial strain on the anion order. 11,29,30 43,44 other perovskite oxynitrides (Fig. 1). Recent density func- The compounds LnTiO2N with Ln = Nd, Ce, Pr, and tional theory (DFT) calculations have indicated that whereas a Eu45 have also been also investigated for their photocatalytic and N-cis configuration is more stable in bulk2 LaTiO N, a non-polar electronic properties. The four oxynitrides show the orthorhom- + − − trans arrangement of nitrides is preferred at the (001) surface of bic GdFeO3-type structure with tilting system a b b and space this perovskite.31 group Pnma, with a disordered distribution of nitride and 22,43 LaTiO2N with a bandgap of 2.1 eV and an absorption edge oxide. In the Ce and Pr compounds, short range anion of 600 nm is one of the most studied perovskite oxynitrides since order has been suggested from electron diffraction experiments. the first report by Domen et al. on its ability to split water under NdTiO2N, CeTiO2N, and PrTiO2N show bandgaps in the range of 32–34 visible light in the presence of sacrificial agents. It showed a 2.0–2.1 eV, close to LaTiO2N. The photocatalytic activity in water low apparent quantum yield that was improved significantly using oxidation of NdTiO2N in the presence of a sacrificial agent is 35,36 CoOx as co-catalyst. The photocatalytic activity improved in similar to that of the lanthanum compound. However, the Pr samples obtained by treatment of La2Ti2O7 crystals in NH3 using and Ce compounds show lower activities, which is ascribed to 37 different fluxes. LaTiO2N is also active as photocatalyst under the presence of localized f-orbital states in the bandgap or near visible light for the reduction of CO2 into CH4 in the presence the valence band maximum that act as electron–hole recombi- 38 44 of H2O.