Mixed anion oxides are emerging materials showing a variety of physical and chemical properties. Among them oxynitrides are widely investigated because of important photocatalytic, dielectric, luminescent and electronic properties. Nitrides show more positive free energies of formation than oxides because of the higher stability of N2 molecule with respect to O2 and the unfavourable electron affinity of nitrogen compared to oxygen. However the stability of oxynitrides is higher than for nitrides, and they easily form from oxides in presence of reactive gases as NH3.
The notable development of new oxynitride materials in the last years is a consequence of the improving of synthetic methodologies. Research in the field is increasingly showing that despite the lower thermodynamic stability of nitrides the N3- anion can be easily stabilized in any structural type shown by oxides. Nitriding in ammonolysis reactions is governed by kinetic factors and for more reducible cations the oxynitrides are isolated as metastable compounds that with prolonged time decompose into complex oxides and binary metal nitrides. Recent examples of solution methods in supercritical ammonia show the obtention of highly crystalline powders and small single crystals. The development of crystal growth methods is challenging but necessary to provide large single crystals for measuring physical properties. Within the group of transition metals the majority of oxynitrides have been reported for the groups 4, 5 and 6. Among them the tantalum compounds are the most investigated because of their photocatalytic and dielectric properties. The stabilization of more reducible later transition metals is difficult under the conventional ammonolysis conditions used for Ti, Zr or Ta oxynitrides. However new oxynitrides of more electronegative Cr, Fe, Mn or Zn have been recently prepared at lower temperatures from adequate precursors in the ammonolysis reaction (Figure) or under high pressure. These show interesting electronic properties such as antiferromagnetism (for LnCrO3-xNx and Sr2FeWO5N), ferromagnetism (for Sr2FeMoO5N) and helicoidal spin order (for MnTaO2N).
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
Synthetic approaches in oxynitride chemistry
Progress in Solid State Chemistry 51, 63-70 (2018)
References in the text:
 Ceravola R, Oró-Solé J, Black AP, Ritter C, Puente Onrech I, Mata I, Molins E, Frontera C, Fuertes A. Topochemical synthesis of cation ordered double perovskite oxynitrides. Dalton Trans., 46, 5128-5132 (2017).