Synthesis and crystal structure of novel Ruddlesden–Popper strontium niobium oxynitrides

The family (SrO)(SrNbO_2-xN)_n is the first example of Ruddlesden-Popper strontium niobium oxynitrides and provides layered structures to obtain mixed valence niobium compounds with a variety of oxidation states and transport properties. We have prepared members with n=1 (Sr₂NbO₃N) and n=2 (Sr₃Nb₂O₅N₂) by solid state reaction of Nb₂O₅ and SrCO₃ at 900–1050 °C under ammonia flow. These compounds crystallize in the I4/mmm space group and show a range of anionic stoichiometry that together with the N/O ratio determines the formal oxidation state for niobium and therefore the resultant physical properties. Samples of the n=1 member show mixed valence Nb⁵⁺/Nb⁴⁺ and are paramagnetic and very resistive at room temperature. This behavior may be understood by considering electron localization due to N/O disorder.