Controlling nitrogen incorporation in (Eu,Ba,Ca)TiO$_3$-derived perovskite-type oxynitrides through Fermi Level Engineering
Defect states directly influence not only functional properties, but also oxynitride formation during ammonolysis. Along with structural stability and charge balance, these defects can cause non-uniform nitrogen incorporation. To address this, the recently proposed Fermi Level Engineering concept provides a powerful alternative of rationalising oxynitride synthesis. By using the electron chemical potential, i.e., the Fermi level, as a unique descriptor, this approach can predict, understand, and manipulate defect formation enthalpies and charge transition levels, subsequently allowing to incorporate defect-property correlations to the concept. This work combines theoretical modelling and experimental characterisation of barium titanium oxynitrides to explain an intrinsic, self-limiting nitridation process. Progressive nitrogen incorporation causes a Fermi level downward shift, leaving further nitrogen substitution on oxygen sites energetically unfavourable, up to 0.03 N per unit formula. We demonstrate that A-site cation substitution in Eu$_{1-x}$Ba$_x$TiO$_3$- and Eu$_{1-x}$Ca$_x$TiO$_3$-derived oxynitrides can be applied to control compensation of the additional negative charge from N$^{3-}$ and keep the Fermi level high. The Eu$^{2+/3+}$ redox couple enables substitution of oxygen by nitrogen through pinning of the Fermi level at the europium charge transition level, at $\sim$ 2.9 eV above the VBM. This allows for higher nitrogen uptake of up to 0.65 per formula unit (taken from hot gas extraction) through a more controlled process. Further incorporation is then limited by an intra-Ti(O,N)$_6$ octahedra distortion due to internal bond-angle variance. These findings demonstrate that Fermi Level Engineering provides a pathway to predict and overcome intrinsic synthetic and thermodynamic limitations in oxynitride synthesis.