Oxynitrides have been explored in recent times for applications like photocatalysis, dielectrics and energy storage devices like supercapacitors. However, they have been reported to have defects and non-stoichiometry during synthesis and post-processing treatments. These defects and non-stoichiometry have been found to impact the functional properties significantly. Hence understanding on these defects is essential.
In this work, ABO2N (A = Ba, Ca, Sr and B = Nb, Ta) perovskite oxynitride systems have been chosen as the material of interest. The current work is aimed at gaining insights on the role of synthesis conditions on the formation of defects and non-stoichiometric phases along with analysing the defect chemistries of these systems. To begin with, a molecular dynamics (MD) study has been carried out on BaTaO2N nanoparticles (2-20 nm) as this material is interesting for both dielectric and photocatalytic applications. This study is mainly focused on the effect of sintering temperatures on stoichiometry of BaTaO2N. A combination of bond valence derived Morse and Coulomb potentials has been used for the simulation. It is found that, even upon heat treatment, stoichiometry of the nanoparticles can be preserved if the particle size is 10nm. This is consistent with experimental observation of TaO phases that are formed after sintering of BaTaO2N.
To further support the above results, thermo-physical properties like specific heat capacity and diffusion constants have been calculated for BaTaO2N nanoparticles. Cp Vs. T shows additional features with size-reduction which are due to the surface vibration modes and formation of vacancies. With increasing size, the average diffusion of atoms decreases due to reduced contribution from surface diffusion and associated mechanisms. This variation is found to be less for tantalum at lower temperatures ( 2000 K). The sluggish behaviour of tantalum is due its high bond dissociation energy with oxygen and nitrogen. This also correlates with the sintering results that show the emergence of tantalum rich secondary phases. These studies can also be extended to other ABO2N oxynitrides.
Defect chemistry studies have been carried out using Mott-Littleton approach. The formation energy of oxygen vacancies has been used as a rough estimate for categorizing ABO2N systems into different applications. Further work involves the calculation of migration energy of oxygen vacancies in these systems to validate the results obtained in the previous step. The defect studies will give insights on modulating surface-active sites, charge carrier mobilities and properties based on ionic conductivity.