Analysis of A₂NX (A = Ca, Sr, Ba) Mixed Anion Nitrides as Thermoelectric Materials

Anion mutation is a promising method to design mixed anion compounds. Varying the ions can tune the charge, ionic radii, ionic mass, electronegativity and polarisability, offering a new dimension to control electronic and physicochemical properties.[1-2] This phenomenon is inaccessible to a single-anion analogue. Due to the bonding heterogeneity, some mixed-anion compounds show ultralow lattice thermal conductivity and are proposed as promising thermoelectric materials.[3-5] In this work, we investigate electronic and thermal transport properties of a series of alkaline-earth metal mixed anion compounds Ca₂NBr, Ca₂NCl, Sr₂NBr and Ba₂NBr.
These four compounds show a diverse array of characteristics. We analyse the LO-TO splitting using phonon dispersions and ionicity via the Integrated Crystal Orbital Bond Index (ICOBI) and electronegativity difference analysis, showing Ba₂NBr to be a particularly efficient thermoelectric. The in-plane and out-of-plane lattice thermal conductivity of Ba₂NBr at 300 K are predicted to be 1.289 and 1.090 and at 1000 K are 0.393 and 0.327 W m^-1 K^-1. Via modal analysis we take an in-depth look at the interplay between these characteristics, among others, in order to elucidate a way forward for the anion mutation framework in thermoelectrics.