Thermodynamic Stability and Anion Ordering of Perovskite Oxynitrides

Abstract

Perovskite oxynitrides (PONs) are a promising class of materials for applications ranging from catalysis to photovoltaics. However, the vast space of single PON materials (ABO3–x N x ) has yet to be fully explored. Additionally, the community needs guidelines that relate PON chemistry and anion ordering to stability to better understand how to design PON materials that resist corrosion and decomposition under operating conditions. Screening this materials space requires identifying candidate PON materials that will be stable under operating conditions, which in turn requires methods to evaluate each material’s stability. Here, we predict the stability of single PON materials using a four-step approach based on density functional theory modeling: (i) enumerate viable cation pairs, (ii) select an energetically favorable prototypical anion ordering, (iii) compute each PON’s energy above the thermodynamic convex hull, and (iv) generate computational Pourbaix diagrams to determine allowable ranges of electrochemical operating conditions. A critical part of our approach is determining a prototypical stable anion ordering for both ABO2N and ABON2 stoichiometries across a variety of A- and B-site cations. We demonstrate a stable anion ordering containing a high degree of cis ordering between B cations and minority-composition anions. We predict 85 stable and 109 metastable PON compounds, with A ∈ {La, Pb, Nd, Sr, Ba, Ca} and B ∈ {Re, Os, Nb, Ta} forming cation pairs that lead to stable PONs less than 10 meV/atom above the thermodynamic convex hull. Computational Pourbaix diagrams for two stable candidates, CaReO2N and LaTaON2, suggest that not all compounds with zero energy above the thermodynamic convex hull can be easily synthesized.

Investigation of O/N Ordering in Perovskite-Type Oxynitrides La1−xYxTa(O,N)3 on Long Range and Short Scale

“…This type of ordering has also commonly been observed in the more extensively studied SrTaO 2 N, where there is a preferential partial ordering with the -B-(O,N)-bonds forming cis-oriented chains in the equatorial planes of the BO 4 N 2 octahedra [5,8,21,22].…”

“…Many oxynitrides, as in the case of LaTa(O,N) 3 , have a perovskite-type crystal structure and characteristic formulas such as ABO 2 N and ABON 2 , in which A is usually a lanthanide, an alkaline, or an alkaline earth cation, and B is a transition metal cation. In the case of a cubic lattice, cation A is coordinated by 12 anions, and cation B is coordinated by six anions, forming a corner-sharing octahedral B(O,N) 6 building block [2,8]. Whether the anions in the ABO 2 N and ABON 2 structures are ordered, and if so, what that order is, remain key questions that are being more and more investigated nowadays, as they can largely alter physical properties [3,4].…”

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

“…In DFT simulations, identifying the most stable surface and surface structure of oxynitrides requires exploring a larger structural space compared to determining the most stable surface and surface structure of oxides. 114–116 Some computational studies have shown that cations can influence the anion order. 117 Different anion orders can lead to significant differences in properties related to catalytic activity, such as optical properties, hole effective mass, and overpotential.…”

“…58,59,122–124 High-throughput screening combined with DFT calculations can be used to explore the vast space of chemical compositions and structures, and to rapidly screen for potential oxynitrides with optimal properties for a given chemical composition. 116,125…”

A Bayesian method for selecting data points for thermodynamic modeling of off-stoichiometric metal oxides

“…The cyclic reduction/re-oxidation of metal oxide (M x O y ) materials forms the backbone of a vast number of chemical processes, such as gas reforming, 1,2 gas separation and pumping, 3–9 energy production and storage, 10–17 and many catalytic processes, particularly those which operate on Mars-van Krevelen-like mechanisms. 18–24 The performance of these materials is directly related to their partial molar reduction thermodynamics, i.e. enthalpies and entropies of reduction as a function of reduction (M x O y − δ ).…”