Defect Landscape of Orthorhombic Ba$_2$In$_2$O$_5$ from First-Principles Calculations: The Role of Oxygen Interstitials

TL;DR

This paper establishes a first-principles defect thermodynamics framework for orthorhombic BIO, focusing on vacancies, oxygen interstitials, Frenkel pairs, and Cr doping. Using DFT with PBE and selective HSE06 benchmarking, it computes defect formation energies, charge transition levels, and self-consistent carrier concentrations as a function of the oxygen chemical potential and temperature, connecting to experimental p_O2 via the relation μ_O(T,p_O2). The results show that V_O and O_i dominate the intrinsic defect landscape, with neutral O_i dumbbells and charged interstitials emerging under different conditions; Cr_In donors are predicted but negative formation energies point to Cr-rich secondary phases, suggesting possible phase evolution under Cr doping. The study demonstrates how p_O2 controls defect populations and μ_e, providing a roadmap for defect and doping engineering in BIO and guiding future investigations into the tetragonal phase and diffusion barriers for oxygen defects.

Abstract

The brownmillerite-type oxide Ba$_2$In$_2$O$_5$ (BIO) is a potential candidate as an electrolyte material for mixed ionic-electronic conduction in solid oxide fuel cells. Despite its structural relation to perovskite oxides, the defect chemistry of BIO has remained largely unexplored. Using Density Functional Theory within the generalized gradient approximation, complemented by selected hybrid-functional calculations, we evaluate the formation energies, charge transition levels, and concentrations as a function of oxygen partial pressure of vacancies, oxygen interstitials, Frenkel pairs, and substitutional Cr doping. Our results reveal that oxygen vacancies and interstitials dominate the intrinsic defect landscape. Among the interstitials, we identify stable dumbbell configurations that remain neutral across the entire band gap. Other interstitial configurations show charged states and become the prevailing compensating defect at high oxygen partial pressures. For extrinsic doping, we find that Cr preferentially substitutes at the tetrahedral In site and behaves as a donor. Its negative formation energies suggest the formation of secondary phases, possibly the tetragonal Cr-doped BIO. These results provide a first picture of the thermodynamics of intrinsic and extrinsic defects in BIO and set the stage for future investigations into the tetragonal phase and the diffusion dynamics of oxygen vacancies and interstitials.

Figures (10)

• Figure 1: Crystal structure of orthorhombic BIO with 36 atoms. Ba, In, and O atoms are identified in green, pink, and red, respectively. The three distinct oxygen sites and the two distinct indium sites are marked.
• Figure 2: Band structure and density of states of orthorhombic BIO (space group Ibm2) computed with the PBE functional.
• Figure 3: Stability diagram of the ternary Ba-In-O system, as derived from the data in Figure \ref{['tab:pd_form_en']}. The defect formation energies are discussed in terms of the chemical potentials at points A to D and point X as reported in Table \ref{['tab:mu_values']}.
• Figure 4: Defect formation energies as a function of the Fermi level position for representative thermodynamic conditions (shown in Figure \ref{['fig:sd']}). The stars on the formation energy curves represent charge transitions.
• Figure 5: Charge transition levels for the defects considered in this work. The energy levels in the valence band (VB) and conduction band (CB) are indicated by the grey shaded areas.