@article{kumar_baker_bowes_cabral_zhang_dickey_irving_lebeau_2021, title={Atomic-resolution electron microscopy of nanoscale local structure in lead-based relaxor ferroelectrics}, volume={20}, url={https://doi.org/10.1038/s41563-020-0794-5}, DOI={10.1038/s41563-020-0794-5}, abstractNote={Relaxor ferroelectrics, which can exhibit exceptional electromechanical coupling, are some of the most important functional materials, with applications ranging from ultrasound imaging to actuators. Since their discovery, their complex nanoscale chemical and structural heterogeneity has made the origins of their electromechanical properties extremely difficult to understand. Here, we employ aberration-corrected scanning transmission electron microscopy to quantify various types of nanoscale heterogeneities and their connection to local polarization in the prototypical relaxor ferroelectric system Pb(Mg1/3Nb2/3)O3–PbTiO3. We identify three main contributions that each depend on Ti content: chemical order, oxygen octahedral tilt and oxygen octahedral distortion. These heterogeneities are found to be spatially correlated with low-angle polar domain walls, indicating their role in disrupting long-range polarization and leading to nanoscale domain formation and the relaxor response. We further locate nanoscale regions of monoclinic-like distortion that correlate directly with Ti content and electromechanical performance. Through this approach, the connections between chemical heterogeneity, structural heterogeneity and local polarization are revealed, validating models that are needed to develop the next generation of relaxor ferroelectrics. Relaxor ferroelectric systems exhibit exceptional electromechanical coupling that arises from a variety of nanoscale chemical ordering. Here, scanning transmission electron microscopy is used to quantify this structural complexity directly.}, number={1}, journal={Nature Materials}, publisher={Springer Science and Business Media LLC}, author={Kumar, Abinash and Baker, Jonathon N. and Bowes, Preston C. and Cabral, Matthew J. and Zhang, Shujun and Dickey, Elizabeth C. and Irving, Douglas L. and LeBeau, James M.}, year={2021}, month={Jan}, pages={62–67} }
 @article{mirrielees_baker_bowes_irving_2021, title={Computational approaches to point defect simulations for semiconductor solid solution alloys}, volume={154}, ISBN={1089-7690}, url={https://doi.org/10.1063/5.0041127}, DOI={10.1063/5.0041127}, abstractNote={Despite their technological importance, studying the properties of alloys with first principles methods remains challenging. In cases of AlxGa1-xN and BaxSrx-1TiO3 (BST), whose most important properties are governed by point defects, explicit simulation can be a computationally demanding task due to the random occupation of Al and Ga on cation sites in AlGaN and Ba and Sr on A-sites in BST. In this work, interpolation between end member compounds is used as a first approximation to defect properties and concentrations in intermediate alloy compositions in lieu of explicit simulation. In AlGaN, the efficacy of Si and Ge as dopants for n-type Al-rich AlGaN is explored by considering self-compensating defects such as multi-donor vacancy complexes and Si and Ge DX configurations. In BST, variation of the high temperature defect chemistry of Mg and Fe is examined. The approach presented here is expected to be generally appropriate for first approximation of defect properties in semiconductors and dielectrics where the alloy is a random solid solution of the end members.}, number={9}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Mirrielees, Kelsey J. and Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2021} }
 @article{ryu_bowes_mcgarrahan_irving_dickey_2021, title={Fermi level pinning in Co-doped BaTiO3: Part I. DC and AC electrical conductivities and degradation behavior}, volume={105}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.18042}, DOI={10.1111/jace.18042}, abstractNote={Abstract We explore the synergistic effects of co‐doping BaTiO 3 with a judicious combination of acceptors and donors to control the point defect chemistry and electrical properties, with the goal of simultaneously limiting the electronic and ionic conductivities over broad temperature and oxygen partial pressure ( p O 2 ) ranges. Specifically, we compare the temperature‐ and p O 2 ‐dependent electrical properties of BaTiO 3 ceramics acceptor‐doped with either Mn or Mg and co‐doped with a Y donor. This study, which is the first of a two‐part series, presents the electrical properties as a function of p O 2 , temperature, and time, focusing on the grain‐interior electrical response. The DC and AC electrical conductivity measurements reveal that co‐doping with Mn and Y can result in (1) increased electrical resistivity over a broad temperature range, (2) p O 2 ‐independent electrical conductivity in oxidizing conditions, and (3) improved time‐dependent dielectric degradation resistance. These behaviors are attributed to a Fermi level pinning effect, as is explained in the companion paper, which presents complementary density functional theory (DFT)‐based grand‐canonical defect chemistry models. The collective experimental and computational studies demonstrate that the p O 2 ‐independent electrical conductivity in the Mn and Y co‐doped BaTiO 3 is attributed to a Fermi level pinning mechanism arising from the multivalent Mn dopant, and the background reservoir of positive charge provided by the predominant substitution of Y on the Ba sites. The enhanced degradation resistance is attributed to a reduced oxygen vacancy concentration relative to the other doping chemistries.}, number={1}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Ryu, Gyung Hyun and Bowes, Preston C. and McGarrahan, John R. and Irving, Douglas L. and Dickey, Elizabeth C.}, year={2021}, month={Jul} }
 @article{bowes_ryu_baker_dickey_irving_2021, title={Fermi level pinning in Co-doped BaTiO3: Part II. Defect chemistry models}, volume={7}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.17938}, DOI={10.1111/jace.17938}, abstractNote={A first-principles informed grand canonical defect chemistry model capable of accounting for non-stoichiometry and partial equilibration of different sub-lattices is developed and used to study Mg and Mn doped, and (Mg+Y) and (Mn+Y) co-doped BaTiO3 to elucidate the role of Mn and Y in improving the resistivity and resistance degradation of BaTiO3 as observed by Ryu et al. in Part I of this series of papers. The model qualitatively captures the behavior of the samples in all conditions, reproducing the observed carrier plateau and increased resistivity of (Mn+Y) co-doped BaTiO3, and expected trends in the concentrations of free oxygen vacancies with doping. These trends reflect the observed differences in degradation characteristics, and help explain the substantially improved degradation resistance of the (Mn+Y) co-doped samples. Our model adds to the mechanism proposed by Yeoh et al. that the Fermi level is pinned by the multivalent character of MnTi in (Mn+Y) co-doped BaTiO3 by giving insight into the role of barium vacancies, the site preferences of the dopants, and defect complexes in this mechanism. These insights provide a set of criteria in the search for sets of co-dopants with similar behaviors.}, number={11}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Ryu, Gyung Hyun and Baker, Jonathon N. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2021}, month={Jul} }
 @article{bowes_wu_baker_irving_2021, title={Modeling the spatial control over point defect spin states via processing variables}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0039972}, DOI={10.1063/5.0039972}, abstractNote={Contemporary models that are used to search for solid-state point defects for quantum-information applications tend to focus on the defect’s intrinsic properties rather than the range of conditions in which they will form. In this work, a first-principles based multi-scale device model is used to explore how the conditions (i.e., growth temperature, doping concentration, unintentional impurity concentration) influence the formation of a neutral aluminum vacancy complexed with an oxygen impurity at a neighboring nitrogen site vAl-1ON in an Si/Mg:AlN homojunction. Varying the donor (Si) concentration is predicted to lead to the greatest change in both the maximum height and shape of the (vAl-1ON)0 profile. The shape is found to depend on the acceptor (Mg) concentration as well, and a critical ratio between the acceptor and unintentional impurities below which the (vAl-1ON)0 center would not form was identified. A detailed analysis of the electrostatic potential, electric field, and defect chemistry obtained with the model was used to reveal the underlying causes of these changes. These results show the potential of varying processing parameters to manipulate the local electronic structure as a means to control the properties of point defects for quantum-information applications.}, number={22}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Jun} }
 @article{wu_bowes_baker_irving_2021, title={Photochromism of UV-annealed Fe-doped SrTiO3}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0068523}, DOI={10.1063/5.0068523}, abstractNote={High-temperature annealing coupled with above bandgap UV illumination is an emerging approach to manipulate defect chemistries and resultant properties of electroceramics. To explore defect-processing-property relationships in these materials, an advanced multiphysics and multiscale model has been developed, which involves (a) high-fidelity first principles simulations of defect energies, (b) grand canonical thermodynamics of defect equilibria, (c) UV-perturbed defect formation energies from Shockley–Read–Hall generation and recombination, and (d) finite-element analyses of electrostatic potential and defect redistribution. Using this model, bottom-up insights into defect mechanisms associated with the UV-induced brown photochromism of Fe-doped SrTiO3 at high temperatures are provided. It is found that UV illumination leads to dissociation of the FeTi-vO complex and reduction in the oxygen vacancy concentration through exchange with the gas reservoir. Changes to these defect populations cause reionization of the FeTi defect from −1 to 0 charge state to maintain charge neutrality. This collectively gives rise to an increased concentration of FeTi0, which is the source of brown chromism. In addition, this model reproduces the experimentally observed electrical resistance degradation of samples annealed in this manner due to the increasing hole concentration in the material with time. The present model itself offers a route to guide and facilitate future efforts in this field.}, number={26}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Dec} }
 @article{baker_bowes_harris_collazo_sitar_irving_2020, title={Complexes and compensation in degenerately donor doped GaN}, volume={117}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0013988}, DOI={10.1063/5.0013988}, abstractNote={Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions.}, number={10}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2020}, month={Sep} }
 @article{wu_bowes_baker_irving_2020, title={Influence of space charge on the conductivity of nanocrystalline SrTiO3}, volume={128}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0008020}, DOI={10.1063/5.0008020}, abstractNote={A grand canonical multiscale space-charge model has been developed to study and predict the electrical properties of polycrystalline perovskites with complex defect chemistries. This model combines accurate data from hybrid exchange-correlation functional density functional theory calculations (defect formation energies, resultant grand canonical calculations of defect concentrations, and ionization states) with finite-element simulation of the electric field and its coupling to defect redistribution and reionization throughout the grain. This model was used to simulate the evolution of the oxygen partial pressure-dependent conductivity of polycrystalline acceptor-doped strontium titanate as the grain size decreases, and the results were compared to previous experiments. These results demonstrate that as the grain size is reduced from the microscale to nanoscale, the experimentally observed disappearance of ionic conductivity and forward shift of the oxygen partial pressure of the n–p crossover are successfully reproduced and explained by the model. Mechanistically, the changes to conductivity stem from the charge transfer from the grain boundary core into the grain interior, forming a space-charge layer near the grain boundary core that perturbs the local defect chemistry. The impact of the grain size on the electrical conductivity and the underlying defect chemistry across the grain are discussed. In addition to the findings herein, the model itself enables exploration of the electrical response of polycrystalline semiconductor systems with complex defect chemistries, which is critical to the design of future electronic components.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Jul} }
 @article{bowes_baker_irving_2020, title={Site preference of Y and Mn in nonstoichiometric BaTiO3 from first principles}, volume={4}, ISSN={["2475-9953"]}, url={https://doi.org/10.1103/PhysRevMaterials.4.084601}, DOI={10.1103/PhysRevMaterials.4.084601}, abstractNote={$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_{3}$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\ensuremath{\circ}}\mathrm{C}$.}, number={8}, journal={PHYSICAL REVIEW MATERIALS}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Aug} }
 @article{bowes_baker_irving_2020, title={Survey of acceptor dopants in SrTiO3: Factors limiting room temperature hole concentration}, volume={103}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.16784}, DOI={10.1111/jace.16784}, abstractNote={Abstract Unintentional impurities often found in strontium titanate (doped or undoped) have hindered efforts to study individual impurities experimentally. To fill this gap, a computational survey of acceptor‐type point defects of common intentional or unintentional impurities (Al, Cu, Fe, K, Mg, Mn, N, Na, Ni, and Zn) is presented. Utilizing defect formation energies from density functional theory using hybrid exchange correlation functionals in a grand canonical model of the defect chemistry, the equilibrium Fermi level ( μ e ) was calculated as a function of processing conditions for pure SrTiO 3 , SrTiO 3 individually doped with each impurity, and SrTiO 3 co‐doped with Al and N. Above a certain concentration, each impurity reduced the maximum predicted hole concentration relative to the intrinsic case. Al, Mg, Zn, K, and Na exhibited similar trends and behaved more like ideal acceptors while N, Ni, Fe, Mn, and Cu were all unique and pinned μ e near or above the mid‐gap in most conditions. Al/N:SrTiO 3 also exhibited similar trends at 800°C for all Al/N ratios, but more variation at 25°C. Additionally, the behavior of Al:SrTiO 3 was not recovered until Al/N = 10 4 . This suggests that to achieve SrTiO 3 with free holes at room temperature, the concentration of most impurities must be controlled.}, number={2}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Feb}, pages={1156–1173} }
 @article{baker_bowes_harris_irving_2019, title={An informatics software stack for point defect-derived opto-electronic properties: the Asphalt Project}, volume={9}, ISSN={["2159-6867"]}, url={http://dx.doi.org/10.1557/mrc.2019.106}, DOI={10.1557/mrc.2019.106}, abstractNote={Computational acceleration of performance metric-based materials discovery via high-throughput screening and machine learning methods is becoming widespread. Nevertheless, development and optimization of the opto-electronic properties that depend on dilute concentrations of point defects in new materials have not significantly benefited from these advances. Here, the authors present an informatics and simulation suite to computationally accelerate these processes. This will enable faster and more fundamental materials research, and reduce the cost and time associated with the materials development cycle. Analogous to the new avenues enabled by current first-principles-based property databases, this type of framework will open entire new research frontiers as it proliferates.}, number={3}, journal={MRS COMMUNICATIONS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Sep}, pages={839–845} }
 @article{baker_bowes_harris_irving_2019, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3 (vol 124, 114101, 2018)}, volume={125}, ISSN={["1089-7550"]}, DOI={10.1063/1.5084251}, abstractNote={First Page}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jan} }
 @article{bowes_wu_baker_harris_irving_2019, title={Space charge control of point defect spin states in AlN}, volume={115}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5099916}, DOI={10.1063/1.5099916}, abstractNote={One barrier to developing quantum information systems based on impurity point defects is that the desirable spin states of the defects are often unstable for Fermi levels obtained at increased impurity concentrations. The space charge induced band bending near the interface of Si/Mg aluminum nitride (AlN) homojunction is investigated computationally as a method to control the concentration, spin state, and position of such point defects. This is done by solving Poisson's equation with the charge density described by a grand canonical defect chemistry model informed by hybrid-functional density functional theory (DFT) calculations. Previous experimental works have found unintentional carbon and oxygen impurities pervade AlN homojunctions. First principles calculations have predicted the neutral complex between an aluminum vacancy and oxygen impurity on a neighboring nitrogen site (vAl-1ON)0 has a spin triplet configuration, which is stable in a region when the Fermi level is below midgap. From defect equilibrium simulations considering 602 possible defects, vAl-1ON was found to be unstable on the Mg-doped side of the homojunction and isolated oxygen impurities are preferred. On the Si-doped side, vAl-1ON forms but as (vAl-1ON)–2, not (vAl-1ON)0. This makes vAl-1ON a prototypical test case for the proposed strategy. Simulations of the Si/Mg:AlN homojunction showed (vAl-1ON)0 is stabilized within 6 nm of the interface in the Si-doped portion. This result indicates space charge induced band bending enables control over the concentration, spin state, and position of point defects, which is critical to realizing point defect based quantum information systems.}, number={5}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jul} }
 @article{long_cai_baker_bowes_bayer_wang_wang_chen_randall_irving_et al._2018, title={Conductivity of iron‐doped strontium titanate in the quenched and degraded states}, volume={102}, ISSN={0002-7820 1551-2916}, url={http://dx.doi.org/10.1111/jace.16212}, DOI={10.1111/jace.16212}, abstractNote={The electrical behavior of iron-doped strontium titanate (Fe:SrTiO3) single crystals equilibrated at 900°C and quenched below 400°C at various oxygen partial pressures () was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO3 annealed and quenched between 1.2 × 10−14 and 2.0 × 10−4 Pa exhibits a conduction activation energy (EA) around 0.6 eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in EA are found to either side of this range; a transition from 0.6 to 1 eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23 eV is found in reducing These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO3 provides insight into the origins of the conductivity activation energies observed in those samples.}, number={6}, journal={Journal of the American Ceramic Society}, publisher={Wiley}, author={Long, Daniel M. and Cai, Biya and Baker, Jonathon N. and Bowes, Preston C. and Bayer, Thorsten J.M. and Wang, Jian‐Jun and Wang, Rui and Chen, Long‐Qing and Randall, Clive A. and Irving, Douglas L. and et al.}, year={2018}, month={Dec}, pages={3567–3577} }
 @article{baker_bowes_irving_2018, title={Hydrogen solubility in donor-doped SrTiO3 from first principles}, volume={113}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5047793}, DOI={10.1063/1.5047793}, abstractNote={Hydrogen contamination of strontium titanate (STO) during processing and usage is a known problem. However, it is relatively little-studied due to the difficulty in quantifying the amount of hydrogen that dissolves in the lattice. Here, we use hybrid exchange-correlation density functional theory calculations as input to a grand canonical thermodynamics framework to estimate hydrogen solubility and site preferences in donor-doped STO. Our results provide clear theoretical evidence that hydrogen contamination in donor-doped STO occurs at a low enough level to essentially ignore. But, this simple conclusion belies hydrogen's rich behavior; unlike many dopants, it is able to easily change its incorporation site in response to changes in processing conditions. Overall, the findings are consistent with prevailing wisdom and suggest that the presented first principles approach could be used for systematic exploration of hydrogen's impact as a function of doping and processing in this and other wide bandgap materials.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @article{bowes_baker_harris_behrhorst_irving_2018, title={Influence of impurities on the high temperature conductivity of SrTiO3}, volume={112}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5000363}, DOI={10.1063/1.5000363}, abstractNote={Abstract}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Bowes, Preston C. and Baker, Jonathon N. and Harris, Joshua S. and Behrhorst, Brian D. and Irving, Douglas L.}, year={2018}, month={Jan} }
 @article{baker_bowes_harris_irving_2018, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3}, volume={124}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5044746}, DOI={10.1063/1.5044746}, abstractNote={Barium titanate (BTO) and strontium titanate (STO) are often treated as close analogues, and models of defect behavior are freely transferred from one material to the other with only minor modifications. On the other hand, it is often reported that B-site vacancies (vB) are the dominant metal vacancy in BTO, while A-site vacancies (vA) dominate in STO. This difference precludes the use of analogous defect models for BTO and STO, begging the question: how similar are the defect chemistries of the two materials? Here, we address this question with density functional theory calculations using a state-of-the-art hybrid exchange correlation functional, which more accurately describes the electronic structure and charge localization than traditional functionals. We find that vA is the dominant metal vacancy in STO but that different combinations of vA, vB, and vB-vO complexes are present in BTO depending on processing and doping. Mechanistically, this occurs for two reasons: thermodynamic differences in the accessible processing conditions of the two materials and energy differences in the bonds broken when forming the vacancies. These differences can also lead to widely differing responses when impurity dopants are intentionally added. Therefore, the response of metal vacancy behavior in BTO and STO to the inclusion of niobium and iron, two typical dopants in these systems, is examined and compared.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @article{baker_bowes_long_moballegh_harris_dickey_irving_2017, title={Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles}, volume={110}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4978861}, DOI={10.1063/1.4978861}, abstractNote={To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Long, Daniel M. and Moballegh, Ali and Harris, Joshua S. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2017}, month={Mar}, pages={122903} }
 @article{xu_bowes_grimley_irving_lebeau_2016, title={In-situ real-space imaging of single crystal surface reconstructions via electron microscopy}, volume={109}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.4967978}, DOI={10.1063/1.4967978}, abstractNote={Crystal surfaces are sensitive to the surrounding environment, where atoms left with broken bonds reconstruct to minimize surface energy. In many cases, the surface can exhibit chemical properties unique from the bulk. These differences are important as they control reactions and mediate thin film growth. This is particularly true for complex oxides where certain terminating crystal planes are polar and have a net dipole moment. For polar terminations, reconstruction of atoms on the surface is the central mechanism to avoid the so called polar catastrophe. This adds to the complexity of the reconstruction where charge polarization and stoichiometry govern the final surface in addition to standard thermodynamic parameters such as temperature and partial pressure. Here we present direct, in-situ determination of polar SrTiO3 (110) surfaces at temperatures up to 900 C using cross-sectional aberration corrected scanning transmission electron microscopy (STEM). Under these conditions, we observe the coexistence of various surface structures that change as a function of temperature. As the specimen temperature is lowered, the reconstructed surface evolves due to thermal mismatch with the substrate. Periodic defects, similar to dislocations, are found in these surface structures and act to relieve stress due to mismatch. Combining STEM observations and electron spectroscopy with density functional theory, we find a combination of lattice misfit and charge compensation for stabilization. Beyond the characterization of these complex reconstructions, we have developed a general framework that opens a new pathway to simultaneously investigate the surface and near surface regions of single crystals as a function of environment.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Xu, Weizong and Bowes, Preston C. and Grimley, Everett D. and Irving, Douglas L. and LeBeau, James M.}, year={2016}, month={Nov} }