All inorganic materials contain point defects in the form of impurities, interstitials, or vacancies. Common imaging techniques fail to capture these defects because they typically substitute into a crystal lattice at parts per million concentrations, well below practical detection limits. It is important to be able to effectively characterize the concentration and nature (valence state, site of substitution, local structure, etc.) of point defects because they often play a primary role in determining the functional properties of a material. Traditional techniques that are used to measure point defects (electron diffraction and absorption) are best suited for high defect concentrations (typically 1 mol% or greater). Trace concentrations (less the 1 mol%) are always present in materials and can be introduced by intrinsic or extrinsic mechanisms. Electron paramagnetic resonance (EPR) spectroscopy is a characterization tool best suited for trace defect concentrations. In the NIST Materials Structure and Data Group, the measurement of point defect chemistry in complex oxides is being advanced by the development of new EPR characterization techniques, and important connections are made between these measurements and complementary advancements in detection of dilute point defects analyzed using electron microscopy and x-ray absorption.
Project Goal: To develop magnetic resonance, x-ray absorption, electron diffraction, and electrical conductivity measurements to better characterize dilute concentrations of point defects in oxide materials and effectively correlate electro-mechanical properties to measured defect chemistry.
Oxide materials are used in diverse applications that include random access memory, solid oxide fuel cells, solid state refrigeration, and micro-actuators to name only a few. Because these applications require an electric bias or mechanical stimulus, ion diffusion is a prevalent process (typically in the form of vacancy diffusion). Fast-ion diffusion can be a desired or engineered phenomenon in the case of oxide fuel cells or oxygen sensors, or ion diffusion can result in local changes to composition resulting in a negative effect on performance and reliability over time. Accurate measurement of the mobility and local interaction of ionic charge carriers in a given system is dependent on effective analysis of the underlying defect chemistry. Better measurement of defect chemistry will facilitate advanced prediction of properties and help design improved materials for a given application.
Materials Measurement Laboratory (MML) and the Center for Nanoscale Science and Technology (CNST) staff published the first experimental evidence of the ability to control the valence and location of a transition metal ion, incorporated as a dopant in a perovskite oxide, by finely tuning the stoichiometry of the host material. The electronic properties of wide-band gap perovskite oxides, which find applications as dielectrics, piezoelectrics, ionic conductors, etc., can be critically influenced by small concentrations of impurities. Common impurities include naturally-abundant transition-metal and alkali ions that are often found at the parts per million level in the raw materials used to synthesize functional oxides. The electrical neutrality condition dictates that the unbalanced charge, introduced by these ions, must be compensated by other charge carriers, such as electrons, holes, or charged vacancies. These extrinsic charge compensation mechanisms have a dominant impact on the functional properties of the material.
The behavior of transition-metal dopants in perovskites is particularly difficult to predict because these ions can assume multiple oxidation states (number of electrons gained by a metal atom chemically bonded to oxygen), which affects their radii and location in the structure. The type of lattice site occupied by the dopant and its oxidation state determine whether the material is a dielectric, an n- or p-type semi-conductor, an ionic conductor, or an electronically conductive material. MML and CNST researchers synthesized a carefully designed set of manganese-doped SrTiO3 samples and used electron paramagnetic resonance (EPR) measurements to demonstrate that the dopant-ion oxidation state, location in the lattice, and charge compensation mechanism can be controlled by slightly modifying the host-lattice stoichiometry.
Specifically, by adjusting the Sr/Ti ratio, the manganese dopant could be substituted as Mn2+ onto the 12-fold coordinated Sr2+ site in a slightly Ti-rich composition or as Mn4+ onto the 6-fold coordinated Ti4+ site in a slightly Sr-rich material. Manganese in SrTiO3 appeared to act as an amphoteric dopant, adjusting its own charge to provide an overall net electrical neutrality with suppressed concentrations of mobile charge carriers. As a prototypical material, SrTiO3 has been the subject of numerous studies, which yielded a wide range of declaredly intrinsic electronic properties. All materials have impurities, and misleading interpretations of results can occur when the intrinsic properties are attributed to a system controlled by extrinsic defects. The results of the MML/CNST study provide a framework for controlling the point-defect chemistry in perovskite oxides to facilitate more clearly interpretable measurements of their electronic and dielectric properties. A complete account of this research can be found in the published manuscripts: R.A. Maier, A-C. Johnston-Peck, and M.P. Donohue, (Magic Dopant) Adv. Fun. Mat., DOI: 10.1002/adfm.201602156 (2016).
(Low Temperature Conductivity I) R.A. Maier and C.A. Randall, “Low-Temperature Ionic Conductivity of an Acceptor-Doped Perovskite: I. Impedance of Single-Crystal SrTiO3,” J. Am. Ceram. Soc., 99  3350–3359 (2016).
(Low Temperature Conductivity II) R.A. Maier and C.A. Randall, “Low Temperature Ionic Conductivity of an Acceptor-Doped Perovskite: II. Impedance of Single-Crystal BaTiO3,” J. Am. Ceram. Soc., 99  3360–3366 (2016).
(Defect Dipole) R.A. Maier, T.A. Pomorski, P.M. Lenahan, and C.A. Randall, “Acceptor-Oxygen Vacancy Defect Dipoles and Fully Coordinated Defect Centers in a Ferroelectric Perovskite Lattice : Electron Paramagnetic Resonance Analysis of Mn2+ in Single Crystal BaTiO3,” J. Appl. Phys., 118 164102 (2015).