Determination of Local Structural and Dynamic Descriptors in Doped Oxides: The Case of Zr-Doped Ceria, a Non-Classical Electrostrictor

Non-classical electromechanical properties, such as large electrostriction stress coefficient and low dielectric constant in Zr-doped ceria and other doped cerium oxides, discovered in the last several years, are the focus of intense research as they indicate the existence of solids with unusually large anharmonicity of chemical bonds. Theoretical models of electrostriction mechanisms rely on the concept of elastic dipoles that are formed due to the local structural distortions around dopants. The structural, dynamic, and electronic descriptors of the pre-existing (at zero field) dipoles (as in aliovalent-doped ceria) and those dynamically formed under the electric field (as in isovalent-doped ceria) are unknown. Here, the characteristics of the local structure and dynamics of the dopant and host bonding environments were obtained by using the temperature-dependent X-ray absorption fine structure measurements. The candidate set of descriptors was identified in a quasiharmonic approximation for the nearest neighboring bonds between Zr (Ce) and the atoms in the first and second coordination shells. The descriptors included static bond length disorder, the anharmonicity of the effective pair potential, and effective force constants for the probed atomic pairs. The observation of significant Zr-O bond anharmonicity confirms previous first-principles calculations. The structural model, emerging from these simulations, emphasizes significant free volume around Zr in the oxygen cube, explaining both the mechanical anomaly (the decrease in Young’s modulus with Zr concentration) as well as correlating at least one descriptor (static disorder) to the measured electrostriction coefficient. This descriptor-based approach can be extended to studies under applied electric fields or stress in this and other materials.