Non‐Classical Electrostriction in Hydrated Acceptor Doped BaZrO₃: Proton Trapping and Dopant Size Effect

Abstract

Point defects such as oxygen vacancies and protonic interstitials are not only essential for ionic conductivity in oxides since they also affect the mechanical and electromechanical properties. These properties of nominally dry and hydrated proton-conducting BaZr 0.85 M 0.15 O 2.925+δ H 2δ (M = Al, Ga, Sc, In, Y, Eu) ceramics are investigated. Doping decreases Young's modulus with increasing ionic radii difference between the dopant and the host. Nominally dry samples show consistently higher Young's moduli than hydrated samples. All samples exhibit large non-classical electrostriction, with a negative electrostriction coefficient M 33 <0. M 33 shows saturation with the field and a non-ideal Debye relaxation with frequency. The low-frequency M 33 value for both dry and hydrated samples shows a similar dependence on dopant radius as Young's modulus. For the hydrated samples, the relaxation frequency increases by a factor >100 in the series Ga-Y, emphasizing the importance of proton trapping, with Y-doped samples having minimal trapping energy. This coincides with the fact that the saturation strain for Y-doped samples is also the smallest. In light of these findings, it is concluded that the present data give strong evidence for the existence of defect-related elastic dipoles in acceptor doped barium zirconate and that the non-classical electrostriction originates in their reorientation under electric field.

Engineering of Electromechanical Oxides by Symmetry Breaking

“…[59] The oxygen-defective materials having both large electromechanical coefficients and low permittivity are regarded as a new class of electrostrictors -similar features were observed in polymer composites, protonated metal oxides, and lead halide perovskites. [59,[62][63][64] There is a remaining debate on the sign of field-induced electrostrictive strain in oxygen-defective oxides. For example, the sign of the field-induced strain was proposed to be negative (i.e., contracts along the direction parallel to the electric field applied) with an oxygen vacancy-mediated lattice contraction (electrostatic interactions).…”

“…[ 56 ] However, exceptionally large electromechanical properties (| M | ≈ 5–100 × 10 −18 m 2 V −2 ) have been found in oxygen‐defective metal oxides such as Gd‐doped CeO 2‐ δ , (Nb,Y)‐stabilized Bi 2 O 3‐ δ , and BaCeO 3‐ δ with intrinsic permittivity of ε r ≈ 20–40 and Q coefficients in the range of ≈40–300 m 4 C −2 . [ 3,60–62 ] These materials do not obey the empirical Newnham's relation (| M | or | Q | versus s/ε), and their electrostriction coefficients are three orders larger than values for typical perovskites including SrTiO 3 , BaTiO 3 , PbTiO 3 , Pb(Ti 1‐ x Zr x )O 3 , and Pb(Mg 1/3 Nb 2/3 )O 3 ‐PbTiO 3 ( Q ≈ 0.02–0.07 m 4 C −2 ). [ 59 ]…”

“…[56] However, exceptionally large electromechanical properties (|M| ≈ 5-100 × 10 −18 m 2 V −2 ) have been found in oxygen-defective metal oxides such as Gd-doped CeO 2-δ , (Nb,Y)-stabilized Bi 2 O 3-δ , and BaCeO 3-δ with intrinsic permittivity of ε r ≈ 20-40 and Q coefficients in the range of ≈40-300 m 4 C −2 . [3,[60][61][62] These materials do not obey the empirical Newnham's relation (|M| or |Q| versus s/ε), and their electrostriction coefficients are three orders larger than values for typical perovskites including SrTiO 3 , BaTiO 3 , PbTiO 3 , Pb(Ti 1-x Zr x )O 3 , and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (Q ≈ 0.02-0.07 m 4 C −2 ). [59] The oxygen-defective materials having both large electromechanical coefficients and low permittivity are regarded as a new class of electrostrictors -similar features were observed in polymer composites, protonated metal oxides, and lead halide perovskites.…”

Observation of Potential-Induced Hydration on the Surface of Ceramic Proton Conductors Using In Situ Near-Ambient Pressure X-ray Photoelectron Spectroscopy

“…It is worth noting that XPS is a surface technique. As observed by Makagon et al, doped BaZrO 3 exhibits nonclassical electrostriction behavior, which means that under large electric field, the location of protons affects the elastic dipole moment in the lattice, imposing changes in the crystal structure. Other characterization methods, such as in situ X-ray diffraction or Raman spectroscopy, could be used under applied external potential to explore the behavior of protons in ceramics.…”

Factors Constituting Proton Trapping in BaCeO3 and BaZrO3 Perovskite Proton Conductors in Fuel Cell Technology: A Review

“…Smaller acceptor dopants with high electronegativity exhibit a pronounced proton trapping effect over larger acceptor dopants with low electronegativity. A DFT study conducted by Makagon et al 115 illustrates a congruent result of increased proton trapping effect by smaller acceptor dopants with suitable trapping energy estimations. While scandium (Sc 3+ , (r = 0.745 Å)) demonstrated a trapping energy of 0.28 eV, gallium (Ga 3+ , (r = 0.62 Å)) showcased a trapping energy of 0.49 eV in the first coordination (closest to the dopant).…”