DFT investigation of (electro)mechanical properties of Y- and Sc-doped BaZrO3

Yttrium doped BaZrO3 combines a high bulk proton conductivity with good chemical stability and is thus considered a promising electrolyte material for protonic ceramic fuel cells (PCFCs), cf. [1] and references therein.... [ view full abstract ]

Yttrium doped BaZrO₃ combines a high bulk proton conductivity with good chemical stability and is thus considered a promising electrolyte material for protonic ceramic fuel cells (PCFCs), cf. [1] and references therein. Acceptor doping e.g. by Y³⁺ in the Zr⁴⁺ site leads to oxygen vacancy formation. Dissociative water absorption into these vacancies forms mobile protonic defects (hydroxide ions at oxygen sites). The accompanying chemical expansion may cause strains that become critical for PCFC. Thus a detailed understanding of its mechanical properties such as Young's modulus is necessary. In addition, hydrated BaZrO₃:Y exhibits unusual electromechanical properties, with an exceptionally high electrostriction coefficient [2]. In this presentation, we discuss the elastic and dielectric properties of Y- and Sc-doped BaZrO₃ calculated by density functional theory (DFT). Comparing experimental Young’s moduli [3] with DFT results shows good consistency, with a considerable modulus decrease with increasing Y doping. The decrease is less pronounced for Sc-doped BaZrO₃. By calculating charged supercells containing exclusively oxygen vacancies, hydroxide ions or acceptor dopants, their effects on the mechanical strength could be separated. Introducing acceptors, oxygen vacancies or protonic defects soften the material. Since the effects of one oxygen vacancy is similar to that of two protonic defects, only small differences are expected between dry and hydrates samples. The less pronounced softening for Sc-doped BaZrO₃ can be rationalized by its smaller increase of lattice constant. Finally, a mechanism is discussed in which the response of point defect clusters to an external electric field could, in principle, explain the observed high strains under applied electric field.

[1] K. D. Kreuer, Annu. Rev. Mater. Res. 33, 333-59 (2003)

[2] N. Yavo, O. Yeheskel, E. Wachtel, A. Frenkel and I. Lubomirsky, Abstract Solid State Ionics Conference, Padua, Italy (2017)

[3] I. Lubomirsky, unpublished results