Impact of point defects on the elastic properties of BaZrO₃: Comprehensive insight from experiments and ab initio calculations

Acceptor doped BaZrO₃ is the prototype of proton conducting perovskites which are of strong interest as electrolytes for intermediate temperature fuel cells. Elastic properties of both dry and hydrated Y-doped BaZrO₃ (1.5–17 mol% Y) were determined using ultrasound time of flight (TOF) measurements, and complemented by ab initio calculations which allow for an analysis of the different contributions. The experimental and theoretical findings are consistent and reveal a strong decrease of the Young's, shear and bulk moduli upon increasing dopant concentration. This decrease is attributed to a combined effect of (i) macroscopic lattice chemical expansion mainly caused by differing ionic radii, and (ii) presence of point defects such as acceptors Acc_Zr ^′ (with decreased cation charge), oxygen vacancies V_O ^••, and protonic defects OH_O ^• (hydroxide ions on oxide ion sites) that locally weaken the chemical bonds in the perovskite structure. The effect from modified lattice parameter is minor relative to the decrease in moduli caused by Acc_Zr ^′, V_O ^••, OH_O ^• weakening the chemical bonds. The elastic moduli differ only slightly between the dehydrated and hydrated samples. The decrease in the elastic moduli with increasing acceptor and oxygen vacancy concentrations is much stronger in Y-doped BaZrO₃ (−5.8% in Y:BaZrO₃ per mol% of vacancies) compared to similar earlier investigations on Gd-doped CeO₂ (−2% in Gd:CeO₂). This result indicates a greater effect of oxygen vacancies on the elastic properties in ABO₃ perovskites with the linear B–O–B bonds as compared to fluorites with strongly bent M-O-M bonds.