Oxygen vacancy ordering and viscoelastic mechanical properties of doped ceria ceramics

Young's, shear and bulk moduli of Ce_1-xSm_xO_2-x/2 (x ≤ 0.55) were studied using ultrasonic time of flight and nanoindentation techniques. Sound velocity measurements, corrected for sample porosity, demonstrate decrease in the unrelaxed ceramic moduli with increasing Sm-content. Room temperature creep under indenter load-hold, as well as time-dependent material stiffness, reveal a transition from prominent anelasticity in the fluorite phase to prominent elasticity in the double fluorite phase. This supports rearrangement of elastic dipoles under anisotropic stress, which occurs more readily when oxygen vacancies are not ordered on the crystal lattice, as the source of ceria anelastic behavior.