Ceramics and glasses
Why does stabilized zirconia (YSZ), ZrO₂ with Y₂O₃ dissolved in it, conduct oxide ions?
- AY³⁺ enters interstitial sites and supplies free electrons
- BY³⁺ substitutes for Zr⁴⁺, and one oxygen vacancy forms for every two Y³⁺
- CY³⁺ substitutes for Zr⁴⁺, and two oxygen vacancies form for every Y³⁺
Show answer and explanation
Answer: B. Y³⁺ substitutes for Zr⁴⁺, and one oxygen vacancy forms for every two Y³⁺
When trivalent Y³⁺ substitutes on a site of tetravalent Zr⁴⁺, each one leaves a deficit of one positive charge. To maintain electrical neutrality, one vacancy of the divalent oxide ion forms for every two Y³⁺ (in Kröger–Vink notation, Y₂O₃ → 2Y′_Zr + V_O•• + 3O_O). At high temperature O²⁻ moves through these oxygen vacancies, so YSZ is used as a solid electrolyte in oxygen sensors and solid oxide fuel cells (SOFCs). Two vacancies per Y³⁺ does not balance the charge, and Y does not enter interstitial sites and release electrons either. Adding Y₂O₃ also serves to stabilize the cubic or tetragonal phase down to room temperature.