Synchrotron X-rays Uncover Crystalline Phase and Fine Structure of Zirconia

Yttria-stabilized zirconia (YSZ) has become indispensable in restorative dentistry; however, the influence of yttria content on its crystalline phases and fine structure remains incompletely understood. This study investigated how variations in yttria content (3-8 mol%) and low-temperature degradation (LTD) influence the crystalline phase composition and local atomic structures of dental zirconia using synchrotron-based X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy. Forty-two sintered disc-shaped specimens were prepared from zirconia powders stabilized with 3-8 mol% yttria (3-8YSZ). Half of the specimens were polished, while the other half underwent autoclaving to induce LTD. Synchrotron XRD showed that 3-5YSZ contained 2 tetragonal (t and t') phases, while 8YSZ contained a cubic (c) phase. The t-phase fraction decreased markedly with increasing yttria content, from 83.1 mass% in 3YSZ to 25.3 mass% in 5YSZ. LTD induced monoclinic (m) and rhombohedral (r) phases in 3YSZ and 4YSZ but not in 5YSZ, indicating improved LTD resistance at higher yttria contents. XAFS showed that the dual-peak white-line profile of zirconia specimens varied with yttria content, reflecting the t-to-t' phase transition, and transformed into a single broadened peak after LTD, consistent with structural disorder induced by phase transformation. Fine structural analysis revealed 2 main coordination shells, Zr-O and Zr-Zr. In the Zr-Zr shell, the Debye-Waller factors changed systematically with yttria content, reflecting structural disorder associated with oxygen vacancies. In conclusion, a combined synchrotron XRD and XAFS approach elucidated the phase composition and atomic-scale structural disorder in dental zirconia across different yttria contents. These insights provide a foundation for the rational design of next-generation translucent dental zirconia, in which crystalline phase composition and local atomic disorder are deliberately tailored to achieve durable clinical performance.