Characteristics and local structure of hafnia-silicate-zirconate ceramic nanomixtures

Zirconate systems having the composition 3HfO₂·15SiO₂·xY₂O₃·(82 - x)ZrO₂, where x = 2, 7 and 12 mol% Y₂O₃, were synthesized by a sol-gel method. The analysis of X-ray diffraction data showed the presence of the t-ZrO₂, m-ZrO₂, m-HfO₂, Y₂SiO₅ and Y₂Si₂O₇ crystalline phases in a ceramic nanomixture. Spectroscopic data show that the increase of the Y₂O₃ content of samples determines the increase of the t-ZrO₂, m-HfO₂ and silicate crystalline phases. Gap energy values decrease almost linearly with increasing Y₂O₃ content of samples. A detailed study of XANES data does not show a significant difference with increasing Y₂O₃ content of the samples suggesting an appreciable stability of the hafnium ions +4 oxidation state and their microvicinity. EXAFS results show that the local structure around the Hf cation is similar to that from the monoclinic crystalline HfO₂ where the Hf-O coordination number tends to 7. The bond lengths of Hf-O shells show small deviations from ∼2.12 Å and the Hf-metal paths become more structured by increasing the Y₂O₃ content of the samples.