Solid-state NMR and X-ray studies of the structural evolution of nanocrystalline zirconia

A combination of X-ray techniques [diffraction and Zr K-edge absorption (EXAFS and XANES)] and multinuclear (¹H, ¹³C, ¹⁷O) solid-state NMR spectroscopy is employed to follow in detail the structural development of nanocrystalline zirconia. ¹⁷O magic-angle spinning NMR spectroscopy of sol - gel produced undoped ZrO₂ shows unequivocally that oxygen sites in the initial gel are monoclinic-like. This result is consistent with X-ray absorption measurements, which also suggest that the structures of the initial amorphous phases of doped and undoped samples produced by the hydroxide-precipitation and sol - gel methods are very similar. On crystallization, the local structure of the crystalline component is tetragonal, but a significant fraction of the sample remains disordered. Heating to higher temperatures results in conversion to monoclinic zirconia in undoped samples at room temperature. For sol - gel-produced ZrO₂, ¹³C NMR shows that loss of all of the organic fragments occurs prior to crystallization. The ¹H NMR experiments determined that the proton content remains significant until well above the crystallization temperature, so that the composition is not accurately described as ZrO₂ until >500°C.