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 ( 1 H, 13 C, 17 O) solid-state NMR spectroscopy is employed to follow in detail the structural development of nanocrystalline zirconia. 17 O magic-angle spinning NMR spectroscopy of sol−gel produced undoped ZrO 2 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 ZrO 2, 13 C NMR shows that loss of all of the organic fragments occurs prior to crystallization. The 1 H NMR experiments determined that the proton content remains significant until well above the crystallization temperature, so that the composition is not accurately described as ZrO 2 until >500 °C.