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 (EX AFS and XANES)] and multinuclear (H-1, C-13, O-17) solid-state NMR spectros copy is employed to follow in detail the structural development of nanocrys talline zirconia. O-17 magic-angle spinning NMR spectroscopy of sol-gel pro duced undoped ZrO2 shows unequivocally that oxygen sites in the initial gel are monoclinic-like. This result is consistent with X-ray absorption measu rements, which also suggest that the structures of the initial amorphous ph ases of doped and undoped samples produced by the hydroxide-precipitation a nd sol-gel methods are very similar. On crystallization, the local structur e of the crystalline component is tetragonal, but a significant fraction of the sample remains disordered. Heating to higher temperatures results in c onversion to monoclinic zirconia in undoped samples at room temperature. Fo r sol-gel-produced ZrO2, C-13 NMR shows that loss of all of the organic fra gments occurs prior to crystallization. The H-1 NMR experiments determined that the proton content remains significant until well above the crystalliz ation temperature, so that the composition is not accurately described as Z rO2 until > 500 degreesC.