Low temperature phase transformation of nanocrystalline tetragonal ZrO2 by neutrons and Raman Scattering studies

The structural properties of three, undoped, nanocrystalline, tetragonal zirconia powders, characterized by 6.9, 13 and 18 mn average crystallite size, respectively, have been studied at low temperature by neutron diffraction and Raman spectrometry. Raman spectrometry has shown that the 6.9 nm sample undergoes a continuous and reversible phase transition, towards the monoclinic structure below 175 K. This transformation could be at the origin of the appearance of components near 22.5 and 35 meV in the inelastic neutron spectrum when the temperature is lowered to 100 K. Such a transformation is not observed for the 13 and 18 nm samples. This transformation seems to depend on the crystallite size and on the kinetics of thermal treatments. This could explain why the transformation of the 6.9 nm zirconia could not be induced by the slow cooling and heating rates of the cryostat used in neutron diffraction. This transition has some similarities with infrared, low temperature measurements on cerium-doped zirconia reported in the literature. The neutron diffraction data have allowed the determination of thermal expansion coefficients: beta (a) varies from 0.76 to 0.78 and beta (b) from 0.80 to 0.91 x 10(-5) K-1 for the 6.9 and the 18 nm zirconia, respectively. (C) 2001 Elsevier Science Ltd. Adl fights reserved.