College Park, Maryland      June 6 - 10 , 2004

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WP71: Acoustic Mode Confinement in Nano-Crystalline Yttria

M.P. Hehlen, F.R. Trouw, J.T. Mang (Los Alamos National Laboratory), R.M. Laine (University of Michigan, Ann Arbor, MI 48109)

Nano-crystalline cubic yttria (Y₂O₃) was prepared by liquid-feed flame spray pyrolysis. The mean particle diameter was 21 nm as determined by small-angle X-ray scattering and assuming polydisperse spheres. Inelastic neutron scattering from nano-crystalline and coarse-grained Y₂O₃ at 15 K was measured with an incident energy of 90 meV (ΔE/E~6%) on the Pharos spectrometer at LANSCE. The phonon density of states (PDOS) of both the nano-crystalline and the coarse-grained Y₂O₃ exhibits a number of well resolved peaks in the 15-75 meV range that are in agreement with optical phonons observed by Raman scattering and infrared absorption. Relative to coarse-grained Y₂O₃, the PDOS of nano-Y₂O₃ is substantially enhanced in the 5-30 meV range by a broad asymmetric band peaking at 10 meV. This additional phonon density is due to acoustic modes becoming confined on the length scale of the nano-particles. The confined acoustic modes are analyzed using a model of homogeneous, elastic spheres as well as molecular dynamics calculations.

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