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Simmons, R.O. Venkataraman, Chitra T. 2011-04-19T14:21:46Z 2011-04-19T14:21:46Z 10000-01-01 1996-05 At low densities the helium solids are highly quantum in nature, with the small atomic mass and weak interactive forces giving rise to large vibrational motions of the atoms about their mean lattice sites. While these distances can be as much as 25 % of the nearest-neighbor distance at the lowest densities, the atoms are significantly more confined ·as the density of the solid is increased. Since 4He is a highly compressible solid, it is possible to examine the density dependence of the an harmonicity of the interatomic potential within accessible experimental conditions. One can then study the decreasing quantum nature of the solid. The research presented in this thesis involves x-ray diffraction studies of the high density phases of 4He to 60 atoms/nm3 • This includes in situ single crystal growth and characterization; measurement of the De bye-Waller factor through the dependence of the scattered intensities on the wave-vector transfer,q; study of the inelastic diffuse scattering to relate it to crystal elastic constants and other dynamical information; and lattice parameter measurements to study thermal defect properties. Analysis of the data indicate that the average displacement amplitudes are reduced to 10 % of the nearest -neighbour distance at 60 atoms/nm3 Related De bye temperatures are within the range of those inferred from neutron dispersion curves and from PIMC calculations. The dynamic diffuse scattering data can be qualitatively understood in terms of the 4He elastic constants. At low q, the 4He diffuse scattering is intense enough to warrant detailed analysis. It appears that both defects and dynamic processes contribute to the diffuse intensity. Estimates of vacancy concentrations in the fcc phase, as measured from lattice parameter shifts, greatly extend the density range over which these Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-04-19T14:21:46Z No. of bitstreams: 1 1996_venkataraman.pdf: 5672405 bytes, checksum: be942acba43af363f9dc30f4d6c47185 (MD5) Made available in DSpace on 2011-04-19T14:21:46Z (GMT). No. of bitstreams: 1 1996_venkataraman.pdf: 5672405 bytes, checksum: be942acba43af363f9dc30f4d6c47185 (MD5) Previous issue date: 1996-05 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-04-19T14:21:46Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:11:59-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Thesis U of I Only 3961396 http://hdl.handle.net/2142/18834 en 1996 Chitra T. Venkataraman lattice dynamics x-ray diffraction helium high-density phases (solids) X-ray diffraction study of the high-density phases of solid 4He Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.