X-ray diffraction study of the high-density phases of solid 4He

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

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Venkataraman, Chitra T.

Contributors dc:contributor

• Simmons, R.O.

• lattice dynamics
• x-ray diffraction
• helium
• high-density phases (solids)

Statement dc:rights

• 1996 Chitra T. Venkataraman

Language dc:language

en

Identifier

3961396

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/18834