Orientational Ordering in Solid Methane at Low Temperatures

Abstract

<p>The solid isotopic methanes (CHnD4-n, 0 ≤ n ≤ 4) are known to exist in at least three solid phases, called phases I, I I and III in order of decreasing temperature domain. There have been many studies, both experimental and theoretical, of the properties of solid methane, and the structures of phases I and II are known. In phase I, the molecules form a face-centered cubic lattice, and are freely, or nearly freely, rotating on the lattice sites. In phase II, the solid has a complex sublattice structure: 3/4 of the molecules arc orientationally ordered and 1/4 are orientationally disordered. The structure of phase III is not known, and one of the main aims of this thesis is to draw conclusions about the orientational order in this phase.</p> <p>The heat capacities of isotopically purified CH2D2, CHD3 and CD4 were measured accurately in the temperature range 0.15K ≤ T ≤ 3.5K where Schottky anomalies were known to occur in the partially deuterated methanes. From the heat capacity measurements, the residual entropies at T= 0K of CH2D2 and CHD3 were determined, and used to deduce that phase III is not classically orientationally ordered, and that it must contain at least three different sublattices . The latter conclusion is in agreement with that of recent theoretical calculations and optical measurements on solid methane. A model is presented which accounts for the low temperature thermal properties of the solids.</p> <p>Nuclear spin symmetry species conversion is known to occur in CH4 and CH3D but the present measurements show that it is not significant in CH2D2 or CHD3. In CD4 on the other hand, longer thermal relaxation is found and it may be ascribable to conversion.</p> <p>The positions and shapes of the Schottky anomalies in the deuterated methanes are used to ma~e estimates of the tunnel level splitting these results are compared with those of recent neutron inelastic scattering experiments for CD4 , and with theoretical estimates for all of ·the deuterated methanes.</p> <p>In a different type of experimental investigation, the relationship between the temperatures of the solid-solid phase transitions and thermally-stimulated luminescence of electron and ultraviolet-irradiated methane (CH 4 and CD 4) was probed. Under certain conditions, a correlation was found to exist between the phase transition temperatures and sharp peaks in the glow curves, but thermal gradients in the samples preclude the use of this method for kinetic analysis of the phase transitions.</p>