Phase transitions in solid Kr-CH4 solutions and rotational excitations in phase II

Classical Cryocrystals

Анотація

The heat capacity C_P of solid Kr–n CH₄ solutions with the CH₄ concentrations n = 0.82, 0.86, 0.90 as well as solutions with n = 0.90, 0.95 doped with 0.002 O₂ impurity has been investigated under equilibrium vapor pressure over the interval 1–24 K. The (T,n)-phase diagram was refined and the region of two-phase states was determined for Kr–n CH₄ solid solutions. The contribution of the rotational subsystem, C_rot, to the heat capacity of the solutions has been separated. Analysis of C_rot(T) at T < 3 K made it possible to estimate the effective conversion times t and the energy gaps E₁ and E₂ between the tunnel levels of the A-, T-and A-, E-nuclear-spin species of CH₄ molecules in the orientationally ordered subsystem, and to determine the effective energy gaps E₁ between the lowest levels of the A- and T- species. The relations t(n) and E₁(n) stem from changes of the effective potential field caused as the replacement of CH₄ molecules by Kr atoms at sites of the ordered sublattices. The effective gaps E_L between a group of tunnel levels of the ground-state libration state and the nearest group of excited levels of the libration state of the ordered CH₄ molecules in the solutions with n = 0.90 (E_L = 52 K) and 0.95 (E_L = 55 K) has been estimated.