Heat capacity of methane–krypton solid solutions. Conversion effect

Abstract

The heat capacity of Kr–nCH4 solid solutions with the concentrations n=1; 5; 10% and of the solid solution Kr–1% CH4–0.2% O2 is studied at 0.7–8 K. The contributions Crot to the heat capacity of the solutions due to the rotation of the CH4 molecules are estimated. The deviations of the measured Crot from the values corresponding to the equilibrium distribution of the CH4 nuclear spin modifications are dependent on the correlation between the characteristic times of conversion and of the calorimetric experiment. The effects of temperature, O2 impurities, and CH4 clusters upon the conversion rate are studied. It is shown that the hybrid mechanism of conversion proposed by Berlinsky and Nijman, which takes into account both intramolecular and intermolecular interactions of the proton spins, is predominant.

Effect of octupole interaction on the rotational motion of rotors in a solid Kr–CD4solution

“…As with the solutions with n = 0.13, 0.5, 0.6, 0.7 [2,10], the rate of temperature relaxation after heating or cooling the calorimeter suggests that the nuclear-spin conversion is rather fast in these solutions.…”

“…The measured heat capacities at equilibrium vapour pressure (to a high extent of accuracy C sol = C p ) of solid (CD 4 ) n Kr 1−n solutions with CD 4 concentrations n = 0.09, 0.17, 0.25 and 0.35 are given in figure 1. The figure also shows the heat capacities measured earlier on (CD 4 ) n Kr 1−n solutions with n = 0.50 [2] and n = 0.13 [10]. It is seen in figure 1 that appreciable changes occur in the temperature dependences C sol (T ) in the solutions with n = 0.50CD 4 [2] (T = 18-21 K) and the solutions with n = 0.25 and 0.35 [8] (T = 13-14 K).…”

“…The limiting Debye temperature at T → 0 K is 0 = 71.6 K, and coincides with 0 = 71.7 K [15] and 0 = 71.9 K [16] within the measurement error. ) n Kr 1−n solutions with n = 0.09 ( ); 0.13 (+) [10]; 0.17 ( ); 0.25 (•); 0.35 ( ); 0.50 ( ) [2].…”

“…In the (CD 4 ) n Kr 1−n system with CD 4 concentrations n 0.05 the rotation of most of the molecules, as in CH 4 -Kr solution, is weakly hindered and their rotational spectrum is close to that of matrix-isolated molecules [7][8][9]11]. However, the rotational motion becomes rather intricate in solution with n > 0.05CD 4 , where the noncentral interaction is essentially important [10]. The temperature dependences of the heat capacity of the rotational subsystem C rot (T ) in concentrated (CD 4 ) n Kr 1−n solutions [2] are drastically different from C rot (T ) in (CD 4 ) n Kr 1−n solutions with n = 0.01, 0.05 CD 4 [7,8] and (CH 4 ) n Kr 1−n solutions with n = 0.1-0.78 CH 4 [1,2,11].…”

“…The experimental heat capacities C rot measured on solution with n = 0.13 [10] at temperatures below 2 K were described as a sum of contributions from molecules in effective weak, moderate and strong molecular fields. The rotors experiencing these fields are weakly hindered (WH-rotors), hindered (H-rotors) and strongly hindered (SH-rotors) respectively.…”