Isotopic effects in the heat capacity of concentrated orientationally disordered solid solutions of methane and deuteromethane in krypton

Abstract

The heat capacity of orientationally disordered solid solutions (CH4)nKr1−n (n=75 and 78 mole% CH4 in the temperature interval ΔT=0.8–20 K) and (CD4)nKr1−n (n=50, 60, and 70 mole% CD4, ΔT=0.6–30 K) is investigated. At liquid-helium temperatures the temperature dependences of the molar heat capacities of the rotational subsystems of the solutions are qualitatively and quantitatively very different. One of the main reasons for the effect is that in the concentrated solutions investigated the CD4 molecules are in a substantially stronger molecular field than the CH4 molecules. This is because in low-energy states the effective octupole electric moment of CD4 molecules, which determines the molecular field, is larger than the effective octupole moment of the more quantum molecules CH4. The weak concentration dependences of the heat capacity of the solutions studied are due to the influence of frustration, which weakens the molecular fields produced at the lattice sites by the surrounding molecules. No evidence of the formation of orientational octupole glasses in the experimental systems was found.

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 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].…”

“…N 2 -Kr) noncentral interaction. The rotation of more quantum CH 4 molecules at liquid helium temperatures remains qualitatively the same even if over 50% of the sites in the first coordination sphere are occupied by molecules [1][2][3]. In contrast, in the case of strong molecular interaction (N 2 , CO in Kr), even one molecule present in the first coordination sphere can cause qualitative changes in the molecule rotation and the rotational spectrum [4][5][6].…”

“…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 distinctions in the low temperature dynamics of the rotational subsystems of CH 4 and CD 4 solutions in Kr are mostly due to the short-range (decreasing as 1/R 7 ) octupoleoctupole interaction of the molecules in the first coordination sphere and to the quantum effects in the rotational motion of the molecules. According to calorimetric data [2], in solid solutions with equal CH 4 and CD 4 concentrations, the CD 4 molecules are subjects to a much stronger molecular field than the CH 4 molecules. This is due to the features of quantum solids: in the ground state CD 4 molecules have a higher effective octupole electric moment and a smaller amplitude of zero orientational vibrations in the solid phase.…”