Isotopic effect in heat capacity of solid concentrated orientationally-disordered solutions of methane and deuteromethane in krypton

The heat capacity of the orientationally-disordered solid solutions (CH4)nKr1-n (n = 75 and 78 mol. % CH4) and (CD4)nKr1-n (n = 50, 60 and 70 mol. % CD4) has been investigated in the intervals ΔT = 0.8-20 K and ΔT 0.6-30 K, respectively. The temperature dependences of the molar heat capacities of rotational subsystems of the solutions exhibit considerable qualitative and quantitative distinctions at helium temperatures. One of the important reason for the effect is that the CD4 molecules of the investigated concentrated solutions are in a significantly stronger molecular field than the CH-4 molecules. This is due to a higher effective octupole electric moment of the CD4 molecules in the ground states and a smaller amplitude of their zero orientational vibrations in the solid phase. The weak concentration dependencies of the heat capacity of these solutions are due to the frustration weakening the molecular fields at the lattice sites that are excited by the surrounding molecules. No evidence has been detected in favor of the formation of orientational octupole glasses in the systems studied