Evidence for low-temperature isotopic ordering in methyl-monodeuterated lithium acetate dihydrate

Highlights: ► Cp measurements of mono-methyl-deuterated LiC2H2DO2·2H2O show transition at 5.5 K. ► Transition is in addition to the 14 K ordering transition. ► Transition is suggested to be due to isotopic ordering. - Abstract: We measured the heat capacities of methyl-monodeuterated lithium acetate dihydrate single crystals (LiC2H2DO2·2H2O) at 2K<T<20K and Raman spectra at ambient conditions. Significant excess heat capacities were found at T = 14.5 K and 5.5 K. The thermodynamic data is compared to that of fully protonated and fully deuterated LiAc and interpreted in terms of phase transitions between a classical dynamically disordered phase above 14.5 K, a phase stable between 14.5 K and 5.5 K in which there is quantum tunnelling of the methyl groups, and an isotopically ordered phase below 5.5 K.