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

Abstract We measured the heat capacities of methyl-monodeuterated lithium acetate dihydrate single crystals (LiC 2 H 2 DO 2 ·2H 2 O) at 2 K T 20 K 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.