A calorimetric investigation of the low-temperature phase transition in NaOD

The heat capacity of NaOD from T=20 to 350 K is reported. An anomaly centered at T=153.2±0.1 K (ΔH=131.1±0.7 J mol−1, ΔS=0.864±0.005 J K−1 mol−1) corresponds to the onset of a low-temperature antiferroelectric phase. This transformation, which likely involves tunneling of the deuterons, does not appear to have an atmospheric pressure analog in NaOH above T=6 K.