Anomalous Specific Heat and Configurational Entropy Change in Sodium Trihydrogen Selenite

Specific heats of sodium trihydrogen selenite, NaH 3 (SeO 3 ) 2 , and its deuterated crystal were measured in their transition temperature regions. The entropy changes in NaH 3 (SeO 3 ) 2 associated with the upper and lower phase transitions are found respectively to be (1.03±0.05) and ∼0.5 cal·mol -1 ·deg -1 , resulting in the total entropy change, ∼1.53 cal·mol -1 ·deg -1 . The entropy change for the lower transition is the value estimated roughly with the help of differential thermal analysis. The entropy change associated with the phase transition in NaD 3 (SeO 3 ) 2 , which undergoes one phase transition in contrast with the two in NaH 3 (SeO 3 ) 2 , is found to be (1.58±0.09) cal·mol -1 ·deg -1 . The number of configurations of hydrogens which can be assumed to form the layer structure is calculated by taking full correlations into account. The entropy change between ordered and disordered states is calculated to be 1.59 cal·Mol -1 ·deg -1 , which is in good agreement with the experimental results. Another calculation for the entropy change by Pauling's method gives 2.18 cal·mol -1 ·deg -1 , which is too large when it is compared with the experimental values.