Calorimetric study of phase transitions in ammonium hydrogen oxalate hemihydrate and its deuterated compound

Article abstract of DOI:10.1016/0022-3697(89)90015-2

The molar heat capacities of ammonium hydrogen oxalate hemihydrate and its fully-deuterated compound were measured from 13 to 300 K. A higher-order phase transition was found in each compound. The temperatures and molar entropies of transition are 145.4K (6.01 J K^-1 mol^-1) and 160.1 K (8.23 J K^-1 mol^-1) in the protonated and deuterated compounds, respectively, where the mole refers to (NH₄HC₂O₄)₂·H₂O or (ND₄DC₂O₄)₂·D₂O. The entropies of the transitions indicate that they are order-disorder transitions, probably related to the ordering of one of the ammonium ions. The pressure coefficient of the transition temperature was calculated by the Ehrenfest relation and the available data of the thermal expansivity of the hydrogenous compound: -2.0 × 10⁷ Pa K^-1 in good agreement with a reported value -1.9 × 10⁷ Pa K^-1 based on a high pressure experiment.