Low temperature heat capacity Study of Fe(PO3)3 and Fe2P2O7

The heat capacities of two iron phosphates, Fe(PO₃)₃ and Fe₂P₂O₇, have been measured over the temperature range from (2 to 300) K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). A phase transition related to magnetic ordering has been found in the heat capacity at T = 8.76 K for Fe(PO₃)₃ and T = 18.96 K for Fe₂P₂O₇, which are comparable with literature values from magnetic measurements. By fitting the experimental heat capacity values, the thermodynamic functions, magnetic heat capacities, and magnetic entropies have been determined. Additionally, theoretical fits at low temperatures suggest that Fe₂P₂O₇ has an anisotropic antiferromagnetic contribution to the heat capacity and a large linear term likely caused by oxygen vacancies. Further data fitting in a series over widened temperature regions found that this linear term exists only below 15 K and disappears gradually from (15 to 17) K.