Low temperature heat capacity study of Fe3PO7 and Fe4(P2O7)3

The low temperature heat capacities of Fe3PO7 and Fe4(P2O7)3 have been measured using a Quantum Design Physical Property Measurement System (PPMS) over the temperature range from (2 to 300) K. Phase transitions due to Fe3+ magnetic ordering have been determined in the heat capacities at temperatures of (164.5 and 47.6) K for Fe3PO7 and Fe4(P2O7)3, respectively, which agrees well with the magnetic measurements reported in the literature. Also, another small transition occurring at around 27 K for Fe4(P2O7)3 has been found for the first time. The thermodynamic functions and magnetic heat capacities have been calculated based on the curving fitting of the experimental heat capacity values. Using the fitted heat capacity results, the standard molar entropies have been calculated to be (219.73 ± 2.42) J · K−1 · mol−1 and (561.03 ± 6.17) J · K−1 · mol−1 for Fe3PO7 and Fe4(P2O7)3, respectively. The calculated magnetic entropy of Fe3PO7 using the magnetic heat capacity suggests that the five 3d-electrons in the Fe3+ are in the t2g orbital with a low spin state according to crystal field theory.