Heat capacities, order–disorder transitions, and thermodynamic properties of rare-earth orthoferrites and rare-earth iron garnets

Rare-earth orthoferrites, RFeO3, and rare-earth iron garnets (RIGs) R3Fe5O12 (R=rare-earth elements) were prepared by citrate–nitrate gel combustion method and characterized by X-ray diffraction method. Isobaric molar heat capacities of these oxides were determined by using differential scanning calorimetry from 130 to 860 K. Order–disorder transition temperatures were determined from the heat capacity measurements. The Néel temperatures (TN) due to antiferromagentic to paramagnetic transitions in orthoferrites and the Curie temperatures (TC) due to ferrimagnetic to paramagnetic transitions in garnets were determined from the heat capacity data. Both TN and TC systematically decrease with increasing atomic number of R   across the series. Lattice, electronic and magnetic contributions to the total heat capacity were calculated. Debye temperatures as a function of absolute temperature were calculated for these compounds. Thermodynamic functions like Cp,mo, Smo, Ho, Go, (HTo-H0o), (HTo-H298.15Ko), -(GTo-H298.15Ko)/T, ΔfHmo, and ΔfGmo have been generated for the compounds RFeO3(s) and R3Fe5O12(s) based on the experimental data obtained in this study and the available data in the literature.

Plot of molar heat capacities (Cp,mo) of R3Fe5O12(s) (R=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) against temperature (T). The inset shows the magnified portion of the heat capacity plot near the transition region indicating nearly same values of Curie temperatures for different R3Fe5O12(s).Figure optionsDownload as PowerPoint slide