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

摘要

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 Neel temperatures (T-N) due to antiferromagentic to paramagnetic transitions in orthoferrites and the Curie temperatures (T-C) due to ferrimagnetic to paramagnetic transitions in garnets were determined from the heat capacity data. Both T-N and T-C 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 C-p,C-m degrees, S-m degrees, H degrees, G degrees, (H-T degrees - H-0 degrees), (H-T degrees - H-298.15K degrees), - (G(T)degrees - H-298.15K degrees) / T, Delta H-f(m)degrees, and Delta fG(m)degrees 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.