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 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. (c) 2007 Elsevier Inc. All rights reserved.