Cosa

ARTICLE IN PRESS

Journal of Solid State Chemistry 181 (2008) 101–121 www.elsevier.com/locate/jssc

Heat capacities, order–disorder transitions, and thermodynamic properties of rare-earth orthoferrites and rare-earth iron garnets
S.C. ParidaÃ, S.K. Rakshit, Ziley Singh
Product Development Section, Radiochemistry and Isotope Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085,India Received 14 August 2007; received in revised form 1 November 2007; accepted 5 November 2007 Available online 13 November 2007

Abstract Rare-earth orthoferrites, RFeO3, and rare-earth iron garnets (RIGs) R3Fe5O12 (R ¼ 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 weredetermined by using differential scanning calorimetry from 130 to 860 K. Order–disorder transition temperatures were determined from ´ the heat capacity measurements. The Neel 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 o o o o these compounds. Thermodynamic functions like C o , S o , Ho, Go, ðH o À H o Þ, ðH o À H o p;m m T 0 T 298:15 K Þ, ÀðG T À H 298:15 KÞ=T, Df H m , and Df G m 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. r 2007 Elsevier Inc. All rights reserved.
Keywords: Rare-earth orthoferrites; Rare-earth iron garnets; Differential scanning calorimetry; Order–disorder transformation; Isobaric molar heat capacity; Lattice heatcapacity; Electronic heat capacity; Magnetic heat capacity; Debye temperature; Thermodynamic properties

1. Introduction Rare-earth iron perovskites (orthoferrites) and rareearth iron garnets (RIGs) are among some of the important magnetic materials and have extensive applications in materials science and technology. The orthoferrties exhibit a great and bewildering array of magnetic properties.The practical applications of orthoferrites and some of their substituted analogues include high-temperature superconductors, electrode materials in solid oxide fuel cells, electrode materials in magneto-hydrodynamic generators, and materials for sensor technology. In recent years, there has been considerable progress in developing materials that do not exist in nature but can be createdartificially by layered deposition of different materials on an atomic or molecular scale to form superlattices. This method has been applied to the formation of ferromagnetic multilayers
ÃCorresponding author. Fax: +91 22 2550 5151.

E-mail address: sureshp@barc.gov.in (S.C. Parida). 0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2007.11.003

fromantiferromagnetic (AF) layers of LaFeO3 and LaCrO3, LaFeO3, LaMnO3, etc. [1–3]. The series of RIGs is probably the most thoroughly investigated one of all ferrites, because of several properties which make the experimental and theoretical studies very rewarding. The RIGs are ferrimagnets and are used in magnetic recording device and show giant magnetorestriction at low temperatures [4]. Similarly, the garnetend members have important applications in laser industry. By substituting various rareearth ions into the garnet lattice one can study the effect of these ions on the macroscopic properties. Within the rareearth iron garnets, there are a variety of magnetic interactions, which provide a detailed test for any proposed theoretical model. Most of the published literature on the physico-chemical...