Heat capacity, thermodynamic and magnetic properties of the pyrochlore-like compounds RE2FeTaO7

Abstract

The isobaric heat capacity of pyrochlore-related compounds RE2FeTaO7 (RE = Sm and Gd) was measured using an adiabatic and a differential scanning calorimetry over a wide temperature range using adiabatic and differential scanning calorimetry. An anomaly on the curves of all the compounds is clearly observed within the low-temperature range, T < 25 K, while at the high temperatures the peculiarities were not detected. The standard thermodynamic functions: heat capacity, entropy, enthalpy change and reduced Gibbs energy of studied RE2FeTaO7 were calculated. The entropy of the anomalous contribution was equal to (14.8 and 19.6) J⋅K−1⋅mol−1 for Sm2FeTaO7 and Gd2FeTaO7, respectively. A comparative study of Y2FeTaO7 and Sm2FeTaO7 confirmed the relationship of low temperature magnetic properties with a heat capacity anomaly. The DC and AC magnetization measurements within the (300–2.3) K temperature range and in magnetic fields up to 5 kOe revealed spin glass (SG) transitions for Y2FeTaO7 and Sm2FeTaO7 at 4 K and 9 K, respectively. Against the background of developing SG in the frustrated Kagome sublattice, an antiferromagnetic transition at T ~ 8 K was detected for Sm2FeTaO7 in a large magnetic field. This transition occurs in Sm-sublattice and is responsible for the large magnetic contribution to the anomalous low temperature heat capacity in Sm2FeTaO7.