THERMODYNAMIC PROPERTIES OF REE STANNATES
Ryumin M.A., Tyurin A.V., Nikiforova G.E., Gavrichev K.S.
Kurnakov Institute of General and Inorganic Chemistry of the RAS
119991, Moscow, Leninsky Prospect, 31
Compounds with the pyrochlore structure are considered potential thermal barrier materials due to their unique characteristics: high melting points, absence of phase transformations over a wide temperature range, and low thermal conductivity. Stannates of rare earth elements (REE) REE2Sn2O7 form a complete series of pyrochlores from La to Lu [1]. A distinctive feature of the pyrochlore structure is the presence of oxygen vacancies. These vacancies are believed to play a key role in reducing thermal conductivity, which is crucial for thermal barrier materials [2,3].
The work defines temperature-time regimes for obtaining stannates by various methods and shows that high temperatures are required for the formation of ceramic samples. Porous ceramic samples of Ln2Sn2O7 stannates with crystallite sizes of ≈100–400 nm were obtained. All obtained stannates were indexed under the assumption of a cubic syngony with parameters characteristic of compounds of the pyrochlore structural type. The chemical composition of the obtained stannates and the absence of impurities were confirmed using X-ray fluorescence spectroscopy.
Heat capacity measurements were performed in the temperature range of 5-350 K using a BKT-3 adiabatic vacuum calorimeter and at high temperatures (330-1300 K) using a Netzsch STA 449F1 Jupiter® differential scanning calorimetry (DSC). The temperature curves for the heat capacity of the studied stannates show no visible anomalies, indicating the absence of phase transitions in the temperature range under study. Based on smoothed values of the rare-earth stannate heat capacity in the range of 0–1300 K, the temperature dependences of standard thermodynamic functions—entropy S0(T), enthalpy increment H0(T)–H0(0), and reduced Gibbs free energy Ф0(T)—were calculated.
1. Kennedy B.J., Hunter B.A., Howard C.J. Structural and Bonding Trends in Tin Pyrochlore Oxides. // J. Solid State Chem. 1997. V. 130. P. 58-65.
2. Bonville P., Hodges J.A., Ocio M., Sanchez J.P., Vulliet P., Sosin S., Braithwaite D. Low temperature magnetic properties of geometrically frustrated Gd2Sn2O7 and Gd2Ti2O7 // J. Phys.: Condens. Matter. 2003. V. 15. P. 7777.
3. Klemens P.G. Phonon scattering by oxygen vacancies in ceramics. // Physica B 1999. Vol. 263. P. 102.
This work was supported by the Ministry of Science and Higher Education of the Russian Federation as part of the State Assignment of the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences.
This research was performed using the equipment of the Joint Research Centre of Physical Methods of Research, Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences.