The ilmenite form of NaNbO3 was synthesized hydrothermally. The ilmenite form of LiNbO3 was synthesized by ion exchanging the ilmenite form of NaNbO3. These transform irreversibly to the stable orthohombic form of NaNbO3 and the lithium niobate form of LiNbO3 at temperatures higher than 900 K. The energetics of the transformation was studied through differential scanning calorimetry and transposed temperature drop calorimetry. The calorimetric results show that the ilmenite form of NaNbO3 is metastable by 5.5 (±1.3) kJ/mole and the ilmenite form of LiNbO3 is metastable by 9.8 (±4.1) kJ/mole. Differential scanning calorimetry suggests that the LiNbO3 transformation occurs in two steps. The enthalpy difference between the ilmenite and the lithium niobate structures for LiNbO3 is compared with the difference in the lattice energies of the two structures predicted by H. J. Donnerberg, S. M. Tomlinson, C. R. A. Catlow, and O. F. Schirmer (Phys. Rev. B40, 11909, 1989) on the basis of an ionic model and is in good agreement with the calculations. This small energy difference lends credence to the LiNbO3 defect model proposed by D. M. Smyth (“ISAF’86—Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics,” p. 115, June 1986). The determination of the metastahility of the ilmenite form of NaNbO3 puts a limit on the tolerance factor for the stability of the perovskite structure.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry