Perovskite oxynitride dielectrics

Abstract

The synthesis, crystal structures and dielectric properties of perovskite oxynitrides of the type LnTiO2N (Ln = La, Nd and mixtures) and ATaO2N (A = Ca, Sr, Ba and mixtures), have been investigated. The end-member oxynitrides and their associated LaxNd1-xTiO2N, CaxSr1-xTaO2N and BaxSr1-xTaO2N solid solutions were successfully prepared by ammonolysis of the appropriate precursor oxides at temperatures in the range 900-1200ºC. The complete range of LaxNd1-xTiO2N solid solution are orthorhombic perovskites, which show a small increase in unit cell parameters with increasing La content across the range. Compositions in the CaxSr1-xTaO2N series are tetragonal for x < 0.6 and orthorhombic for x > 0.6. BaxSr1-xTaO2N solid solutions are tetragonal for x < 0.5 and cubic for x > 0.5. It was impossible to form any perovskite solid solution in the BaTaO2N-CaTaO2N series. Attempts were made to densify these oxynitrides by hot-pressing and spark plasma sintering, but a characteristic of all these oxynitride structures is that they are stable up to relatively low temperatures (1250oC) with decomposition occurring before any sintering can be achieved. A reducing environment is necessary to prevent oxidation, but this must not be too reducing to promote conversion of the transition metal into a lower oxidation state. As a result, fully dense samples were not obtained and dielectric property measurements could not be made on these samples. As an alternative, pure single phase LaTiO2N, NdTiO2N and LaxNd1-xTiO2N solid solutions were prepared as » 3 μm thick surface layers on dense pellets of the corresponding La2Ti2O7, Nd2Ti2O7 and (LaxNd1-x)2Ti2O7) oxides by ammonolysis. The bulk dielectric properties of these VI coated samples were then measured by LCR bridge techniques. The presence of an oxynitride layer significantly increased the measured dielectric constant of all samples, compared with the pure oxides, but a significantly higher dielectric loss was also observed. This lossy behaviour is believed to be due to the presence of a more conductive region of reduced La2Ti2O7 (of typical composition 2 2 7 La TiIV TiIIIO -x x -z ) situated immediately below the oxynitride layer, produced during synthesis by the presence of hydrogen in the nitriding ammonia atmosphere. The dielectric constant of all these oxynitrides was also measured in particulate form by impedance analysis of slurries and the dielectric constant calculated by a method of mixtures. Compared with the parent oxides, the dielectric constants were noticeably larger, ranging from 100-1500. These values are in reasonable agreement with the limited amount of data available in the literature, and show that this group of materials merits further exploration, providing easier synthesis routes can be developed, which also result in low loss final materials being obtained.