Synthesis and Stability of Perovskite Oxynitrides

Abstract

In the field of materials, there is continuous search for new compounds with enhanced performance for a variety of applications. Historically, perovskite oxides have been of particular interest, due to the ease of compositional-property manipulation and availability of inexpensive manufacturing processes. A new class of ceramic materials with a perovskite crystal structure has been synthesized by substituting nitrogen for oxygen in the anion lattice, e.g. A+2B+5O2N (A=Ca, Sr, Ba and B = Nb, Ta), A+3B+5ON2 (A=Ln and B = Ta, Nb) and A+3B+4O2N (A=Ln and B = Ti, Zr). Of interest in present work are applications as capacitors and lead-free piezoelectrics. Kim et al. characterized permittivity, dielectric loss and conductivity of ATaO2N (A=Ca,Sr,Ba) and BaNbO2N powder compacts (~45% porosity).1 Results indicated high permittivity for BaTaO2N (κ ~4870) and SrTaO2N (κ ~2870) that was relatively constant over a range of temperature (273K to 373K) and frequency (0.1 kHz to 1MHz). The origin of the high permittivity is not fully understood, since both compounds possess non-polar crystal symmetries and therefore, cannot exhibit permanent nonzero dipole moments. Various hypotheses have been proposed. Kim et al. contend that anion disorder varies the local structure thus inducing Ta+5 displacements and dipoles. Alternatively, formation of internal barrier layers was considered, but Kim et al. reason that the impedance results do not support this model. In contrast, Marchand et al. report that the dielectric response of a BaTaO2N polycrystalline pellet, deduced from infrared reflection spectra, is not high.2 The contradicting results merit further study and a property-material relationship as undetermined. In addition, dielectric loss was high (~0.2) and material had insufficient density (~55%) for use in application. Dense, low defect concentration oxynitride ceramic are essential for electrical property measurements. However, to control purity, while creating dense ceramic, requires an understanding of oxynitride synthesis and stability. Therefore, current work created a fundamental knowledge base for synthesizing oxynitride compounds that allowed large-batch production of phase-pure oxynitride powders. Unfortunately, investigations into sintering oxynitride powders identified material instabilities at high temperatures causing difficulties in forming dense ceramic. This led to development of a liquid precursor process for fabricating thin-film oxynitrides. Results indicate an insulating and dense thin-film that allowed a true characterization of electrical properties.