Solving the puzzle of the dielectric nature of tantalum oxynitride perovskites

1 Introduction

There were inconsistencies between the reported physical properties and the understanding of strontium and barium tantalum oxynitride perovskites. In the present manuscript both experimental findings and theoretical calculations are considered to clarify existing inconsistencies and a model structure is designed that can account for previous discrepancies.

2 The large dielectric permittivity reported for tantalum oxynitride perovskite ceramics

Dielectric properties were first reported for the oxynitride perovskites AMO₂N (A = Ba, Sr, Ca; M = Ta, Nb) after firing the compact samples (produced via cold isostatic pressing) under a flow of ammonia at T = 1020 °C for 2 h [1]. Among these materials, BaTaO₂N and SrTaO₂N exhibited unexpectedly high bulk dielectric constants of approximately 4900 and 2900, respectively, and their niobium derivatives showed metallic conduction. The high permittivity of these tantalum compounds together with the moderate temperature dependence was assumed to indicate ferroelectric-like behavior, although there was no experimental evidence for their ferroelectricity. The same research group also investigated BaTaO₂N thin films grown epitaxially on SrTiO₃ substrates by pulse laser deposition, and reported dielectric permittivities ranging from 200 to 240 with a slight frequency dependence [2]. The large variations in the permittivity values determined in the above two investigations were not fully explained.

Later studies examined the effects of the processing conditions on various ceramics in detail [3]. The addition of an alkaline earth carbonate was found to be necessary to compensate for the partial loss of A from the ATaO₂N compounds during densification above 1400 °C. A portion of the nitrogen contents in these materials was also lost during sintering, even when performed under nitrogen gas at a pressure of 0.2 MPa. Post-annealing of the ceramics in an ammonia flow was required to recover their original orange color, although the results of this annealing were greatly affected by the porosity of the ceramic. During this process, the ceramic surface readily recovered its stoichiometric composition and original orange color although the interior remained black or dark red. Samples having low densities had lower dielectric constants, ɛ. The value of ɛ increased continuously from 60 at a relative density (RD) of 55% to 400 at RD of 83% depending on their RD after the post-ammonolysis [4]. In addition, a slightly conductive, nonstoichiometric impurity phase was found to remain in the SrTaO₂N ceramics even after post-annealing in an ammonia flow. This residual impurity increased the apparent polarization of the ceramic such that it showed a large dielectric constant. It was unclear as to whether or not the oxynitride perovskites were in a paraelectric or ferroelectric state at room temperature during these prior studies.

3 The ferroelectric responses of tantalum oxynitride perovskites

The ferroelectric response of a tantalum oxynitride perovskite was first observed in a study in which piezoresponse force microscopy (PFM) was used to assess an orange surface layer peeled off from a black SrTaO₂N ceramic [5]. The most obvious piezoresponse was obtained at ±4 V and was lowered with duration. The local hysteresis loop shown in Figure 1(a) was observed over a range of ±12 V using a platinum electrode deposited on the same thin surface layer. Current leakage began at the coercive electric field and became significant above the range of ±12 V. It was possibly induced by a residual electronic effect related to crystal structure imperfections in the SrTaO₂N sample. Based on first principles analyses, the ferroelectric response of this ceramic was attributed to the presence of low-energy displacements having opposite polarization directions in the –Ta–N– coiled chain motif composed of TaO₄N₂ octahedra, which had a cis-configuration and exhibited relaxer-type behavior [6]. Similar ferroelectric responses have also been observed for BaTaO₂N ceramics [7]. The ferroelectric properties of these free-standing samples were completely different from those reported for thin films of epitaxially grown strained SrTaO₂N [8]. Based on PFM observations, the latter specimens were found to have inhomogeneous film surfaces. In addition, density functional theory (DFT) calculations indicated that the cis-type relaxer-like matrix contained small domains of a trans-type classical ferroelectric phase.

Figure 1:

(a) Local hysteresis curves acquired from a post-annealed SrTaO₂N ceramic surface layer and (b) the ferroelectric phase signal obtained from a BaTaO₂N crystal. Reprinted with permission from refs. [5, 9], respectively. Copyrights 2016 and 2019 by the American Chemical Society.

Reddish BaTaO₂N crystallites up to 3.1 μm in size have been grown from the reaction between BaTaO₂N powder and a BaCN₂ flux [9]. Figure 1(b) represents PFM data of the small crystals which demonstrate the appearance of ferroelectricity in conjunction with complete phase inversion. A PFM phase hysteresis loop and a butterfly amplitude curve originating from the ferroelectricity of the sample were clearly observed at 120 °C with an applied voltage of ±100 V, associated with a large coercivity of 50–60 V. Weak secondary harmonic wave generation (SHG) was also apparent at a wavelength of 532 nm in response to irradiation with a 1064 nm laser beam having a diameter of approximately 100 μm [9]. The observation of SHG suggested the presence of non-centrosymmetric regions in the BaTaO₂N crystals.

4 Theoretical support for ferroelectricity in tantalum oxynitride perovskites

Theoretical calculations based on Crystal Orbital Hamilton Population (COHP) chemical bonding analyses were performed for these oxynitride perovskites while preserving local site symmetries [10]. These analyses have shown that structures with cis-type local symmetry are much more stable than those with trans-type symmetry. DFT calculations clearly demonstrated that the crystallographic unit cells at T = 0 K can best be described as being orthorhombic with space group Pmc2₁ so as to optimize metal-nitrogen bonding. This result was in contrast to the average structures determined from experimental work, which comprised the cubic space group P m 3 ‾ m for BaTaO₂N, the tetragonal space group I4/mcm for SrTaO₂N and the orthorhombic space group Pnma for CaTaO₂N. The cubic structure of BaTaO₂N at room temperature was assumed to result from the random distributions of oxygen and nitrogen atoms throughout the Wyckoff sites, based on assessments of the molecular dynamics.

The orthorhombic Pmc2₁ space group is non-centrosymmetric, exhibiting the SHG phenomenon and may possibly have a ferroelectric domain structure. Virgin BaTaO₂N crystals are assumed to show no polarization at room temperature because the Pmc2₁ domains in this material are arranged in various directions and so, on average, have a cubic P m 3 ‾ m macroscopic structure. The oxygen and nitrogen atoms are not simply randomized in the crystal structure as expected in molecular dynamics. However, this compound can exhibit ferroelectric polarization after its poling treatment under an electric field exceeding the coercive field. The ferroelectricity is understood even in a classical way, not in the relaxer behavior proposed previously [6].