Phase Relations in Ba_{𝟔−𝟑𝑥}Ln_{𝟖+𝟐𝑥}Ti₁₈O₅₄ (Ln = Nd & Sm) Electroceramics

Abstract

A careful, systematic investigation of Ba_6−3𝑥Ln_8+2𝑥Ti₁₈O₅₄ (BLnTss) ceramics has been performed in order to understand the relationship between composition, microstructure evolution, and microwave dielectric properties. In this paper, we report the effects of composition, morphology, and sintering time on the phase relations and properties of BLnTss (Ln = Nd, Nd/Sm, Sm) ceramics. The microwave dielectric properties of the materials are reported in addition to phase characterisation and structural analysis via X-ray diffraction and field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. BLnTss, 𝑥=0.33, ceramics with high Sm content are found to experience a severe degradation of Qf and changes in 𝜏_cf trending, associated with the onset of globular and needle-like grain morphology and a Ba-Ti rich phase. 𝑥=0.67 ceramics with high Nd content are found to exhibit a secondary phase (Nd₂Ti₂O₇) upon prolonged sintering which resulted in beneficial changes to Qf and 𝜏_cf without affecting 𝜀_𝑟. Two BLnTss ceramics compositions with near-zero 𝜏_cf were successfully synthesised with high Qf and 𝜀_𝑟 values.

1. Introduction

Dielectric ceramics are fundamental building blocks of modern microwave telecommunications technology, being widely used as resonators in filters, phase shifters, and dielectric resonator antennas [1]. Ba_6−3𝑥Ln_8+2𝑥Ti₁₈O₅₄ solid solutions (BLnTss, Ln=lanthanide) in ceramic form have attracted significant interest since their discovery in 1968 [2] and subsequent investigation in 1981 [3] due to their high dielectric permittivity (𝜀_𝑟), comparatively high quality factor (Qf), and moderate temperature coefficient of resonant frequency (𝜏_cf) at microwave frequencies [4].

The tungsten bronze-type structure of these BLnTss ceramics is composed of corner sharing blocks of TiO₆-octahedra of perovskite-like structure. These perovskite-like blocks are in turn corner-connected to one another leading to three distinct potential locations for Ba and Ln cation insertion, forming channels running parallel to the b-axis; 12-coordinate rhombohedral sites (A1-sites within the perovskite-like block regions) preferentially occupied by Ln³⁺ cations, 15-coordinate pentagonal sites (A2-sites) preferentially filled with Ba²⁺, and 9-coordinate trigonal sites (C-sites) that remain vacant due to their restrictive size [5]. In order to form the upper limit of the Ba_6−3𝑥Ln_8+2𝑥Ti₁₈O₅₄ solid solution, Ba²⁺ substitutes for Ln³⁺ and vacancies on A1-sites, while the lower limit of the solid solution is determined by excessive vacancy formation in the A1- and A2-sites which destabilises the BLnTss structure. The dielectric properties of BLnTss ceramics vary substantially with the Ba/Ln ratio as well as with the selection of the lanthanide, as each results in a distortion of the ideal structure and its resultant properties [6]. BLnTss ceramics with Ln=La, Pr, and Nd have been reported to display a positive 𝜏_cf while Ln=Sm, Eu, and Gd display negative 𝜏_cf [5]. BLnTss compounds combining lanthanides with opposite 𝜏_cf characteristics have been found to form a final product with a near-zero 𝜏_cf when mixed in correct proportions [7], resulting in high commercial demand for compounds of this type [8].

It is noteworthy that the BLnTss system does not have a well-defined crystallisation temperature [9] and its X-ray diffraction patterns are rather too complicated to determine the phase composition, with common secondary phases “masked” by superposition of main diffraction peaks. As a result, there is some uncertainty surrounding the solid solubility limits for both the Ln=Sm and Ln=Nd analogues of Ba_6−3𝑥Ln_8+2𝑥Ti₁₈O₅₄ [9]. In this paper we report the effects of composition, sintering temperature, and time on the properties of BLnTss (Ln=Nd, Sm, and Nd-Sm) ceramics. A careful, systematic investigation is performed to build a relationship between composition, microstructure evolution, and microwave dielectric properties. This investigation, particularly, focuses on changes in morphology and the effects of prolonged annealing, which had previously been shown to result in unexplained cell parameter and dielectric properties variation deserving of further attention [10].

2. Materials and Methods

Ba_6−3𝑥(Nd_1−𝑦Sm_𝑦)_8+2𝑥Ti₁₈O₅₄ electroceramics were synthesized via solid state reaction using BaCO₃ (>2N, May and Baker), Nd₂O₃ (3N, Pi-KEM), Sm₂O₃ (3N, Pi-KEM), and TiO₂ (4N, PiKEM, 20 nm). The reported solid solution range of BLnTss (Ln=Sm, Nd) compounds is approximately 0.25(5)≤𝑥≤0.75(5) [8, 11–13]; therefore the compositions investigated in this paper were 𝑥=0.33, 0.5, 0.67, henceforth referred to as compositions A, B, and C, respectively. To observe the effects of varying the size of the lanthanide, for each composition A, B, and C, samples for 𝑦=0, 0.2, 0.4, 0.6, 0.8, and 1 were synthesised, henceforth referred to as A0, A2, A4, and so forth. All starting materials were thoroughly dried prior to weighing, with Nd₂O₃ and Sm₂O₃ undergoing an additional high temperature (1000°C) treatment prior to weighing to minimise stoichiometric errors due to the formation of Ln_𝑥O_𝑦(OH)_{3𝑥−2𝑦}.

Reagents were mixed in appropriate ratios, ball-milled for 12 hours under ethanol (polyoxymethylene canisters, stabilised ZrO₂ balls) and then dried in an oven to vaporise residual milling solvent. These powders were then sieved to a particle size <125 μm and calcined at 1100°C for 4 hours in air. Individual cylindrical samples were formed via 5 tonne anisotropic pressing using polyvinyl alcohol as a binding agent. Batches of each composition were subsequently sintered at temperatures between 1250 and 1375°C on platinum foil within an alumina crucible. To promote solid-state reaction characteristics and mitigate potential compositional variation effects, nanoscale TiO₂ was used as a starting reagent, and a minimum sintering period of 4 hours was selected [14, 15]. The optimum sintering temperature was found to be 1375°C with samples of each composition sintered for 4 hours (e.g., A0-4) and 60 (e.g., A0-60) hours to observe the effects of prolonged heat treatment. Densification was determined via the Archimedes method.

Phase composition was investigated via XRD (Siemens D-5000). Field emission scanning electron microscopy (FESEM) was used to characterise both morphology (Zeiss UltraPLus FESEM) and phase distribution (Hitachi 4300SE/N FESEM). Quantitative analyses were undertaken via electron probe microanalysis (EPMA on Hitachi: 15 kV, 600 pA) with Nd₂Ti₂O₇, SmP₅O₁₄, and BaSO₄ used as calibration standards.

A network analyser (Agilent E5062A) was used to characterise microwave dielectric properties based on the TE₀₁₁ resonant mode. 𝑄 values were calculated using “QZERO for Windows”. The dielectric constant (𝜀_𝑟) was obtained using a 3D finite element analysis (FEA) eigenmode solver. The temperature coefficient of resonant frequency (𝜏_cf) was calculated via the relation 𝜏_cf=Δ𝑓/𝑓Δ𝑇 (ppm/K).

3. Results and Discussion

All A-Series samples were initially identified as single phase via XRD. Sintered surface morphology displayed a significant increase in density with prolonged sintering time (Figures 1(a) and 1(b)) and increasing Sm content (Figures 1(a), 1(c), and 1(d)). All A-series samples displayed the formation of columnar-type grains characteristic of BLnTss ceramics, in good agreement with previous reports [9, 10]. Smooth “globular” shaped grains (indicated in Figure 1(c)) were evident in A-series samples with 𝑦≥0.4, and needle-like characteristics were observed for some grains within the 𝑦=1.0 sample (as indicated in Figure 1(d)). SEM backscatter imaging displayed distinct differential contrast between the columnar grains typical of BLnTss and the needle-like grains observed for A10-4 (Figure 1(e)) and A10-60, indicating the presence of a secondary phase.

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(b)

(c)

(d)

(e)

Figure 1 

SEM images of 𝑥=0.33 samples sintered at 1375°C for either 4 or 60 hours. Secondary electron images of (a) A0-4, (b) A0-60, (c) A6-4, and (d) A10-4 and matching backscattered electron images for (e) A10-4.

EPMA compositional analysis of both the main phase and the contrasting phase was undertaken on the “as-sintered” surface of the A10-60 sample. Due to the restrictive grain size of the darker contrast regions, an exact composition could not be determined; however these areas indicated increased levels of Ba and Ti and a significant reduction in Sm content when compared with the main phase (Table 1 (a)-(b)). In order to better identify the phase nature of the needle-type morphology grains, the A10-60 sample was polished and reexamined.

Table 1 

EPMA quantitative results for A10-60 sample, for “as-sintered” (a, b) and polished surfaces (c, d).

When polished, there was no detectable secondary phase in the A10-60 sample at low-to-medium magnification, with spot and area analyses confirming the nominal stoichiometry within the limits of experimental error. At high magnification, FESEM images were compared with the corresponding backscattered images (Figure 2(a)), to reveal darker patches almost indistinguishable at maximum backscatter detector contrast. Due to their small surface area, EPMA quantitative analysis of these darker areas was not feasible; however spot analysis (Table 1 (c)-(d)) concentrated on such grain regions and surface mapping (Figures 2(b)–2(d)) of the sample revealed the existence of regions rich in Ba and Ti, with reduced Sm content when compared to the main phase. These results, in good agreement with the EPMA compositional analysis of the as-sintered surface, indicate the presence of a Ba/Ti-rich secondary phase, either extremely poor in or completely devoid of Sm, and not previously reported for this composition. This secondary phase was unable to be further defined, as grain size limited the effectiveness of EPMA analysis, while the complex nature of the main phase X-ray diffraction pattern concealed this Bi/Ti-rich secondary phase.

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(b)

(c)

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Figure 2 

(a) SEM backscatter image and matching EPMA surface elemental mapping for (b) Ba, (c) Sm, and (d) Ti content for the A10-60 sample revealing the Ba- and Ti-rich secondary phase contained within the A10-60 sample. Brighter contrast in (b), (c), and (d) indicates a higher proportion of that element in comparison with darker areas.

Figure 3(a) presents the microwave dielectric properties of the A-series samples. For 𝑦=1, both the A10-4 and A10-60 samples were found to correlate well with the reported dielectric properties for single-phase Sm-BLnTss, with an extremely low Qf, a moderate 𝜀_𝑟, and a highly negative 𝜏_cf measured [16]. The Qf and 𝜏_cf of the A-Series samples display a nonlinear deterioration in Qf and a considerable decrease in 𝜏_cf with increasing Sm content. The negative trend in 𝜏_cf observed as Sm content increases originates from the contrasting contributions of the positive 𝜏_cf Nd-BLnTss analogue and the negative 𝜏_cf Sm-BLnTss analogue. The primary driver behind these differing 𝜏_cf contributions is attributed to the TiO₆ octahedral tilting resultant from Nd and Sm positioning within the perovskite-like matrix of BLnTss [17].

(a)

(b)

Figure 3 

Microwave dielectric properties of Ba6−3𝑥(Nd1−𝑦Sm𝑦)8+2𝑥Ti18O54 samples (a) A-Series (𝑥=0.33) and (b) C-Series (𝑥=0.67), sintered for 4 hours (red, solid line) and 60 hours (black, dashed line) at 1375°C. Error bars are displayed for all measurements; however in some cases error range is smaller than data points.

It was found that Qf significantly deteriorated for 𝑦≥0.4 compositions, and the near-linear slope of 𝜏_cf was altered for the same range of compositions (Figure 3(a)). These changes appear to correlate well with the appearance of noncolumnar grain morphologies for these samples (Figures 1(c)-1(d)). Qf begins to degrade as globular grains become apparent, with deleterious effects amplifying with increasing proportions of noncolumnar grain shapes. It is therefore suggested that this degradation in Qf is primarily due to the heterogeneous nature of the phase composition and morphologies in these samples, (i.e., significant variations in composition and a strong deleterious effect of the Ba/Ti-rich second phase). The composition and dielectric characteristics of this Ba/Ti-rich secondary phase are worth investigating further.

The dielectric constant was found to increase slightly with increased Sm content (𝑦-value). Generally speaking, the introduction of smaller isovalent lanthanides (Sm) within the tungsten bronze-type structure results in a lower dielectric permittivity, as the smaller average cell volume decreases the magnitude of potential off-centre Ti ion displacements within the TiO₆ octahedral framework [9]. Accordingly, increased Sm content should lower the dielectric permittivity of BLnTss ceramics. Our results exhibit a different trend, however, with the dielectric permittivity of BLnTss increasing with Sm content. As dielectric permittivity is also highly dependent on the extent to which the sample has undergone densification and crystallisation [1], this is attributed to similar polarisation of both end-members as reported previously [16] and a significant increase in densification observed in the Sm-rich compositions.

With increased sintering time, all A-series samples demonstrated an improved Qf as well as a change in value of 𝜏_cf for each composition. An improved linearity of the 𝜏_cf transition from positive to negative is also observed with increased sintering time. Improvements in Qf are attributed to an increase in crystallisation, homogenisation of grain size and composition, and a reduction in grain-boundary defects. This is evidenced by improved densification, a reduction in the proportion of globular and needle-like grains, and an observed change in the distribution of the Ba/Ti-rich secondary phase with prolonged sintering. The dielectric constant of the BLnTss ceramics remains almost unchanged as a function of sintering time, suggesting that the samples have been well sintered during the 4-hour sinter and have primarily undergone compositional homogenisation over the prolonged sintering period.

C-Series BLnTss samples sintered for 4 hours (Figure 3(b)) also displayed a nonlinear degradation in Qf, a slight increase in 𝜀_𝑟, and a considerable decrease in 𝜏_cf with increasing Sm content. Prolonged sintering of C-Series samples was shown to significantly improve crystallisation of Nd-rich samples, while little change was observed for the Sm-Rich samples. With prolonged sintering there were two significant changes in dielectric properties observed for the C0-60 and C2-60 samples: a significant improvement in Qf and a distinct change in the 𝜏_cf characteristics.

All C-Series samples were initially identified as single phase via XRD. Further investigation via SEM analysis revealed that while the Nd-rich 4-hour samples were single phase, the 60-hour samples displayed segments of distinctly different backscatter contrast (Figure 4) for low Sm content compositions (C0-60 to C6-60). The higher the proportion of Nd in the 60-hour samples, the higher the amount of observed secondary phase. EPMA analysis of these areas confirmed the presence of a secondary phase (Nd₂Ti₂O₇).

Figure 4 

Polished BSE SEM image of C0-60 displaying secondary phase (Nd2Ti2O7).

Nd₂Ti₂O₇ has been reported to have a lower permittivity than BLnTss (𝜀_𝑟=36), a higher quality factor (Qf=  16,400), and a highly negative temperature coefficient (𝜏_cf=−118 ppm/K) [18]. Improvements in Qf for C-Series Nd-rich samples undergoing extended sintering are therefore attributed to improved crystallisation, a reduction in grain-boundary defects due to increased grain size, improved homogeneity of the main BLnTss phase composition, and excretion of a high-Qf secondary phase. The observed changes in 𝜏_cf with prolonged sintering indicate that the highly negative 𝜏_cf of Nd₂Ti₂O₇ compensates for the positive 𝜏_cf of BLnTss, Ln=Nd. Significant improvements in both 𝜏_cf and Qf are therefore achieved through the formation of Nd₂Ti₂O₇ as a secondary phase.

Prolonged sintering of BLnTss (Ln=Nd) compounds is therefore shown to provide a simplified, one-step method for embedding Nd₂Ti₂O₇ into a dense BLnTss matrix. This result effectively enables tuning of the 𝜏_cf of BLnTss (Ln=Nd) compounds to near-zero values, whilst improving the density and compositional homogeneity of the primary phase. One multiphase composition with near-zero 𝜏_cf was identified for BLnTss prolonged sinter samples; C2-60 with a Qf of 8343, 𝜀_𝑟=75, and a 𝜏_cf of +3.0 ppm/K. While previous reports state that Ln₂Ti₂O₇ is one of the precursor compounds of BLnTss [9, 15], there has been no suggestion until now that Nd-BLnTss is metastable for the 𝑥=0.67 composition. It is suggested that previous investigations have not revealed the metastable nature of Nd-BLnTss 𝑥=0.67, due to their use of microscale reagents and shorter sintering periods. It is proposed that this study, through the use of nanoscale TiO₂ and extended sintering periods, enhanced solid-state reaction characteristics exposing the metastable nature of Nd-BLnT 𝑥=0.67.

BLnTss is reported to have the lowest structural strain for the C-Series composition [16], with apparent bond valence values closest to their ideal values for this composition [14]. Through phase segregation of a Nd- and Ti-rich secondary phase, the BLnTss primary phase would move toward a more Ba-rich composition, significantly increasing the global instability index (GII) of the compound based on previous reports [14]. Details of this are worth further investigation.

For B-Series samples, similar trends to the A and C-series samples were observed with regard to improved density and crystallisation with increased Sm content and sintering time. One composition with near-zero 𝜏_cf was identified, B8-60 with a Qf of 8044, 𝜀_𝑟=76.5, and a 𝜏_cf of +3.8 ppm/K.

4. Conclusions

𝑥=0.33 ceramics with high Sm content were found to experience a severe degradation of Qf and extreme negative trending of 𝜏_cf corresponding with the onset of globular and needle-like grain morphology and the formation of a Ba/Ti-rich secondary phase at BLnTss grain-boundary regions. 𝑥=0.67 ceramics with high Nd content were found to exhibit a secondary phase (Nd₂Ti₂O₇) upon prolonged sintering which resulted in beneficial changes to Qf and 𝜏_cf without affecting 𝜀_𝑟. Two ceramics compositions with near-zero 𝜏_cf were synthesised; 𝑥=0.5, 𝑦=0.8 (Qf=8044, 𝜀_𝑟=76.5, and 𝜏_cf=+3.8 ppm/K) and 𝑥=0.67, 𝑦=0.2 (Qf=8343, 𝜀_𝑟=75, and 𝜏_cf=+3.0 ppm/K).

Acknowledgments

Amanda L. Snashall, Yun Liu, and Ray L. Withers thank the Australian Research Council for financial support in the form of ARC Linkage Grants. Amanda L. Snashall is grateful for the funding supplied via her Australian Postgraduate Award Industry scholarship.