Phase Relations in Ba 6 − 3 x Ln 8 + 2 x Ti 18 O 54 ( Ln = Nd & Sm ) Electroceramics

A careful, systematic investigation of Ba 6−3x Ln 8+2x Ti18O54 (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 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 (Nd 2 Ti 2 O 7 ) upon prolonged sintering which resulted in beneficial changes to Qf and cf without affecting εr. Two BLnTss ceramics compositions with near-zero cf were successfully synthesised with high Qf and


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 6−3 Ln 8+2 Ti 18 O 54 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 (  ), comparatively high quality factor (Qf), and moderate temperature coefficient of resonant frequency ( cf ) at microwave frequencies [4].
The tungsten bronze-type structure of these BLnTss ceramics is composed of corner sharing blocks of TiO 6octahedra of perovskite-like structure.These perovskitelike 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 Ln 3+ cations, 15-coordinate pentagonal sites (A2-sites) preferentially filled with Ba 2+ , 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 6−3 Ln 8+2 Ti 18 O 54 solid solution, Ba 2+ substitutes for Ln 3+ and vacancies on A1sites, 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  cf while Ln = Sm, Eu, and Gd display negative  cf [5].BLnTss compounds combining lanthanides with opposite  cf characteristics have been found to form a final product with a near-zero  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 6−3 Ln 8+2 Ti 18 O 54 [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].
Reagents were mixed in appropriate ratios, ball-milled for 12 hours under ethanol (polyoxymethylene canisters, stabilised ZrO 2 balls) and then dried in an oven to vaporise residual milling solvent.These powders were then sieved to a particle size <125 m 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 TiO 2 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.
A network analyser (Agilent E5062A) was used to characterise microwave dielectric properties based on the TE 011 resonant mode. values were calculated using "QZERO for Windows".The dielectric constant (  ) was obtained using a 3D finite element analysis (FEA) eigenmode solver.The temperature coefficient of resonant frequency ( cf ) was calculated via the relation  cf = Δ/Δ (ppm/K).

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  ≥ 0.4, and needle-like characteristics were observed for some grains within the  = 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.
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.
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   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. Figure 3(a) presents the microwave dielectric properties of the A-series samples.For  = 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   , and a highly negative  cf measured [16].The Qf and  cf of the A-Series samples display a nonlinear deterioration in Qf and a considerable decrease in  cf with increasing Sm content.The negative trend in  cf observed as Sm content increases originates from the contrasting contributions of the positive  cf Nd-BLnTss analogue and the negative  cf Sm-BLnTss analogue.The primary driver behind these differing  cf contributions is attributed to the TiO 6 octahedral tilting resultant from Nd and Sm positioning within the perovskite-like matrix of BLnTss [17].
It was found that Qf significantly deteriorated for  ≥ 0.4 compositions, and the near-linear slope of  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 (-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 TiO 6 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  cf for each composition.An improved linearity of the  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  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  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 (Nd 2 Ti 2 O 7 ).
Nd 2 Ti 2 O 7 has been reported to have a lower permittivity than BLnTss (  = 36), a higher quality factor (Qf = 16,400), and a highly negative temperature coefficient ( 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 grainboundary 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  cf with prolonged sintering indicate that the highly negative  cf of Nd 2 Ti 2 O 7 compensates for the positive  cf of BLnTss, Ln = Nd.Significant improvements in both  cf and Qf are therefore achieved through the formation of Nd 2 Ti 2 O 7 as a secondary phase.
Prolonged sintering of BLnTss (Ln = Nd) compounds is therefore shown to provide a simplified, one-step method for embedding Nd 2 Ti 2 O 7 into a dense BLnTss matrix.This result effectively enables tuning of the  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  cf was identified for BLnTss prolonged sinter samples; C2-60 with a Qf of 8343,   = 75, and a  cf of +3.0 ppm/K.While previous reports state that Ln 2 Ti 2 O 7 is one of the precursor compounds of BLnTss [9,15], there has been no suggestion until now that Nd-BLnTss is metastable for the  = 0.67 composition.It is suggested that previous investigations have not revealed the metastable nature of Nd-BLnTss  = 0.67, due to their use of microscale reagents and shorter sintering periods.It is proposed that this study, through the use of nanoscale TiO 2 and extended sintering periods, enhanced solid-state reaction characteristics exposing the metastable nature of Nd-BLnT  = 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  cf was identified, B8-60 with a Qf of 8044,   = 76.5, and a  cf of +3.8 ppm/K.

Conclusions
= 0.33 ceramics with high Sm content were found to experience a severe degradation of Qf and extreme negative trending of  cf corresponding with the onset of globular and needle-like grain morphology and the formation of a Ba/Tirich secondary phase at BLnTss grain-boundary regions. = 0.67 ceramics with high Nd content were found to exhibit a secondary phase (Nd 2 Ti 2 O 7 ) upon prolonged sintering which resulted in beneficial changes to Qf and  cf without affecting   .Two ceramics compositions with near-zero  cf were synthesised;  = 0.5,  = 0.8 (Qf = 8044,   = 76.5, and  cf = +3.8ppm/K) and  = 0.67,  = 0.2 (Qf = 8343,   = 75, and  cf = +3.0ppm/K).

Figure 2 :
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.

Table 1 :
Analysis represents a composite of the main phase and the dark contrast region due to restrictive grain size.