# Effect of sintering additives on the densification and dielectric properties of Sr0.5Ba0.5Nb2O6 ceramics synthesized by a soft-chemistry method

Roberto Köferstein, Stefan Ebbinghaus

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Roberto Köferstein, Stefan Ebbinghaus. Effect of sintering additives on the densification and dielectric properties of Sr0.5Ba0.5Nb2O6 ceramics synthesized by a soft-chemistry method. Journal of Solid State Chemistry, 2022, 316, pp.123564. 10.1016/j.jssc.2022.123564. hal-03780547

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# Effect of sintering additives on the densification and dielectric properties of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics synthesized by a soft-chemistry method

Roberto Köferstein *, Stefan G. Ebbinghaus

Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Straße 2, 06120, Halle, Germany

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Keywords:

Sintering aid

Permittivity

Strontium barium niobate

Band gap

Lithium niobate

Relaxor

## ABSTRACT

Nano-crystalline Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> powders with addition of LiNbO<sub>3</sub> or LiF as sintering additives were prepared by a soft-chemistry synthesis using polyethylene glycol. Calcination at 600 °C results in nanocrystalline powders (d<sub>cryst.</sub> ≈ 30 nm) which were sintered between 1000 and 1300 °C to ceramic bodies. By addition of 10 and 20 mol % LiNbO<sub>3</sub>, the sintering temperature was reduced by about 200 K and the activation energy of the initial stage of sintering decreases from 386 to 271 kJ mol<sup>-1</sup>. The sintering aid improves the grain growth and dense ceramic bodies were obtained after sintering at 1125 °C for 1 h. A higher LiNbO<sub>3</sub> content favors the formation of a pillar-like microstructure. XRD patterns of ceramics sintered above 1000 °C show only reflections of the Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> phase indicating an incorporation of LiNbO<sub>3</sub> in the Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> structure. Dielectric measurements reveal a diffuse phase transition and a relaxor-like behavior. The phase transition temperature (T<sub>m</sub>) depends on the sintering conditions and is between 117 and 127 °C for 10 mol% LiNbO<sub>3</sub>, which is very close to pure Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, while addition of 20 mol% shifts T<sub>m</sub> to around 200 °C and the transition becomes slightly more diffuse. The optical band gap of the samples is ≈ 3.3 eV and depends slightly on the sintering conditions. We also tried LiF as sintering aid, but this leads to the formation of considerable amounts of secondary phases in the ceramics and relative densities of only 82%.

## 1. Introduction

As a lead-free and eco-friendly alternative to e.g. PZT (lead zirconate titanate), strontium barium niobate, Sr<sub>x</sub>Ba<sub>1-x</sub>Nb<sub>2</sub>O<sub>6</sub> (SBN), is an interesting candidate because of its ferroelectric, pyroelectric, piezoelectric, electro-optic, and photorefractive properties [1-4]. Due to its ferroelectric behavior, SBN is also used in magnetoelectric composites [5-7]. Furthermore, some authors report about photocatalytic activities [8,9]. Sr<sub>x</sub>Ba<sub>1-x</sub>Nb<sub>2</sub>O<sub>6</sub> is ferroelectric in the x range from 0.25 to 0.75 and crystallized in the tetragonal tungsten-bronze (TTB) structure. Its ferroelectric properties are sensitive to the composition and preparation procedure [10,11]. With rising strontium content an increasing relaxor behavior can be observed [12]. For Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, the permittivity maximum (T<sub>m</sub>), which reflects the ferroelectricity, ⇌ paraelectric phase transition, is between 110 and 130 °C for single crystals [3,11], while for polycrystalline samples values between 40 and 177 °C have been reported [13-17].

Usually, Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> is synthesized by the conventional mixed-oxide method which leads to coarse-grained powders. Additionally, few soft-chemistry syntheses have been reported to produce sub-micro or

nano-sized powders [14,18-21]. Compacts of strontium barium niobate reveal moderate densification behaviour. Thus, sintering to dense ceramic bodies requires high temperatures and long soaking times [14-22]. The densification behavior and the grain growth can generally be improved using sintering additives [23,24]. For example, Wang et al. [25] used SiO<sub>2</sub> as additive to decrease the sintering temperature. Due to the addition of SiO<sub>2</sub>, T<sub>m</sub> is shifted to lower temperatures. On the other hand, some authors utilized V<sub>2</sub>O<sub>5</sub> as additive (liquid phase sintering) and observed an increase of T<sub>m</sub> [26-28]. Furthermore, addition of Bi<sub>2</sub>O<sub>3</sub>, PbO and Nb<sub>2</sub>O<sub>5</sub> enhance the densification and influence the dielectric behavior marginally [27].

In this work, we investigated the influence of LiNbO<sub>3</sub> and LiF on the sintering and dielectric behavior of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>. The samples were synthesized by a one-pot soft-chemistry method leading to nano-scaled preceramic powders. Phase evolution and microstructure of ceramics bodies were monitored by XRD and SEM. The sintering behavior was investigated up to 1350 °C. Additionally, the samples were characterized by diffuse reflectance and dielectric measurements.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: roberto.koeferstein@chemie.uni-halle.de (R. Köferstein).</footnote>
## 2. Experimental

### 2.1. Material preparation

All samples were synthesized according to the one-pot soft-chemistry method as described earlier [14]. The amount of the added sintering additive is related to the amount of $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$. For the addition of $LiNbO_{3}$, the initial compositions of the samples were as follows: 1 mol $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ + x mol $LiNbO_{3}$ with x = 0.1 and 0.2 (= 3.75 and 7.50 wt%), abbreviated as SBN+LN-x.

In a typical reaction, 0.02 mol $NbCl_{5}$ (Alfa Aesar, 99%) was dissolved in 25 ml 2-methoxyethanol. 50 ml polyethylene glycol (PEG400) and 52 g citric acid were added under stirring on a heating plate at 150 °C until a clear solution resulted. Equimolar amounts of $SrCl_{2}·6H_{2}O$ (Merck, ≥ 99%) and $BaCl_{2}·2H_{2}O$ (Fluka, ≥ 99%) were solved together in 10 ml 1,2-ethanediol and $Li(CH_{3}COO)·2H_{2}O$ (Chemapol, purum) was solved in 5 ml 1,2-ethanediol. The amounts of the Sr-, Ba-, and Li-compounds are calculated as $n_{Sr} = n_{Ba} = n_{Nb}/(2[2+x])$ and $n_{Li} = n_{Nb}·x/(2+x)$ (x = 0.1 and 0.2; molar fraction of $LiNbO_{3}$ related to the amount of $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$). Afterwards, both the (Sr,Ba)- and Li-solutions were added to the hot Nb-solution. The reaction mixture was continuously stirred on the heating plate at 250 °C for about 3–4 h until it turned to a white highly viscous solution. The resulting (Sr,Ba,Nb,Li)-solutions were calcined at 600 °C for 2 h (rate 10 K min⁻¹) in static air leading to white nanoscaled powders. Ceramic bodies were fabricated from these powders by mixing with 10 wt% of a saturated aqueous polyvinyl alcohol (PVA) solution as pressing aid and uniaxially pressed at about 71 MPa into pellets (green density: 2.2 g cm⁻³). These pellets were sintered on a $ZrO_{2}$ fibre mat between 1000 and 1300 °C (heating-/cooling rate 5 K min⁻¹) with a soaking time of 1 h to the final ceramic bodies. These had a thickness of about 1–1.6 mm and diameters of 4–6 mm.

For comparative purposes, pure $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ (abbreviated as SBN) was also prepared according to the aforementioned synthesis.

The common sintering additive LiF was also tested. For this purpose, $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$, prepared as mentioned above, was calcined at 1100 °C for 2 h (rate 10 K min⁻¹) and mixed in a planetary mill with 76 mol% LiF (= 5.0 wt%, Sigma-Aldrich, 98%) in a polyamide jar using propane-2-ol and $ZrO_{2}$-balls. After drying, the white powder was pressed and sintered to ceramics as described above. The samples were abbreviated as SBN+LF.

### 2.2. Characterization

X-ray powder diffraction patterns were recorded at room temperature on a Bruker D8-Advance diffractometer, equipped with a one-dimensional silicon strip detector (LynxEye™) using Cu-Kα radiation and a counting time of 1 s per data point. Crystallite sizes and strain parameters were calculated from XRD line broadening (integral peak breadth) using the software suite WinXPOW [29] applying the Scherrer and Wilson equations. Differential thermoanalytic (DSC) investigations in flowing synthetic air (50 ml min⁻¹) were performed with a heating rate of 10 K min⁻¹ using a Netzsch STA 449F5 system. Atom absorption spectroscopy measurements were carried out with an Agilent AA 280FS. Scanning electron microscope images were collected with a Phenom ProX SEM in backscattered electron mode (BSE). For impedance investigations, the ceramic bodies with a thickness of 1.2–1.4 mm were sputtered on both sides with 100 nm thick gold electrodes using a Cressington Sputter Coater 108auto. An Impedance Analyzer 4192A (Hewlett Packard) was used for frequency- and temperature dependent impedance measurements.

## 3. Results and discussion

LiF is used as a common sintering additive for various ceramic materials [30–34], therefore we tested LiF to reduce the sintering temperature of $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$. Additions of 76 mol% (= 5 wt%) LiF to

$Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ leads to an enhanced densification at low sintering temperatures with relative densities of 80(1) and 82(1) % after sintering at 1050 °C and 1150 °C, whereas at 1200 °C a reduction to 77(1) % was observed. XRD investigations (Fig. S1, supporting information) of SBN+LF ceramics sintered at 1050 °C and 1200 °C leads to a mixture of $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$, $Li_{4}Sr_{3.06}Nb_{6}O_{20}$ (JCPDS #01-089-7516), $Li_{2}O$ (JCPDS #01-012-0254), and $Ba_{3}Nb_{2}O_{8}$ (JCPDS #01-072-0192). The fraction of secondary phases was calculated to be 25–30 wt%. For that reason, LiF is not suitable as sintering aid for $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$.

### 3.1. $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ + $LiNbO_{3}$

#### 3.1.1. Synthesis and powder characterization

In the following we used $LiNbO_{3}$ as sintering aid. 1 mol $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ + x mol $LiNbO_{3}$ (SBN+LN-x) with x = 0.1 and 0.2 were synthesized by a one-pot synthesis. To obtain white nanoscaled powders, the viscous (Sr,Ba,Nb,Li)-solutions were calcined at 600 °C for 2 h. The XRD patterns (Fig. 1) show only reflections of tetragonal $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ (JCPDS #01-074-6520) and rhombohedral $LiNbO_{3}$ (JCPDS #01-078-0250), however an increasing background level around 2θ = 30° indicates an amorphous fraction in the samples. The specific surface areas (BET method) were determined to be 32(3) m² g⁻¹ for x = 0.1 and 24(2) m² g⁻¹ for x = 0.2, which correspond to equivalent primary particle sizes of 35(3) and 47(3) nm. The volume-weighted average crystallite size of the $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$ phase for SBN+LN-0.1 and SBN+LN-0.2 is 31(3) and 29(3) nm, respectively.

#### 3.1.2. Sintering, microstructure, and phase composition of the ceramics

Non-isothermal dilatometric measurements of SBN and SBN+LN-x (x = 0.1 and 0.2) powder compacts up to 1350 °C in flowing air are shown in Fig. 2. The SBN sample starts to shrink slightly at about 1100 °C while at about 1220 °C the shrinkage increases significantly. The shrinking rate achieves a maximum at 1325 °C with -1.08% min⁻¹. The SBN+LN-x compacts with the sintering additive show an improved shrinkage behavior. A first slight shrinkage begins around 700 °C, whereas a considerable densification process starts at 1150 °C (SBN+LN-0.1) and 1000 °C (SBN+LN-0.2), respectively. The shrinkage maxima are shifted to lower temperatures of 1300 °C and 1209 °C with values in both cases of -1.28% min⁻¹ as shown in the inset of Fig. 2. The shrinkage rates of the three samples points to viscous flow (sliding process of whole grains) as a dominant shrinkage mechanism, because diffusion processes cause

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Room-temperature XRD patterns of SBN+LN-x (= $Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$+x-LiNbO$_{3}$) powders after calcining at 600 °C for 2 h (heating rate 10 K min⁻¹): (a) x = 0.1, (b) x = 0.2. The inset shows a magnification of the XRD pattern for x = 0.1 recorded with a prolonged counting time of 10 s per data point.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Non-isothermal dilatometric measurements up to 1350 °C (heating rate 5 K min⁻¹) in flowing air of (a) SBN, (b) SBN+LN-0.1, (c) SBN+LN-0.2 of compacted powders calcined at 600 °C. The inset shows the shrinkage rates.</figcaption></figure>

typical shrinkage rates between 10⁻⁴ and 10⁻¹ % min⁻¹ [35–38]. Furthermore, DSC measurements of SBN+LN-0.2 up to 1350 °C did not show any indication of the formation of a liquid phase (Fig. S2, supporting information) indicating that the densification process is caused by solid-state phase sintering. The activation energy of the initial state of sintering can be calculated from the non-isothermal dilatometric measurements applying the equation for the constant heating rate (CHR) method (1) [39,40].

$$T^2 \left( \frac{d(\Delta L)}{dt} \right) = \frac{2aE_a}{nR} \cdot \frac{\Delta L}{L_0} \quad (1)$$

where, T = temperature, ΔL L₀⁻¹ = relative shrinkage, t = time, a = heating rate, n = constant depending on the mechanism of initial stage of sintering, Eₐ = activation energy, R = universal gas constant. As mentioned above, the initial stage of sintering can be described by the viscous flow mechanism (n = 2 [39]). Thus, Eₐ can be calculated from the slope by plotting the left hand-side of equation (1) versus ΔL L₀⁻¹ (as depicted in Fig. 3). For Sr₀.₅Ba₀.₅Nb₂O₆ the activation energy for the initial stage of sintering was calculated as 386(22) kJ mol⁻¹, which is significantly lower than the activation energy of 825 kJ mol⁻¹ found for coarse-grained Sr₀.₅Ba₀.₅Nb₂O₆ synthesized by the conventional

<figure><img src="image_6.png" /><figcaption>Fig. 3. Plot of T²[d(ΔL L₀⁻¹)dt vs. ΔL L₀⁻¹. For the sake of clarity, every 30th data point is marked by a symbol. Solid lines are the fit results.</figcaption></figure>

mixed-oxide method [41]. By addition of LiNbO₃ the activation energy decreases to 300(18) and 271(15) kJ mol⁻¹ for SBN+LN-0.1 and SBN+LN-0.2, respectively.

Based on these results, for further investigations the nanocrystalline powders were pressed to pellets and sintered in static air at temperatures between 1000 and 1300 °C for 1 h (heating-/cooling rate: 5 K min⁻¹). The bulk densities of the white-yellow ceramics bodies were calculated from their weight and geometric dimensions. The obtained densities are related to the single crystal density of Sr₀.₅Ba₀.₅Nb₂O₆ [42]. Dense ceramics (relative density ≥90%) of pure Sr₀.₅Ba₀.₅Nb₂O₆ can be only obtained after firing above 1300 °C. In contrast, by the addition of LiNbO₃ densities above 90% can be obtained already at 1175 °C (x = 0.1) and 1125 °C (x = 0.2) as shown in Fig. 4. Increasing temperatures raise the relative bulk densities further up to 96(1) %. The SBN+LN-0.2 ceramics show a very slight decrease in density after firing at 1300 °C due to a change in the microstructure as described below. In summary, the use of the sintering additive LiNbO₃ results in dense ceramics at significantly lower temperature and shorter sintering time, compared to pure Sr₀.₅Ba₀.₅Nb₂O₆.

Typical microstructures of SBN+LN-x ceramics are shown in Fig. 5. The average grain size (Øli) was estimated by counting of at least 700 grains using the linear intercept method [43]. Ceramics with x = 0.1 consist of globular and irregular shaped grains. Their sizes are between 0.5 and 3.7 µm (Øli = 1.7(4) µm), 1–7 µm (Øli = 3.0(5) µm), and 1–18 µm (Øli = 6(2) µm) after sintering at 1150, 1200, and 1300 °C, respectively. SBN+LN-0.2 samples show globular/irregular grains after sintering at 1100 °C and 1150 °C with grains sizes between 0.7 and 5.2 µm (Øli = 2.3(2) µm) and 1–10 µm (Øli = 4.8(4) µm). On the other hand, sintering at 1200 °C and 1300 °C leads to a change of the microstructure. After firing at 1200 °C, we find a bimodal grain size distribution with irregular grains from 1.2 to 6 µm and few grains of about 8 µm. In addition small pillars with lengths between 1.5 and 4 µm and widths ranging from 3 to 19 µm as well as large rectangular micro crystals up to 200 µm were observed (Fig. S3, supporting information). After sintering at 1300 °C the ceramic bodies exclusively consist of pillars with dimensions similar to the ones after firing at 1200 °C and larger rectangular crystals with lengths up to 400 µm as shown in Fig. 5d.

The phase composition after sintering was examined by XRD. Fig. 6 shows the XRD patterns of SBN+LN-x sintered ceramics. After sintering at 900 °C reflections of tetragonal Sr₀.₅Ba₀.₅Nb₂O₆, and traces of orthorhombic SrNb₂O₆ (JCPDS #00-028-1243) are observed. Additionally, the sample with x = 0.2 shows the presence of rhombohedral LiNbO₃. The LiNbO₃ and SrNb₂O₆ reflections disappear after sintering at 1000 °C for both x = 0.1 and 0.2, as visible in Fig. 6c and d. Up to the highest sintering temperature of 1300 °C the XRD patterns only show reflections of the Sr₀.₅Ba₀.₅Nb₂O₆ phase (Fig. 6e,f). Even a prolonged counting time of

<figure><img src="image_7.png" /><figcaption>Fig. 4. Bulk densities of ceramic bodies after sintering at various temperatures for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. SEM-BSE images of selected SBN+LN-x ceramic bodies sintered at different temperatures (T_s) for 1 h: (a) T_s = 1150 °C, (b) T_s = 1150 °C (c) T_s = 1300 °C, and (d) T_s = 1300 °C.</figcaption></figure>

10 s per step does not reveal any traces of secondary phases (Fig. S4, supporting information) suggesting the formation of a solid solution between Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> and LiNbO<sub>3</sub> in the tetragonal tungsten-bronze structure which will be the subject of further investigations. The proposed formation of a solid solution is additionally supported by the absence of the melting peak of LiNbO<sub>3</sub> around 1250 °C [44,45] in the DSC measurement as mentioned above. The solid solution can be written as Li<sub>y</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-y/2</sub>Nb<sub>2</sub>O<sub>6</sub>, where y represents the molar fraction of LiNbO<sub>3</sub> which can be calculated from the x parameter as y = x/(1+x). Solid solutions between SBN and (K<sub>1/3</sub>Na<sub>2/3</sub>)NbO<sub>3</sub> has been reported by several authors [46–48]. Because of the volatility of lithium salts at high temperatures, sintering often leads to a partial sublimation of Li<sub>2</sub>O [49–51]. For our samples, atom absorption spectroscopy indicates that the loss of lithium accounts to about 6–7% after sintering at 1300 °C for 1 h.

The optical band gap energies were obtained from diffuse reflectance spectra using the Kubelka–Munk theory. The optical band gap (E_g) can be described by equation (2) [52]:

$$F(R) \cdot h\nu = k(h\nu - E_g)^{1/n} \qquad (2)$$

(F(R) – Kubelka–Munk function, k – energy-independent constant, E_g – optical band gap, n – exponent reflecting the type of transition with n = 2 for direct allowed transitions and n = 0.5 for indirect allowed transitions). As described elsewhere, Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> has an indirect band gap of 3.40–3.29 eV, depending on the sintering temperature [14,53]. Thus, E_g can be calculated by plotting of (F(R) · hν)<sup>0.5</sup> versus hν and extrapolating the slope to F(R) → 0 (Fig. 7). Using this method, the band gap energies of SBN+LN-x ceramics with x = 0.1 were determined as 3.39(4) and 3.32(2) eV after sintering at 1100 and 1150 °C, whereas sintering at

1200 and 1300 °C gave values of 3.26(3) eV. However, in x = 0.2 ceramics, sintered above 1100 °C, an additional absorption feature appears at lower energies in the range between 2.4 and 3.1 eV, most likely caused by defects due the incorporation of Li<sup>+</sup> into the Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> structure [54–56]. Because of the overlapping of these two absorption regions, we used a baseline approach according to Makula et al. [47] to derive the band gap as seen in Fig. 7. The so obtained band gaps are 3.34(2) eV after sintering at 1100 °C and 3.36(6) eV for x = 0.2 samples sintered between 1150 and 1300 °C. As a conclusion, the use of the sintering additive LiNbO<sub>3</sub> does not lead to a considerable change of the band gap compared to pure Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> [14].

### 3.1.3. Impedance spectroscopy

Fig. 8 shows the evolution of the relative permittivity (ε<sub>r</sub>′) and dissipation factor (tan δ) with temperature for ceramic bodies with x = 0.1 at 1 kHz. Samples fired at 1150, 1200, and 1300 °C show diffuse ferroelectric ↔ paraelectric phase transitions with broad maxima at 127(1), 117(1), and 118(1) °C, close to pure Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> [3,14]. The ε<sub>r</sub>′ values at these maxima increases with sintering temperature from 1362(123) to 3135(282) most likely caused by increasing grain sizes and densities. Fig. 9 shows the temperature (T_m) of the permittivity maxima depending on frequencies. T_m is shifted towards higher temperatures with increasing frequencies due to the relaxor-like behavior of the samples [57,58]. The shift of T_m in the range from 1 kHz to 3 MHz is 6(2) K for the sample sintered at 1150 °C and increases to 11(2) K after sintering at 1300 °C (inset in Fig. 9).

The evolution of the permittivity depending on temperature for SBN+LN-0.2 ceramics can be seen in Fig. 10a. The samples with x = 0.2 show a shift of T_m towards higher temperatures. Sintering at 1100 °C does not lead to a clear phase transition at 1 kHz, whereas frequencies
<figure><img src="image_9.png" /><figcaption>Fig. 6. Room-temperature XRD patterns of SBN+LN-x ceramics after sintering at different temperatures (T<sub>s</sub>) for 1 h (heating-/cooling rate 5 K min<sup>-1</sup>).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. (F(R) · hν)<sup>0.5</sup> versus hν of SBN+LN-x ceramics sintered at 1200 °C for 1 h. (a) x = 0.1, (b) x = 0.2. The inset shows the diffuse reflectance spectra.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Temperature dependence of the real part of the relative permittivity (ε'<sub>r</sub>) and tan δ at 1 kHz of SBN+LN-0.1 ceramic bodies sintered for 1 h at: (a) 1150 °C, (b) 1200 °C, and (c) 1300 °C. The uncertainty of the data is smaller than the symbol size and is lower than 5%.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Temperature dependence of the relative permittivity (ε'<sub>r</sub>) and the dissipation factor (tan δ) at various frequencies for SBN+LN-0.1 sintered at 1200 °C for 1 h. The inset shows the temperature of the permittivity maximum (T<sub>m</sub>) depending on frequency and sintering temperature for samples sintered at: (a) 1150 °C, (b) 1200 °C, and (c) 1300 °C. The uncertainty of the data is smaller than the symbol size and is lower than 5%.</figcaption></figure>

above 1 kHz reveal a maximum at 213(2) °C (Fig. 10b and Fig. S5, supporting information). The permittivity maximum of the dense ceramic, sintered at 1150 °C (x = 0.2), appears at 196(2) °C with ε'<sub>r</sub> = 3071(277) at 1 kHz and shifts to 200(2) °C at 3 MHz with ε'<sub>r</sub> = 2580(232) (Fig. S6, supporting information). The development of ε'<sub>r</sub> for samples (x = 0.2) sintered at 1200 and 1300 °C shows only at frequencies ≥100 kHz a clear permittivity maximum at 223(2) and 221(2) °C corresponding to the diffuse ferroelectric ↔ paraelectric phase transition (Figs. S7 and S8, supporting information). Moreover, both the considerable shift of T<sub>m</sub> to higher temperatures and the reduction of the permittivity maxima after sintering from 1150 to 1300 °C correlates with the appearance of a pillar-like microstructure as also observed in pure Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> [14]. Frequency-dependent dielectric measurements at room temperature (23 °C) in the range from 0.5 kHz to 10 MHz are shown in Fig. 11. The samples show decreasing permittivity values with rising frequency, while the dissipation factor (tan δ) slightly increases. The ε'<sub>r</sub> values of the x = 0.1 ceramics (Fig. 11a) rise with sintering temperature caused by an increase in density and grain sizes. However, a different behavior can be observed for x = 0.2 ceramics (Fig. 11b). Sintering from 1100 to 1150 °C results in larger permittivities, while firing at 1200 and 1300 °C leads to significant lower ε'<sub>r</sub> values, due to the change in microstructure after
<figure><img src="image_13.png" /><figcaption>Fig. 10. Temperature dependence of the real part of ε' and tan δ at 1 kHz (a) and 100 kHz (b) of SBN+LN-0.2 ceramic bodies sintered for 1 h at: (a) 1100 °C, (b) 1150 °C, (c) 1200 °C and (d) 1300 °C. The uncertainty of the data is smaller than the symbol size and is lower than 5%.</figcaption></figure>

sintering at these temperatures as mentioned above. The ceramics sintered at 1200 °C (x = 0.2) consist of globular/irregular grains along with pillar-like grains which effects higher ε' values compared to those sintered at 1300 °C, which only show pillar-like grains. Decreasing permittivity values by changing from a globular/irregular to a pillar-like microstructure were also observed in Sr0.5Ba0.5Nb2O6 ceramics [14].

The dielectric behavior of a relaxor ferroelectric can be expressed by the following Curie-Weiss type law (3) [59]:

$$ \frac{1}{\epsilon} - \frac{1}{\epsilon_m} = \frac{(T - T_m)^\gamma}{C} \qquad (3) $$

where, ε_m is the permittivity maximum, T_m is the temperature at ε_m, C is a constant, and the exponent γ is the diffuseness coefficient. Thus, γ can be obtained from a plot of ln(ε⁻¹ - ε_m⁻¹) versus ln(T - T_m). x = 0.1 ceramics reveal diffuseness coefficients between 1.7 and 1.8, which is comparable with pure Sr0.5Ba0.5Nb2O6 [14], whereas x = 0.2 samples show a slightly larger γ value of 1.8–1.9. Larger γ values indicate an increasing degree of lattice disorder most likely due to formation of a solid solution between LiNbO3 and Sr0.5Ba0.5Nb2O6 [60,61].

The discussion above is based on the model of a lossy capacitor and does not consider a possible dc-conductivity. At elevated temperatures the ceramics show considerable dc-conductivities suggesting a semi-conducting nature. As reported elsewhere [62] for a single resistance-capacitor (RC) element, the specific impedance (ρ*) is described by equation (4):

<figure><img src="image_14.png" /><figcaption>Fig. 11. Frequency dependence of the real part of ε' and the dissipation factor at 23 °C for SBN+LN-x ceramic bodies sintered at the indicated temperatures. (a) x = 0.1, (b) x = 0.2. The uncertainty of the data is smaller than the symbol size and is lower than 5%.</figcaption></figure>

$$ \rho^* = \frac{\rho_{dc}}{1 + (i\omega\tau)^\beta} \qquad (4) $$

(β – constant phase shift (CPE) coefficient, τ = ρdcεε₀). The high-temperature impedance data were modelled using equivalent circuits consisting of one or two parallel resistance-capacitor (RC) elements. For x = 0.2 ceramics sintered at 1200 and 1300 °C, the impedance data reveal two semicircle arcs, which can be described by two parallel connected RC-elements suggesting two different relaxation processes (Fig. 12). The two obtained relative permittivities for each of these ceramics are 13·10³ and 27·10⁴ sintered at 1200 °C, whereas values of and 9·10³ and 17·10⁴ were found after sintering at 1300 °C. The smaller values can be related to bulk contributions, while the larger values most likely corresponds to grain boundary effects, as, according to Irvine et al. [63], the bulk permittivity is at least one order of magnitude lower than grain boundary permittivity. Cole-Cole plots for the other ceramic samples (Fig. S9 and Fig. S10, supporting information) show single semicircle arcs indicating a single relaxation process. Because of the calculated permittivities between 1.6·10³ and 3.5·10³ these values can be assigned to the bulk effect, showing that grain boundary contributions play a negligible role in these samples.

Due to the addition of LiNbO3, we observed an increase of the permittivity maximum up to 50% and a considerable shift of T_m for x = 0.2, compared to pure Sr0.5Ba0.5Nb2O6 [14].

## 4. Conclusion

Sr0.5Ba0.5Nb2O6 ceramics with the addition of 10 and 20 mol%
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Cole-Cole plots measured at the indicated temperatures of SBN+LN-0.2 ceramic bodies sintered for 1 h at (a) 1200 °C and (b) 1300 °C. Fits were carried out using two parallel connected RC-elements. Lines are the respective fit results, while symbols correspond to experimental data. The uncertainty of the data is smaller than the symbol size and is lower than 5%.</figcaption></figure>

LiNbO₃ as sintering aid were synthesized by a one-pot method using PEG400. Calcining at 600 °C results in white nanocrystalline powders with crystallite sizes of 31(3) and 29(3) nm and specific surface areas of 32(3) and 24(2) m² g⁻¹. Dilatometric measurements on powder com-pacts show that the sintering starts at significant lower temperatures with addition of LiNbO₃. The activation energies for the initial stage of sintering were calculated as 300(18) and 271(15) kJ mol⁻¹ for 10 and 20 mol% LiNbO₃, respectively, while Sr₀.₅Ba₀.₅Nb₂O₆ without any sintering additive reveals an activation energy of 386(22) kJ mol⁻¹. Dense ceramic bodies were obtained after sintering at 1125 °C, which is about 200 K lower than for Sr₀.₅Ba₀.₅Nb₂O₆ ceramics without any sintering aid. XRD and DSC measurements indicate the incorporation of LiNbO₃ into the Sr₀.₅Ba₀.₅Nb₂O₆ structure by forming a solid solution during sintering. Temperature and frequency-dependent dielectric investigations show a diffuse ferroelectric ⇌ paraelectric phase transition and a relaxor behavior. The dielectric maximum (T_m) slightly depends on the sintering temperature. T_m is around 120–130 °C for the addition of 10 mol% LiNbO₃ and shifts to about 200 °C for 20 mol%.

LiNbO₃ as sintering additive considerably reduced the sintering temperatures of SrₓBa₁₋ₓNb₂O₆ ceramics without the formation of a liquid phase. Due to the additive, the phase transition temperature (Tₘ) and the grain sizes can be easily tuned. LiNbO₃ is also a potential candidate as sintering aid in SrₓBa₁₋ₓNb₂O₆ based magnetoelectric composites.

### CRediT authorship contribution statement

**Roberto Köferstein:** Conceptualization, Data curation, Formal analysis, Investigation, Project administration, Resources, Validation, Visualization, Writing – original draft, Writing – review & editing.
**Stefan G. Ebbinghaus:** Conceptualization, Project administration, Resources, Supervision, Validation, Writing – original draft.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

We are grateful to Dr. F. Oehler for AAS investigations.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jssc.2022.123564.

### References

[1] A.M. Glass, Investigation of the electrical properties of Sr₁₋ₓBaₓNb₂O₆ with special reference to pyroelectric detection, J. Appl. Phys. 40 (1969) 4699–4713.

[2] D. Rytz, B.A. Wechsler, R.N. Schwartz, C.C. Nelson, C.D. Brandle, A.J. Valentino, G.W. Berkstresser, Temperature dependence of photorefractive properties of strontium-barium niobate (Sr₀.₆Ba₀.₄Nb₂O₆), J. Appl. Phys. 66 (1989) 1920–1924.

[3] T. Lukasiewicz, M.A. Swirkowicz, J. Dec, W. Hofman, W. Szyrski, Strontium–barium niobate single crystals, growth and ferroelectric properties, J. Cryst. Growth 310 (2008) 1464–1469.

[4] R.R. Neurgaonkar, L.E. Cross, Piezoelectric tungsten bronze crystals for SAW device applications, Mater. Res. Bull. 21 (1989) 893–899.

[5] S. Tiwari, S. Vitta, Magnetoelectric and magnetodielectric coupling and microwave resonator characteristics of Ba₀.₅Sr₀.₅Nb₂O₆/CoCr₀.₄Fe₁.₆O₄ multiferroic composite, Sci. Rep. 8 (2018), 11619.

[6] X.M. Chen, Y.H. Tang, I.-W. Chen, Z.C. Xu, S.Y. Wu, Dielectric and magnetoelectric characterization of CoFe₂O₄/Sr₀.₅Ba₀.₅Nb₂O₆ composites, J. Appl. Phys. 96 (2004) 6520–6522.

[7] R. Köferstein, F. Oehler, S.G. Ebbinghaus, Fine-grained magnetoelectric Sr₀.₅Ba₀.₅Nb₂O₆–CoFe₂O₄ composites synthesized by a straightforward one-pot method, Mater. Chem. Phys. 278 (2022), 125616.

[8] G. Han, S. Cao, B. Lin, UV photocatalytic activity for water decomposition of SrₓBa₁₋ₓNb₂O₆ nanocrystals with different components and morphologies, J. Chem. (2017), 2163608.

[9] K. Yoshida, H. Masai, Y. Takahashi, R. Ihara, T. Fujiwara, Fabrication of Sr₀.₅Ba₀.₅Nb₂O₆-precipitated microstructured ceramics for photocatalytic application, J. Ceram. Soc. Jpn. 119 (2011) 731–735.

[10] S.-I. Kang, J.-H. Lee, J.-J. Kim, H.Y. Lee, S.-H. Cho, Effect of sintering atmosphere on densification and dielectric characteristics in Sr₀.₅Ba₀.₅Nb₂O₆ ceramics, J. Eur. Ceram. Soc. 24 (2004) 1031–1035.

[11] A.A. Ballmann, H. Brown, The growth and properties of strontium barium metaniobate SrₓBa₁₋ₓNb₂O₆, A tungsten bronze ferroelectric, J. Cryst. Growth 1 (1967) 311–314.

[12] W.H. Huang, Dwight Viehland, R.R. Neurgaonkar, Anisotropic glasslike characteristics of strontium barium niobate relaxors, J. Appl. Phys. 76 (1994) 490–496.

[13] S.H. Kshirsagar, A.N. Tarale, S.R. Jigajeni, D.J. Salunkhe, P.B. Joshi, Effect of Ni doping on ferroelectric, dielectric and magneto dielectric properties of strontium barium niobate ceramics, Indian J. Pure Appl. Phys. 53 (2015) 119–124.

[14] R. Köferstein, F. Oehler, S.G. Ebbinghaus, Investigations of nano-crystalline Sr₀.₅Ba₀.₅Nb₂O₆ and bulk ceramics synthesized by a polymerization method using PEG400, J. Eur. Ceram. Soc. 39 (2019) 1156–1163.

[15] M. Said, T.S. Velayutham, W.C. Gan, W.H. Abd Majid, The structural and electrical properties of SrₓBa₁₋ₓNb₂O₆ (SBN) ceramic with varied composition, Ceram. Int. 41 (2015) 7119–7124.

[16] P.K. Patro, A.R. Kulkarni, C.S. Harendranath, Microstructure and dielectric properties of strontium barium niobate ceramics synthesized by partial coprecipitation, J. Eur. Ceram. Soc. 23 (2003) 1329–1335.

[17] T. Chen, S.Y. Wu, X.Q. Liu, X.M. Chen, A novel sol–gel route to synthesize (Sr₀.₅Ba₀.₅)Nb₂O₆ ceramics with enhanced electrocaloric effect, J. Adv. Dielectr. 7 (2017), 1750012.

[18] A.B. Panda, A. Pathak, P. Pramanik, Low temperature preparation of nanocrystalline solid solution of strontium–barium–niobate by chemical process, Mater. Lett. 52 (2002) 180–186.

[19] S.R. Dhage, R. Pasricha, V. Ravi, Synthesis of Sr₀.₅Ba₀.₅Nb₂O₆ by urea method, Mater. Lett. 59 (2005) 1053–1055.

[20] R. Pasricha, V. Ravi, Synthesis of Sr₀.₅Ba₀.₅Nb₂O₆ by citrate gel method, Mater. Chem. Phys. 94 (2005) 34–36.

[21] C.M. Dudhe, S.B. Nagdeot, C.P. Chaudhari, Ferroelectric domains in Sr₀.₅Ba₀.₅Nb₂O₆ (SBN50) at nanolevel, Ferroelectrics 482 (2015) 104–112.

[22] S.B. Deshpande, H.S. Potdar, P.D. Godbole, S.K. Date, Preparation and ferroelectric properties of SBN:50 ceramics, J. Am. Ceram. Soc. 75 (1992) 2581–2585.

[23] R. Köferstein, L. Jäger, M. Zenkner, H.-P. Abicht, Preparation and sintering behaviour of a fine grain BaTiO₃ powder containing 10 mol% BaGeO₃, J. Mater. Sci. 43 (2008) 832–838.

[24] A. Talimian, H.F. El-Maghaby, M. Michálkova, D. Galusek, Sintering and grain growth behaviour of magnesium aluminate spinel: effect of lithium hydroxide addition, J. Eur. Ceram. Soc. 41 (2021) 5634–5643.
[25] L. Wang, S. Wanmei, M. Kunchang, L. Shijun, Effect of silica sintering additive on the sintering behavior and dielectric properties of strontium barium niobate ceramics, J. Eur. Ceram. Soc. 29 (2009) 1427-1432.

[26] Q.W. Huang, L.H. Zhu, J. Xu, P.L. Wang, H. Gu, Y.B. Cheng, Effect of V₂O₅ on sintering behaviour, microstructure and dielectric properties of textured Sr₀.₄Ba₀.₆Nb₂O₆ ceramics, J. Eur. Ceram. Soc. 25 (2005) 957-962.

[27] S. Nishiwaki, J. Takahashi, K. Kodaira, Effect of additives on microstructure development and ferroelectric properties of Sr₀.₃Ba₀.₇Nb₂O₆ ceramics, Jpn. J. Appl. Phys. 33 (1994) 5477-5481.

[28] A.Y. Oral, M.L. Mecartney, Properties of sol-gel derived strontium barium niobate ceramics and the effect of V₂O₅ additive, J. Mater. Sci. 36 (2001) 5519-5527.

[29] Program WinXPOW v2.11, Stoe & Cie GmbH, Darmstadt, 2004.

[30] U. Intatha, S. Eitssayeam, K. Pengpat, K.J.D. MacKenzie, T. Tunkasiri, Dielectric properties of low temperature sintered LiF doped BaFe₀.₅Nb₀.₅O₃, Mater. Lett. 61 (2007) 196-200.

[31] S. Balabanov, D. Permin, T. Evstropov, P. Andreev, L. Basyrova, P. Camy, M. Baranov, X. Mateos, P. Loiko, Hot pressing of Yb:Y₂O₃ laser ceramics with LiF sintering aid, Opt. Mater. 119 (2021), 111349.

[32] Y. Lai, H. Su, G. Wang, X. Tang, X. Huang, X. Liang, H. Zhang, Y. Li, K. Huang, X.R. Wang, Low-temperature sintering of microwave ceramics with high Qf values through LiF addition, J. Am. Ceram. Soc. 102 (2019) 1983, 1903.

[33] J.P. Guha, H.U. Anderson, Reaction during sintering of barium titanate with lithium fluoride, J. Am. Ceram. Soc. 69 (1986) C193-C194.

[34] J.M. Huassonne, G. Desgardin, Ph Bajolet, B. Raveau, Barium titanate perovskite sintered with lithium fluoride, J. Am. Ceram. Soc. 66 (1983) 801-807.

[35] R. Köferstein, L. Jäger, M. Zenkner, T. Müller, H.-P. Abicht, Shrinkage mechanism and phase evolution of fine-grain BaTiO₃ powder compacts containing 10 mol% BaGeO₃ prepared via a precursor route, Mater. Chem. Phys. 112 (2008) 531-535.

[36] W. Schatt, Sintervorgänge, VDI-Verlag, Düsseldorf, 1992, pp. 78-100.

[37] W. Schatt, E. Friedrich, Dislocation activated sintering processes, Powder Metall. 28 (1985) 140-144.

[38] W. Schatt, B. Vetter, E. Friedrich, Non-isothermal shrinkage of compacts, Powder Metall. 34 (1991) 179-182.

[39] L.A. Pérez-Maqueda, J.M. Criado, C. Real, Kinetics of the initial stage of sintering from shrinkage data: simultaneous determination of activation energy and kinetic model from a single nonisothermal experiment, J. Am. Ceram. Soc. 85 (2002) 763-768.

[40] J.L. Woolfrey, M.J. Bannister, Nonisothermal techniques for studying initial-stage sintering, J. Am. Soc. 55 (1972) 390-394.

[41] J.-T. Sgiue, T.-T. Fang, The sintering behavior of CeO₂-doped strontium barium niobate ceramics, J. Eur. Ceram. Soc. 22 (2002) 1705-1709.

[42] G.W. Marks, L.A. Monson, Effect of certain group IV oxides on dielectric constant and dissipation factor of barium titanate, Ind. Eng. Chem. 47 (1955) 1611-1620.

[43] M.I. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443-446.

[44] R.L. Byer, J.F. Young, R.S. Feigelson, Growth of high-Quality LiNbO₃ crystals from the congruent melt, J. Appl. Phys. 41 (1970) 2320-2325.

[45] B.A. Scott, G. Burns, Determination of stoichiometry variations in LiNbO₃, and LiTaO₃, by Raman powder spectroscopy, J. Am. Ceram. Soc. 55 (1972) 225-230.

[46] Y.B. Yao, C.L. Mak, B. Ploss, Phase transitions and electrical characterizations of (K₀.₅Na₀.₅)₂(Sr₀.₆Ba₀.₄)₅-xNb₁₀O₃₀ (KNSBN) ceramics with ‘unfilled’ and ‘filled’ tetragonal tungsten-bronze (TTB) crystal structure, J. Eur. Ceram. Soc. 320 (2012) 4353-4361.

[47] Y. Jiang, R. Guo, A.S. Bhalla, Single crystal growth and ferroelectric properties of α(Ba₁-xSrx)Nb₂O₆; β(Na₁-yKy)NbO₃ solid solutions, J. Appl. Phys. 84 (1998) 5140-5146.

[48] R.R. Neurgaonkar, W.K. Cory, J.R. Oliver, W.W. Clark III, G.L. Wood, M.J. Miller, E.J. Sharp, Growth and ferroelectric properties of tungsten bronze Ba₂ₓSrₓNkyNb₂O₇, yNa₃Nb₅O₁₅ (BSKNN) single crystals, J. Cryst. Growth 84 (1987) 629-637.

[49] A.Z. Simões, A.H.M. González, A.A. Cavalheiro, M.A. Zaghete, B.D. Stojanovic, J.A. Varela, Effect of magnesium on structure and properties of LiNbO₃ prepared from polymeric precursors, Ceram. Int. 28 (2002) 265-270.

[50] R. Köferstein, Thermoanalytical, optical, and magnetic investigations on nanocrystalline Li₀.₅Fe₂.₅O₄ and resulting ceramics prepared by a starch-based soft-chemistry synthesis, J. Solid State Chem. 287 (2020), 121380.

[51] E. Antolini, Lithium loss from lithium cobalt oxide: hexagonal Li₀.₅Co₀.₅O to cubic Li₀.₀₆Co₀.₉₅O phase transition, Int. J. Inorg. Mater. 3 (2001) 721-726.

[52] R. Köferstein, T. Buttlar, S.G. Ebbinghaus, Investigations on Bi₂₅FeO₄ powders synthesized by hydrothermal and combustion-like processes, J. Solid State Chem. 217 (2014) 50-56.

[53] J. Hormadaly, S.N. Subbarao, R. Kershaw, K. Dwight, A. Wold, Preparation and comparison of the photoelectronic properties of Sr₂Nb₂O₇ and Ba₀.₅Sr₀.₅Nb₂O₆, J. Solid State Chem. 33 (1980) 27-32.

[54] V. Sa-Yakanit, H.R. Glyde, Urbach tails and disorder, comments cond, Mat. Phys. 13 (1987) 35-48.

[55] P. Makula, M. Pacia, W. Macyk, How to correctly determine the band gap energy of modified semiconductor photocatalysts based on UV-Vis spectra, J. Phys. Chem. Lett. 9 (2018) 6814-6817.

[56] F.P. Koffyberg, K. Dwight, A. Wold, Interband transition of semiconducting oxides determined from photoelectrolysis spectra, Solid State Commun. 30 (1979) 433-437.

[57] L.E. Cross, Relaxor ferroelectrics: an overview, Ferroelectrics 151 (1994) 305-320.

[58] T. Tsurumi, K. Soejima, T. Kamiya, M. Daimon, Mechanism of diffuse phase transition in relaxor ferroelectrics, Jpn. J. Appl. Phys. 33 (1994) 1959-1964.

[59] K. Uchino, S. Nomura, Critical exponents of the dielectric constants in diffused phase-transition crystals, Ferroelectrics 44 (1982) 55-61.

[60] X. Qin, D. Shihu, S. Tianxiu, W. Homgni, Diffuse phase transition of Ba₀.₉₂Ca₀.₀₈(Ti₀.₈₂Zr₀.₁₈)O₃ based ceramics, Ferroelectrics 426 (2012) 282-289.

[61] L.E. Cross, Relaxor Ferroelectrics, Ferroelectrics 76 (1987) 242-267.

[62] F. Oehler, H.-T. Langhammer, S.G. Ebbinghaus, Preparation and dielectric properties of CaTaO₂N and SrNbO₂N ceramics, J. Eur. Ceram. Soc. 37 (2017) 2129-2136.

[63] J.T.S. Irvine, D.C. Sinclair, A.R. West, Electroceramics: characterization by impedance spectroscopy, Adv. Mater. 2 (1990) 132-138.
Supporting Information

# Effect of sintering additives on the densification and dielectric properties of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics synthesized by a soft-chemistry method

Roberto Köferstein* and Stefan G. Ebbinghaus

Institute of Chemistry, Martin Luther University Halle-Wittenberg,

Kurt-Mothes-Straße 2, 06120 Halle, Germany.

* Corresponding author

Tel.: +49-345-5525630; Fax: +49-345-5527028.

E-mail address: roberto.koeferstein@chemie.uni-halle.de

The following abbreviations were used:

SBN+LF – Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> + 0.76 LiF

SBN+LiN-0.1 – Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> + 0.1 LiNbO<sub>3</sub>

SBN+LiN-0.2 – Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> + 0.2 LiNbO<sub>3</sub>
<figure><img src="image_16.png" /><figcaption>**Fig. S1:** Room-temperature XRD patterns of SBN-LF ceramics after sintering for 1 h at (a) 1050 °C, (b) 1200 °C.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. S2:** DSC measurement of SBN+LN-0.2 compact in flowing air.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. S3:** SEM-BSE image of a SBN+LiN-0.2 ceramic body sintered at 1200 °C for 1 h.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. S4:** Room-temperature XRD patterns of SBN+LN-x ceramics after sintering at 1300 °C for 1 h (heating-/cooling rate 5 K min⁻¹): (a) x = 0.1 (b) x = 0.2. The patterns were recorded with a prolonged counting time of 10 s per data point.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. S5:** Temperature dependence of ε'_r and tan δ (inset) at various frequencies for a SBN+LN-0.2 ceramic body sintered at 1100 °C for 1 h.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. S6:** Temperature dependence of ε'<sub>r</sub> and tan δ (inset) at various frequencies for a SBN+LiN-0.2 ceramic body sintered at 1150 °C for 1 h.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Fig. S7:** Temperature dependence of ε'<sub>r</sub> and tan δ (inset) at various frequencies for a SBN+LiN-0.2 ceramic body sintered at 1200 °C for 1 h.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. S8:** Temperature dependence of ε'<sub>r</sub> and tan δ (inset) at various frequencies for a SBN+LiN-0.2 ceramic body sintered at 1300 °C for 1 h.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Fig. S9:** Cole-Cole plots measured at the indicated temperatures of SBN+LiN-0.1 ceramics sintered for 1 h at different temperatures: (a) 1150 °C (b) 1200 °C, and (c) 1300 °C. Fits were carried out using one parallel connected RC-elements.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Fig. S10:** Cole-Cole plot measured at the indicated temperature of SBN+LiN-0.2 ceramic sintered at 1150 °C for 1 h. The fit was carried out using one RC-elements.</figcaption></figure>