# Impact of Li-substitution on dielectric, structural and sintering behavior of Lix(Sr0.5Ba0.5)1-x/2Nb2O6 ceramics

Roberto Köferstein, Jonas Jacobs, Stefan G Ebbinghaus

► To cite this version:

Roberto Köferstein, Jonas Jacobs, Stefan G Ebbinghaus. Impact of Li-substitution on dielectric, structural and sintering behavior of Lix(Sr0.5Ba0.5)1-x/2Nb2O6 ceramics. Journal of the European Ceramic Society, 2025, 45 (15), pp.117567. 10.1016/j.jeurceramsoc.2025.117567. hal-05101469

HAL Id: hal-05101469

https://hal.science/hal-05101469v1

Submitted on 6 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Impact of Li-substitution on dielectric, structural and sintering behavior of Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics

Roberto Köferstein*, Jonas Jacobs, Stefan G. Ebbinghaus

<figure><img src="image_2.png" /></figure>

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

<figure><img src="image_4.png" /></figure>

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

## ARTICLE INFO

### Keywords:

Strontium barium niobates

Niobates

Relaxor

Sintering

Dielectrics

Impedance spectroscopy

## ABSTRACT

Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics were prepared by a solid state synthesis and sintered at 1300 °C. XRD investigations reveal single phase samples up to a substitution limit of x = 0.275. For higher lithium contents, the appearance of LiNbO<sub>3</sub> as secondary phase was observed. The density of the ceramics increases with lithium content. The microstructures show globular grains which turns to pillar-like grains with rising lithium substitution. Dielectric investigations confirm relaxor ferroelectric behavior and the diffuse phase transition is shifted towards higher temperatures with increasing lithium content from 129 °C (x = 0) to 221 °C (x = 0.275). The diffuseness coefficient increases with the lithium content. From high-temperature XRD measurements the length of the tetragonal cell parameter c decreases with increasing temperature, reaches a minimum at about 150 –230 °C (depending on x) and afterward increases with increasing temperature. Additionally, a significant shift of the Nb(2) position with temperature can be observed.

## 1. Introduction

Lead-free Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> based materials show excellent pyroelectric, piezoelectric, electro-optic, and photorefractive properties [1–3]. Additionally, strontium barium niobates can be used as catalyst in water splitting reactions and advanced oxidation processes [4–7]. Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> crystallized in the unfilled tetragonal tungsten bronze structure [(A1)<sub>0.4</sub>(A2)<sub>0.8</sub>C<sub>0.8</sub>(B1)<sub>0.4</sub>(B2)<sub>1.6</sub>O<sub>6</sub>] which consists of corner-shared NbO<sub>6</sub>-octahedra (B1/B2) building tetragonal (A1), pentagonal (A2) and trigonal (C) channels along [001] (Fig. 1) [8,9]. The A1 sites (coordination number (CN): 12) are filled by Sr<sup>2+</sup>, the A2 sites (CN: 15) are occupied both by Sr<sup>2+</sup> and Ba<sup>2+</sup>, while the smaller C sites (CN: 9) are empty. Additionally, 1/6 of the A1 and A2 sites are vacant [10,11]. The ferroelectric state in Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> is present in the range from y = 0.25–0.75 [12]. The ferroelectric properties depend on the composition (y) and the synthesis method. Relaxor behavior for polycrystalline Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics and single crystals has been reported for y > 0.4 [3,13–18]. The relaxor properties can be varied by partial ionic substitution on the A- and B-sites, respectively [19–21]. In relaxor materials the diffuse ferroelectric ↔ paraelectric phase transition extends over a temperature range (Curie range) and does not show a sharp transition in dielectric measurements in contrast to normal ferroelectric materials, therefore a well-defined Curie Temperature (T<sub>C</sub>)

does not exist. The diffuseness of the phase transition is due to the existence of polarized nano-regions in a paraelectric matrix even well above the temperature of the permittivity maximum (T<sub>m</sub>) up to the so-called burns temperature (T<sub>B</sub>) [22–24]. The diffuse phase transition (T<sub>m</sub>) in Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> samples, lie between roughly 40 and 200 °C, depending on y [3, 10, 18, 25, 26]. Moreover, the dielectric response, sintering behavior, and microstructure of Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics can be varied by partial substitution of Sr<sup>2+</sup>/Ba<sup>2+</sup> by e.g. alkaline metal ions [21,27–31]. Recently, we have found a considerably increase in density and of the magnetoelectric coefficient in NiFe<sub>2</sub>O<sub>4</sub>–Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> composites due to the partial replacement of Sr<sup>2+</sup>/Ba<sup>2+</sup> by Li<sup>+</sup> [32]. However, systematic investigations on the influence of lithium substitution on the dielectric, structural and sintering behavior of strontium barium niobate have not been reported.

The aim of this work is to investigate the impact of the lithium content in Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub> on the ferroelectric, structural, and sintering behaviour up to the substitution limit. The specimens were obtained from a conventional solid state synthesis. Phase evolution and microstructure of the ceramics were monitored by XRD and SEM. Dielectric and diffuse reflectance UV-Vis measurements were used to determine the influence of lithium on the phase transition temperature and optical band gap, respectively. High-temperature XRD measurements reveal the structural change during the phase transition.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: roberto.koeferstein@chemie.uni-halle.de (R. Köferstein).</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** The tetragonal tungsten bronze structure of strontium barium niobate viewed along [001].</figcaption></figure>

# 2. Experimental

## 2.1. Material preparation

$Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ (≡ $(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x/2}(LiNbO_3)_x$ solid solution) ceramic samples were prepared by a solid state synthesis. Stoichiometric amounts of $Nb_2O_5$ (abcr, 99.995 %), $BaCO_3$ (Alfa Aesar, Puratronic, 99.997 %), $SrCO_3$ (Alfa Aesar, Puratronic, 99.994 %), and $Li_2CO_3$ (Acros Organics, 99.999 %) were mixed in polyamide jars for 4 h using a planetary mill, $ZrO_2$-balls, and propan-2-ol. After drying the mixtures were calcined in static air between 1200–1250 °C for 3–5 h to ensure complete calcination (Fig. S1, supporting information). The white calcined powders were then mixed with 10 wt% of a saturated aqueous polyvinyl alcohol (PVA) solution as a pressing aid and uniaxially pressed at about 110 MPa into pellets (green density: 3 g cm⁻³). These pellets were placed on a $ZrO_2$ fibre mat and fired to ceramic bodies at 1300 °C for 1 h (heating-/cooling rate 5 K min⁻¹).

## 2.2. Characterization

Room-temperature X-ray powder diffractions were carried out on a Bruker AXS D8-Advance diffractometer, equipped with a one-dimensional silicon strip detector (LynxEye™) using Cu-Kα radiation in the range of $2\theta = 10-140°$ with a step size of 0.0105° and a counting time of 1 s per data point. Temperature dependent XRD measurements were conducted on a STOE STADI MP diffractometer with monochromatic Mo-Kα₁ radiation equipped with a DECTRIS MYTHEN2 1 K detector and a STOE HT1 capillary furnace. Samples were contained in 0.8 mm external diameter capillaries, and the temperature range of 50–300 °C was scanned in steps of 10 °C. Diffraction patterns were obtained in the range of $2\theta = 5-51°$ with a point resolution of 0.015° and a total acquisition time of 2 h. Rietveld refinements were carried out with the software FullProf [33]. Scanning electron microscope images were collected with a Phenom ProX SEM in backscattered electron mode (BSE). An Impedance Analyzer 4192 A (Hewlett Packard) was used for impedance measurements. For this purpose, ceramic samples were sputtered on both sides with 100 nm thick gold electrodes using a Cressington Sputter Coater 108auto. Diffuse reflectance spectra were recorded at room temperature using a Perkin Elmer UV-Vis

spectrometer Lambda 19 with $BaSO_4$ as white standard.

# 3. Results and discussion

## 3.1. Phase composition, sintering, and optical band gap

The preceramic powders were pressed to pellets and sintered in static air at 1300 °C for 1 h (heating-/ cooling rate: 5 K min⁻¹) to ceramic bodies which were used for further investigations. Fig. 2 shows room-temperature XRD patterns of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramic samples depending on the lithium content (x). Up to x = 0.275 we observed only reflections of the tetragonal (SG: *P4bm*) $Sr_{0.5}Ba_{0.5}Nb_2O_6$ phase (PDF #01-084-4102), without any traces of a secondary phase, like e.g. $LiNbO_3$, suggesting the formation of a solid solution between $Sr_{0.5}Ba_{0.5}Nb_2O_6$ and $LiNbO_3$ yielding $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ in the tetragonal tungsten bronze structure. On the other hand, the formation of a weak $LiNbO_3$ reflection (PDF #01-078-0250) at about 2θ = 23.7° for x = 0.3 indicates a solution limit at about x = 0.275. Rietveld refinements in space group *P4bm* [10,11] (Fig. S2, supporting information) show a decrease of both lattice parameters with increasing lithium content (Fig. 3), because of the smaller ionic radius of $Li^+$ compared to $Sr^{2+}$ and $Ba^{2+}$ [34]. The contraction with rising x of the shorter c-axis up to 0.46(1) % is significantly greater than that of the a-axis (up to 0.21(1) %) (Fig. S3, supporting information).

The bulk densities of the white ceramic bodies were calculated from their weight and geometric dimension. The relative bulk densities were related to the theoretical (single crystal) densities (Tab. S1, supporting information) [16,35–37]. A seen in Fig. 4, the densities of the $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramic bodies increase with increasing lithium content (x). The slight drop in density for x > 0.2 is most likely due to the formation of a mainly pillar-like microstructure (see below) [29].

SEM (BSE mode) images of the microstructure of selected ceramics are shown in Fig. 5 (see also Fig. S4, supporting information). After sintering, samples up to a lithium substitution level of x < 0.15 consist only of globular and irregular shaped grains, respectively. The average grain size ($\Theta_{li}$) of those grains was determined by counting of at least 1000 grains per sample using the linear intercept method [38]. The globular-/irregular grains grow from 1.5–14 µm ($\Theta_{li}$ = 6.7(3) µm) for

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Room temperature XRD patterns of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics sintered at 1300 °C for 1 h. The inset shows a magnification of the pattern for x = 0.3.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Evolution of the tetragonal lattice parameters of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ depending on the lithium content ($x$) at room temperature. The uncertainties of the data are smaller than the symbol size.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 4. Relative bulk densities of ceramic bodies in dependence of $x$ after sintering at 1300 °C for 1 h (heating-/cooling rate 5 K min$^{-1}$). The inset shows the absolute bulk densities. The uncertainties of the data in the inset are smaller than the symbol size.</figcaption></figure>

$x=0$ to 2.2–26 µm ($\Theta_{li} = 10(1)$ µm) for $x=0.15$. The grain size range represents the smallest and largest grains in the sample. Besides these globular grains, we also observed the appearance of pillar-like grains and large crystals at $x \ge 0.15$. At a lithium level of $x=0.15$ the pillar-like grains have dimensions of about 4–8 × 6–16 µm. For $x=0.25$ the pillars grow to 4–14 × 5–50 µm along with the appearance of sporadic large grains up to 50 × 170 µm, whereas the globular-/irregular grains have dimensions of about 4–83 µm ($\Theta_{li} = 16(3)$ µm). The fraction of the pillar-like grains and large grains increases considerably with $x$, therefore for $x=0.275$ the microstructure of the ceramic sample completely consists of pillars between 3–25 × 5–74 µm and large grains up to 50 × 125 µm (Tab. S2, supporting information). Thus, the incorporation of $Li^+$ promotes an anisotropic grain growth resulting in the formation of pillar-like grains and abnormal grain growth, which was also observed in potassium and sodium substituted calcium/strontium barium niobates as well as in $Sr_{0.5}Ba_{0.5}Nb_2O_6$ ceramics fired at high temperatures [17,21,28,39]. According to Lee et al. [20,40] the formation of pillar-like and especially large grains is most likely caused by a stoichiometry gradient between grain and grain-boundary leading to $Nb_2O_5$- and probably lithium-rich grain-boundary regions with a locally reduced melting point.

From diffuse reflectance UV-Vis spectra (Fig. S5, supporting information) we obtained the optical band gap energies of the samples applying the Kubelka-Munk theory. For that purpose, the measured reflectance data ($R$) were converted into the absorption coefficient ($\alpha$).

Assuming the scattering factor ($s$) is wavelength independent $\alpha$ is proportional to the Kubelka-Munk function $F(R)$ as given by Eq. 1 [41–43]:

$$F(R) = \frac{\alpha}{s} = \frac{(1 - R)^2}{2R} \quad (1)$$

The optical band gap can be calculated by Eq. 2 [41]:

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

where, $k$ – energy-independent constant, $E_g$ – optical band gap, $n$ – exponent reflecting the type of transition. As described elsewhere, $Sr_{0.5}Ba_{0.5}Nb_2O_6$ has an indirect band gap, thus the exponent is set to $n=0.5$ [17,44]. Generally, the band gap energy can be estimated from a plot of $(F(R) \cdot h\nu)^{1/2}$ versus $h\nu$ and extrapolating the slope to $F(R) \to 0$. However, an additional absorption feature appears at lower energies most likely caused by defects in the crystal structure [45–47]. Because of the overlapping of these two absorption regions, we used a baseline approach to determine $E_g$ according to Makula et al. [47] as seen in the inset of Fig. 6. The obtained band gap energy of 3.33(3) eV for $x=0$ agrees very well with previous values for $Sr_{0.5}Ba_{0.5}Nb_2O_6$ [17,48]. Moreover, it can be seen in Fig. 6 the incorporation of $Li^+$ into the $Sr_{0.5}Ba_{0.5}Nb_2O_6$ lattice does not change the band gap energy significantly.

## 3.2. Dielectric and high-temperature XRD measurements

The temperature-dependence of the relative permittivity ($\epsilon_r$) and dissipation factor (tan$\delta$) for $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramic bodies at 1 MHz is shown in Fig. 7. The temperature of the permittivity maximum ($T_m$) is shifted towards higher temperatures with $x$ from 129(3) °C ($x=0$) to 221(4) °C ($x=0.275$) (Fig. 8). $Sr_{0.5}Ba_{0.5}Nb_2O_6$ is a relaxor ferroelectric and the broad permittivity maximum suggests a diffuse ferroelectric ⇌ paraelectric phase transition. Additionally, $T_m$ of the $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ samples shifts towards higher temperatures with increasing frequency (Fig. S6, supporting information). $Li^+$ preferably occupies the C site, however $Li^+$ can also occupy the A1 and A2 sites [49]. As pointed out by Abrahams et al. [50] the temperature of the dielectric maximum depends on the displacement of the $Nb^{5+}$ ions inside the oxygen octahedra. Thus, the incorporation of $Li^+$ leads to an increase of the octahedron distortion and $Nb^{5+}$ displacement resulting in rising $T_m$ values with $x$. Additionally, we also observe an increase of the permittivity maximum up to a lithium substitution of $x=0.175-0.2$. The same trend is true for the permittivity values at room temperature (inset in Fig. 8). The low permittivity values for ceramics with $x=0$ and 0.05 are also a consequence of the lower bulk densities and thus higher porosity [51]. The observed significant drop of the permittivity maximum for samples with $x>0.2$ gets along with the formation of a mainly and complete pillar-like microstructure, respectively. Since a decrease of the permittivity maximum upon formation of a pillar-like microstructure was also observed in pure $Sr_{0.5}Ba_{0.5}Nb_2O_6$ ceramics [17] we suppose that the significant change in microstructure connected with the formation of new grain boundaries is most likely the main reason for the decrease of the dielectric response.

The dielectric behavior of a relaxor ferroelectric can be expressed by the modified Curie-Weiss law (3) [52]:

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

where, $\epsilon_m$ is the permittivity maximum, $T_m$ is the temperature at $\epsilon_m$, $C$ is a constant, and the exponent $\gamma$ is the diffuseness coefficient. $\gamma$ can be derived from a plot of $\ln(\epsilon^{-1}-\epsilon_m^{-1})$ versus $\ln(T-T_m)$ (Fig. S7, supporting information). It is commonly assumed that $\gamma=1$ for a normal ferroelectric material and $\gamma=2$ for an ideal relaxor ferroelectric one. The derived diffuseness coefficient rises from 1.63(5) ($x=0$) to 1.94(5) ($x=0.275$) (Fig. 9) indicating an increase of the diffuse nature of the phase transition caused by an increase of lattice disorder with rising
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** SEM-BSE images of selected $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramic bodies sintered at 1300 °C for 1 h. a) $x=0$, b) $x=0.1$, c) $x=0.2$, d) $x=0.275$. The inset in (d) shows a very large irregular grain.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Optical band gap energies (indirect allowed) of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics sintered at 1300 °C for 1 h. The inset exemplarily shows $(F(R \cdot h\nu))^{1/2}$ versus $h\nu$ for a ceramic sample with $x=0.2$.</figcaption></figure>

lithium content [53,54].

At higher temperatures, the $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics reveal a considerable dc-conductivity indicating a semiconducting behavior. In contrast, the ceramics samples show low conductivities at

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Temperature dependence of the relative permittivity ($\epsilon_r$) and dissipation factor (tan $\delta$) at 1 MHz for $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics sintered at 1300 °C.</figcaption></figure>

room temperature ($\sigma_{DC} \ll 10^{-8} \text{ S cm}^{-1}$), thus the high-temperature impedance data were fitted with an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase-shift element. The specific complex impedance ($Z^{*\text{spec}}$) for a single RC
<figure><img src="image_12.png" /><figcaption>Fig. 8. Evolution of the temperature of the permittivity maximum (Tm) depending on the composition x at 1 MHz. The inset shows the permittivity values depending on x at Tm (a) and at room temperature (b).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 9. Diffuseness coefficient versus lithium content (x) for ceramic samples sintered at 1300 °C.</figcaption></figure>

element is described by Eq. 4 [55]:

$$Z_{\text{spec}}^{*} = \frac{\rho_{\text{DC}}}{1 + (i\omega\tau)^{\beta}} \quad (4)$$

(β is the constant phase-shift (CPE) coefficient, ρDC is the specific DC resistivity, and τ = ρDCεε₀). Cole-Cole plots of the specific impedance of the samples can be successfully described by one RC element (Fig. S8, supporting information). The calculated specific DC resistivities (ρDC) of the specimen up to about x = 0.2 is one magnitude higher than the one of unsubstituted Sr₀.₅Ba₀.₅Nb₂O₆ samples. However, for higher lithium levels (x > 0.2) the resistivity values decrease significantly (Fig. 10), which correlates with the change in microstructure from globular-like to pillar-like grains (see above).

We carried out temperature dependent XRD measurements to examine the evolution of the cell parameters during the diffuse transition from the ferroelectric to the paraelectric phase. Fig. 11 shows the relative length change, related to the initial values at 50 °C, of the cell parameters with temperature. All samples reveal a similar behavior. The lattice parameter a (Graph 11a) increases almost linearly with temperature due to the expected thermal expansion. The shorter c-axis (Graph 11b) shows a completely different behavior. The lattice parameter c decreases with temperature, reaches a minimum and afterwards c increases. The minimum tends to shift to higher temperatures with

<figure><img src="image_14.png" /><figcaption>Fig. 10. Development of the specific DC resistivity with lithium content (x) of Liₓ(Sr₀.₅Ba₀.₅)₁-x/Nb₂O₆ ceramics.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 11. Relative length change of cell parameters (a and c) and cell volume (inset) with temperature of Liₓ(Sr₀.₅Ba₀.₅)₁-x/Nb₂O₆ samples sintered at 1300 °C derived from powder XRD measurements. The relative change is related to the values at 50 °C. The uncertainties of the data are smaller than the symbol size.</figcaption></figure>

increasing lithium substitution (x) analogous to the dielectric maximum (Tm) described above. The positive temperature coefficient of c at high temperatures is mainly dominated by the expected increasing atomic oscillation with temperature.

As seen in Fig. 12 (see also Fig. S9, supporting information) the minimum of the lattice parameter c is near the maximum of the permittivity, indicating the polarization is along the c-axis. A
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Evolution of the c-axis (relative length change $\Delta c/c_{50}^{\circ}c$) and permittivity depending on temperature for $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics with $x=0$ (a) and $x=0.275$ (b). The relative length change $\Delta c/c_{50}^{\circ}c$ derived from powder XRD measurements is related to the value at 50 $^{\circ}$C. The uncertainties of the data are smaller than the symbol size.</figcaption></figure>

corresponding thermal behavior of the cell parameters was reported for $Sr_yBa_1-yNb_2O_6$ with $y=0.61$ and $0.82$ [56,57]. Dilatometric measurements on single crystals of $Sr_{0.5}Ba_{0.5}Nb_2O_6$ by Shorrocks et al. [58] reveal also a negative temperature coefficient for the lattice parameter $c$ below the diffuse phase transition temperature. Moreover, our data show that the beginning of the shrinkage of the c-axis is shifted to higher temperatures with rising lithium content suggesting that the incorporation of lithium stabilizes the ferroelectric phase.

It is known that the ferroelectric $\rightleftharpoons$ paraelectric phase transition in displacive ferroelectric niobates is accompanied by the displacement of the niobium atoms [50]. Fig. 13 shows the shift of the z-coordinate of both crystallographic independent niobium atoms Nb(1) and Nb(2) with increasing temperature for the two samples with $x=0$ and $x=0.275$. The Nb(2) position (Wyckoff site: 8d) in both samples shows a gradually decreasing trend with rising temperature while the fractional coordinate of Nb(1) (Wyckoff site: 2b) increases only slightly, suggesting that the displacement of Nb(2) is mainly responsible for the ferroelectric (relaxor) behavior in accordance with PDF analysis by Pásciak et al. [8]. The x- and y-positions do not change significantly with temperature (Fig. S10, supporting information). Similar results have been reported for $Sr_yBa_{1-y}Nb_2O_6$ with $y=0.33$ and $0.67$ [59].

## 4. Conclusion

Ferroelectric $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics were prepared by conventional solid state synthesis. A solid solution is formed up to a lithium content of $x=0.275$ whereas higher lithium levels lead to the formation of $LiNbO_3$ as secondary phase. The incorporation of lithium leads to an improved densification of ceramic bodies, rising grain sizes and the gradual formation of pillar-like grains, while leaving the optical band gap unaffected. Dielectric measurements reveal a relaxor behavior and the diffuseness of the ferroelectric $\rightleftharpoons$ paraelectric phase transition

<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Temperature-dependent shift of the fractional z-coordinate of both niobium sites in $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics with $x=0$ (a) and $x=0.275$ (b). The vertical blue dashed line represents the temperature of the permittivity maximum.</figcaption></figure>

increases with $x$. The temperature of the permittivity maximum ($T_m$) shifts towards higher temperatures with higher lithium contents ($x$) indicating a stabilization of the ferroelectric phase at higher temperatures and enabling a tunability of $T_m$ between 129–221 $^{\circ}$C. The DC resistivity moderately increases with lithium incorporation, whereas higher lithium levels caused declining resistivity values. XRD investigations show a decrease of the cell parameter $c$ in all samples with rising temperature. The length of the c-axis reaches a minimum near the phase transition region and then increases with temperature. In contrast, the cell parameter $a$ continuously increases with temperature. The Nb(2) site shows a shift in the z-position with temperature, which is most likely linked with the ferroelectric behavior of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$.

## CRediT authorship contribution statement

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

## 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.
# Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jeurceramsoc.2025.117567.

## References

[1] 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 (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>), J. Appl. Phys. 66 (1989) 1920-1924.

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

[3] A.M. Glass, Investigation of the electrical properties of Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> with special reference to pyroelectric detection, J. Appl. Phys. 40 (1969) 4699-4713.

[4] J. Dai, Z. Fan, Y. Long, W. Yue, F. Huang, Y. Jiao, Y. Deng, Y. Chang, D. Wang, Remarkably boosting piezocatalytic performance of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> piezoceramics via size optimization and oxygen vacancy engineering, Adv. Funct. Mater. 34 (2024) 2408754.

[5] J. Dai, N. Shao, S. Zhang, Z. Zhao, Y. Long, S. Zhao, S. Li, C. Zhao, Z. Zhang, W. Liu, Enhanced piezocatalytic activity of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> nanostructures by engineering surface oxygen vacancies and self-generated heterojunctions, ACS Appl. Mater. Interfaces 13 (2021) 7259-7267.

[6] G. Han, S. Cao, B. Lin, UV photocatalytic activity for water decomposition of Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> nanocrystals with different components and morphologies, J. Chem. (2017) 2163608.

[7] D. Fan, R. Chong, F. Fan, X. Wang, C. Li, Z. Feng, A tetragonal tungsten bronze-type photocatalyst: ferro-paraelectric phase transition and photocatalysis, Chin. J. Catal. 37 (2016) 1257-1262.

[8] M. Pásciak, P. Ondrejkovic, J. Kulda, P. Vaněk, J. Drahokoupil, G. Steciuk, L. Palatinus, T.R. Welberry, H.E. Fischer, J. Hlinka, E. Buixaderas, Local structure of relaxor ferroelectric Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> from a pair distribution function analysis, Phys. Rev. B 99 (2019) 101402.

[9] R.R. Neurgaonkar, W.K. Cory, J.R. Oliver, E.J. Sharp, G.L. Wood, G.J. Salamo, Growth and optical properties of ferroelectric tungsten bronze crystals, Ferroelectrics 142 (1993) 167-188.

[10] P.B. Jamieson, S.C. Abrahams, J.L. Bernstein, Ferroelectric tungsten bronze-type crystal structures. I. Barium Strontium Niobate Ba<sub>0.27</sub>Sr<sub>0.75</sub>Nb<sub>2</sub>O<sub>5.78</sub>, J. Chem. Phys. 48 (1968) 5048-5057.

[11] J. Schefer, D. Schaniel, V. Petříček, T. Woike, A. Cousson, Manfred Wöhlecke, reducing the positional modulation of Nb<sub>6</sub>O<sub>4</sub>Octahedra in Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> by increasing the barium content: a single crystal neutron diffraction study at ambient temperature for x = 0.61 and x = 0.34, Z. Krist. 223 (2008) 399-407.

[12] A.A. Ballmann, H. Brown, The growth and properties of strontium barium metaniobate Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub>, a tungsten bronze ferroelectric, J. Cryst. Growth 1 (1967) 311-314.

[13] B. Jimenez, C. Alemany, J. Mendiola, E. Maurer, Phase transition in ferroelectric ceramics of the type Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, J. Phys. Chem. Solids 46 (1985) 1383-1386.

[14] M.-S. Kim, P. Wang, J.-H. Lee, J.-J. Kim, H.Y. Lee, S.-H. Cho, Site occupancy and dielectric characteristics of strontium barium niobate ceramics: Sr/Ba Ratio dependence, Jpn. J. Appl. Phys. 41 (2002) 7042-7047.

[15] J. Zhang, G. Wang, F. Gao, C. Mao, F. Cao, X. Dong, Influence of Sr/Ba ratio on the dielectric, ferroelectric and pyroelectric properties of strontium barium niobate ceramics, Ceram. Int. 39 (2013) 1971-1976.

[16] S.S. Rathore, S. Vitta, Enhanced dielectric constant and relaxor behavior realized by dual stage sintering of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, AIP Conf. Proc. 1591 (2014) 133-135.

[17] R. Köferstein, F. Oehler, S.G. Ebbinghaus, Investigations of nano-crystalline Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> and bulk ceramics synthesized by a polymerization method using PEG400, J. Eur. Ceram. Soc. 39 (2019) 1156-1163.

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

[19] J.S. Kim, S.Y. Cho, M.S. Jang, Ferroelectric and relaxor behavior of Fe-doped Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>-Fe<sub>2</sub>O<sub>6</sub> ceramics with a tungsten-bronze structure, J. Korean Phys. Soc. 51 (2007) 692-696.

[20] M.-S. Kim, J.-H. Lee, J.-J. Kim, H.Y. Lee, S.-H. Cho, Microstructure and dielectric characteristics of tungsten bronze structured SBN70 ceramics: effect of Nb<sub>2</sub>O<sub>5</sub> Content, J. Eur. Ceram. Soc. 22 (2002) 2107-2113.

[21] P. Yang, B. Yang, S. Hao, L. Wei, Z. Yang, Variation of electrical properties with structural vacancies in ferroelectric niobates (Sr<sub>0.53</sub>Ba<sub>0.47</sub>)<sub>2.5-0.5x</sub>Na<sub>x</sub>Nb<sub>5</sub>O<sub>15</sub> ceramics, J. Alloy. Compd. 685 (2016) 175-185.

[22] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977, p. 285 (et seqq).

[23] G. Burns, F.H. Dacol, Glassy Polarisation behavior in ferroelectric compounds Pb (Mg<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub> and Pb(Zn<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>, Solid State Commun. 48 (1983) 853-856.

[24] E. Dul'kin, On the question about the burns temperature in KTa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub> ferroelectrics, Ferroelectrics 467 (2014) 1-5.

[25] R.R. Neurgainkar, W.K. Cory, Progress in photorefractive tungsten bronze crystals, J. Opt. Soc. Am. B 3 (1986) 274-282.

[26] M. Said, T.S. Velayutham, W.C. Gan, W.H. Abd Majid, The structural and electrical properties of Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> (SBN) ceramic with varied composition, Ceram, Inter 41 (2015) 7119-7124.

[27] X.-J. He, Y.-C. Zhang, L. Li, J. Zhang, Z.-B. Gu, S.-T. Zhang, Relaxor-normal ferroelectric transition in (1-x)Sr<sub>0.75</sub>Ba<sub>0.25</sub>Nb<sub>2</sub>O<sub>6</sub>-xNaNbO<sub>3</sub> ceramics, Appl. Phys. Lett. 120 (2022) 182902.

[28] Y.B. Yao, C.L. Mak, B. Ploss, Phase transitions and electrical characterizations of (K<sub>0.5</sub>Na<sub>0.5</sub>)<sub>2x</sub>(Sr<sub>0.6</sub>Ba<sub>0.4</sub>)<sub>1-x</sub>Nb<sub>10</sub>O<sub>30</sub> (KNSBN) ceramics with ‘unfilled’ and ‘filled’ tetragonal tungsten-bronze (TTB) crystal structure, J. Eur. Ceram. Soc. 32 (2012) 4353-4361.

[29] R. Köferstein, S.G. Ebbinghaus, 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, J. Solid State Chem. 278 (2022) 123564.

[30] J. Guo, X. Yang, Y. Yang, S. Pang, S.-T. Zhang, B. Yang, Different effect of Li<sup>+</sup> Na<sup>+</sup>, K<sup>+</sup>-doping on crystal structure and electrical property of Sr<sub>0.6</sub>Ba<sub>0.4</sub>Nb<sub>2</sub>O<sub>6</sub>, J. Mater. 10 (2024) 1071-1079.

[31] Y. Jiang, R. Guo, A.S. Bhalla, Single Crystal Growth and Ferroelectric Properties of a(Ba<sub>1-x</sub>Sr<sub>x</sub>)Nb<sub>2</sub>O<sub>6</sub>:p(Na<sub>1-x</sub>K<sub>x</sub>)NbO<sub>3</sub> Solid Solutions, J. Appl. Phys. 84 (1998) 5140.

[32] R. Köferstein, S.G. Ebbinghaus, Improvement of the magnetoelectric response in NiFe<sub>2</sub>O<sub>4</sub>-Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> composites using LiNbO<sub>3</sub> as sintering additive, J. Eur. Ceram. Soc. 43 (2023) 6137-6144.

[33] J. Rodriguez-Carvajal, Recent advances in magnetic structure determination by neutron powder diffraction, Phys. B 192 (1993) 55-69.

[34] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751-767.

[35] 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.

[36] M. Zenkner, L. Jäger, R. Köferstein, H.-P. Abicht, Synthesis and characterization of nanoscaled Ba(Ti<sub>1-x</sub>Y<sub>x</sub>Sn<sub>3</sub>Ge<sub>5</sub>)O<sub>3</sub> powders and corresponding ceramics, Solid State Sci. 10 (2008) 1556-1562.

[37] S.C. Abrahams, P. Marsh, Defect structure dependence on composition in lithium niobate, Acta Cryst. B 42 (1986) 61-68.

[38] M.I. Mendelson, Average grain size in polycrystalline ceramics, J. Am. Ceram. Soc. 52 (1969) 443-446.

[39] B. Yang, P. Yang, L. Wei, Z. Wang, Z. Yang, X. Chao, Z. Yang, Structural nodulation and electrical properties in ferroelectric niobates (Ca<sub>0.28</sub>Ba<sub>0.72</sub>)<sub>2.5-0.5x</sub>K<sub>x</sub>Nb<sub>5</sub>O<sub>15</sub> (0.0<sub>x</sub><5<0.6) Ceramics, Ceram. Int. (2015) 13988-13997.

[40] H.-Y. Lee, R. Freer, The mechanism of abnormal grain growth in Sr<sub>0.6</sub>Ba<sub>0.4</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics, J. Appl. Phys. 81 (1997) 376-382.

[41] O. Schevcvi, W.B. White, The optical absorption edge of rare earth sesquihydrides and alkaline earth - rare earth sulfides, Mater. Res. Bull. 18 (1983) 1059-1068.

[42] P. Kubelka, F. Munk, Ein Beitrag Zur Optik der Farbanstriche, Z. Techn. Phys. 11 (1931) 593-601.

[43] R. Köferstein, T. Buttlar, S.G. Ebbinghaus, Investigations on Bi<sub>25</sub>FeO<sub>40</sub> powders synthesized by hydrothermal and combustion-like processes, J. Solid State Chem. 217 (2014) 50-56.

[44] B. Andriyevsky, A. Patryn, K. Dorywalski, C. Cobet, M. Piasecki, I. Kityk, N. Esser, T. Lukasiewicz, J. Dec, Electronic and optical properties of strontium barium niobate single crystals, Ferroelectrics 426 (2012) 194-205.

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

[46] V. Sa-Yakanit, H.R. Glyde, Urbach tails and disorder, Comments Cond. Mat. Phys. 13 (1987) 35-48.

[47] 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.

[48] J. Hormadaly, S.N. Subbarao, R. Kershaw, K. Dwight, A. Wold, Preparation and comparison of the photoelectronic properties of Sr<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub> and Ba<sub>0.5</sub>Sr<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, J. Solid State Chem. 33 (1980) 27-32.

[49] S.C. Abrahams, P.B. Jamieson, J.L. Bernstein, Ferroelectric tungsten bronze-type crystal structures. III. Potassium lithium niobate K<sub>(6-x-y)</sub>Li<sub>(4+x)</sub>Nb<sub>(10+y)</sub>O<sub>30</sub>, J. Chem. Phys. 54 (1971) 2355-2364.

[50] S.C. Abrahams, S.K. Kurtz, P.B. Jamison, Atomic displacement relationship to curie temperature and spontaneous polarization in displacive ferroelectrics, Phys. Rev. 172 (1968) 551-553.

[51] C.-C. Hsieh, M.L. Mcartney, W.B. Harrison, M.R.B. Hanson, B.G. Koeppke, Microstructure and electrical properties of fast-fired lead zirconate-titanate ceramics, J. Mater. Sci. Lett. 8 (1989) 1209-1216.

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

[53] X. Qin, D. Shihu, S. Tianxiu, W. Homgni, Diffuse phase transition of Ba<sub>0.92</sub>Ca<sub>0.08</sub>(Ti<sub>0.82</sub>Zr<sub>0.18</sub>)O<sub>3</sub> based ceramics, Ferroelectrics 426 (2012) 282-289.

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

[55] F. Oehler, H.-T. Langhammer, S.G. Ebbinghaus, Preparation and dielectric properties of CaTaO<sub>2</sub>N and SrNbO<sub>2</sub>N, Ceram. J. Eur. Ceram. Soc. 37 (2017) 2129-2136.

[56] S. Podlozhenov, H.A. Graetsch, J. Schneider, M. Ulex, M. Wöhlecke, K. Betzler, Structure of strontium barium niobate Sr<sub>1-x</sub>Ba<sub>x</sub>Nb<sub>2</sub>O<sub>6</sub> (SBN) in the composition range 0.32 ≤ x ≥ 0.82, Acta Cryst. B 62 (2006) 960-965.

[57] J. Schefer, D. Schaniel, V. Pomjakushin, U. Stuhr, V. Petříček, Th Woike, M. Wöhlecke, M. Imlau, Structural properties of Sr<sub>0.61</sub>Ba<sub>0.39</sub>Nb<sub>2</sub>O<sub>6</sub> in the temperature range 10-500 K investigated by high-resolution neutron powder diffraction and specific heat measurements, Phys. Rev. B 74 (2006) 134103.

[58] N. Shorrocks, R.W. Whatmore, S.T. Liu, The electro-elastic and SAW properties of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>, J. Phys. D Appl. Phys. 15 (1982) 2469-2481.

[59] C. Li, Y. Zhang, J. Liu, H.A. Graetsch, Long range and local structure of SrxBa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub> (x = 0.33 and 0.67) across the ferroelectric-relaxor transition, Chem. Mater. 32 (2020), 1844(1853).
# Impact of Li-substitution on dielectric, structural and sintering behavior of
$Li_x(Sr_{0.5}Ba_{0.5})_{1-x}Nb_2O_6$ ceramics

Roberto Köferstein*, Jonas Jacobs, and Stefan G. Ebbinghaus

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

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

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

<footnote>* Corresponding author</footnote>
<table><caption>**Tab. S1:** Theoretical bulk densities of $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$</caption><thead><tr><th>Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub></th><th>Calculated theoretical bulk densities* of the ceramics according to [1,2] (g cm<sup>-3</sup>)</th></tr></thead><tbody><tr><td>x = 0</td><td>5.37</td></tr><tr><td>x = 0.05</td><td>5.35</td></tr><tr><td>x = 0.10</td><td>5.34</td></tr><tr><td>x = 0.15</td><td>5.32</td></tr><tr><td>x = 0.175</td><td>5.31</td></tr><tr><td>x = 0.20</td><td>5.30</td></tr><tr><td>x = 0.25</td><td>5.29</td></tr><tr><td>x = 0.275</td><td>5.28</td></tr></tbody></table>

* The theoretical bulk densities represent the maximum achievable densities.

[1] G. W. Marks and L. A. Monson, Ind. Eng. Chem. 47 (1955) 1611; M. Zenkner et al., Solid State Sci. 10 (2008) 1556.

[2] S. S. Rathore and S. Vitta, AIP Conf. Proc. 1591 (2014) 133; S. C. Abrahams and P. Marsh, Acta Cryst. B 42 (1986) 61.

<table><caption>**Tab. S2:** Grain sizes of ceramics</caption><thead><tr><th>Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub></th><th>Grain size (µm)</th></tr></thead><tbody><tr><td>x = 0</td><td>irregular-/globular: 1.5–14* (Ø<sub>li</sub> = 6.7(3))</td></tr><tr><td>x = 0.05</td><td>irregular-/globular: 1.6–14 (Ø<sub>li</sub> = 7.1(4))</td></tr><tr><td>x = 0.1</td><td>irregular-/globular: 2.0–17 (Ø<sub>li</sub> = 8.5(5))</td></tr><tr><td>x = 0.15</td><td>irregular-/globular: 2.2–26 (Ø<sub>li</sub> = 10(1))<br>pillars: 4–8<sub>W</sub>×6–16<sub>L</sub></td></tr><tr><td>x = 0.175</td><td>irregular-/globular: 2.0–23 (Ø<sub>li</sub> = 10(1))<br>pillars: 4–10<sub>W</sub>×6–27<sub>L</sub></td></tr><tr><td>x = 0.2</td><td>irregular-/globular: 1.8–27 (Ø<sub>li</sub> = 12(1))<br>pillars: 4–11<sub>W</sub>×5–25<sub>L</sub></td></tr><tr><td>x = 0.25</td><td>irregular-/globular: 4–83 (Ø<sub>li</sub> = 16(3))<br>pillars: 4–14<sub>W</sub>×5–50<sub>L</sub></td></tr><tr><td>x = 0.275</td><td>no irregular-/globular grains<br>pillars: 3–25<sub>W</sub>×5–74<sub>L</sub>, large crystals up to 50<sub>W</sub> × 125<sub>L</sub></td></tr></tbody></table>

Ø<sub>li</sub>: average grain size of the irregular-/globular shaped grains estimated by the linear intercept method [1].

* The range represents the smallest and largest grains.

L: length; W: width

The fraction of the pillar-like grains increases with $x$.

[1] M. I. Mendelson, J. Am. Ceram. Soc. 52 (1969) 443.
<figure><img src="image_18.png" /><figcaption>**Fig. S1:** Room temperature XRD patterns of selected $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ preceramic powders calcined between 1200–1250 °C. The inset shows a magnification of the pattern for $x = 0.3$ recorded with a prolonged counting time.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. S2:** Rietveld refinements (Cu-K<sub>α1+α2</sub> radiation) of selected Li<sub>x</sub>(Sr<sub>0.5</sub>Ba<sub>0.5</sub>)<sub>1-x/2</sub>Nb<sub>2</sub>O<sub>6</sub> ceramics sintered at 1300 °C for 1 h.

a) x = 0 (R<sub>p</sub> = 4.16%, R<sub>wp</sub> = 5.55%, χ<sup>2</sup> = 4.53), b) x = 0.1 (R<sub>p</sub> = 3.82%, R<sub>wp</sub> = 5.02%, χ<sup>2</sup> = 3.09), c) x = 0.2 (R<sub>p</sub> = 3.74%, R<sub>wp</sub> = 4.96%, χ<sup>2</sup> = 3.23), d) x = 0.275 (R<sub>p</sub> = 3.98%, R<sub>wp</sub> = 5.28%, χ<sup>2</sup> = 3.10).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. S3:** Relative length changes of the tetragonal lattice parameters depending on the lithium content (x) for Liₓ(Sr₀.₅Ba₀.₅)₁-x/Nb₂O₆ ceramics sintered at 1300 °C for 1 h. The relative length change is related to the lattice parameters for the sample with x = 0.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. S4:** SEM-BSE images of selected $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramic bodies sintered at 1300 °C for 1 h. a) $x = 0.05$, b) $x = 0.15$, c) $x = 0.175$, d) $x = 0.25$. The insets in b) and d) shows the microstructure recorded in a topographical mode.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. S5:** Diffuse reflectance spectra of selected powdered $Li_x(Sr_{0.5}Ba_{0.5})_{1-x}Nb_2O_6$ ceramics sintered at 1300 °C for 1 h. For the sake of clarity only selected spectra are shown and every 20th data point is represented by a symbol.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. S6:** Temperature dependence of $\epsilon_r$ at different frequencies demonstrated for $Li_{0.1}(Sr_{0.5}Ba_{0.5})_{0.95}Nb_2O_6$ ceramics sintered at 1300 °C for 1 h.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Fig. S7:** Plot of ln($\epsilon^{-1}-\epsilon_m^{-1}$) as a function of ln(T-T_m) for selected Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6 ceramics sintered at 1300 °C for 1 h. Lines are the fit results, while symbols correspond to experimental data.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Fig. S8:** Cole-Cole plots measured at the indicated temperature for Liₓ(Sr₀.₅Ba₀.₅)₁⁻ₓ/₂Nb₂O₆ ceramics sintered at 1300 °C for 1 h. Fits were carried out using the shown equivalent circuit. Lines are the fit results, while symbols correspond to experimental data. Errors bars are omitted for clarity. The uncertainties of the experimental data are between 5–10 %.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Fig. S9:** Relative length change of the c-axis together with the development of the permittivity depending on temperature for $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics, sintered at 1300 °C with $x = 0.05$ (a), $x = 0.1$ (b), $x = 0.15$ (c), $x = 0.175$ (d), $x = 0.2$ (e), and $x = 0.25$ (f). The relative length change $\Delta c/c_{50^{\circ}C}$ is related to the value at 50 °C. The uncertainties of the data are smaller than the symbol sizes.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Fig. S10:** Shift of the fractional x- and y-coordinates of the Nb(2) site (Wyckoff site: 8d) with temperature for $Li_x(Sr_{0.5}Ba_{0.5})_{1-x/2}Nb_2O_6$ ceramics with $x = 0$ (a) and $x = 0.275$ (b).</figcaption></figure>