Comprehensive Structural Analysis Identifies the Relationships Between the Electrical Characteristics of Environmentally Friendly NBTMn-BAl-NaNb Ceramics

The conventional solid-state method was selected to prepare the eco-friendly (1-x)0.98(Na_0.5Bi_0.5)(Ti_0.995Mn_0.005)O₃−0.02BiAl₂O₃-xNaNbO₃(x = 0 & 0.01): NBTMn-BAl & NBTMn-BAl-NaNb ceramics. The raw oxide materials (AR-grade)Bi₂O₃ (99%), TiO₂ (98%) Al₂O₃ (94%), MnO₂ (99%), Nb₂O₅ (99.5%) and carbonate Na₂CO₃ (99.8%) were mixed in stoicmetric ratio according to their weight percentage. These mixed oxides and carbonate powders were ball milled in ethyl alcohol for 10 h @ 300 rpm. These dried powders were calcined at 800 °C for 3 h, milled further for 10 h @ 300 rpm and added to 4 wt% Polyvinyl alcohol (PVA)(4 gm of PVA mixed in 100 ml of de-ionized water) for dry powders before making a circular disc of radius (=10 mm). Finally, these circular discs were sintered at 1150 °C for 3 h in the air medium with a uniaxial heating rate of 5 °C min⁻¹. The structural phase purity of these of NBTMn–BAl and NBTMn–BAl-NaNb ceramic powders was confirmed with the help of an X-ray diffractometer (XRD, Cu Kα radiation).The surface morphology of these compounds was carried out with the help of a field emission scanning electron microscope. Raman spectroscopy was carried out with the help of Brukar Technologies, USA at room temperature. In order to characteristics of ferroelectric loops, the samples were thinned down up to 0.3 mm and the measurement was performed on printed electrodes on both sides with an ion-sputter coater. The room temperature polarization loops and strain curves were measured by P-E loop test system (PolyK Technologies, USA). Temperature dependent dielectric measurements were performed on the silver coated sintered ceramic discs from 30 °C–500 °C with a frequency variation of 10 kHz by using an impedance analyzer (E4980A; Agilent technologies company, Palo Alto, CA).

The structural information (X-ray diffraction) of lead-free NBTMn–BAl–NaNb ceramics can be seen in Fig. 1. These patterns demonstrate that both compounds crystallized into pure perovskite structures without any secondary phases coexisting from 20° to 80°. The morphological structures show that there are no superlattice reflections owing to the oxygen octahedral tilt. Figure 1 displays the enhance part of the (110)_PC pseudocubic representation for verifying the effect/substitution of ${NaNb}$ over the NBTMn–BAl. The shift of the Bragg peak towards the lower angle side of the (110)_PC Bragg peak suggests diffusion of ${NaNb}$ into the NBTMn–BAl. These ceramics' structural existence is compared to standard JCPDS card no: 01–070–9850, which has been found to be in good accord with the rhombohedral structure with R3c space group. ^19,20 Since the rhombohedral structure is non-centrosymmetric and polar in order, these polarities will be switched parallel to the A-site and B-site cations, in addition to the orientations of the (111) Bragg peaks. As a result, increasing the ${NaNb}$ concentration/quantity will result in the disruption of the long-range polar phase.

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Further structural refinement of the XRD reports was carried out for these compounds using the Full prof program ²¹ to improve the structural stability and quantitative phase fractions. In accordance with the initial structural pattern, it has rhombohedral symmetry with R3c space group. As a consequence of this disappointing finding/observation, our group examined dual phase models such as R3c main contribution and other phases such as additional phase contribution P4mm tetragonal phase. After investigating the features using several dual phase models, these structures demonstrated that it could be suitable with rhombohedral (R3c) and tetragonal (P4mm) can be seen in Fig. 2. As seen in the Table I, the level of fit showed good and reliable fitting parameters. The qualitative phase analysis showed a further phase contribution, P4mm, in addition to the dominant R3c phase. A combination NBTMn–BAl experienced a small amount of P4mm (8%), which may not disturb the polar order of the R3c phase. The replaced ${NaNb}$ enhances the contribution of P4mm considerably more than double (19%), altering the long range polar order of the R3c phase. This confirmation could be due to a decrease in the coercive field (E_C) observed in the polarization studies.

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The room temperature Raman scattering spectra of NBTMn–BAl and NBTMn–BAl-NaNb ceramics can be seen in Fig. 3. The identified band exhibits a typical perovskite structure. The entire spectrum has been divided into four bands, A, B, C, and D. These band locations correlate nicely to the typical perovskite vibrational modes. The structural stretching of the structure can be seen by the shift of small frequencies in all vibrational modes of the NBTMn–BAl-NaNb as compared to the NBTMn–BAl. This claim turns out to be in great indicating with the structural refinement of the NBTMn–BAl-NaNb, which incorporates significant contributions from when compared to the NBTMn–BAl.

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The surface morphology of NBTMn–BAl and NBTMn–BAl-NaNb appears in Fig. 4. Both surfaces are pore free according to this surface morphology. Each grain is readily recognized by its distinctive grain border and uniform grain distribution all through all morphological microstructures. There is no evidence of phase segregation at the grain boundaries. The Archimedes method confirms that both the ceramics are sufficiently density (>95% of theoretical density: 5.976 g cm⁻³). In the presence of ${NaNb},$ a slight fluctuation in grain size throughout the entire surface showed the compound's even distribution. The line intercept technique (ASTM International) can be used to evaluate the grain size distribution of these compounds. The Gaussian distribution function was employed to optimize the average grain size, as shown in Fig. 3. For NBTMn–BAl and the NBTMn–BAl-NaNb, the average grain size is 2.6 μm and 1.5 μm, respectively. The reduction of grain size aids in the increase of the breakdown strength of the compounds. Higher break down strength aids in increasing density and withstanding higher electric fields. ^22–24

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The temperature dependent dielectric constant (ε') and loss (tanδ) of the NBTMn–BAl and NBTMn–BAl-NaNb ceramics from RT to 400 °C at a frequency of 10 kHz are illustrated in Fig. 5. The observed curves proved to be similar to typical NBT-based lead-free ceramics. Figure demonstrates the significant phase transition features at T_d and T_c temperature. The loss curves show the same substantial trend as the NBTMn–BAl and NBTMn–BAl-NaNb curves. As compared to the NBTMn–BAl, the replacement of ${NaNb}$ decreases dielectric loss. As, ${NaNb}$ is substituted in the NBTMn–BAl, the T_d decreases (by 10 °C) as compared to the NBTMn–BAl. Because of its diffuse phase transition, both of these ceramics exhibited typical ferroelectric relaxor behavior. ^25,26 The dielectric constant fails to follow the Curie-Weiss the formula at this phase transition. The diffusiveness coefficient determines whether the material acts ferroelectrically or relaxoelectrically. The calculated values are found to be fairly comparable in this instance, and minor increases for the NBTMn–BAl-NaNb in compared to the NBTMn–BAl result in a more diffusive dielectric response. Figure 5 illustrates the obtained outcomes.

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The P-E hysteresis loops for NBTMn–BAl and NBTMn–BAl-NANb ceramics have been evaluated at room temperature under electric fields 60–70 at 1 Hz. In the presence of electric fields, both compounds exhibit well saturated hysteresis loops with remarkable residual polarization (P_r) and saturation polarization (P_s). The coercive field (E_C) causes a slight decrease in the presence of ${NaNb},$ as seen by the effect of ${NaNb}$ substitution, resulting in domain wall clamping. The value of ${P}_{r}$ slightly increases in presence of NaNb close to change in ${P}_{r}:$ ${\rm{\Delta }}{P}_{r}$ = 4 μC cm⁻². The coercive field decreases in presence of NaNb, ${\rm{\Delta }}{E}_{C}=10\,{\rm{kV}}/{\rm{cm}}.$ These loops show ferroelectric behavior, and the broad square loops may be approximated utilizing the equation below.

$$\begin{eqnarray*}&&{R}_{sq}=\left(\displaystyle \frac{{P}_{r}}{{P}_{s}}\right)+\left(\displaystyle \frac{{P}_{1.1{E}_{C}}}{{P}_{r}}\right)\end{eqnarray*}$$

R_sq is the hysteresis loop's squareness, whereas P_r is the residual polarization at field zero. P_s indicates the saturated polarization gained at some finite field strength below the dielectric break down, whereas P_{1:1 EC} denotes the polarization obtained at 1.1_Ec. R_sq equals 2 for an ideal square loop. R_sq values of 1.94 and 1.83 for NBTMn–BAl and NBTMn–BAl-NaNb, respectively, correspond with ideal square loops. ^27–29 The presence of both phases, specifically rhombohedral and tetragonal, explains for the modest drop in R_sq value for NBTMn–BAl-NaNb. This is in line with the good cooperation with the structural refinement of NBTMn–BAl-NaNb. The diagrams in Fig. 6 indicate apparent shifts in polarization with different fields for NBTMn–BAl and NBTMn–BAl-NaNb.

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Figure 7 shows bipolar and unipolar strain curves of NBTMn–BAl and NBTMn–BAl-NaNb ceramics at room temperature and 1 Hz frequency. These curves involve butterfly-shaped loops, and the derived strain value from the graph shows the strong ferroelectric property. The small rise in NBTMn–BAl-NaNb strain value illustrates the influence of a replacement in NBTMn–BAl. This significant change in NBTMn–BAl-NaNb is primarily caused by the secondary phase's contribution. Because of the higher dielectric constant, the positive strain (S_pos) rose and the negative strain (S_neg) decreased for NBTMn–BAl-NaNb as opposed to NBTMn–BAl.

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Table I. The structural refined parameters and lattice constants of ${\boldsymbol{NBTMn}}{\boldsymbol{-}}{\boldsymbol{BAl}}$ and ${\boldsymbol{NBTMn}}{\boldsymbol{-}}{\boldsymbol{BAl}}{\boldsymbol{-}}{\boldsymbol{NaNb}}$ ceramics.

R3c	NBTMn–BAl	NBTMn–BAl-NaNb
a Å	9.5460	9.4460
b Å	5.5183	5.4783
c Å	5.5120	5.6120
$\alpha =\gamma =90^\circ \,\beta =125.257^\circ$
P4mm	NBTMn–BAl	NBTMn–BAl-NaNb
a Å	3.8844	3.4444
b Å	3.8844	3.5344
c Å	3.9449	3.4149
$\alpha =\gamma =\beta =90$

We utilized a solid state approach to successfully synthesis high density NBTMn–BAl and NBTMn–BAl-NaNb ceramics in this study. As seen in Fig. 2, evident that, structural characteristics quantitatively bring forth additional phase contribution. Raman spectroscopy confirmed this structural confirmation of addition phase by small blue shift. The ferroelectric examines demonstrated that the synthesized compounds exhibited a strong ferroelectric character, ${P}_{r}$ slightly increases ${\rm{\Delta }}{P}_{r}$ = 4 μC cm⁻² and coercive field changes ${\rm{\Delta }}{E}_{C}=10\,{\rm{kV}}/{\rm{cm}}$ in presence of NaNb, which was also visible from the strain measurements. These structural, morphological, and electrical features in the presence of NaNb have been discovered to be useful /beneficial for today's eco-friendly applications.

The author A. A. Ansari thanks the Researchers supporting project number (RSP2023R365), King Saud University, Riyadh, Saudi Arabia.

The authors declare that all the data generated or analyzed during this study are included in this manuscript.