Synthesis, structural modification, and dielectric property enhancement of manganese and tin-doped bentonite composites for high-performance energy storage applications

2.1. Materials

BN [CAS Number: 1302-78-9, Molecular weight: 1801 g/mol], SnCl₂ [CAS Number: 7772-99-8, Molecular weight: 189.62 g/mol], and MnCl₂ [CAS Number: 7773-01-5, Molecular weight: 125.84 g/mol] were obtained from Sigma-Aldrich (USA). Deionized water was used to prepare all aqueous solutions.

2.2. Composites preparation

Mn-BT and Sn-BT composites were prepared through the incipient wet impregnation method, following the procedure described in our previous work [7]. First, BT was dispersed in distilled water under constant stirring at room temperature. A precise amount of MnCl₂ and SnCl₂ salts (5 wt%) were then dissolved in the solution, followed by vigorous stirring for 3 h to ensure homogeneity. The mixture was subsequently dried at 393 K for 12 h. After drying, the obtained solid material was ground into a fine powder using a mortar and pestle. Finally, 0.30 g of the powdered composite was pressed into pellets using a hydraulic die-set for dielectric measurements.

2.3. Characterization techniques

The X-ray diffraction (XRD) patterns were obtained utilizing a Philips X’Pert-Pro diffractometer. The analysis used Cu-Kα radiation with a wavelength of 1.5418 Å, operating at 40 kV and 40 mA. Data were collected over a 2θ range of 10° to 65° with a scan step size of 0.02° and scan speed of 12°/min. The optical investigation was performed through an Ocean Optics (2000 USB) spectrometer covering the wavelength width of 200–1100 nm. On the other hand, a Shimadzu FTIR-8400 spectrophotometer was employed for the Fourier transform infrared (FT-IR) measurements in the wavenumber range 500 – 4000 cm^-1. Additionally, we investigated the dielectric and electrical properties in the temperature range 25-120°C and in the frequency range from 1 kHz to 1 MHz. The samples were circular films, roughly 0.5 cm radius and approximately 1 mm thick. In our investigation, copper electrodes were attached to the films. For impedance measurements, copper electrodes were attached to both sides of the films using a thin adhesive layer to ensure good electrical contact. This method was preferred over using conductive paste (such as silver paste), which could potentially seep into the sample and produce unwanted signals in the impedance spectrum.

3.1. XRD analysis

Figure 1 depicts the XRD patterns for the pure BT, Mn-BT, and Sn-BT at room temperature. One can observe the presence of the M, Si, and Quartz (Q) phases within the three samples. For pure BT, the prominent peaks at 2θ values of approximately 19.78°, 27.99°, 35.35°, 54.16°, and 61.98° correspond to M. Diffraction peaks at 21.93° were attributed to the presence of Si (JCPDS no. 27-0605) [10]. The peak at approximately 26.60° indicates the presence of Q [7]. The additional diffraction peaks in the undoped BT are most likely caused by impurities like cristobalite, feldspar, and illite, consistent with the findings of previous BT investigations [11]. The same diffraction peaks were observed in Mn-BT and Sn-BT samples with a slight shift in 2θ values. This peak shift in 2θ values suggests structural modifications due to the interaction of metal ions with the BT. For Mn-BT, the peaks shift towards higher 2θ values, which suggests a decrease in the interlayer spacing (d-spacing). This contraction may result from the incorporation of Mn²⁺ ions, leading to stronger electrostatic interactions and a reduction in the distance between clay layers. Conversely, in Sn-BT, the peaks shift towards lower 2θ values, indicating an expansion of the interlayer spacing. This expansion could be attributed to the larger ionic radius of Sn²⁺ compared to Mn²⁺, which may disrupt the M structure and increase the basal spacing. Additionally, the shift may be due to the aggregation of clay layers caused by a shortage of water in the interlayer region, which might be a consequence of adding Mn and Sn metals [12,13]. All the diffraction peaks have been summarized in Tables 1–3 for BT, Mn-BT, and Sn-BT, respectively. To quantify these structural modifications, the shifts in 2θ values (Δ2θ) for key diffraction peaks indicate noticeable changes in the crystal structure of BT upon doping with Mn and Sn. For the M peak initially observed at approximately 19.78°, a shift of +0.062° is recorded for Mn-BT, whereas a slight decrease of -0.026° is observed for Sn-BT. Similarly, the Si peak at around 21.94° shifts by +0.123° for Mn-BT but exhibits a more pronounced decrease of -0.465° for Sn-BT. The Q peak near 26.61° experiences a minor shift of +0.056° for Mn-BT and +0.015° for Sn-BT, indicating minimal structural distortion. Conversely, the M peak at approximately 27.99° shifts by -0.282° for Mn-BT and -0.341° for Sn-BT, further suggesting variations in interlayer spacing and potential structural rearrangements induced by the incorporation of metal ions. Further, no distinct peaks corresponding to Mn or Sn were detected in the diffraction patterns of Mn-BT and Sn-BT, suggesting the successful inclusion of Mn and Sn ions into the BT structure. According to Scherrer’s equation, the average crystallite size of the BT, Mn-BT, and Sn-BT materials increased from 16.07 nm, 17.58 nm, and 19.88 nm, respectively. Similar behavior was noted in reference [9] for BT doped with Fe, Zn, Cd, and Cu metals.

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Figure 1:

XRD spectra of BT, Mn-BT, and Sn-BT samples at room temperature.

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3.2. SEM analysis

Figures 2 and 3 illustrate a detailed characterization of Mn-BT and Sn-BT, respectively, using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). For the Mn-BT sample, the surface morphology shows a rough and irregular texture with multiple layers and flakes, indicative of its natural, non-uniform formation, which highlights the varying particle sizes and shapes, with some particles appearing larger and more irregular. The porous texture is evident, which is characteristic of BT and contributes to its adsorption properties. The layering and aggregation observed suggest a stratified structure, typical of clay minerals. However, for the Sn-BT compound, the surface morphology reveals a rough, granular surface with clusters of particles dispersed throughout the field of view. These clusters appear to be irregular in shape and size, indicating a heterogeneous distribution typical of BT materials. The image also shows cracks and voids within the structure, suggesting a porous nature, which is a key characteristic of BT and contributes to its high surface area and adsorption capacity. The detection of all constituents within the samples, such as Si, Al, Mn, N, Mg, Na, Fe, Ca, F, Zn, Cl, and O, indicates that no elements were lost throughout the experiment. Consequently, the composition of doped BT is mainly dominated by large amounts of Si and Al, with tiny quantities of Fe, Mg, Ca, and Na. In addition, all the elements are distributed homogeneously across the surface of the compounds. It is noteworthy that the SEM analysis for pure BT was investigated in our previous work [8].

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Figure 2:

SEM analysis of Mn-doped BT. The image displays the morphological and elemental characterization. (a) shows the SEM micrograph of (Mn/BT) revealing the surface morphology and mapping of all presents elements, such as (b) aluminum, (c) calcium, (d) iron, (e) carbon, (f) manganese, (g) magnesium (h) silica, (i) sodium, (j) nitrogen, (k) sulphur, and (l) Oxygen. (m) are elemental full mapping images illustrating the distribution of each elements and their combinations within the nanocomposite structure.

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Figure 3:

SEM analysis of Sn-doped BT. The image displays the morphological and elemental characterization. (a) shows the SEM micrograph of (Sn/BT) revealing the surface morphology and mapping of all presents elements, such as (b) aluminum, (c) carbon, (d) calcium, (e) chlorine, (f) fluorine, (g) iron (h) zinc, (i) silica, (j) magnesium, (k) nitrogen, (l) sodium, and (m) oxygen. (n) are elemental full mapping images illustrating the distribution of each elements and their combinations within the nanocomposite structure.

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3.3. FT-IR analysis

Figure 4 shows the FT-IR spectra of the prepared samples in the wavenumber range 500 - 4000 cm^-1. Distinct absorption bands characteristic of crystalline impurities and those related to the clay phase can be identified. The spectra for all three samples display prominent bands around 3625 cm^-1 and 3415 cm^-1, indicative of the stretching vibrations of OH groups, particularly those associated with water molecules in the clay structure [14]. A notable band around 1632 cm^-1 corresponds to the H-O-H deformation vibrations of water molecules adsorbed between the clay layers [14]. Additionally, the strong absorption band around 1121 cm^-1 is attributed to the Si-O stretching vibrations in the tetrahedral layers, while the band around 915 cm^-1 is linked to the Al-O-H bending vibrations, characteristic of M [15].

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Figure 4:

-FT-IR spectra of BT, Mn-BT and Sn-BT samples at room temperature.

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Furthermore, the deformation vibrations of Si-O-Al manifest in M with a peak at 550 cm^-1 [16]. It is noteworthy that specific bands appearing at around 1437 cm^-1 are assigned to Na-M [17]. However, there are distinct differences in the absorption bands between the samples. In Mn-BT, there are slight shifts and variations in the intensity of the OH stretching and Si-O stretching bands compared to pure BT, indicating interactions or substitutions involving Mn ions. These shifts suggest that Mn incorporation alters the local electronic structure by modifying the bonding environment and electron density distribution around oxygen atoms. Similarly, the Sn-BT shows shifts and intensity changes in the same regions, reflecting the influence of Sn ions on the clay’s structure. These modifications arise due to differences in electronegativity and ionic radii between Mn²⁺/Sn²⁺ and the original ions they replace, leading to changes in dipole moments and local polarization effects. The differences in the clay’s structure highlight the effect of replacing metal ions on the structural and vibrational properties of the BT samples.

3.4. UV-Vis analysis

The absorbance spectra of BT, Mn-BT, and Sn-BT were recorded over the wavelength range of 200 to 1100 nm (Figure 5a). Notably, all three samples exhibit a strong absorbance peak in the UV region, specifically around 200 to 300 nm, which is characteristic of Si–O and the Al–O charge-transfer electronic within the clay minerals [18,19]. The BT sample shows a distinct absorbance peak at approximately 210 nm, indicating the presence of structural features and possibly impurities that absorb strongly in this region. Comparatively, the Mn-BT and Sn-BT samples demonstrate similar absorbance behavior but with slightly higher intensity and slight shifts in the peak positions. This behaviour suggests an increased absorbance due to the incorporation of Mn and Sn ions.

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Figure 5.

(a) Absorbance spectra; (b) band gap analysis at room temperature of BT, Mn-BT and Sn-BT samples.

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The band gap energies of our materials were determined by applying the Tauc plot method based on the Tauc Eq. (1) [20]:

α represents the optical absorption coefficient, h is Planck’s constant, ʋ denotes the frequency of the light, E_g stands for the band-gap energy, and A is a material-dependent constant. Figure 5(b) displayed the Tauc plot for BT, Mn-BT, and Sn-BT.

The extrapolation of the linear portion of the curve to the energy axis (x-axis) gives the bandgap energy (E_g) for each sample. From the graph, the intercepts of the linear fits for BT, Mn-BT, and Sn-BT are around 4.14 eV, 4.01 eV, and 4.07 eV, respectively. This indicates that the doping of BT with Mn and Sn slightly reduces the bandgap energy compared to pure BT. The decrease in bandgap energy suggests that doping introduces new energy levels within the bandgap, which can enhance the material’s electronic and optical properties [21]. These impurity levels facilitate electronic transitions at lower energies, contributing to the observed bandgap narrowing. Additionally, lattice distortions induced by the dopants may further influence the electronic structure. While charge transfer interactions cannot be entirely ruled out, the gradual nature of the bandgap reduction suggests that impurity levels are the dominant factor. A slight reduction in the bandgap allows the material to absorb light over a broader wavelength range, facilitating easier excitation of electrons from the valence band to the conduction band under lower photon energy. This improvement in optical response is particularly beneficial for applications in optoelectronic devices, photocatalysis, and sensors, where enhanced light interaction is desirable [22]. Our findings on the influence of metal substitution on BT’s band gap energy are consistent with previous research reported by M. Kaur et al. [23], where the incorporation of MgFe₂O₄ nanoparticles into BT reduced the band gap.

3.5. Dielectric analysis

3.5.1. Dielectric permittivity

The dielectric analysis represents a significant source of valuable information on conduction processes, as it can be used to understand the origin of dielectric losses and the electrical and dipolar relaxation times [24]. Figure 6 illustrates the real part of the permittivity ${\epsilon }^{\prime }$ of BT, Mn-BT, and Sn-BT versus frequency at various temperatures. All the samples have the same behaviour. As frequency increases, the ${\epsilon }^{\prime }$ decreases consistently across all temperatures. This trend is typical in dielectric materials, where higher frequencies result in reduced permittivity due to the inability of dipolar entities to align with the rapidly oscillating electric field [25]. At lower frequencies, the permittivity values are higher, indicating better alignment of dipoles with the electric field, contributing to higher dielectric constant values [26].

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Figure 6:

Frequency evolution of ${\epsilon }^{\prime }$ at various temperatures of (a) BT, (b) Mn-BT, (c) Sn-BT, (d) frequency evolution of ${\epsilon }^{\prime }$ at room temperature.

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Debye relaxation describes the dielectric response of material that pertains to an ideal dielectric subjected to an external alternating electric field, without the mutual interaction of the dipole population. It is generally described by the expression (Eq. 2):

(2)

${\epsilon }^{*}={\epsilon }_{\infty }+\frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+i\left(2\pi F\tau \right)}$

Where ${\epsilon }_s$ is the static permittivity, ${\epsilon }_{\infty }$ is the high-frequency permittivity, $\tau$ is the characteristic relaxation time of the material. However, most dielectric materials do not follow the Debye expression and are often described by a more general expression of permittivity known as the Cole-Cole law (Eq. 3) [27]:

(3)

${\epsilon }^{*}={\epsilon }^{\prime }+i{\epsilon }^{″}={\epsilon }_{\infty }+\frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+{\left(i2\pi F\tau \right)}^{1-\alpha }}-i\frac{{\sigma }_{dc}}{2\pi {\epsilon }_{0}F}$

Where ${\sigma }_{dc}$ is the direct current electrical conductivity and ${\epsilon }_0=8.85\times {10}^{-12} \text{F}{\text{.m}}^{-1}$ is the vacuum permittivity. α represents the distribution of relaxation times (0 < α < 1) and can be interpreted as a dispersion factor around the average relaxation time. From the Eq. (4), the real part of the permittivity may be expressed as:

(4)

${\epsilon }^{\prime }\left(F\right)=\frac{({\epsilon }_s-{\epsilon }_{\infty }){\left(2\pi F\tau \right)}^{1-\alpha }\cos \left((1-\alpha )\frac{\pi }{2}\right)}{1+2{\left(2\pi F\tau \right)}^{1-\alpha }\cos \left((1-\alpha )\frac{\pi }{2}\right)+{\left(2\pi F\tau \right)}^{2(1-\alpha )}}+\frac{{\sigma }_{dc}}{2\pi F{\epsilon }_0}$

We observe that Equation II.16 consists of two terms. The first term corresponds to thermal polarization, while the second term is related to electrical conduction [28]. The solid lines on the curves in Figure 6 represent the simulated results for each temperature based on a Cole-Cole model. This indicates a strong correlation between experimental and calculated results. Tables 4–6 show the fit parameters, such as εₛ, ε_∞, α, τ, and σ_dc for BT, Mn-BT, and Sn-BT, respectively. Specifically, the increase in relaxation time (τ) for Mn-BT and Sn-BT suggests a shift in polarization mechanisms, while the variations in σ_dc indicate changes in charge transport properties. These findings are consistent with previous studies on doped dielectric materials [29-33]. As shown in Figure 6(d), the dielectric constants for Mn-BT and Sn-BT are significantly higher than that of pure BT, indicating that doping with Mn and Sn enhances the dielectric properties of BT​.

Table 4: The fit parameters based on Cole-Cole model of pure BT.

T (°C)	${\epsilon }_s$ (F.m-1)	${\epsilon }_{\infty }$ (F.m-1)	α	τ (s)	${\sigma }_{dc}$ (S.m-1)
25	92.99	3.00	0.71	2.09×10-6	3.61×10-6
30	115.26	17.09	0.75	2.27×10-6	2.52×10-6
40	115.81	17.00	0.74	1.47×10-6	2.84×10-6
50	96.96	0.65	0.76	1.26×10-6	2.59×10-6
60	93.19	1.81	0.74	1.04×10-6	2.84×10-6
70	86.22	8.78	0.72	1.11×10-6	2.59×10-6
80	67.20	27.80	0.72	1.05×10-6	1.19×10-6
90	65.27	29.73	0.70	1.31×10-6	1.12×10-6
100	63.18	31.82	0.68	1.73×10-6	1.17×10-6
110	61.38	33.62	0.65	2.30×10-6	1.24×10-6
120	58.03	36.97	0.69	4.63×10-6	9.66×10-7
130	57.16	37.84	0.77	6.16×10-5	4.07×10-7

Table 5: The fit parameters based on Cole-Cole model of Mn-doped BT.

T (°C)	${\epsilon }_s$ (F.m-1)	${\epsilon }_{\infty }$ (F.m-1)	α	τ (s)	${\sigma }_{dc}$ (S.m-1)
25	85.61	9.39	0.74	7.20×10-4	3.98×10-7
30	78.42	16.58	0.65	6.35×10-6	1.38×10-6
40	78.64	16.36	0.66	1.89×10-6	2.60×10-6
50	78.13	16.87	0.69	1.62×10-6	2.30×10-6
60	77.03	17.97	0.73	1.56×10-6	1.90×10-6
70	75.81	19.19	0.73	1.28×10-6	2.36×10-6
80	73.97	21.03	0.74	1.08×10-6	3.58×10-6
90	73.95	22.05	0.77	1.43×10-6	4.82×10-6
100	88.39	7.61	0.91	3.70×10-6	6.18×10-6
110	85.11	9.89	0.89	4.12×10-6	5.47×10-6
120	76.26	18.74	0.79	1.49×10-5	5.42×10-6
130	76.06	18.94	0.70	6.89×10-6	6.46×10-6

Table 6: The fit parameters based on Cole-Cole model of Sn-doped BT.

T (°C)	${\epsilon }_s$ (F.m-1)	${\epsilon }_{\infty }$ (F.m-1)	α	τ (s)	${\sigma }_{dc}$ (S.m-1)
25	74.44	20.56	0.75	2.98×10-4	1.48×10-6
30	80.87	14.13	0.75	8.77×10-5	1.00×10-6
40	79.55	15.45	0.75	5.36×10-5	1.16×10-6
50	78.59	16.41	0.75	2.95×10-5	1.22×10-6
60	75.65	19.35	0.71	5.22×10-6	2.78×10-6
70	73.59	21.41	0.69	2.81×10-6	4.79×10-6
80	72.08	22.92	0.69	2.74×10-6	5.22×10-6
90	70.55	24.45	0.71	3.70×10-6	5.42×10-6
100	68.45	26.55	0.72	3.55×10-6	6.93×10-6
110	66.83	28.17	0.73	4.16×10-6	7.51×10-6
120	65.26	29.74	0.74	5.39×10-6	7.83×10-6
130	65.87	29.13	0.80	7.38×10-5	9.30×10-6

At 25°C and 4×10⁴ Hz, the dielectric constant of pure BT is 4.46, while Sn-BT exhibits a value of 12.56. Similarly, Mn-BT shows a dielectric constant of 16.30. These results highlight the effectiveness of Mn and Sn doping in enhancing the dielectric response of BT. This remarkable improvement suggests that Mn-BT and Sn-BT composites have great potential for advanced applications in high-performance capacitors and energy storage devices [34,35]. In comparison with other energy storage technologies, supercapacitors offer high power density, rapid charge-discharge cycles, and extended lifespan, making them superior for applications requiring fast energy storage and release. Unlike conventional capacitors, which rely solely on electrostatic charge storage, or batteries, which involve slow faradaic reactions, supercapacitors benefit from both mechanisms, allowing enhanced energy efficiency. The enhanced dielectric permittivity observed in Mn-BT and Sn-BT composites directly correlates with improved charge storage capacity. A higher dielectric constant increases the ability of the material to store an electric charge, thereby enhancing the energy density of the capacitor. This suggests that Mn-BT and Sn-BT composites could contribute to improving the performance of supercapacitors by enabling greater charge accumulation and reducing energy losses. A detailed comparison with other energy storage technologies can be found in recent studies [36-40]. Figure 7 depicts the temperature evolution of ${\epsilon }^{\prime }$ at distinctive frequencies for BT, Mn-BT, and Sn-BT. The observed trend suggests a pronounced decrease in ${\epsilon }^{\prime }$ with increasing temperature, signifying a transition toward a more conductive state at higher thermal energy levels. This can be attributed to the enhanced mobility of charge carriers within the samples lattice at elevated temperatures, leading to a diminished capacity for storing electrical energy. Furthermore, the higher ${\epsilon }^{\prime }$ values observed at lower frequencies indicate a more prominent contribution from relaxation phenomena, where the materials have sufficient time to reorient their internal dipoles in response to the alternating electric field. This response becomes progressively weaker at higher frequencies as the dipoles encounter difficulty in synchronizing with the rapidly fluctuating external field, leading to a decrease in ${\epsilon }^{\prime }$.

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Figure 7:

Temperature evolution of ${\epsilon }^{\prime }$ at various frequencies of (a) BT, (b) Mn-BT, (c) Sn-BT.

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3.5.2. Dielectric loss

Figure 8 depicts the trend of dielectric losses as a function of frequency at various temperatures for BT, Mn-BT, and Sn-BT samples.

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Figure 8:

Frequency evolution of $Tan\delta$ at various temperatures of (a) BT, (b) Mn-BT, (c) Sn-BT.

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Tan δ are notably high at low frequencies and elevated temperatures but decrease significantly at higher frequencies regardless of the temperature. This unique pattern in dielectric loss is consistent with Koop’s phenomenological theory [41].

3.6. Conductivity analysis

Electrical conductivity measurement in BT samples is a key property that affects their use in various applications, including environmental and material sciences. By examining the electrical behavior of undoped and metal-doped BT, researchers can gain insights into charge transport mechanisms and the impact of dopants on conductivity, leading to improved material performance and new applications. Figure 9 illustrates the electrical conductivity ${\sigma }_{tot}$ versus frequency at temperature range 25-130°C for BT, Mn-BT, and Sn-BT samples. Across all samples, the conductivity shows a gradual increase with rising frequency, followed by a sharp rise beyond the hopping frequency (F > F_h). This rise in conductivity at elevated frequencies could be assigned to electron hopping between grain sites, as described by Almond-West law (Eq. 5) [42,43]:

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Figure 9:

Frequency evolution of ${\sigma }_{AC}$ at various temperatures of (a) BT, (b) Mn-BT, (c) Sn-BT.

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Here, ${\sigma }_0$ is a pre-exponential factor that determines the strength of polarizability, F_h denotes the hopping frequency, which distinguishes the direct current conduction from the dispersive regime, and p serves as the universal parameter, affecting the strength of the connection between mobile charges and the lattice (0 < p < 1). If p $\le$1, the electron transition occurs through a sudden jump via translation. In contrast, for p >1, the electron transition is confined to neighboring sites [44]. By applying Eq. (5) to fit the conductivity data (solid line in Figure 9), the exponent p was obtained. Figure 10 depicts the thermal evolution of the exponent p for BT, Mn-BT, and Sn-BT. It is evident that p rises with increasing temperature before declining, indicating that the non-overlapping small polaron tunneling (NSPT) and correlated barrier hopping (CBH) models apply to all samples [45-47]. The same behavior was observed in our previous work Ce and Zn-doped BT [8]. However, doping significantly reduces the p values, suggesting that Mn and Sn incorporation alter the charge transport pathways. The lower p values in Mn-BT and Sn-BT imply a decrease in the effective energy barrier for charge hopping, which may be attributed to the introduction of additional localized states that facilitate charge mobility.

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Figure 10:

Temperature evolution of exponent p of BT, Mn-BT and Sn-BT samples.

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