Studies on structural, dielectric and optical properties of (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ lead-free ceramics

Abstract

(Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ (BCNTSNO₃) lead-free ceramics with compositions (x = 0.75, 0.8 and 0.85) were synthesized through the traditional solid-state reaction method. X-ray powder diffraction analysis showed the formation of a single phase compound crystallized in tetragonal space group P4mm. The evolution of Raman spectra displayed a disorder introduced into the structure, which favors a ferroelectric relaxor behavior. The dependence of the dielectric properties on temperature exhibited two composition ranges with different behaviors. Ferroelectric relaxor properties were observed for the compositions x < 0.85 and the classical ferroelectric behavior for x = 0.85. Lead free (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ ceramics exhibited larger dielectric constants than those of parent crystal NaNbO₃, suggesting that it is a good candidate for lead-free ceramics in several industrial applications. Using UV-Vis spectroscopy, the optical band gap energy of ceramics (Ba_0.95Ca_0.05)_1−x (Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ is found at 2.89, 2.92, and 3.05 eV for x = 0.75, 0.8 and 0.85, respectively.

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1. Introduction

With the recent growing demand for global environmental protection and sustainable development, researchers' interests have turned to lead-free materials such as ferroelectrics with the ABO₃ perovskite structure.^1–3 Due to their multifunctional properties,^4–6 good thermal resistance as well as chemical stability, nature-friendly ferroelectrics may be good candidates for industrial applications. These materials have similar or better physicochemical properties than toxic ferroelectrics. Ferroelectrics are typical materials that exhibit spontaneous polarization, in which their direction is reversed by reversing the external electric field. However, based on their behavior, ferroelectric materials have been extensively used in multiple fields including multilayer capacitors, electromechanical systems, energy storage and ultrasonic transducers.^7–11 Recently, the coupling of ferroelectric polarization with light absorption makes the ferroelectric perovskites promising candidates for applications in photoelectrics.¹²

Barium titanate (BaTiO₃) is a classical ferroelectric used for the preparation of ceramics, which is widely explored given its excellent electrical, dielectric and ferroelectric properties. From the viewpoint of the nature of ferroelectric phase transitions between order-disorder and displacive scenarios, it presents a very large optical band gap energy of about 3.26 eV.¹³ In attempts to improve the properties of BaTiO₃, dopants are generally added to the A and/or B sites of the material. In this case, the BaTiO₃–CaTiO₃–BaSnO₃ ternary system was studied.¹⁴ The latter may display multiphase coexistence at room temperature such as a Morphotropic phase boundary (MPB) or polymorphic phase boundary (PPB), where good piezoelectric property is expected with a high value of T_C.

Sodium niobate NaNbO₃ (NN) is an antiferroelectric (AFE) perovskite with an orthorhombic structure in space group Pbma at room temperature.¹⁵ It has attracted attention owing to its successive phases structure transition.¹⁶ In addition, NaNbO₃ perovskite is an important photocatalysis semiconductor with a band gap energy of 3.4 eV.^17,18 As a result, NN is also a promising candidate in many areas of optical applications involving energy storage, optical waveguides, ultrafast optical switching, non-volatile ferroelectric memories and semiconductor photoélectrodes.¹⁹ Moreover, in the field of photocatalysis, it is used for the separation of water, purifying the air and the decomposition of dyes.^20,21 Furthermore, the dielectric behavior of NN has been widely investigated. It exhibits a phase transition at T_C around 913 K with a maximum real permittivity of 1750 at T_C.²² NaNbO₃ (NN) is a anti-ferroelectric at room temperature¹⁷ and it transforms to ferroelectric substance, if mixed with a small amount of KNbO₃,⁷ LiNbO₃,²³ BaSnO₃,²⁴ and SrSnO₃.²⁵ However, only several studies have been conducted on the ceramics formed BaTiO₃–NaNbO₃ system with the aim to examine the effect of cation substitution in A and B sites, including significant changes in the dielectric and structural properties.

The mixture or solid solution of anti-ferroelectric and ferroelectric materials usually results in the formation of multifunctional material having good ferroelectric and piezoelectric properties.²⁶ In this study, antiferroelectric NN and ferroelectric ternary systems BaTiO₃–CaTiO₃–BaSnO₃ are synthesized to develop a new lead free ferroelectric solid solution with general composition (1 − x)[(Ba_0.95Ca_0.05)(Ti_0.8Sn_0.2)] Na_xNb_xO₃. The ceramics (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ with x = 0.75, 0.8 and 0.85 were synthesized by means of the conventional solid-state reaction method. The structural and physical properties were explored by X-ray diffraction, dielectric measurements and Raman spectroscopy. The optical properties under UV-visible irradiation were reported.

2. Experimental details

The compounds (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃, (x = 0.75, 0.8 and 0.85), were prepared through the conventional solid-state reaction method. The used precursors are as follows: carbonates (BaCO₃, CaCO₃ and Na₂CO₃) and oxides (SnO₂, TiO₂ and Nb₂O₅) with 99.9% purity of Aldrich brand. The mixed powders are milled in an agate mortar for 1 hour to homogenize the mixture and reduce the size of the grain size. The homogenized powders are calcined between 1000 and 1100 °C for 12 h in an electric furnace to evaporate CO₂. The calcined powders are ground, again for 1 h and then shaped into pellets of 8 mm in diameter and 1 mm thick to undergo second heat treatment (sintering) at a higher temperature ranging between 1250 and 1400 °C for 2 h.

To examine the phase purity and evolution of the crystal symmetry of the prepared ceramics, the X-ray diffraction measurements were carried out on sintered pellets at room temperature. The intensity determining was performed by a step of 0.02 in 2θ and a counting time of 10 s by means of a Philips X'pert PW 3040/00 diffractometer using the Kα radiation (λ = 1.5406 Å). Cell parameters and line profile were refined by a global method using the Fullprof program based on Rietveld method.²⁷

The dielectric study was carried out on sintered pellets of diameter 7 mm and thickness 1 mm. The impedance spectroscopy measurements were performed as a function of temperature (50–600 K) and frequency (0.1 Hz to 200 kHz) using a Wayne-Kerr 6425 component analyzer.

The Raman spectra were recorded in the wavenumber range of 50–1000 cm⁻¹ on a Horiba Lab-ram HR 800 Raman spectrometer equipped with a He–Ne laser of 633 nm emission line with the power of 15 mW and a CCD detector.

The linear optical properties were measured at room temperature, using a UV-3101PC scanning spectrophotometer in the 200–1000 nm wavelength range.

3. Results and discussion

3.1 X-ray diffraction analysis

The X-ray patterns of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics of compositions (x = 0.75, 0.8 and 0.85) indicate that all the samples have pure perovskite-type structures without secondary phases, implying that the Ba²⁺, Ca²⁺, Sn⁴⁺, and Ti⁴⁺ ions are well incorporated on the A and B sites of NaNbO₃ lattice. The Rietveld refinements of the XRD patterns exhibit a tetragonal phase with space group P4mm at room temperature. This symmetry is characterized by a (002)/(200) peak splitting between 45° and 46°. As an example, the difference diffraction pattern for x = 0.75 is shown in Fig. 1.

Fig. 1 X-ray diffraction pattern at room temperature of BCNTSNO ceramic with composition corresponding to x = 0.75.

The corresponding results and the estimated values of the refinement parameters of the title ceramics are summarized in Table 1. One can notice that, when x increases, the cell parameters a, b, c and cell volume v increase. These results can be related to the increase of the ionic radii of the cations substituted in A and B sites.

Table 1 The refinement results of X-ray pattern and cell parameters of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ with different structure

(Ba0.95Ca0.05)1−xNax(Ti0.8Sn0.2)1−xNbxO3	X = 0.75	X = 0.8	X = 0.85
Phase	Tetragonal	Tetragonal	Tetragonal
Space group	P4mm	P4mm	P4mm
Lattice parameters	a = b = 3.916 Å	a = b = 3.919 Å	a = b = 3.924 Å
	c = 3.946 Å	c = 3.9447 Å	c = 3. 945 Å
	v = 60.498 Å^3	v = 60.567 Å^3	v = 60.685 Å^3
〈rA〉	1.279	1.259	1.239
〈rB〉	0.635	0.637	0.638
t	0.924	0.917	0.910

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The values of the ionic radii (r_Na⁺) = 1.18 Å and (r_Nb⁵⁺) = 0.64 Å coordinated with 8 and 6 respectively for NN lower than those of (r_Ba²⁺) = 1.61 Å, (r_Ca²⁺) = 1 Å, (r_Sn⁴⁺) = 0.69 Å and (r_Ti⁴⁺) = 0.605 Å in coordination with 6 in BTS.²⁸ Moreover, these results are in relation with the variations of the average radii in site A 〈r_A〉 and in site B 〈r_B〉. The following relations govern these radii:

〈r_A〉 = x(r_Na⁺) + (1 − x)[0.95(r_Ba²⁺) + 0.05(r_Ca²⁺)]

〈r_B〉 = x(r_Nb⁵⁺) + (1 − x)[0.8(r_Ti⁴⁺) + 0.2(r_Sn⁴⁺)]

According to the calculations, we notice that the ionic radius 〈r_A〉 decreases from 1.27 to 1.239 Å, respectively, for x = 0.75 and 0.85. As it appears, in this case, the ionic radius in A site should increase. However, the ionic radius 〈r_B〉 of the B-site increases with a small step from 0.635 to 0.638 Å. Therefore, the increase of the lattice parameters as well as the cell volume as a function of compositions could be accounted for in terms of the increase of 〈r_B〉 rather than 〈r_A〉.

It is interesting to find out the regularities, which govern the formation of perovskite compounds and use them to further guide the exploration of new materials. In the early 1920s, Goldschmidt²⁹ proposed a tolerance factor “t” to study the stability of perovskite, defined as:

where, r_A, r_B and r_O are ionic radii of A-site cation, B-site cation and oxygen, respectively. In general, the perovskite structure is stable for t values obeying 0.75 < t < 1.06 and the symmetry is high as long as the t value is close to 1. This parameter is suggestive of how far the atoms can move from the ideal packing and be still belonging to the perovskite structure. The tolerance factor was calculated in the order of t = 0.9, which is plotted in Table 1. Hence, we can deduce that the structure of all the ceramics is stable.

3.2 Raman spectroscopy

The Raman spectra of the BCNTSNO ceramics at various compositions (x = 0.75, 0.8 and 0.85) at room temperature, were measured to further explore the evolution of the structure of BCNTSNO ceramics, as shown in Fig. 2.

Fig. 2 Room temperature Raman spectra of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics with x = 0.75, 0.8 and 0.85 compositions.

The presence of three mains regions, which are detected in the parent compounds BaTiO₃ and NaNbO₃ is evident in Fig. 2.^30,31 The region (I), the modes at wavenumbers <170 cm⁻¹ is associated with external vibrations of NbO₆ octahedron, which is connected to the vibrations of the perovskite A-site). The region (II), the 170–450 cm⁻¹ modes related to O–Nb–O bending; zone (III), the 450–800 cm⁻¹ host modes related to the NbO₆ octahedral vibrations. These two regions are attributed to the internal vibration of the NbO₆ octahedron. To get the accurate change affected in the position, intensity and half-width of all bands with the increasing of x, Raman spectra are fitting with Fityk program, using pseudo-Voigt function. An example of the Raman spectrum fitting for x = 0.75 in the 50–1000 cm⁻¹ spectral range is given in Fig. 3. The assignments of principal bands are reported in Table 2.

Fig. 3 Deconvolution of Raman spectra of the BCNTSNO_0.75 ceramic at room temperature.

Table 2 Wavenumbers of the Raman modes and their assignments

NaNbO3 (ref. 31)	Ba0.1(Sn0.5Ti0.5)0.1Na0.9Nb0.9O3 (ref. 34)	Assignments
		x = 0.75	x = 0.8	x = 0.85
64	62	60	60.0	61	Na^+
—	—	66	66	66	Na^+
76	—	70	71	75	Na^+
—	—	—	—	—
95	—	—	84	—
—	—	99	100	101	Na^+
126	105	—	—	—
	—	—	—
146
—	—	143	145	144	Ba^2+
157	154	—	—	—
—	—	—		—
180	—	173	174	177	ν6 (Nb–O)
—	—	—	—	—
186	—	—	—	—
—	—
223	222	215	214	219	ν5 (Nb–O)
258	240	257	262	263
279	275	—	—	—
375	—	311	324	326	E(TO + LO)
433	430	441	445	434	ν4 (Nb–O)
560	—	551	552	561	ν2 (Nb–O)
				—
605	601	598	601	606	ν1 (Nb–O)
	867	835	849	863	ν1 + ν5(Sn–O)

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The bands situated between 60–101 cm⁻¹ are related to Na⁺ translational movements, while the vibrational band located at 144 cm⁻¹ is associated with the Ba²⁺ stretching vibration.³² A small shift in the intensity and frequency is observed for these bands with increasing composition x.

Accordingly, the Raman modes observed at approximately 177 and 263 cm⁻¹ are attributed to the ν₆ and ν₅ modes, respectively, which are in turn assigned to the (Nb–O) bending vibration of the NbO₆ octahedra. Moreover, the existence of the mode B₁ + E(TO + LO) around 326 cm⁻¹ confirmed the presence of the tetragonal phase in all ceramics, as was detected by XRD studies.³³ The band detected at 434 cm⁻¹ is related to asymmetric bending vibration ν₄(Nb–O).²⁵ However, the intense Raman band of the prepared lead-free ceramics in the 598–606 cm⁻¹ region corresponds to a shoulder in the 551–561 cm⁻¹ region. These bands ν₁ are and ν₂ are assigned to symmetric and asymmetric stretching modes of the NbO₆ octahedra. Finally, the mode at 863 cm⁻¹ is attributed to ν₁ + ν₅ vibration modes combination, associated with the Sn–O stretching vibration.³¹ A most significant change is observed in the characteristics of the internal bands on increasing the degree of substitution, except for the increased splitting and shifting of the peak on the right side to higher frequencies. Also, we notice that these bands become more broadening with the increase of the composition x, which implied that the internal mode was sensitive to the effect of substitution in the NaNbO₃.

Additionally, it can also be seen from the Raman spectra that with incasing the composition x, no peaks are observed indicating the absence of formation of a new phase and consequently the formation of solid solutions, which confirms the results found by DRX. When the composition x increases, all the bands display a decreasing intensity and shifted to the high frequency. This behavior can be connected with the disorder of ionic charge created on the A and B sites of NaNbO₃ and distortion of NbO₆ octahedra, as a consequence of Ba, Ca, Sn, and Ti substitutions.

3.3 Dielectric study

The dependence of relative permittivity of (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ ceramics for x = 0.75, 0.8 and 0.85 on temperature at some particular frequencies is illustrated in Fig. 4a and b. The dielectric characterization of the prepared ceramics shows an evolution from relaxor ferroelectric behavior to a classic ferroelectric under the effect of the degree of substitution of the cation Na²⁺ by (Ba²⁺, Ca²⁺) and Nb⁵⁺ by (Ti⁴⁺, Sn⁴⁺) in A and B sites, respectively. Meanwhile, for all the prepared lead-free ceramics a signal dielectric peak is observed, corresponding to ferroelectric paraelectric transition.

Fig. 4 Thermal variation of the real part of the dielectric permittivity of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ with compositions at different frequencies (a) x = 0.75 and (b) x = 0.85.

For the compositions x < 0.85 (Fig. 4a), when the frequency increases, the temperature of the maximum of the real part note T_m moves towards the highest temperatures, whereas the magnitude of permittivity decreases. As a result, large dispersion is observed around and below T_m as a function of frequency in all samples, strongly resembling the behavior in relaxor ferroelectrics.³⁴ The homovalents and heterovalents substitutions at sites A and B for our ceramics and the composition fluctuation, give rise to the relaxing behavior, and thus favoring the strong heterogeneity of the composition with various Curie points.³⁵ Furthermore, the creation of the Coulomb interactions at long-distance prevents the formation of ferroelectric microdomains in favor of the formation of the polar nanoregions PNRs.³⁶ Under these conditions, the order becomes a short distance and each nano-domain freeze at a given temperature. This phenomenon only demonstrates the nature of the transition, whereas the dielectric relaxation observed for temperatures lower than T_m remains unjustified. This dielectric dispersion is explained by the dynamics of the nano-domains.³⁷ The relaxor characteristics of compositions x = 0.75 and 0.8 are depicted in Table 3.

Table 3 Values of factor: , T_m, ΔT_m, and γ for the compositions x = 0.75 and x = 0.8 of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃

x	(10 kHz)	Tm (K) (10 kHz)	ΔTm (K)	Tdev (K)		γ
0.75	1666	308	8	376	0.40	1.87
0.8	4148	324	6	404	0.10	2

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For x ≥ 0.85 (Fig. 4b), the maximum value of the real part decreases with the increase in the frequency but the temperature of Curie (T_C) remains constant, which justifies the classical ferroelectric behavior of the ceramics.

From the Fig. 4, it could be seen that when x increases from 0.75 to 0.8 and 0.85 the values of the maximum dielectric constant of (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ ceramics increases gradually from 2106 to 4421 and 5224. This maximum is higher than the value of the pure NN ( = 1750),¹³ suggested that the introduction of Ba, Ca, Sn and Ti at A and B sites can increase the maximum of the dielectric constant, which is can be caused by the grain size effect.

Additionally, it is worth noting that the temperature of ferroelectric to paraelectric transition shifted to higher temperature from 298 K to 320 K and 388 K, which are mostly smaller than the transition temperature of the NN (T_C = 913 K)^20,22 and the dielectric peak at this temperature becomes less broadened with increasing level x. These results display that the substation in the A and B sites of NaNbO₃ induced that the (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ ceramic transformed from a relaxor ferroelectric to a classical ferroelectric.

In this context, departing from the dielectric results we infer that a substitution level of Ba, Ca, Sn and Ti modify the NaNbO₃ ceramic from antiferroelectric behavior to ferroelectric behavior, showing an important enhancement important of the dielectric constant, with a decrease of the Curie temperature around room temperature. Comparing, our results with the pure NaNbO₃, it is obvious that the studied ferroelectric ceramics with high dielectric permittivity around room temperature show interesting as a potential candidate for lead-free ceramics in several industrial applications such as for non-volatile random access memory technology information storage in electronic devices.

Fig. 5a and b show the thermal evolution of the imaginary part of the permittivity and the dielectric loss for a classical ferroelectric ceramic with composition x = 0.85 as an example. We may notice that the dielectric losses are about 5%.

Fig. 5 Thermal evolution of the imaginary part of the permittivity (a) and dielectric loss (b) for (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics with x = 0.85.

Moreover, for a normal ferroelectric, the thermal evolution of dielectric constant above the Curie temperature in the paraelectric region is described by a Curie–Weiss law equation:

where T_C refers to the Curie temperature and c to the Curie–Weiss constant.

Fig. 6a and b, disclose as an example the thermal evolution of the inverse of at 10 kHz for x = 0.75 (relaxor) and 0.85 (classical), respectively. The fits to the experimental data are obtained by eqn (2). The relaxor ceramics x = 0.75, presents a deviation from the Curie–Weiss law at a deviation temperature T_dev that is defined as the temperature from which it starts to deviate from the Curie–Weiss law. However, the evolution of the inverse of constant dielectric for x = 0.85 obeys the Curie Weiss law implying that the transition from the paraelectric phase to the ferroelectric phase is clearly of the second order and the ferro-electricity is of classical type.³⁸

Fig. 6 Variation of the inverse of the real part of the dielectrique permittivity versus temperature for BCNTSN: (a) x = 0.75 and (b) x = 0.85 ceramics.

Modified Curie law by Uchino and Nomura is used to describe the dielectric relaxation behavior of the (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ ceramics. Generally, the modified Curie–Weiss law can be described by the following equation described as.³⁹

where c denotes the Curie constant and 1 < γ < 2 is defined as the diffuseness degree indicator of the character of the phase transition.⁴⁰ Linear relationships are observed in the plot of as a function of log(T − Tm) at 10 kHz frequency for the compositions x = 0.75 and x = 0.8 as shown in (Fig. 7a and b). The slope of linear fitting the experimental data to eqn (3) allows determining the values of γ diffusion factor, as listed in Table 3. We observe that the values of γ increase monotonously with the increasing x, indicating a shift toward more diffusive phase transition.⁴¹ These results are in good agreement with those reported in the prototype relaxor PbMg_1/3Nb_2/3O₃, 1.6 (ref. 42) and Na_1−xSr_x(Sn_0.25Ti_0.75)_xNb_1−xO₃ lead-free⁴³ ceramic.

Fig. 7 Variation of as a function of ln(T − T_max) at 10 kHz of (a) x = 0.75 and (b) x = 0.8 of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics.

3.4. Optical properties

The absorbance spectra of all compounds at room temperature in the wavelength range (200–1000 nm) are plotted in Fig. 8. We noticed a decrease of the absorption with the increase of wavelengths whose characteristic peaks are shifted towards smaller wavelengths with the increase in NaNbO₃ content, indicating a blue shift, which is suggestive of the increase in the band gap.⁴⁴ This corresponds to a typical behavior for numerous semiconductors and can refer to many reasons such as internal electric fields within the crystal and inelastic scattering of charge carriers by phonons.⁴⁵ Furthermore, this behavior may be attributed to the quantum confinement arising from the lowering of particle size.⁴⁶

Fig. 8 Variation of absorbance at room temperature.

These compounds absorb in the UV-visible region with the same order of absorption bands. These bands can be attributed to the inter-band transition which is formed by the 4d orbitals of NbO₆ (bottom of conduction band) to 2p orbitals of O₂ (top of valence band). The band in the region λ ≥ 400 nm can be associated with an interband transition in relation to the Nb⁵⁺ cation.^18,47

3.5 Optical band gap and Urbach energies

The optical band gap energy (E_g) values of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics are determined using the Kubelka–Munk model as:⁴⁸where F(R) is the Kubelka–Munk function and R is the reflectance of the sample. The absorption coefficient (α) is calculated by using the Kubelka–Munk function in terms of:where t is the width of the sample.

The relation used for the determination of the optical band energy is the following:

αhν = B(hν − E_g)ⁿ (6)

where B is a constant and hν is the photon energy calculated using the equation:

The value of n depends on the nature of the electronic transition responsible for absorption. It is equal to 1/2 and 3/2 for allowed and forbidden direct transitions respectively and to 2 and 3 for allowed and forbidden indirect transitions respectively.⁴⁹ Since the perovskite suffers from allowed direct transition, the value of the index n is assumed to be 1/2.²⁵

Fig. 9 highlights the plot of (αhν)² versus (hν) of all samples. By extrapolating the linear part of the curve to the x-axis; we determine the value of direct band gap energy E_g.

Fig. 9 Plots of (αhν)² versus photon energy (hν (eV)) for (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics with x = 0.75, 0.8 and 0.85 compositions.

The calculated values of the optical band gap of the (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics are found to be 2.89, 2.92 and 3.05 eV for x = 0.75, 0.8 and 0.85, respectively, these values are portrayed in Table 4. Obviously, a little blue shift of the optical band gap occurs when the NaNbO₃ content increases, as the pure NN presents a large band gap (E_g = 3.4 eV). This phenomenon may be caused by the Burstein–Moss effect.⁵⁰ Furthermore, we can notice that the band gap of our ceramics is smaller than that of NaNbO₃. This difference can be assigned to the effect of the degree of substitution of the cation Na²⁺ by (Ba²⁺, Ca²⁺) and Nb⁵⁺ by (Ti⁴⁺, Sn⁴⁺) in A and B sites, respectively, which are expected to perturb the electronic structure of the system.

Table 4 Calculated values of optical band gap (E_g), Urbach energy (E_U) and Cauchy parameter for BCNTSNO ceramics with composition corresponding to x = 0.75, 0.8 and 0.85

(Ba0.95Ca0.05Ti0.8Sn0.2)1−xNaxNbxO3	Eg (eV)	EU (eV)
0.75	2.89	0.348
0.8	2.92	0.350
0.85	3.05	0.375

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The narrower band gap might be assigned to an increase in unit cell volume as it can result in a distortion of the unit cell and therefore can produce a decrease in the overlap between O-2p and metal (d) states.⁵¹ The forbidden band of ceramics (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ in the order of 3.00 eV, may be useful in photocatalytic and solar cell applications.

The Urbach energy (E_U), corresponds to the degree of disorder in the material characterizing the local defects. It can be calculated from the following equation:⁵²

where α₀ is a constant, hν is the photon energy and E_U is the Urbach energy. Taking the logarithm of the two sides of the above equation:

The values of Urbach energy (E_U) can be, therefore, obtained from the slope of the straight line of plotting ln(α) against the incident photon energy (hν) Fig. 10. The values of the Urbach energy (E_U) of (Ba_0.95Ca_0.05)_1−xNa_x(Ti_0.8Sn_0.2)_1−xNb_xO₃ ceramics with x = 0.75, 0.8 and 0.85, are respectively 0.348, 0.350 and 0.375 eV (Table 4). It is worth noting that the Urbach energy increases when the NaNbO₃ content increases. As this energy is associated with the microstructural disorder of the network, it can be suggested that the increase in NaNbO₃ content leads to an increase in the number of disorders of our ceramics.

Fig. 10 Plots of ln α versus photon energy (hν) of BCNTSNO ceramics with x = 0.75, 0.8 and 0.85 compositions.

4. Conclusions

Lead free ceramics (Ba_0.95Ca_0.05)_1−x(Ti_0.8Sn_0.2)_1−xNa_xNb_xO₃ (x = 0.75, 0.8 and 0.85), prepared by the conventional solid-solid reaction method, is crystallized in tetragonal structure with space group P4mm. Lattice parameters increase with increasing NaNbO₃ concentration. These results can be related to the increase of the ionic radii of the substituted cations.

The Raman spectroscopy exhibits a disorder introduced into this composition, which is related to the internal modes of the NbO₆ group, and implied that the internal mode was sensitive to the effect of substitution. As a result, this disorder favors a ferroelectric relaxor behavior. The temperature dependence of the dielectric properties of the prepared ceramics reveals that a doping level of Ba, Ca, Sn and Ti modify the NaNbO₃ ceramics from antiferroelectric to ferroelectric, implying a decrease in the Curie temperature, with an increase of the dielectric constant. Moreover, the dependence of the dielectric properties on temperature exhibited two behavior a relaxor behavior for the compositions x = 0.75 and 0.8 and a classical ferroelectric behavior for the compositions x = 0.85. The enhancement of the dielectric properties of the NaNbO₃ was marked by the shift of the curie temperature to room temperature and the increase of the maxim of permittivity due to (Ba²⁺, Ca²⁺) and (Ti⁴⁺, Sn⁴⁺) substitutions.

Finally, UV-Vis spectra disclose that all lead-free ceramics absorb in the UV-visible region with a blue shift in wavelength along with an increase in NaNbO₃ concentration. The Tauc model is used to determine the optical gap energy. The analysis of data indicates the existence of optical indirect gaps, estimated to be in the order of 3 eV, which is much smaller than that NaNbO₃ ceramic. Therefore, the improvement of the dielectric and optical properties makes our lead-free ferroelectric material suitable for device memory, in photocatalytic and in solar cell applications.

As perspectives for this work, we can consider making the ferroelectric polarization (P–E) hysteresis and the electrical measurements.

Conflicts of 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.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

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