Structural, optical and microwave dielectric properties of Ba(Ti1−xSnx)4O9, 0 ≤ x ≤ 0.7 ceramics

Sn-doped BaTi4O9 (BT4) dielectric ceramics were prepared by a mixed oxide route. Preliminary X-ray diffraction (XRD) structural study shows that the ceramic samples have orthorhombic symmetry with space group (Pnmm). Scanning electron microscopy (SEM) shows that the grain size of the samples decreases with an increase in Sn4+ content. The presence of the metal oxide efficient group was revealed by Fourier transform infrared (FTIR) spectroscopy. The photoluminescence spectra of the ceramic samples reported red color ~ 603, 604, 606.5 and 605 nm with excitation energy ~ 2.06, 2.05, 2.04 and 2.05 eV for Sn4+ content with x = 0.0, 0.3, 0.5, and 0.7, respectively. The microwave dielectric properties of these ceramic samples were investigated by an impedance analyzer. The excellent microwave dielectric properties i.e. high dielectric constant (εr = 57.29), high-quality factor (Qf = 11,852), or low-dielectric loss (3.007) has been observed.


Scientific Reports
| (2021) 11:17889 | https://doi.org/10.1038/s41598-021-97584-x www.nature.com/scientificreports/ devices, etc. During the densification of these titanates at the very high sintering temperature, enhanced dielectric properties are observed due to the phase and compositional defects (fluctuations) (i.e. because of the partial reduction of Ti 4+ to Ti 3+ ion) 17 . The aim of the present work, to achieve a material with enhanced structural and dielectric properties for device application. In this report, we describe the synthesis (i.e.via the mixed oxide method) and the structural, optical, and microwave dielectric properties of Sn-doped BaTi 4 O 9 ceramics. The calcined powders and sintered pellets obtained were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), impedance analyzer, and FTIR (Fourier transform infra-red). The microwave dielectric properties of ceramic samples are discussed in terms of their physical and chemical characteristics.

Materials and methods
The starting raw materials along with purity grades were: BaCO 3 (Merck, Germany, 99.9%), TiO 2 (Aldrich Chemical Company, Inc., U.S.A, 99.9%), and SnO 2 (Strem, Chemicals, U.S.A, 99.9%) used to make the solid solutions of Ba(Ti 1−x Sn x ) 4 O 9 , 0 ≤ x ≤ 0.7 by using mixed oxides route. These raw materials were weighted according to stoichiometric ratio and mixed for 12 h in distilled water by using horizontal ball milling. Then the slurry was dried in a microwave oven at 100 °C for one day and calcined at 1100 °C for 3 h in a nickel crucible in the air atmosphere with a heating-cooling rate of 10 °C/min. The calcined powders were grinded for 60 min with a mortar and pistol manually to avoid agglomerations. Then pressed 0.6-0.8 gm of powder in cylindrical pellets of thickness 2 mm and diameter 10 mm by using a hydraulic press (CARVER, USA) with a pressure of 80 MPa. Thereafter, these green pellets were sintered at 1320 °C temperature in the open air for 4 h with a heating-cooling rate of 10 °C/min. The XRD patterns of the compounds were recorded at room temperature using an X-ray powder diffractometer (JDX-3532, JEOL, Japan) with Cu Ka radiation (k = 1.5405 Å) in a wide range of Bragg angles (20° ≤ 2θ ≤ 60°) at a scanning rate of 2 deg min -1 . The experimental density of the samples was measured by Archimedes' principle using a density meter (MD-3005, Germany). Scanning electron microscopy (SEM, JEOL 7600F) was used to study the microstructure of the dense pellets. The optical properties of these ceramics were done by using the Fourier transformation infrared spectroscopy (FTIR, Perkin-Elmer) and PL spectroscopy. Dielectric properties were measured at microwave frequencies by LCR meter (Agilent 4287 A).

Results and discussions
XRD analysis. Figure  It can be noted from Fig. 1 that the peaks shifted toward the lower 2θ values and representing the cell volume expansion with increasing the Sn 4+ contents. This might be due to the inhomogeneity, micro-strain, or maybe due to the substitution of the relatively larger cation ions of Sn 4+ (~ 0.69 Å) for the smaller cation of Ti 4+ (~ 0.64 Å) 18 . The calculated lattice parameters (i.e. 'a' , 'b' , and 'c') increases with Sn 4+ content. This increase in the lattice parameters may result in the phase transition from orthorhombic to a tetragonal structure. The average crystallite size of these samples was calculated by using Debye Scherer's formula 19 . The observed average crystallite size and lattice parameters are given in Table 1.
where 'θ' is the brags angle, 'λ' is the wavelength of the incident radiation, and 'β' is the full-width of halfmaximum (FWHM). The average crystallite size of these samples was lie in the range of 30-90 nm.
(1) Crystallite size, D = 0.9 βCosθ   Fig. 2). It is clear from the figure that two types of surface morphologies are present in all samples (i.e. rod-like and spherical-like particles). The relative densities are increased, as the Sn 4+ doping content increased in the base sample. The existence of cavities in the denser pellets confirms the existence of porosity. Thus, an increase in the relative density and decrease in the porosity was observed as the Sn content increased 20 . The porosity of these samples was calculated using Eq. (2) shown in Table 2.   Optical properties. Figure 3 represents the FTIR spectra of Ba(Ti 1−x Sn x ) 4  where E = optical excitation energy, h = Plank's constant (~ 6.63 × 10 -34 Js) c = velocity of light (3 × 10 8 m/s) and λ is the emission wavelength. Emission at photoluminescence peak of the samples were recorded at ~ 603, 604, 606.5 and 605 nm with excitation energy ~ 2.06, 2.05, 2.04 and 2.05 eV for x = 0.0, 0.3, 0.5, and 0.7 content of Sn 4+ dopant, respectively. Photoluminescence is a multi-photonic process that is an optical energy emission occurred in the optical range by many vibrational modes within the samples 25 . Within the energy band-gap, the photoluminescence process confirmed that due to localizing state the order/disorder structure may be affected directly. Hence, the structural order/disorder may be increased with increasing the energy band gap 26 . It was recorded that a broad-emission spectrum was located at ~ 604 nm and have an optical excitation energy (~ 2.06 eV) which was smaller than the energy band gap of extremely ordered BaTi 4 O 9 ceramics located at ~ 558 nm (~ 2.23 eV) which may be due to the absence of oxygen vacancy 27 . In the photoluminescence spectrum, the red color may occur due to the oxygen vacancy.
Microwave dielectric properties. The variation of the relative permittivity (ε r ) and tangent loss (tanδ) values of Ba(Ti 1−x Sn x ) 4 O 9 , 0 ≤ x ≤ 0.7 sintered ceramics versus frequency at room temperature is shown in Fig. 5. Due to increasing frequency, the values of ε r and tanδ decrease exponentially in the samples. The high value of ε r at resonant frequency (f o ) can be described based on: (1) According to Maxwell-Wagner's model, the dielectric materials are consist of fine conductive grains which are surrounded by grain boundaries. Large polarization is caused by the motion of charge carriers from grain to the grain surface. (2) The ionic polarization (3) The majority are due to crystal defects, vacancies, and grain defects etc 28,29 .
The increase in ε r values with Sn 4+ contents may be recognized by the substitution of a larger ionic radius of Sn 4+ (~ 0.69 Å) cation for a smaller ionic radius of Ti 4+ (~ 0.64 Å) cation 30 . To increasing the bond length of complex perovskite (i.e. AB 4 O 9 ) the larger ionic radius cation may be substituted at B-site cation. The Sn 4+ contents   www.nature.com/scientificreports/ greatly affected the microwave dielectric properties due to high ionic polarization 31,32 . The maximum dielectric constant is obtained at x = 0.5 at maximum relative density (i.e. at low porosity). Because charge carriers need a medium to propagate and hence dielectric constant decreases with increasing material porosity 33,34 . Dielectric loss decreases with frequency due to the space charge polarization in all samples. At the lowest frequency, the maximum tangent loss occurs due to the presence of defects, impurities, and porosity in the ceramic samples 35 .
In general, the microwave dielectric properties of ceramics are dependent on intrinsic and extrinsic factors. The intrinsic properties are due to the interaction of materials phonons with the applied ac field. Thus the intrinsic properties also depend upon the crystal symmetry as observed in many single crystals 36 . The extrinsic properties are due to the imperfection in the crystal structure such as dopants or impurity atoms, grain boundaries, vacancies, order-disorder, secondary phases, etc [36][37][38][39][40] . Mostly, extrinsic factors are process-dependent and can be optimized. In this report, the sintering of these ceramic samples was done at a very high temperature (i.e. > 1300 °C for 4 h). The sintering at a high temperature may be causing the partial reduction of Ti 4+ to Ti 3+ ions. When Sn 4+ is doped, it helps to maintain Ti 4+ due to the following reaction: and, thus control the reduction of Ti 4+ to Ti 3+ ions.
Therefore, at high-temperature sintering, a sintered layer acts as a shield that prevents the transport of oxygen to the core. Due to the lack of oxygen at the core, oxygen vacancies or titanium interstitials are produced. The presence of vacancies in the lattice is responsible for damping of phonon modes and hence maybe leads to enhancement of dielectric properties and Q-factors 36,41,42 .
The variation of quality factor (Q f ) and relative density (%) of Ba(Ti 1−x Sn x ) 4 O 9 sintered ceramics as a function of composition (x) is shown in Fig. 6. Initially, the quality factor decreases from 9264.49 to 5681.16 with increasing Sn 4+ content (from 0 to 0.3). The observed decrease in the value of Q f may be accepted due to the substitution of larger Sn 4+ cation ion on the B-site cation, contributing to harmonic vibrational modes [43][44][45] and another reason may be the phase transition. Additionally, an increase in Sn 4+ content leads to a high Q f value, which may occur due to: (1) The phonon modes of B-site harmonic, and (2) Relative density of the ceramic samples.
The variation of ac conductivity of Ba(Ti 1−x Sn x ) 4 O 9 , 0 ≤ x ≤ 0.7 sintered ceramics versus frequency is shown in Fig. 7. It is clear from the graph that ac conductivity depends upon the frequency and does not show any significant variation at the lowest frequency. The maximum value of ac conductivity at a lower frequency may be due to the rising state of localization in the hopping process. By the application of electric field, the hopping frequency of the charge carrier increased which result in the highest value of ac conductivity towards the highfrequency region 46 .

Conclusion
Solid solutions of Ba(Ti 1−x Sn x ) 4 O 9 , (0 ≤ x ≤ 0.7) ceramics were fabricated through a mixed-oxide route. The average crystallite size of these ceramic samples lies in the range of 30-90 nm with phase change from orthorhombic (space group = Pnmm) to Tetragonal (P4mm) structure. Sintered ceramics attained 99.5% of the theoretical density at content (x = 0.5) and fine grain growth with uniformity was achieved. Photoluminescence confirmed that the present state of localization within the band-gap may affect the structural order/disorder. The dielectric Sn 4+ + Ti 3+ ⇋ Sn 3+ + Ti 4+ www.nature.com/scientificreports/ properties of sintered ceramic samples showed ε r = 57.29, and high Q f = 11,852. The increase in ac conductivity is due to the hopping mechanism. Based on the above-obtained results, these ceramic materials can be used for filter applications.

Data availability
The data of this study are available from the corresponding author upon reasonable request.