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K. Yukesh Kumar, [N. Sivakumar](https://orcid.org/0000-0002-6113-2993), G. M. Bhalerao, G. Anbalagan, [Kentaro Tashiro](https://orcid.org/0000-0001-7424-0830), Ali Alsulmi

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[Enlightening the effect of strontium in the BiVO4–TiO2 diphase composites for dielectric devices](https://mdr.nims.go.jp/datasets/865ea19a-ec20-49ee-93dc-b1f78952bf99)

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Enlightening the effect of Strontium in the BiVO4-TiO2 diphase composites for dielectric devices K. Yukesh Kumara, N. Sivakumara,, G.M. Bhaleraob, G. Anbalaganc, Kentaro Tashirod, Ali Alsulmie aFunctional Materials Research Laboratory (FMRL), Department of Physics, Sri Sairam Engineering College, Chennai-600 044, Tamil Nadu, India. bUGC-DAE Kalpakkam Node, Kokilamedu-603 104, Tamil Nadu, India. cDepartment of Nuclear Physics, University of Madras, Chennai-600 025, Tamil Nadu, India. dNational Institute for Materials Science, Tsukuba-305-0044, Japan. eDepartment of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia. *Corresponding Author E-mail id: sivakumar.phy@sairam.edu.in Abstract Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) composite materials were prepared by the conventional solid state reaction method at optimized temperature of 750℃. The effect of Strontium (Sr) element in the stoichiometric system of BiVO4-TiO2 on structural phase, micro-morphology, elemental composition, band energy and dielectric properties were systematically analyzed for the first time. All the composite materials synthesized with different concentrations of Sr have two structural phases, monoclinic structure which belongs to BiVO4 whereas tetragonal structure belongs to the rutile TiO2. The relative dielectric permittivity (ɛr) gradually increased from ~ 61 to 122 with the increase of Sr concentration in the Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) composite. The temperature dependent ac electrical conductivity (σac) of the prepared materials was estimated in the frequency range from 50 to 3 MHz. The microwave dielectric properties of the prepared composite materials are examined at 3.9 GHz and results were discussed in detail for their potential dielectric applications. Keywords: Solid state reaction; Powder X-Ray Diffraction; Surface morphology; UV analysis; Dielectric constant; Microwave dielectrics 1. Introduction In this modern era, owing to the drastic usage of cellular phones in day-to-day life, researchers are finding the new paths in the field of wireless communication technologies. The conversion of the microwaves as a carrier wave in the wireless communication technologies that have developed in the area of miniaturization of device fabrication has been a difficult task for the researchers. The dielectric oxide ceramic materials brought a drastic improvement in the wireless communication industries by reducing the size and cost of filters, dielectric resonators, Manuscript Click here to access/download;Manuscript;Manuscript.docx 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:sivakumar.phy@sairam.edu.inhttps://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136753&guid=efa8b28b-0038-4d38-8a37-935a05743a9d&scheme=1https://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136753&guid=efa8b28b-0038-4d38-8a37-935a05743a9d&scheme=1oscillators, and antennas for the application usage from cellular phones to global positioning systems (GPS). Also, the wireless communication technologies have their own special features and functions due to their reduced size and weight for the comfortable usage of mankind.  Generally, dielectric resonators are an electromagnetic component that shows resonance, which is used for the narrow range of frequencies. They exhibit high permittivity and low dissipation factor. These developments of dielectric resonators play an important role in modern wireless communication technologies. Additionally, the ceramic dielectric resonators have developed more miniaturized components than the traditional microwave materials and possess a high-quality factor. The ceramic dielectric resonators have some unique features like cost advantage, lesser dimension, less weight, stable and efficient device performance, tenability, and ruggedness. Also, the temperature variations of the resonant frequency have excellent output, which is a significant requirement for circuit designers. Dielectric resonator filters are generally used to differentiate the wanted frequency from most of the unwanted signal frequencies in the transmitter and receiver end. These potential dielectric resonators depend on the three major factors, such as high relative permittivity (ɛr), high quality factor (Qf) and near-zero temperature coefficient of resonant frequency (TCF). The less permittivity dielectric materials are used in the field of millimeter wave communication, as well as for substrates in the development of microwave integrated circuits. The dielectric materials with permittivity in the range of 25–50 are used for the satellite communication and cellular base station. The dielectric materials of permittivity > 50 are used in the field of mobile phone manufacturing, particularly in the area of developing miniaturization [1]. Low-temperature co-fired ceramics (LTCC) is another important factor that plays a dominant role in developing an integrated microwave device in the modern communication system. In general, dielectric material is fired with an Ag electrode at the temperature below its melting point (961°C) for the development of cost-effective electrodes [2-4]. Some materials have significant microwave dielectric properties which are suitable for the LTCC technologies but they are limited due to their high sintering temperature. Bi-based oxides and V2O5 ceramic dielectric materials are effectively used for LTCC applications, which are sintered at a temperature below 900°C [5–10].  In this aspect, we have identified Bi2O3-V2O5 as a potential candidate for its dielectric applications, as they have significant properties like as paraelectricity, ferroelectric, conductivity, and ionic conductivity, as well as for its  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 acoustic-optical and photocatalytic applications [11–17]. In addition, it has a low sintering temperature, which suits for many LTCC device applications [18]. Also, the Bi2O3-V2O5 system has excellent microwave dielectric properties such as high relative permittivity (ɛr) ~68 and high quality factor (Qf) ~ 6500, with a high negative value of temperature coefficient of resonant frequency (τf) in the range from -243 to -260 ppm/°C [19]. Jun Hong Noh et al. discussed the potential microwave dielectric properties of TiO2, like high relative permittivity (ɛr) ~ 100 and quality factor (Qf) ~ 14,000, with a high positive value of temperature coefficient of resonant frequency (τf) of +400 ppm/°C [20]. The addition of ZnO in Bi2O3-V2O5 shows high dielectric permittivity (ɛr) ~500 and loss ~ 101 at 100Hz frequency [21]. Chen et al. investigated the microwave dielectric properties of a Li0.5Re0.5-WO4 system that has relative permittivity (ɛr) ~71.8 and quality factor (Qf) ~ 7482 at low firing temperature below 800°C [22]. Oliveira et al. demonstrated the dielectric properties on the addition of TiO2 in the Bi2O3 V2O5 system, which shows less dielectric constant (ɛr) ~38.2 as well as loss (Tanδ) ~ 7 x 10-2 [23]. In the present work, we prepared BiVO4-TiO2composite materials with the different Sr concentration at the A+(Bi) site via the cost-effective solid-state synthesis method. Also the crystal structure, phase identification, microstructure features, chemical composition, dielectric and microwave dielectric parameters were systematically studied and compared the results of the Bi (1-2/3x) SrxVO4-TiO2(x = 0, 0.1, 0.15) composites for the first time. 2. Experiment  2.1. Preparation of Bi (1-2/3x) Srx VO4-TiO2 (x = 0, 0.1, 0.15) composite materials A conventional solid state reaction technique was employed to synthesize the pure BiVO4-TiO2 (BVT) and Sr doped Bi (1-2/3x) SrxVO4-TiO2(x = 0.1, 0.15) composite materials. The dopant, Sr = 0.1 wt % and Sr = 0.15 wt % in the Bi (1-2/3x)SrxVO4-TiO2(x = 0.1, 0.15) composite material were named as 1SBVT and 2SBVT respectively. The starting materials, Bismuth Oxide (Bi2O3), Vanadium Pentoxide (V2O5), Titanium Oxide (TiO2) and Strontium Carbonate (SrCO3) with high purity (99.9%) of fine powder are purchased commercially from sigma Aldrich. Initially the Bi2O3, V2O5 and TiO2 were pre-heated to 500 ℃ for 6 hours and SrCO3 pre-heated to 150 ℃ to avoid the absorption of foreign particles and activate the materials for the reaction process. The stoichiometric ratio of the reactant powders BVT were mixed, crushed and grinded well to get a fine powder with the support of mortar and pestle for 6 hours. The fine powders were then transferred into a crucible and calcinated at 750 ℃ for 6 hours, and  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 then bring back to room temperature using a box furnace. The calcinated powders were again grinded well with a mortar and pestle for about 5 hours to obtain a fine composite powder. The final powder was pressed into a circular disc shaped pellet with the dimension of 10 mm diameter and 5 mm thickness. The pellets were then sintered at 750 ℃ for 4 hours. The same procedure was followed to prepare the 1SBVT and 2SBVT composite materials, respectively.  2.2. Characterization details  The powder X-ray diffraction was recorded in the 2θ region from 5⁰ to 60⁰ using XPERT-PRO X-Ray Diffractometer with Cu-Kα radiation (λ = 1.5406 Å) to identify the crystalline structure of the prepared composite materials. The surface morphology features and elemental compositions of the prepared materials were examined by the Scanning Electron Microscope system equipped with energy dispersive X-ray (EDX) setup. The elemental composition of the prepared composite materials was identified by Peek Seeker Raman Spectrophotometer recorded in the range from 100 to 1000 cm-1.  Spectral Studies were carried out to estimate an optical band gap value of the prepared composite material with JASCO V-760 Spectrophotometer, which was used to record the UV- visible spectrum in the region, 200 to 800 nm. The sintered pellets were coated with conducting silver paste on both the sides for the better electrical conductivity. The prepared pellets were subjected to HOIKI IM3536 LCR Meter impedance analyzer to study the electrical behaviors of the prepared composite materials. The prepared pellets were subjected to Microwave Vector Network Analyzers to study the microwave dielectric properties at 3.9 GHz. The TE001 mode was utilized to determine the dielectric constant. The temperature coefficient of resonant frequency, TCF (τf) was evaluated within a temperature range of 28–60°C. The TCF value was then calculated using the following formula,  𝜏𝑓 = 𝑓60− 𝑓28𝑓28(60−28)     (1) Where f60 and f28 are the resonant frequency at 60 °C and 28 °C, respectively 3. Results and discussion 3.1. Powder X-Ray Diffraction (PXRD) Analysis The powder X-ray Diffraction patterns of the prepared composite materials of Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) are shown in Fig. 1. The prominent reflection peaks are observed at the Bragg’s angles 2θ = 18.67⁰,  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 28.94⁰, 30.54⁰, 34.49⁰, 35.22⁰, 39.78⁰, 42.46⁰, 46.03⁰, 46.71⁰, 47.30⁰, 50.31⁰, 54.58⁰, 58.53⁰ and 59.26⁰. The corresponding (h k l) planes are identified as (110), (121), (040), (200), (002), (211), (051), (132), (240), (042), (202), (013), (321) and (123) respectively. The identified planes are well-matched with the XRD patterns of JCPDS Card No.14-068, which belongs to the monoclinic structure of BiVO4 with space group of I2/a. The remaining peaks around at 27.5⁰, 36.4⁰ and 54.1⁰ with (110), (101) and (211) respectively belongs to the tetragonal structure of rutile TiO2 (JCPDS Card No. 21-1276) are well-matched with the previously reported XRD pattern of TiO2 [24, 25]. The observation of two phases in the prepared materials is due to the formation of tetrahedral coordination site by the V5+ ions with ionic radius 0.355 Å. But Ti4+ naturally prefers to form the octahedral coordination sites.  Due to the chemical and structural unstable nature of Ti4+ions, they do not fit with the Tetrahedral sites of the V5+ of BiVO4 [2]. Also, it is observed that the XRD peak intensities of Sr doped BiVO4-TiO2 materials are reduced when compared with the pure BiVO4-TiO2composite material. The reduction in XRD peak intensities is likely due to changes in crystallinity caused by the incorporation of Sr ions into the BiVO4-TiO2 structure. Scherer’s equation [26] is used to calculate the average crystallite size D (nm) of the prepared composite materials.          D = 𝑘𝜆𝛽𝐶𝑂𝑆𝛳     (2) where k is the Boltzmann constant, θ indicates the position of the diffraction peak, λ is the wavelength of the X-Ray source and β is the full width at half maximum (FWHM) of the peaks observed. The estimated crystallite size values of BVT, 1SBVT and 2SBVT are found to be 66 nm, 54 nm and 48 nm respectively. The average crystallite size of the prepared composite materials was found to decrease in size gradually from pure BVT to Sr doped BVT samples. The Williamson–Hall method has been utilized to analyze the broadening of XRD peaks, which is used to understand the size-induced and strain-induced molecular effects. This approach helps to determine the strain present in the synthesized composite materials. The Williamson–Hall equation can be expressed as: βcosθ = 𝐾𝜆𝐷  4ɛ sinθ             (3) β represents the full width at half maximum (FWHM), λ is the X-ray wavelength (1.5406 Å), θ is the Bragg angle, kis a constant (0.9) referred to as the shape factor, D denotes the crystallite size, and ε represents the strain. Figs. 2(a-c) illustrate the plots of βcosθ versus 4sinθ for the samples BVT, 1SBVT and 2SBVT. The linear fit of these plots are utilized to calculate the strain values. The estimated strain values are 1.63 x 10-4, 2.01x 10-4 and 2.31x  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10-4 for the samples BVT, 1SBVT and 2SBVT respectively. The increase in strain observed in BiVO4-TiO2 due to the addition of Sr which develops distortions in the crystal structure. The bulk density was calculated by the Archimedes method for the prepared composite materials. The values were found to be 5.72 (g/cm3), 5.94 (g/cm3), and 6.03 (g/cm3) for the samples BVT, 1SBVT and 2SBVT respectively. It is noted that the increase of density is owing to the addition of strontium in BVT system. The porosity of the prepared composite materials BVT, 1SBVT and 2SBVT were measured to be 7.53%, 2.04% and 1.97% respectively.  The theoretical density of the for the prepared composite materials were calculated using the formula  Theoretical density (g/cm³), ρ = 𝑍.  𝑀𝑁𝐴 .𝑉      (4) where Z is the number of formula units per unit cell, M is the molar mass of the compound (g/mol), NA = Avogadro's number (6.022×1023 atoms/mol) and V is the volume of the unit cell (cm³). The values were found to be 6.50 (g/cm3), 6.28 (g/cm3), and 6.17 (g/cm3) for the samples BVT, 1SBVT and 2SBVT respectively. Also, the relative density (g/cm³) is calculated for the prepared composite materials by the formula     𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙  𝐷𝑒𝑛𝑠𝑖𝑡𝑦  (5) The values were found to be 88%, 94.6% and 97.73%for the samples BVT, 1SBVT and 2SBVT respectively. 3.2. Scanning Electron Microscope with EDX Analysis Figs. 3(a-c) show the surface and micro morphology features of the prepared composite materials of Bi(1-2/3x)SrxVO4-TiO2 (x = 0, 0.1, 0.15). These images revealed that the prepared composite materials are agglomerated with irregular shapes containing small porous in-between the grains and surface shows two distinct grain colours. The white colour represents the BiVO4 grains and the dark black colour indicates the rutile TiO2 grains respectively [25]. Thus, the SEM study confirms the presence of two phases, as observed in the PXRD studies. Figs. 4(a-c) represent the histogram plots (plotted with ImageJ Software) of the Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) composite materials to estimate the average grain size. The average grain size values of pure BiVO4-TiO2, 0.1 wt% Sr doped and 0.15 wt% Sr doped BiVO4-TiO2 composite materials are determined to decrease in particle size (area) and the values are determined to be 1.43 μm, 1.35 μm, 1.09 μm respectively. It is found that the grain size decreases with the  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 increase of Sr doping concentration in BiVO4-TiO2, which is in analogous to the crystallite size decrement as observed from the PXRD results. Also, the Figs. 5(a-c) show the EDX micrograph of the composite products, which explains the presence of elemental compositions. In addition, Table 1 provides the detail of the weight percentage of the chemical elements in the prepared composite materials for further understanding. 3.3. FT-Raman Spectral analysis Fig. 6 show the Raman spectrum of Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) composite materials. The corresponding Raman vibrational assignments are listed in Table 2.  The Raman vibrational peak at 124 cm-1 corresponds to O-Ti-O bond [27]. A sharp peak at 208 cm-1 belongs to Bi-O bond [28-29]. The observed medium Raman band between 329 and 365 cm-1 corresponds to symmetrical bending of VO4+[30]. A wide Raman band near 609 cm-1 attributed to stretching bond of O- Ti-O [27]. The high intensity Raman peak observed at 820 cm-1 represents the V-O Stretching bond [30]. The doping of Sr in the BiVO4-TiO2 composite system shows Raman peaks remind at the same position. So the fundamental vibrations of BiVO4-TiO2 system not altered by the Sr dopant.  3.4. UV – Vis. Spectral analysis   Fig. 7 describes the UV- Visible absorption details of the prepared composite materials Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) that are measured in the range 200 to 1000 nm. The lower cut off wavelength is observed for all the materials around at 430 nm, and no further absorption occurred in the visible region. A small absorption in the BiVO4-TiO2 system at 282 nm in the UV region is due to the π → π* transition [31, 32]. All the Sr doped composite materials have the same absorption in the same position. Fig. 8 (a-c) explains the optical band gap details of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composites respectively. The optical bandgap energy values of the prepared composite materials of BVT, 1SBVT and 2SBVT are calculated from the optical absorption coefficient (α) using an absorbance data using the relation,     α = 2.303𝑑 log 1𝑇     (6) where T is the Transmittance and d is the thickness of the sample.  The relation between the optical absorption coefficient (α) and the photon energy (E= h) is given by,  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 αh = A(h – Eg)m      (7) where A is the material’s constant, h is the Planck’s constant,  is the optical frequency, Eg is the optical bandgap energy and ‘m’ is an index. The bandgap energy is calculated using the plot drawn between the (hν) vs. (αhν)2. The prepared composite material possesses a direct transition and the optical bandgap values are found to be 2.45 eV, 2.48 eV and 2.53 eV corresponds to BVT, 1SBVT and 2SBVT respectively. The strontium doped composite materials (1SBVT & 2SBVT) show slightly increased bandgap energy due to the quantum confinement effect. That is, when the crystallite size as well as particle size decreases, the bandgap energy increases. 3.5. Dielectric analysis  Particular composite materials are having unique properties such as microstructure and lower dielectric constant that find their usefulness in frequency response and AC conductivity for the development of the technological growth in various fields particularly in the fabrication of the resonators, antennas and filters for the application of mobile device and satellite broadcasting. The size requirements of electronic components are getting smaller and smaller in the latest wireless communication gadgets tends way to miniaturization and integration of the devices [22, 33-38]. Through the study of dielectric properties of the materials, we can explore more insights about the atomic dislocation and various polarization mechanisms which include electronic, ionic, orientation and space polarization for the device developments. Generally, the materials having high dielectric constant values tend to dissipate significant power when used for higher frequency application [39]. Figs. 9 (a-c) describe the dielectric constant and Figs. 10 (a-c) shows dielectric loss of the prepared composite materials. We studied the dielectric properties of the materials in the frequency range of 1 KHz – 3 MHz with various temperatures starting from room temperature to 70℃ in the steps of 10⁰C. It is observed that the gradual decrease in the dielectric constant of the prepared composite materials with the increase of frequency and stabilizes at higher frequencies. Initial higher dielectric constant at the lower frequency range is attributed to the electrode interface effect. Local electronic displacement within the materials creates polarization that occurs along the direction of the applied field. However, at higher frequencies, the molecular dipoles do not respond to the applied electric field. These various polarizations lead to the decrease in the dielectric constant until it becomes stabilized at the higher frequencies [40]. The dielectric constant (ɛr) of the materials is calculated using the relation,   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65      𝜀𝑟 = 𝐶𝑝 𝜀0𝑑𝐴     (8) where Cp is the capacitance in parallel, ɛo is the permittivity of free space, A is the area of the sample surface and d is the thickness of the sample. The dielectric constant (ɛr) of the prepared composite sample are found to be ~61, ~95 and ~120 corresponding to BVT, 1SBVT and 2SBVT composites respectively. The mentioned dielectric values are recorded at the frequency of 3MHz with temperatures RT, 50℃, 60℃, and 70℃. The increase in dielectric constant with the increase of addition of strontium in BiVO4 – TiO2 is due the increase in strain, reduced crystallite size which gradually increases the polarization. The dielectric constant values calculated for all the samples at different temperature shows small variation with the rise of temperature. This is due to the fact that the thermal excitation energy by the bound charge carriers, which gradually increases the polarization that leads to the increase in the dielectric constant [41]. The dielectric loss values of the prepared ceramic material are found to be 0.025, 0.195 and 0.047 which are recorded at frequency 3MHz corresponding to the BVT, 1SBVT and 2SBVT composites respectively. The loss factor gradually increases with the addition of the strontium leads to increase in conductive behavior due to the relaxation of the charges at grain boundaries. The ac electric conductivity (σac) has been calculated by the relation  σac = ω ɛo ɛr Tan δ           (9) where ω is the angular frequency (2πf), Tan δ is the loss factor. The graph is plotted against the function of log f and ac electric conductivity (σac) for various temperatures with the frequency range of 1 KHz to 3 MHz are shown in the Figs. 11 (a-c). The conductivity of the prepared composite materials is 8.9  10-5, 7.6  10-4, 3.5  10-4 S.m-1 recorded at frequency 3MHz corresponding to BVT, 1SBVT and 2SBVT composites respectively. The conductivity follows the equation given by σ(ω) = σdc+ P ωƞ     (10) The above equation is the Johscher’s power law (42) where σdcis the dc conductivity and ƞ is the exponent of a power law which represents the mobile ions which measures the interaction of charge carriers along with the lattice. The conductivity versus frequency spectrum (Fig. 11) shows the increase of ac-conductivity with frequency.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 This increase in ac-conductivity with higher frequency and temperature supports the negative temperature coefficient of resistance (NTCR) behavior of the synthesized sample. The variation in conductivity of (Bi (1-2/3x) SrxVO4-TiO2 (x = 0, 0.1, 0.15) with the inverse of the absolute temperature at different frequencies are presented in Fig 12(a-c). The ac-conductivity can be calculated by using the equation σac = σ0 exp ( 𝐸𝑎𝑘𝑇)     (11) where k and σ0 are the Boltzmann constant and the pre-exponential factor, respectively. The activation energy Ea can be calculated with the slope using the relation, Ea = -2.303  slope  1000 X k. The activation energy of the prepared polycrystalline material of BVT is 0.0134eV, 0.0184eV and 0.0033eV at frequencies 1 KHz, 100 KHz and 1 MHz respectively. Also for 1SBVT composite is 0.0127eV, 0.0159eV and 0.002eV at frequencies 1 KHz, 100 KHz and 1 MHz respectively and for 2SBVT is 0.0156eV, 0.0756eV and 0.00154eV at frequencies 1 KHz, 100 KHz and 1 MHz respectively. The low activation energy in metal oxides used in dielectric studies indicates that the material is likely to demonstrate favorable dielectric properties with minimal energy barriers for charge movement or dipole orientation. This phenomenon revealed that the prepared materials are well-suited for various applications such as capacitors, sensors, and other electronic devices. 3.6. Microwave dielectric properties analysis Fig. 13 show the microwave dielectric permittivity and temperature coefficient resonant frequency (τf) of the prepared composite materials (Bi (1-2/3x) SrxVO4-TiO2 (x = 0, 0.1, 0.15). Fig. 14 shows the quality factor of the prepared composite materials (Bi (1-2/3x) SrxVO4-TiO2 (x = 0, 0.1, 0.15). The measured values of microwave dielectric permittivity (ɛr) at 3.9 GHz are 68, 73, and 77 which correspond to the BVT, 1SBVT and 2SBVT composites respectively. The ɛr increases with the increase in strontium content. In this constant, the dielectric permittivity (ɛr) is depended upon the density, ionic polarizability, secondary phase, etc. [43]. This enhancement in permittivity is mainly attributed to the higher polarizability introduced by strontium doping, which modifies the crystal lattice structure. The quality factor (Qf) value of ceramics can be affected by intrinsic factors (such as vibration modes and packing fraction) and extrinsic factors (including density, average grain size, and phase composition) [44-45]. However, the quality factor (Qf) exhibited a decline with increased doping, from 5229 for BVT to 5112 and 4463 for 1SBVT and 2SBVT, respectively, which indicates higher microwave losses due to increased structural imperfections and grain boundary effects.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 The temperature coefficient of frequency (τf) values showed improvement with Sr doping, shifting from −134 ppm/°C for BVT to −118 ppm/°C and −110 ppm/°C for 1SBVT and 2SBVT, respectively. This reduction in the absolute TCF values suggests that strontium doping reduces thermal strain effects, thereby enhancing frequency stability. Strain analysis revealed an increase in strain with higher doping levels, while the grain size and crystalline size decreased. The reduced grain size contributes to additional grain boundaries, which are associated with increased microwave losses, as reflected in the declining quality factor values. Similarly, X-ray diffraction (XRD) analysis revealed a decrease in crystalline size with Sr doping, likely due to restricted grain growth during the sintering process. The reduction in crystallite size, smaller particle dimensions, and increased strain will enhance polarization, which raises the dielectric constant. However, these changes also introduce defects and grain boundary conduction, contributing to higher dielectric loss. Additionally, the relative density of the samples improved significantly with doping, increasing from 88% for BVT to 94.6% for 1SBVT and 97.73% for 2SBVT. The increased relative densification will minimize porosity and enhance the polarization mechanics which tends to enhanced dielectric properties but also introduces microstructural changes that negatively affect the quality factor (Qf). Overall, Sr doping in BVT ceramics enhances dielectric permittivity, make them ideal for applications in resonators, filters, antennas, and frequency-stable microwave devices. 4. Conclusion The system Bi(1-2/3x)SrxVO4-TiO2(x = 0, 0.1, 0.15) is successfully developed by the traditional solid-state method. The prepared composite materials are identified with two phases, confirmed by the powder x-ray diffraction. The SEM with EDX test used to understand the surface morphology and elemental composition of the prepared composite materials. The FT-Raman spectrum is further confirmed the elements present in the system. The dielectric constant of the prepared material is found to increase from 62 to 120 with the increase of strontium dopant concentration. The microwave dielectric permittivity (εr) improved significantly with doping, from 68 to 77. Also, the temperature coefficient of frequency (τf) is enhanced from −134 ppm/°C for BVT to −118 ppm/°C and −110 ppm/°C for the doped samples 1SBVT and 2SBVT respectively, demonstrating better frequency stability. These findings provide a pathway for optimizing these materials for use in resonators, filters, and frequency-stable microwave devices, contributing to advancements in communication and electronic systems.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Acknowledgement Dr. N. Sivakumar and Dr. Ali Alsulmi acknowledge the Researchers Supporting Project (RSP2025R78), King Saud University, Riyadh, Saudi Arabia for the partial financial support.  This work was partially carried out using the facilities of UGC-DAECSR. 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Authors Dr. N. Sivakumar and Dr. Ali Alsulmi have received research support from King Saud University, Saudi Arabia under RPS Scheme. Also, financially supported by UGC-DAE CSR through a Collaborative Research Scheme (CRS) with project number CRS/2021-22/04/627 Competing Interests Authors K. Yukesh Kumar, G.M. Bhalerao, G. Anbalagan and Kentaro Tashiro declare they have no financial interests. Authors Dr. N. Sivakumar and Dr. Ali Alsulmi have received research support from King Saud University, Saudi Arabia under RPS Scheme. Author Dr. N. Sivakumar received financial support from UGC-DAE CSR through a Collaborative Research Scheme (CRS). Author Contributions All authors contributed to the study conception and design. Material preparation was done by K. Yukesh Kumar. Data collection and analysis were performed by K. Yukesh Kumar. The first draft of the manuscript was written by N. Sivakumar and all the other authors commented on previous versions of the manuscript. Dr. Kentaro Tashiro supported for the dielectric interpretation. Finally it was validated by G.M. Bhalerao and G. Anbalagan. All authors read and approved the final manuscript. Research Data Policy and Data Availability Statements Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65  Fig.1. Powder XRD patterns of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials               Figure Click here to access/download;Figure;Figures.docxhttps://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136754&guid=507892c6-3946-4b00-8857-f2f40951159f&scheme=1https://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136754&guid=507892c6-3946-4b00-8857-f2f40951159f&scheme=1 Fig.2. Williamson Hall of (a) pure BiVO4-TiO2, (b) 0.1 wt% Sr doped and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials    Fig. 3 FE-SEM images of (a) pure BiVO4-TiO2, (b) 0.1 wt% Sr doped and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials (a) (b) (c)    Fig. 4 Histogram grain size distribution of (a) pure BiVO4-TiO2, (b) 0.1 wt% Sr doped and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials                           Fig. 5 EDX elemental compositional analysis of (a) pure BiVO4-TiO2, (b) 0.1 wt% Sr doped and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials           (a) (b)   (c)  Fig. 6 FT-Raman spectra of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials   Fig. 7 UV Vis. absorbance spectra of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials            Fig. 8 Optical band gap of (a) pure BiVO4-TiO2 (BVT), (b) 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials (a) (b) (c)     Fig. 9 Dielectric constant plots of (a) pure BiVO4-TiO2 (BVT), (b) 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials    Fig. 10 Dielectric loss plots of (a) pure BiVO4-TiO2 (BVT), (b) 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 (2SBVT) composite materials     Fig. 11 Electrical Conductivity plots of (a) pure BiVO4-TiO2 (BVT), (b) 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials   Fig. 12 Activation energy Ea plots of (a) pure BiVO4-TiO2 (BVT), (b) 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials  Fig.13 Dielectric permittivity and Temperature Co-efficient resonant frequency of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and 0.15 wt% Sr doped BiVO4-TiO2 composite materials           Fig. 14 Quality factor of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials  Table 1:  Weight percentage of the chemical elements of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials. Element BVT   Weight % 1SBVT   Weight % 2SBVT   Weight % Bi 60.7 ± 0.32 58.9 ± 0.45 58.4 ± 0.48 V 14.3 ± 0.25 14.1 ± 0.25 14.8 ± 0.33 O 12.8 ± 0.61 11.4 ± 0.84 10.7 ± 0.83 Ti 12.2 ± 0.16 10.5 ± 0.33 10.3 ± 0.23 Sr - 4.6 ± 0.04 5.8 ± 0.08                  Table 2:  Raman vibrational assignments of pure BiVO4-TiO2 (BVT), 0.1 wt% Sr doped (1SBVT) and (c) 0.15 wt% Sr doped BiVO4-TiO2 composite materials. Peak Positions ( cm-1) Assignment Table Click here to access/download;Table;Tables.docxhttps://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136755&guid=ec61b183-0e0a-4a3d-8581-e9223faf2bec&scheme=1https://www2.cloud.editorialmanager.com/jmse/download.aspx?id=1136755&guid=ec61b183-0e0a-4a3d-8581-e9223faf2bec&scheme=1124  O-Ti-O  208  Bi-O  329  Symmetrical of VO4+ 365  Asymmetrical bending mode of VO4+ 609  γ (O-Ti-O) 820  V-O