Preparation, Structural Analysis, and Tunability of Optical and Dielectric Characteristics of Mn–modified SrLaLiTeO6 Double Perovskite


 SrLaLiTe 1- x Mn x O 6 ( x = 0.02, 0.04, 0.06, 0.08, 0.10) double perovskites have been prepared using solid state method. Studies on structural by applying X–ray diffraction (XRD) characterization found that all compounds formed in monoclinic, P2 1 /n symmetry with reduction of lattice parameters and unit cell volume as dopant concentration increased. The formation of Te 6+ /Mn 6+ –O–Li + octahedral structure can be confirmed with the presence of peaks at certain wavenumbers indicating vibrations of Te – O or Mn – O bonds. As dopant concentration increased, field emission scanning electron microscope (FESEM) characterization found that the increasing trend of formation in grains sizes from x = 0.02 to x = 0.08, and its effects towards dielectric properties which were conducted by electrochemical impedance spectroscopy (EIS) studies were discussed in this paper. Other discussions included were regarding the significant effect of dopant towards optical band gap, E opt and absorption frequencies of prepared compounds compared to pristine compound indicating its promising potential for optoelectronic device application.


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Research and studies regarding perovskites have been instigated extensively due to its promising 14 abilities such as superconducting, conductivity, magnetoresistance and ferroelectric. Pertaining to these sites for substitutions with additional A'-or B'-sites compared to conventional perovskites, they can 23 have substantial advantages for instance higher Curie temperature (TC) such as Sr2CrReO6 and 24 Ba2FeMoO6 with TC above 300 K [12,13] compared to perovskites like RCu3Mn4O12 and Sr0.9 Sn0.1TiO3 25 that possesses TC of 50 K and 200 K, respectively [14,15]. The field has gradually broadened as 26 tellurium based double perovskites, AA'BTeO6 has been studied and reported to have relatively good 27 dielectric properties [16][17][18][19]. Variety of discussions which consisted of densification, grain size or 28 polarizability aspects of A-site cations has been included in these studies. Double perovskites in 29 AA'BB'O6 form that have 1:1 B-site ordering has been reported to have potential in dielectric application 30 [19]. Furthermore, double perovskites in AA'BB'O6 configuration and form polar P21 space group 31 symmetry can be related to existence of ferroelectric or good dielectric properties [20]. Since 32 SrLaLiTeO6 claimed to consist of the perfect 1:1 B site ordering of Te 6+ /Li + and monoclinic P21/n 33 structure [21] that is similar to that in the SrBiLiTeO6, BaBiLiTeO6, BaBiNaTeO6 and BaLaNaTeO6 34 [18,19], it could be suggested that SrLaLiTeO6 has good dielectric abilities. 35 Apart from that, the properties of double perovskites materials mainly depend on B-site cations 36 arrangements which able to modify the structural phase transition, electrical or magnetic properties.

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The position of B-and B'-cations in octahedral are alternating if the size difference is large meanwhile 38 random placement of the B-and B'-cations take place whenever the size difference is small. Study 39 regarding Ba2ZnWO6 double perovskite in microwave frequencies points out that small dope of insulator 40 and larger size cation (Ca 2+ ) into B-site of perovskite could affect its microwave dielectric properties 41 [22]

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Polycrystalline powders of SrLaLiTe1-xMnxO6 (x = 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized using 67 a solid-state reaction method. High-purity (≥99.99%) of strontium carbonate (SrCO3), lithium carbonate 68 (Li2CO3), lanthanum oxide (La2O3), manganese (III) oxide (Mn2O3) and tellurium dioxide (TeO2) 69 powders from Sigma-Aldrich are used as raw materials. The chemical powders have been mixed at 70 stoichiometric ratios with total mass of 3.5 g. The samples then grinded in agate mortar by pestle for 1 71 h to achieve good homogeneity. After grinding process, the mixed powder sample then pressed into 72 pellet at pressure of 4-5 KPa using hydraulic press. The pellet was then placed on an alumina crucible  Graphical User Interface (i.e. EXPGUI) software [26,27] were used for Rietveld refinement [28] prior 80 visualized in Visualisation for Electronic Structural Analysis (VESTA) program. Peak shape was 81 modelled by pseudo-Voight function refined together with cell parameter, scale factor, zero factor and thoroughly with potassium bromide (KBr) and the FTIR reflectance spectra were recorded in FTIR-84 Raman Drift Nicolet 6700 equipment ranging from 400 to 1500 cm -1 . The surface morphology of the 85 sintered pellets and constituent elements was obtained by conducting FESEM and energy dispersive X-86 ray (EDX) characterizations by using SU 8000, Hitachi, Japan equipment. The dielectric and modulus 87 with frequency range of 50 Hz to 1 MHz studies were collected by using a HIOKI 3532-50 LCR Hi 88 Tester connected to a computer while keeping the electrode pellets in sandwich geometry. Optical study 89 performed by using Lambda 750, Perkin Elmer, Waltham, USA equipment for 2 to 5 hv range.  Table 1 shows the complete parameters that were obtained from the refinement.   [29]. The calculated τ were 117 presented in Table 1       SrLaLiTeO6 [21]. Literature [40][41][42] has reported that the Eopt is associated with the presence of impurity energy 210 levels within the band gap of the materials. As wider Mn energy bands (impurity bands) formed with higher 211 dopant concentration, this change will reduce the distance between bands and consequently reduce the Eopt. On 212 the other hand, reduction in structural distortion when Mn 6+ doped into pristine compound is the other reason to 213 reduce Eopt. Opposite explanation regarding effect of distortion onto band gap has been discussed in other report 214 [21]. Besides, oxygen vacancies are the other possible factor which are able to promote the formation of impurity 215 energy levels within the band gap [42] in all compounds. It is accepted that oxygen vacancies can contribute to 216 the results in this work. However, the monoclinic structure as well as lattice parameters of all compounds are 217 comparable to the stoichiometric in SrLaLiTeO6 on previous report [21]. Hence, it is suggested that differences 218 in oxygen content among the compounds are small and should not have major influence on the measured 219 properties. There is other report that reveal the same trend of Eopt of other compound of doping with Mn 2+ 220 realized [43]. The absorbance spectra in Fig. 5(b) shows that all compounds' optical band gaps are in visible light range, makes SrLaLiTe1-xMnxO6 possible for photovoltaic applications with further study to increase the 222 electronic conductivity.   reduce the phonon modes in structure, hence, causing space charge polarization [44]. ' drop in low frequencies 253 was in alignment with high losses in Tan δ at the same frequencies as in Figure 6(b).

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Tan δ represents the energy loss in the compoundsin the midst of electric field alternation. DC conduction loss 255 might be another factor in assisting the drop of the ′ . Nevertheless, the gradient in plot of ln ′ against ln ω 256 (not shown) did not show magnitude close to (-1) to prove the presence of this factor. Apparently, there are no 257 clear difference in frequencies below than 100 Hz range after doping in each compound. However, at 258 frequencies higher than 100 Hz, medium and light sized dipoles did contribute in ′ response. They are mainly 259 comprised of orientational, ionic and electronic dipoles. ′ values keep on decreasing as frequencies increased.

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Nonetheless, the slowly decrease of ′ response in Mn 0.08 compared to other compounds can be pertained to 261 the probability of higher abundance of medium sized of electrical dipoles compared to light ones. This variation 262 is related to smallest Tan δ in the compound at frequencies above 400 kHz. Compared to pristine SrLaLiTeO6 263 [39], there is enhancement of ′ value in all Mn 6+ doped SrLaLiTeO6.

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The variation of ′ response can be explained in terms of its interrelation with average grain size in inset of 265 Figure 6(a). Larger grains size can form larger medium sized dipoles and probability of higher amount of 266 medium sized dipoles. As a consequence, production of larger electrical dipole moments can take place in contrast to smaller grains. Accordingly, higher polarization effect can take place. This then suit to explain the 268 relationship between variation in ′ and average grain size. Mn 0.08 was clearly possessed higher ′ at higher 269 frequencies might be due to this factor. Other reports that are related to this interrelation between ′ and grain 270 size has been recorded before [45].