Positron Annihilation Spectroscopic Investigation on the Origin of

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Positron Annihilation Spectroscopic Investigation on the Origin of Temperature Dependent Electrical Response in Methylammonium Lead Iodide Perovskite Joydeep Dhar, Sayantan Sil, Arka Dey, Partha Pratim Ray, and Dirtha Sanyal J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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The Journal of Physical Chemistry Letters

Positron Annihilation Spectroscopic Investigation on the Origin of Temperature Dependent Electrical Response in Methylammonium Lead Iodide Perovskite Joydeep Dhar,† Sayantan Sil,† Arka Dey,† Partha Pratim Ray*,† and Dirtha Sanyal* , ‡ †

‡

Department of Physics, Jadavpur University, Kolkata 700032, India.

Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064, India.

Corresponding Author’s E-mail: [email protected] Phone : +091-33-23184462 Fax

: +091-33-23346871

ACS Paragon Plus Environment

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ABSTRACT Organic-inorganic hybrid perovskite has appeared as one of the leading materials for realizing solution based high performing optoelectronic devices. The charge properties in this class of material are quite intriguing and still need to be carefully investigated. The temperature dependent electrical property of methylammonium lead iodide (CH3NH3PbI3) has been investigated by employing positron annihilation spectroscopy (PAS) which unambiguously reveals the gradual formation of open volume defects with the enhancement in temperature. The high temperature ionic conductivity is due to the generation of both, cationic (CH3NH3+) and anionic (I-) vacancies possibly because of the elimination of methylammonium iodide (CH3NH3I) as identified from the coincidence Doppler broadening (CDB) of the positron annihilation spectroscopy. Further, the evolution of temperature dependent defect density and corresponding electrical responses has been correlated with the structural phase transitions of CH3NH3PbI3. This is the first ever report of temperature dependent PAS measurement on hybrid lead halide perovskites to understand the nature and the origin of its electrical characteristics arising due to the variation in temperature.

TOC GRAPHICS


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Since the inception of methylammonium lead halide as light harvester for photovoltaic (PV) devices, it has occupied the imagination of researchers owing to phenomenal rise in the power conversion efficiency (PCE) crossing the benchmark value of 20% in a short period of five years.1 Significant low exciton binding energy (~0.2 eV),2 very high charge carrier mobility (>100 cm2V-1s-1)3 and exceptionally large exciton diffusion length over 150 µm3 are few of such unique qualities which led to rapid progress in device performances. In addition, the internal quantum efficiency of perovskite solar cell reaches nearly 100% suggesting efficient exciton dissociation and negligible charge recombination characteristics of this class of material.2 In spite of steady growth achieved in understanding the underlying physics of material properties, the lacuna lies in realizing the charge transport characteristics of hybrid perovskite, as ion motion significantly affects its electrical properties. Earlier studies suggest that the electrical response from organic inorganic hybrid lead halide is having both electronic and ionic components and they behave as a mixed conductor.4 Therefore, it is of paramount importance to identify the origin of ionic conductivity observed under a given potential. Moreover, in recent time the ionic transport is under intense focus, because the ion motion is believed to be primarily responsible for exhibiting acute hysteresis during photo-current measurement and in consequence, also held to be accountable for ambiguity in device performances.5-7 Thus, electric poling or light soaking before device measurement greatly manipulates the photovoltaic performances and supposed to be due to the ion movement phenomenon in organic inorganic lead halides.8 Different studies have suggested that the cation and/or anion vacancy mediated ion movement is the origin of its ionic conductivity.9-10 Contradictory views on methylammonium or iodide ion migration have been proposed due to close energy requirement for respective defect formations as predicted from theoretical calculations.11-14 Huang and coworkers reported the


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migration of methylammonium using photo-thermal induced resonance (PTIR) technique15 and also the presence of iodide vacancy at a relatively higher temperature 330 K.16 Our recent work has demonstrated that the nature of defects in CH3NH3PbI3 depends upon the synthetic procedure and there is a probability of methylammonium and/or iodide defects formation.17 The influence of ion movement on charge carrier mobility was reported by Chin et al. as measured in field effect transistor (FET) device configuration.18 When temperature was raised, the field effect electron and hole-mobilities gradually decrease due to screening of the charge carriers by the mobile ions. But, the variation in electrical conductivity with respect to temperature showed steady non-ohmic like increase for CH3NH3PbI3 in its tetragonal phase.19 It indicates that the rise in conductivity is probably due to increase in the ionic current assisted by the increase in the number of lattice defect working as the ion transport pathway. Therefore, it is necessary to identify the nature of the defect state, responsible for steady rise in ionic conductivity with the increase in temperature. On the other side, recent studies have also investigated the thermal degradation behaviour of CH3NH3PbI3 and observed that the decomposition becomes faster in presence of moisture.20-25 In a solar cell device, using microscopic techniques the diffusion of iodide and lead ion in CH3NH3PbI3 have been shown at a relatively lower (50 °C) and higher (175 °C) temperature, respectively.26 Most of the studies were performed microscopically but the atomic level picture remains quite unclear. Moreover, the effect of annealing during preparation of CH3NH3PbI3 sample from its precursors (PbI2 and CH3NH3I) while fabricating perovskite solar cell was not considered from the point of formation of intrinsic lattice defects. Therefore, the effect of temperature on the perovskite stability is a crucial aspect required a careful attention. In addition, considering practical usability of PV device, the thermal stability profile of a solar cell material is essential and pertinent.


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It is reported that the tetragonal phase at room temperature transits into the cubic phase at 327 K with gradual enhancement in temperature.19 Structural phase transition is an important phenomenon from the point of electrical conductivity is concerned; because it can change the electronic configuration leading to the alteration in the band gap, exciton binding energies, and density of charge carriers which result in the variation in charge carrier mobility.18, 27-28 Recently various temperature dependent experimental techniques e.g. neutron scattering,29 spectroscopic methods like terahertz,30 photoluminescence,28 calorimetric,31 infrared31 and Raman32 have been used to follow the structural phase transition in organo lead halide perovskite. In most of the studies, the change in electronic properties was principally monitored as a function of temperature. Only a few has reported the variation in electrical performances of CH3NH3PbI3 in its different structural phases, but without any in depth analysis.19 In recent years, positron annihilation spectroscopy (PAS), a unique nuclear solid state technique has been employed to study structural phase transition in different inorganic semiconductors.33-36 In the conventional PAS energetic positron (obtained from the beta decay of a β+ emitting nuclei, e.g., 22Na) have been injected directly into the studied material,37-38 where it thermalizes (energy ~ 25 meV) within ~ 1 to 3 ps. Being the anti-particle of electron, positrons annihilate with electron of the studied material by emitting mostly two oppositely directed 511 keV gamma rays. The positron annihilation rate (λ) which is inverse of the positron lifetime (τ) is directly proportional to the electron number density (ne) of the studied material. Normally the electron density at the cation defect site is comparatively less than the electron density of the bulk of the material, thus positron trapped in a cation defects elapses little longer time, as a result one expect a longer positron lifetime from a defect site.37-38 Thus by measuring the sub nanosecond positron annihilation lifetime (PAL) the different electron density regions in a material


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can be studied. The other positron annihilation technique is the Doppler broadening of the annihilated 511 keV gamma ray spectroscopy (DBAR). Employing this technique one can study the electron momentum distribution in a material.37-40 The technique is based on the detection of the Doppler shifts of the annihilated 511 gamma ray photons due to finite energy (momentum) of the electrons (positron is thermalized). One can study the annihilation of positron with the core electrons of an element by drastically reducing the background (which normally comes from the Compton continuum of other gamma ray) by using two HPGe detectors in coincidence. This is normally named as coincidence Doppler broadening (CDB) spectroscopy and is a powerful technique to study the chemical nature of a particular defect.40-41 The DBAR spectra have been analyzed by the so-called line-shape parameter (S-parameter). It is defined as the ratio of the positrons annihilated with the lower momentum electrons with the annihilation of positrons with higher momentum electrons. Thus the S-parameter has been defined as the ratio of counts in the central area (|511 keV – E| ≤ 0.85 keV) with and the counts in the total area (|511 keV–E| ≤ 4.25 keV) under the 511 keV photopeak.40-41 These positron annihilation spectroscopic techniques (PAL, DBAR and CDB) are very useful to characterize chemical nature of defects as well as its role in different structural phase transition in different materials.33-36 In the present hybrid lead halide perovskites sample the temperature dependent Doppler broadening S-parameter measurement have been carried out to study the change in the open volume defect in different crystallographic phases providing a tool to diligently follow the defect mediated ionic transport as well as to identify the phase transition. In this article, we have investigated the nature of lattice defects with the evolution of temperature at two different crystallographic phases namely, tetragonal and cubic and the variation in defect density with temperature by means of PAS. The conductivity values measured for CH3NH3PbI3 along with the S-parameter precisely correlates


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with the structural changes that take place with the variation of temperature. Thus, the influence of temperature on the ion transport is clearly reflected from the parallel temperature dependent conductivity and S-parameter measurement. The investigated material, CH3NH3PbI3 was synthesized following previously reported procedure in which equimolar methylammonium iodide (CH3NH3I) and lead iodide (PbI2) were ground over an hour.19 The powder X-ray diffraction pattern confirms the formation of CH3NH3PbI3 in tetragonal phase and is devoid of any residual peak due to the starting material (Figure 1a).42 The onset of UV-vis absorption spectrum from which the optical band gap of 1.55 eV is calculated has been shown in Figure 1b. The field emission scanning electron microscopic (FESEM) image (Figure 1c) shows that the particles are nanometers in size and mostly aggregated. Transmission electron microscopic (TEM) image (Figure 1d) also corroborates the nanoparticulate morphology observed in FESEM. The selected area electron diffraction (SAED) pattern (Figure 1e) suggests the polycrystalline nature of the perovskite nanocrystallites. Figure 1f shows the 3-dimensional (3-D) view of nanoparticles after deposited on the glass substrate.


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Figure 1. PXRD pattern (a), UV-vis absorption onset (b), FESEM image (c), bright field TEM image (d), SAED pattern (e) and AFM image (3-D view) (f) of CH3NH3PbI3 perovskite synthesized by mechanical grinding procedure.17 As mentioned previously, the electrical characteristics of organo lead halide perovskite have been investigated with respect to temperature (Figure 2A). The conductivity was measured in the temperature range of 220 to 340 K. The variation observed in electrical conductivity was quite interesting as there were distinct changes one can clearly identify in two different crystallographic phases, i.e. tetragonal and cubic (Figure 2B) while monitoring the temperature dependent electrical responses. It is well reported in literature that defect mediated ionic conductivity plays a significant role in governing electrical properties of CH3NH3PbI3.4 The interplay between ionic and electronic conductivity ultimately determines the final outcome of its electrical characteristics with the evolution of temperature and we see a non-ohmic type currentvoltage plot for CH3NH3PbI3.19 At 200 K, the electrical conductivity was about 1 × 10-8 S/cm and it remained almost the same up to temperature 250 K. Then, conductivity slowly increases with the increase in temperature till 300 K, followed by a steep rise in conductivity by almost two orders to 8.9 × 10-7 S/cm up to the tetragonal to cubic phase transition temperature at 327 K. There was a small dip in the conductivity values around the phase transition temperature, but then onwards conductivity gradually increases when temperature was raised. Thus, we can easily determine the phase transition temperature by monitoring the conductivity with respect to temperature. The variation in conductivity with temperature was similar to that of temperature dependent heat capacity profile, as it was observed previously.31,

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The observed variation in conductivity is well

understood when the defect analysis was performed utilizing PAS techniques.


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Figure 2. (A) Temperature dependent conductivity and positron annihilation Doppler broadening S-parameter for CH3NH3PbI3 perovskite sample. (B) The schematic representation of the corresponding two crystal structures appeared during temperature variation for CH3NH3PbI3. The variation in S-parameter with temperature for CH3NH3PbI3 is plotted in Figure 2A. It is interesting that the S-parameter followed similar trend to that of conductivity; i.e. small increase initially and then a sharp increase above the sample temperature of 290 K attaining a maximum value at 333 K. Therefore, this observation suggests that the electrical response at higher temperature arises due to the defect mediated enhanced ionic motion since electronic conductivity falls off at the elevated temperature.18 To ensure the repeatability the temperature dependent DBAR measurements have been carried out three times with three sets of newly prepared samples. With further heating the S-parameter decreases a little around 350 K and then further increases. Interestingly, around 327 K there is structural phase transition (tetragonal to cubic phase transition) in these perovskite. The increase in S-parameter indicates either increase of positron annihilations with lower momentum electrons or decrease of positron annihilations with higher momentum electrons. Increase in S-parameter indicates the increase in open volume defects in the sample.35, 44-45 The present S vs. T graph indicates the number of defects increases with temperature. Since the conductivity also increases with increasing sample temperature,


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present study clearly establish a correlation between defects and the observed conductivity in this perovskite material. The decrease in S-parameter beyond 333 K i.e., in the cubic phase indicates the sudden decrease of defects due to reorientation of the crystal structure or in other word it indicate a significant role of these defects for such structural phase transition. Defect mediated structural phase transition has also been studied by temperature dependent DBAR measurement in AgBiS2 and other related system.33-36 Now to identify the chemical nature of the defects we employed positron lifetime and CDB measurements at two different temperatures, 300 and 350 K (both annealed and in-situ).

Figure 3. The typical positron lifetime spectra at two different temperatures (300 and 350 K_annealed)


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Table 1. Positron life-time component of CH3NH3PbI3 at 300 and 350 K_annealed Sample

τ1 (ps)

I1 (%)

τ2 (ps)

I2 (%)

τ3 (ps)

I3 (%)

300 K

160 ± 3

16 ± 1

326 ± 5

76 ± 1

1050 ± 50

8 ± 0.5

350 K

161 ± 3

6±1

309 ± 5

86 ± 1

1105 ± 50

8 ± 0.5

The positron annihilation lifetime spectrum at two different temperature, 300 and 350 K have been recorded (Figure 3). Both the lifetime spectra have been fitted with the computer programme PATFIT 88.44 The best fit has been obtained for (variance of fit 0.85 and 0.91 per channel, respectively) three components fitting yielding a very long lifetime of ~ 1100 ps. The long component (τ3) is due to formation of positronium either at grain boundary or in large voids inside the polycrystalline perovskite samples. The shortest component (τ1) is the free annihilation of positron in the bulk of the material, while the intermediate component (τ2) is the positron lifetime at the defect site. Its intensity represents the relative amount of defect present in the sample. The cation type vacancy is present at both the temperatures with similar positron lifetime values (> 300 ps) indicating identical nature of the defect sites. We attribute this cationic defect to the CH3NH3+ vacancy, since formation of Pb2+ requires larger amount of energy.11, 13-14 All the lifetime components are listed in the Table 1. The intensity of the intermediate lifetime component (I2) is increased from 76 % to 86 % due to heating the sample at 350 K, suggests the formation of more defects at 350 K temperature, which are in agreement with the S-parameter data. Another interesting point in Table 1 is the value of τ2 decreases due to annealing at 350 K, which suggests the overall electron density at the defect site little increases.


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Figure 4. Ratio of area normalized CDB spectrum at 350 K with area normalized CDB spectrum recorded at 300 K; in situ (a) and annealed (b) To identify the exact nature of the defect, coincidence Doppler broadening of the annihilation radiation measurement has been performed at two different temperatures (300 and 350 K). The experiment was performed in situ as well as on annealed sample. The so called “ratio curve” has been constructed to understand the nature of the defect. Figure 4 represents the so called “ratio curve” which is the ratio between area normalized 350 K CDB spectrum with area normalized 300 K CDB spectrum. For both samples i.e. in situ (Figure 4a) and annealed (Figure 4b), the nature of the ratio curve was very close to each other. For the sample measured in situ, we had two significant dips one at momentum value of 9 ×10-3 moc and other at a dip around 14 ×10-3 moc, whereas a broad peak spread across 8 to 15 ×10-3 moc was obtained in the “ratio-curve” for the annealed sample. Using the relation E = pL2/2mO the energy of the core electrons participating in the annihilation process in this momentum range have been identified as 5d electron of Pb (~ 20 eV), and 2s electron of N (~ 37 eV) and corresponding momentum values are 9 ×10-3 moc and 12 ×10-3 moc respectively. The other core electrons participating in the


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annihilation process in this momentum range are 5s (~ 20 eV) and 4d electrons (~ 50 eV) of I with momentum values 9×10-3 moc and 14 ×10-3 moc respectively. This clearly suggests that at 350 K the positrons are less annihilating with the electrons of Pb (5d), N (2s) and I (5s, 4d). Thus CDB results suggest formation of cation like vacancy from Pb2+ and CH3NH3+ at higher temperature. But, from our previous positron lifetime data we have observed only one cation vacancy related defect site at both the temperatures and this was due to the creation of CH3NH3+ vacancy.17 Therefore, we can exclude the formation of Pb2+ defects at 350 K and assign those dips in CBD spectrum due to the lower positron annihilation with N (2s) and I (5s, 4d) electrons. The formation of both CH3NH3+ and I- defects suggests probable sublimation of CH3NH3I during heating and this process becomes faster above 100 °C as reported by Supasai et al.46 Our experiment also confirms that the defects formation is irreversible in nature since the “ratiocurves” for in situ and annealed sample were almost identical in nature. It indicates that the defects formed during perovskite sample preparation by heating the two precursor materials (PbI2 and CH3NH3I) remained unperturbed even after cooling back the sample at room temperature. But, the crystal structure remains the same for the annealed sample to that of nonheated sample as confirmed from the PXRD pattern (Figure 5) of the CH3NH3PbI3 before and after heating at 350 K (no peak due to PbI2). Supasai et al. observed the formation of PbI2 only when the sample was heated above 100 °C.46 Hence, our results suggest that though the detectable amount of PbI2 formation i.e. significant amount of defects form through CH3NH3PbI3 decomposition above 100 °C, but at the molecular level the lattice defects form even at a lower temperature (70-80 °C) which is required for perovskite synthesis.


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Figure 5. PXRD pattern for CH3NH3PbI3 recorded at 300 K before and after heating at 350 K In summary, the present temperature dependent positron annihilation spectroscopic investigation of CH3NH3PbI3 based perovskite indicates the formation of both cation (CH3NH3+) and anion (I-) like vacancy at higher temperature which are mainly responsible for the increase in conductivity with temperature. The close resemblance of the temperature dependent S-parameter with the variation in conductivity in CH3NH3PbI3 suggesting ionic current contributes predominately on the electrical response at elevated temperature. Though the crystal structure changes reversibly with respect to temperature, but the formation of lattice defects in this class of material is irreversible in nature. Thus, annealing the sample during synthesis of CH3NH3PbI3 is indirectly responsible for the current-voltage hysteresis which in principle has origin on the lattice defects.


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EXPERIMENTAL SECTION Synthesis of CH3NH3PbI3. Firstly, CH3NH3I was synthesized as follows. Hydroiodic acid (HI, 16 mL 57% in water from Alfa-Aeser) was added drop-wise into the methylamine (CH3NH2, 18.2 mL 33% in Methanol from Spectrochem) and stirred at ice-cold condition for 2 h under N2 atmosphere. After evaporating the solvent at 60 °C the solid powder was obtained. The powder was washed several times, initially with ethyl acetate and then with diethyl ether. It was vacuum dried overnight at 60 °C. The CH3NH3PbI3 sample was synthesized by grinding CH3NH3I and PbI2 (99.9985% from Alfa-Aeser) (1:1) in a mortar over 1 h. Preparation and characterization of samples. The powdery samples were used for recording powder X-ray diffraction (PXRD) pattern, UV-vis absorption spectrum and for pellet formation. For field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging, the powdery samples were dispersed in diethyl ether and drop-casted on silicon wafer, carbon coated copper grid and glass substrate, respectively. Pellets of 12 mm diameter were prepared by uniaxial cold pressing with the pressure of 100 MPa. Thickness of the pellets varied within 450-500 µm and was measured by Mitutoyo digital micrometer with maximum precession of 1 µm. Pellets were used for measuring dc conductivity and for positron annihilation spectroscopic study. The PXRD pattern was obtained in Bruker D8 (Cu Kα =1.5418 Ǻ) diffractometer under ambient conditions. UV-vis spectra were recorded in Shimadzu 2401PC spectrophotometer. FESEM analysis were performed in FEI Inspect F-50 scanning electron microscope.


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TEM images were collected from JEOL JEM 2100 operated at 200 kV. AFM image has been taken in Bruker Multimode 8 instrument. In the present positron annihilation lifetime spectroscopic study (PALS), about 10 µCi (microCi)

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NaCl source sealed in 1.5 µm thick nickel foil has been used as a positron source. The

sealed source was sandwiched between two identical plane faced samples (12 mm diameter × 500 µm thick pellet). The conventional fast-fast coincidence assembly using two gamma ray detectors (25 mm long and 25 mm tapered to 13 mm diameter BaF2 scintillator optically coupled with XP2020 Q photomultiplier tube) have been used for the present positron lifetime measurement system.47 The spectrometer has a timing resolution (full width at half maximum) of ~ 220 ps measured by the prompt gamma ray of 60Co source. About ten million total coincidence counts have been recorded in a multichannel analyzer. The recorded lifetime spectrum have been analyzed by the computer code PATFIT-8848 with proper source corrections. The other positron annihilation technique is the Doppler broadening of positron annihilation spectroscopy.44 Although positrons are thermalized but electron has some momentum; depending on the electron momentum (pel), the 511 keV annihilated γ-rays are Doppler shifted by an amount (±∆E = pLc/2) in the laboratory frame where pL is the component of pel along the direction of measurement. This Doppler broadening can be precisely measured with two high purity germanium detectors in the standard nuclear γ-γ coincidence technique.49 For the CDBS measurement with ± ∆E selection and very high peak to background ratio (better than 105:1) two identical HPGe detectors (model number PGC 1216sp of DSG, Germany of 12 % efficiency and having energy resolution of 1.15 keV at 514 keV of 85Sr) were used.41 About 2 × 107 counts have been recorded in a dual ADC based multiparameter data acquisition system (Model number MPA-3 of FAST ComTec, Germany).


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For the temperature dependent DBAR and CDB studies the sample-source-sample sandwich has been placed on the cold-head of a closed cycle helium cryogenerator (Advanced research systems Ins., USA made Model No. ARS-2HW) system. The Si diode temperature sensor is placed near the sample and a LakeShore temperature controller (model number 335) with a temperature stability of ± 0.1 K has been used to control the temperature from 6 to 350 K.50 For the DBAR measurements in the temperature range 340 K to 370 K the sample-source-sample has been placed in a vacuum oven type furnace.33-34,

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For the temperature dependent electrical

resistivity measurement sample has been mounted on the cold head of the ARS-2HW closed cycle helium cryo-generator. For dc conductivity measurement silver electrodes were deposited on both sides of the pellets for the purpose of electrical contact. The Keithley constant current source (model number 2400) and a Keithley nanovoltmeter (model number 2182) have been used for the electrical conductivity measurement. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Joydeep Dhar and Sayantan Sil acknowledge University Grants Commission (UGC) for providing Dr. D. S. Kothari Postdoctoral Fellowship and NET-Junior Research Fellowship, respectively. The support from FIST and PURSE programm of Department of Science and Technology (DST) from Government of India is highly acknowledged.


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