Gadolinium and Thulium Doping to Perovskite Structured Strontium Titanates at Strontium and Titanium Sites

Low temperature sol gel novel technique has been employed, in the work here, to fabricate gadolinium and thulium doped strontium titanates with nominal composition; Sr1-aGdaTi1-bTmbO3-d for various values of ‘a’ and ‘b’. The synthesized ceramics were cubic structured, phase pure, having well developed morphology and desired composition as revealed by XRD and FESEM/ EDX characterization techniques. The electrical and dielectric properties of the prepared samples were studied and various parameters like dielectric constant, loss factor, a c conductivity etc. were determined. Furthermore, these properties enhance with gadolinium and thulium doping at strontium and titanium sites respectively.


INTRODUCTION
Nowadays, fuel cells are emerging as a leading technology in the field of power generation owing to their better efficiency and environment friendliness. A fuel cell produces electric current via electrochemical reaction using hydrogen, oxygen as fuels and producing water as by product. Out of various types of fuel cell used nowadays, solid oxide fuel cells (SOFCs) are preferred due to various limitations suffered by counter fuel cells like requirement of pure fuel, expensive catalyst, corrosive electrolyte, high temperature requirement etc 1-2 .
Among various conventional materials employed in fabrication of SOFCs i.e. Ni-Cu/YSZ (nickel/copper-yttria stablized zirconia), rare earth doped CeO 2 (ceria), perovskite structured oxides etc. Lanthanum chromites, Strontium titanates etc.; both in pure and derived forms, are leading candidates in perovskite structured oxides [3][4] . The investigation of electrical properties of strontium titanates doped at 'Sr' and 'Ti' sites reveals the enhancement in the ionic conductivity with doping 5-6 . The doped strontium titanates have been widely employed in fabrication of various components of SOFCs i.e. anode, cathode and electrolytes. These oxides offers many advantages over other materials like high stability in oxygen, brief communication carbon, and sulfur containing atmospheres, remarkably active for methane (CH 4 ) oxidation at high temperature in the absence of excess steam, high conductivity (mainly in doped forms) stable dimensionally and chemically upon redox cycling etc [7][8][9] .
In this study, gadolinium and thulium have been doped at strontium and titanium sites respectively for various compositions via sol gel route; furthermore, structural, morphological and electrical/dielectric properties of the fabricated samples were studied. .5H 2 O} as metal precursors; propanol, hydrochloric acid (HCl) and distilled water as solvents. Here, in this series, three samples were synthesized for various combinations of 'a' and 'b' i.e. a = 0, b = 0; a = 0.05, b = 0.005 and a = 0.1, b = 0.005, and were named as SGTT0, SGTT1 and SGTT2 respectively employing the sol gel technique 10 . The calcination and then sintering of derived powder samples were carried out at 600 o C and 1250 o C respectively for 2 h; afterwards the powders were pelletized and pellets were further heated at 600 o C for 1 hour.
X-ray diffraction, SEM and dielectric characterization of the prepared samples were carried out using XPERT-PRO X-ray diffractometer, Nova Nano FE-SEM (FEI) and LCR HiTester respectively.

XRD Analysis
The Fig. 1 includes the XRD patterns of SGTT samples for all the compositions; the peaks are well defined, properly indexed based on cubic structure symmetry mathematical method 11 and phase pure. Similar XRD patterns have been observed for SrTiO 3 as reported in literature 10 . All the three XRD patterns are almost identical with slight shift to the large angle with increase in Gadolinium content; the replacement of strontium ions (ionic radius 132 pm) with smaller gadolinium ions (ionic radius 107.8 pm) may be responsible for that 8,12 . The lattice parameter values for all the three samples, calculated using mathematical method for cubic crystals 11 , are included in Table 1. The 'a' values decrease with increase in gadolinium content due to replacement of larger strontium ions by smaller gadolinium ions. The crystallite size of prepared samples, calculated for peak (110) using Sherrer's equation 13 , was between 46 to 52nm.  Cole-Cole plots for all the three samples at temperature 400 o C are included in Fig. 6. The grain effects in the prepared samples can be depicted from the presence of semicircle in the plots. The semicircle radius decreases with temperature as well as with doping indicating the decrement in resistance 23 . Dielectric constant measurement done in temperature range 50-400 o C and frequency range 1Hz to 1 MHz revealed the decrease in dielectric constant value with frequency and increase in it with temperature. Fig. 3 includes the frequency variation of dielectric constant at temperature 400 o C for all the samples; clearly doped samples have higher values than the pure one [14][15][16][17] .

CONCULUSION
Gadolinium and thulium doped strontium titanates have been synthesized in this work. XRD and SEM/EDX analysis revealed the formation of desired samples with required cubic structure. Doping with 'Gd' led to decrease in lattice parameter as expected; furthermore the doping led to decrease in grain size revealing the better morphology. Also, the dielectric constant, conductivity etc. showed improvement with 'Gd' ad 'Tm' doping.