Study of Suppression of Superconducting Transition Temperature of Y1-XPrxBa2Cu3O7-δ System

The effect of Pr-doping on the transition temperature (Tc) and structural properties of YBa2Cu3O7-δ (YBCO) superconductor have been investigated. The sol-gel method has been employed for the synthesis of Y1xPrxBa2Cu3O7-δ (YPBCO) samples with the compositions (x=0.0, 0.05, 0.10, 0.20 and 0.30). The broadening in X-ray diffraction (XRD) peaks has been used to calculate the micro-strain by the Williamson-Hall (W-H) plot. The broadening in XRD peaks and micro-strain are found to increase with increase in Pr concentration x which could be representatives for some disorder in Cu-O planes. The resistance-temperature measurements of as prepared YPBCO samples show the monotonic suppression of Tc with increase in Pr concentration from x=0 to x=0.30. The experimental Tc values were in good agreement with theoretical Tc values calculated by the inclusion of disorder effects along with magnetic pair breaking and hole filling effects. It has been suggested that disorder in Cu-O planes might be strain-induced. The disorder effects along with magnetic pair breaking and hole filling effects are responsible for the suppression of Tc in YPBCO system. Introduction YBCO was the first superconductor to have transition temperature (Tc) above boiling temperature of Liquid Nitrogen 1. Its superconducting properties are not affected much by the substitution of Y with the rare earth (RE) elements like La, Nd, Sm, Eu and Gd etc. however, Praseodymium (Pr) is the exception2,3. The Resistance-temperature measurements of Y1xPrxBa2Cu3O7-δ system clearly indicate the influence of Pr-doping on electrical properties of YBCO4. The Pr-doped YBCO has potential applications as superconducting wires due to its enhance critical current density (Jc) in the magnetic field at liquid nitrogen temperature. The superconducting transition temperature (Tc) decreases monotonically with increase in Pr concentration in YPBCO system. Different mechanisms based on Pr valence have been suggested for the monotonic suppression of Tc in YPBCO system with increasing x. The magnetic property measurements6 revealed the tetravalent (+4) state of Pr, and suggested that hole filling effect is responsible for suppression of Tc in YPBCO system. The optical measurements7,8 indicate the trivalent state (+3) of Pr, and believed that superconducting pair breaking by the local moment of the Pr ion is responsible for suppression of Tc in YPBCO system9. In trivalent state (+3) of Pr, The degradation of Tc in YPBCO system with increasing Pr concentration follows the Abrikosov-Gorkov relation10. The variation of Tc (in Kelvin) with x and y in (Y1-x-yCay)PrxBa2Cu3O6.95+0.02 system can be represented by11 The equation (1) suggested that the Tc values depend on two terms (i) the linear term due to magnetic pair breaking mechanism and, (ii) the quadratic term due to hole filling effect. Further, the theoretical Tc (in Kelvin) of Y1-xPrxBa2Cu3O7-δ system for various values of x has been calculated by J. J. Neumeier et al11 using Abrikosov-Gorkov10 (A-G) relation Where and. The equation (2) contains only linear term in x which is attributed to magnetic pair breaking mechanism. The Tc (in Kelvin) for Y1-xPrxBa2Cu3O7-δ system according to Dole12 is given by The equation (3) also suggested the degradation of Tc due to magnetic pair-breaking mechanism and hole filling effects. The Gill et al.13 reported the variation of Tc (in Kelvin) with x in Y1-xPrxBa2Cu3O7-δ system by the relation Where initial quadratic and linear terms have same dependence as given by equation (3) but third square root term is due to disorder in Cu-O planes after doping with Pr. In the present work, we synthesized the series samples of Y1xPrxBa2Cu3O7-δ superconductor with various values of x. The samples were characterized by various techniques and the results are presented in this paper. Experimental The samples of Y1-xPrxBa2Cu3O7-δ (x=0, x=0.05, x=0.10, x=0.20 and x=0.30) were prepared by using sol-gel method. The appropriate amounts of Y(No3)3, Ba(No3)2, Cu(No3)2 and Pr(No3)3 were mixed in the solution of water and ethyl alcohol. The mixture was stirred at 50-70°C until it became gel. It was dried in oven, then grinded and put in the furnace at 500 °C to eliminate moisture and nitrates for 5 h. The material was then regrinded and calcined in air at 900 °C for about 20 h. It was cooled and pressed in to pellets. Subsequently, the pellets were sintered at 930 °C for 20 hours in the presence of oxygen. The transition temperature Tc of as prepared samples was determined from resistance-temperature measurements by means of standard four probe method. The structure of the samples was characterized using XRD technique. The size and surface morphology of grains were determined from SEM measurements. Results and Discussions Fig. 1 shows the variation of relative resistance with temperature in Y1-xPrxBa2Cu3O7-δ (x=0, x=0.05, x=0.10, x=0.20 and x=0.30) samples. The transition temperature Tc for various


Introduction
YBCO was the first superconductor to have transition temperature (T c ) above boiling temperature of Liquid Nitrogen 1 . Its superconducting properties are not affected much by the substitution of Y with the rare earth (RE) elements like La, Nd, Sm, Eu and Gd etc. however, Praseodymium (Pr) is the exception 2,3 . The Resistance-temperature measurements of Y 1-x Pr x Ba 2 Cu 3 O 7-δ system clearly indicate the influence of Pr-doping on electrical properties of YBCO 4 . The Pr-doped YBCO has potential applications as superconducting wires due to its enhance critical current density (J c ) in the magnetic field at liquid nitrogen temperature. The superconducting transition temperature (T c ) decreases monotonically with increase in Pr concentration in YPBCO system. Different mechanisms based on Pr valence have been suggested for the monotonic suppression of T c in YPBCO system with increasing x. The magnetic property measurements 6 revealed the tetravalent (+4) state of Pr, and suggested that hole filling effect is responsible for suppression of T c in YPBCO system. The optical measurements 7,8 indicate the trivalent state (+3) of Pr, and believed that superconducting pair breaking by the local moment of the Pr ion is responsible for suppression of T c in YPBCO system 9 . In trivalent state (+3) of Pr, The degradation of T c in YPBCO system with increasing Pr concentration follows the Abrikosov-Gorkov relation 10 . The variation of T c (in Kelvin) with x and y in (Y 1-x-y Ca y )Pr x Ba 2 Cu 3 O 6.95+0.02 system can be represented by 11 The equation (1) suggested that the T c values depend on two terms (i) the linear term due to magnetic pair breaking mechanism and, (ii) the quadratic term due to hole filling effect. Further, the theoretical T c (in Kelvin) of Y 1-x Pr x Ba 2 Cu 3 O 7-δ system for various values of x has been calculated by J. J. Neumeier et al 11 using Abrikosov-Gorkov 10 (A-G) relation Where and. The equation (2) contains only linear term in x which is attributed to magnetic pair breaking mechanism.
The T c (in Kelvin) for Y 1-x Pr x Ba 2 Cu 3 O 7-δ system according to Dole 12 is given by The equation (3) also suggested the degradation of T c due to magnetic pair-breaking mechanism and hole filling effects. The Gill et al. 13 reported the variation of T c (in Kelvin) with x in Y 1-x Pr x Ba 2 Cu 3 O 7-δ system by the relation Where initial quadratic and linear terms have same dependence as given by equation (3) but third square root term is due to disorder in Cu-O planes after doping with Pr.
In the present work, we synthesized the series samples of Y 1- x Pr x Ba 2 Cu 3 O 7-δ superconductor with various values of x. The samples were characterized by various techniques and the results are presented in this paper.

Experimental
The samples of Y 1-x Pr x Ba 2 Cu 3 O 7-δ (x=0, x=0.05, x=0.10, x=0.20 and x=0.30) were prepared by using sol-gel method. The appropriate amounts of Y(No 3 ) 3 , Ba(No 3 ) 2 , Cu(No 3 ) 2 and Pr(No 3 ) 3 were mixed in the solution of water and ethyl alcohol. The mixture was stirred at 50-70°C until it became gel. It was dried in oven, then grinded and put in the furnace at 500 °C to eliminate moisture and nitrates for 5 h. The material was then regrinded and calcined in air at 900 °C for about 20 h. It was cooled and pressed in to pellets. Subsequently, the pellets were sintered at 930 °C for 20 hours in the presence of oxygen. The transition temperature T c of as prepared samples was determined from resistance-temperature measurements by means of standard four probe method. The structure of the samples was characterized using XRD technique. The size and surface morphology of grains were determined from SEM measurements. Fig. 1 shows the variation of relative resistance with temperature in Y 1-x Pr x Ba 2 Cu 3 O 7-δ (x=0, x=0.05, x=0.10, x=0.20 and x=0.30) samples. The transition temperature T c for various ReseaRch PaPeR concentrations of Pr is calculated using fig.1, eq. (2), 3 and 4 are shown in table-1 and found in good agreement with reported values 12,14 . The variation of experimental T c with Pr concentration x is shown the fig. 2. The monotonic suppression of T c with increase in Pr concentration was observed. Further, the various values of transition temperature calculated from equations (2), (3) & (4) along with experimental transition temperature is plotted against Pr concentration x in fig. 3. The transition temperatures calculated from eq. (4) are found in good agreement with experimental T c values. From fig. 3, this is evident that there is some disorder present in the system.        Here, in order to find the strain broadening each XRD peak fitted with Lorentzian profile. The micro-strain, ε and the average grain size, D can be calculated from Williamson-Hall (W-H) plot for the X-ray diffraction peak broadenings 18 ,

Results and Discussions
Where λ is wavelength of incident X-rays. In fig. 7, plotted against and linearly fitted to find micro-strain ε. The microstrain ε increases with increase in Pr concentration x as shown the fig. 8. Further, fig. 9 shows that transition temperature T c of YPBCO system decreases monotonically with increase in micro-strain ε. The variation of T c with micro-strain ε has not been reported elsewhere and, suggested that micro-strain ε is a good variable to define the suppression of T c along with magnetic pair-breaking and hole filling effects. A closer correlation can be found between micro-strain ε and transition temperature T c compared with the correlation between the disorder present in Cu-O planes and transition temperature T c .   From these results, it can be said that the disorder in Cu-O planes might be strain-induced. Fig. 10

Conclusions:
The effects of Pr-doping on the superconducting transition temperature T c and XRD patterns of YBCO samples have been investigated. The monotonic suppression of T c was observed with increase in Pr-doping. The broadening in XRD peaks was found to increase with increase in Pr-doping and, has been used to calculate the micro-strain ε. We have observed an increase of ε and a decrease of T c with increase in Pr-doping. Our experimental T c values were in good agreement with theoretical T c values calculated from equation (4) which includes the disorder term along with the magneticpair breaking and hole filling terms. Probably, the theoretically predicted disorder in Cu-O planes was strain-induced. The more experimental studies will be required to correlate the micro-strain and disorder effects in YPBCO system. The present work emphasizes on the strain-induced disorder effect, along with the other effects, to explain the suppression of T c in Pr-doped YBCO superconductor.