The phase formation and the variation of the normal phonon frequencies of high temperature superconductors YBa2Cu3O7−d and GdBa2Cu3O7−d upon doping Tb and Ce for Y and Gd, respectively, have been investigated using XRD and Raman spectroscopy measurements. It is found that the increase of doping content causes the formation of impurity phases that can be detected in the XRD and Raman spectra, and results in the suppression of superconductivity. Moreover, analysis of the Raman peaks reveals that substitutions of Tb and Ce for Y and Gd in the parent structure are restricted to low concentrations in favor of impurity island formation.
Since the discovery of high-T c superconductivity [1], there has been an explosion of experimental and theoretical studies on these materials. A large number of these studies are restricted to the ef- fects of substitution of rare earth (R) elements for Y that is mostly helpful in understanding the superconducting properties and their dependences on different parameters. Among various isomorphic substitutions possible in YBa 2 Cu 3 O 7Àd , the substitution of Pr [2– 4], Ce [5,6], and Tb [7–9], for Y have received the most attention. The reason for this interest is that these substitutions suppress the transition temperature of superconductivity drastically. The Y 1Àx Pr x Ba 2 Cu 3 O 7Àd system is particularly interesting since it is iso- structural with the YBa 2 Cu 3 O 7Àd superconductor, yet the supercon- ductivity is strongly suppressed as a function of Pr concentration [4].
Despite the large number of experimental and theoretical re- ports on doping YBCO with these elements, their destructive effect on superconductivity is still not completely understood. These studies include a number of Raman and infrared spectroscopic investigations that provide an improved understanding of the impurity phases [10], the pair breaking mechanism [11], and the superconducting mechanism itself by either measurement of the superconducting gap [12] or by identification of a strong electron–phonon interaction [13]. Such studies could be very informative because vibrational spectroscopy is an excellent technique for the investigation of low energy elementary excitations in these materials [14]. In order to find a world-class theory for supercon- ductivity, the effects of various parameters, such as doping and oxygen content, on the occurrence or elimination of this phenom- enon is examined. There are evidences showing that electron–pho- non interaction plays a major role in the occurrence of high-T c superconductivity [13,15–17]. See also Ref. [18] and the references therein. In this work, we have made an attempt to investigate the effects of Tb and Ce doping on the phonon spectra of the high tem- perature superconductors YBCO and GdBCO, respectively.
Polycrystalline samples of Y 1Àx Tb x Ba 2 Cu 3 O 7Àd with Tb content x = 0, 0.1, 0.3, 0.5, 0.6, and Gd 1Àx Ce x Ba 2 Cu 3 O 7Àd with Ce content x = 0, 0.1, 0.3, 0.5, 0.6 were prepared by the solid-state reaction method. For synthesizing the Y series, 99.9% pure Y 2 O 3 , Tb 4 O 7 , BaCO 3 , and CuO powders were mixed in atomic ratios of (Y:Tb):Ba:Cu of 1:2:3. Appropriate amounts of the 99.9% pure Gd 2 O 3 , CeO 2 , BaCO 3 , and CuO in the stoichiometric ratios were mixed for synthesizing the Gd series. Each mixture was ground and calcinated at 850 °C in air for 24 h. The calcination process was repeated at least twice to ensure sample homogeneity. Then, both of the compounds were pressed into pellets and sintered for 36 h at 950 °C and 930 °C for Y series and Gd series, respectively. Then, the samples were cooled to 450 °C in an oxygen atmosphere, retained there for 5 h, and finally cooled down to room tempera- ture at a cooling rate of 1 °C/min.
Routine X-ray diffraction measurements were carried out at room temperature using a Philips h/2h diffractometer with Cu K a radiation (k = 1.5406 Å) for Y 1Àx Tb x Ba 2 Cu 3 O 7Àd polycrystalline samples, and Co radiation (k = 1.7887 Å) for Gd 1Àx Ce x Ba 2 Cu 3 O 7Àd polycrystalline samples. The cell parameter refinements were per- formed using the Fullprof software [19].
Raman scattering studies were performed on fresh surfaces. Ra- man spectra were obtained using a LAB RAM HR800 spectrometer. For Raman excitation the 532 nm line from frequency doubled NdYAG laser was used. The laser was focused to a spot 2 lm in diameter. The Raman scattering measurements were carried out in the spectral region from 200 cm À1 and 700 cm À1.
The X-ray diffraction patterns of Y 1Àx Tb x Ba 2 Cu 3 O 7Àd samples with x = 0, 0.1, 0.3, 0.5, and 0.6 are illustrated in Fig. 1. All samples have the YBCO123 phase which is responsible for the superconduc- ting state. The (2 0 0) and (0 2 0) peaks near 2h $ 47° in the x = 0, 0.1, and 0.3 samples are characteristic of the orthorhombic phase. The peak intensities for the YBCO structure are affected by Tb dop- ing, where the intensities of the planes are larger for the pure sam- ple as compared with the doped samples. Upon doping, new secondary phases, like TbBaO 3 and BaCuO 2 , appear in the spec- trum. Larger amounts of secondary phases are accompanied by les- ser amounts of superconducting phase, which is clearly seen from the growth of the impurity peaks and the depression of the (1 0 3) peaks of the superconducting phase.
From the XRD pattern, it is inferred that the solubility of Tb at the Y site is very limited. Single phase compounds result only for x < 0.3, whereas the sample with higher Tb content (x > 0.3) shows an extra peak at 2h $ 29.5° corresponding to the TbBaO 3 secondary phase.
XRD and Raman analyses were performed to probe the physical properties of the high temperature superconducting YBa 2 Cu 3 O 7Àd and GdBa 2 Cu 3 O 7Àd compounds upon addition of Tb and Ce rare earth elements. The XRD analysis shows that in addition to the orthorhombic 123 phase, some nonsuperconducting peaks, which are mainly due to the BaRO 3 (R = Tb, Ce) and BaCuO 2 secondary phases, are also formed that suppress the superconducting transi- tion temperature. The Rietveld analysis of the XRD patterns reveals a variation in the structural bond formation. In the case of Tb sub- stitution, the c-axis of the YBCO unit cell compresses, while in the case of Ce substitution in GdBCO, the c-axis compression is drastic.
Raman spectroscopy confirms the XRD phase formations results and shows the dramatic drop in the intensity of the superconduc- ting phase, indicating that part of the 1:2:3 phase has decomposed into RBaO 3 (R = Tb, Ce) phases, which compete with the formation of the 1:2:3 structure, thus revealing that the incorporation of Ce and Tb into 1:2:3 is limited to low concentrations. The conductivity in the Y 1Àx Tb x Ba 2 Cu 3 O 7Àd pellets is carried along the percolating paths of the Tb poor 1:2:3 phase, embedded in the insolating perv- oskite TbBaO 3 . At relatively high Tb doping values, the secondary phase formation causes disconnecting of the superconducting grains, and the formation of the superconducting phase (123 phase) is destroyed, consequently the transition temperature for superconductivity disappears.
The Raman data depict that the intrinsic Raman mode near 500 cm À1 which is attributed to the O(4) stretching vibration, shows variations in the vibrational frequency as a function of dop- ing. The O(4) stretching mode softens nearly $13 cm À1 on going to 60% increase in the Tb doping content. In the case of Ce doping, the 507 cm À1 mode is seen to harden nearly $4 cm À1 on going to 10% increase in the Ce doping content.