Pb-doped Crystals Research Articles

Optical and X-ray absorption spectroscopy in lead doped lithium fluoride crystals

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb LIII and LI edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 °C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

Metallicity and positive magnetoresistance induced by Pb substitution in a misfit cobaltatecrystal

We have synthesized single crystals of the misfit-layered cobalt oxide,[Bi1.5Pb0.5Sr2O4−δ][CoO2]1.86, with quadruple NaCl-type layers, using a flux method, and measured their transport properties.From structural refinements, it is found that the modulation in the BiO layer observed in[Bi1.74Sr2O4−δ]RS[CoO2]1.82 issuppressed by Pb substitution. The in-plane resistivity, thermopower, and Hall coefficient are 4.3 mΩ cm, 101 µV K−1,and 1 × 10−2 cm3 C−1 at 300 K, respectively; these are consistent with those of themisfit-layered cobalt oxides. All of these values are smaller than those of[Bi1.74Sr2O4][CoO2]1.82,indicating that the carrier concentration is larger than that of the undoped crystal. Moreover, the lowT upturnof resistivity observed for most of the cobalt misfit oxides is replaced by a metallic behavior, followinga T2 dependence, indicating strong correlations in the Pb-doped crystal. Also, themagnetoresistance, usually negative for misfit oxides, is replaced by a small positivemagnetoresistance. Doping with Pb is thus an efficient way to suppress the lowT localized behavior usually observed for misfits.

Growth and Superconducting Properties of Pb-Free and Pb-Doped Bi-2223 Crystals

High quality single crystals of both Pb-free and Pb-doped Bi-2223 (with sizes up to 3/spl times/2/spl times/0.1mm/sup 3/) were grown by means of a newly developed "Vapor-Assisted Travelling Solvent Floating Zone" technique in an image furnace. This technique allowed us to grow large Pb-doped Bi-2223 crystals for the first time. Post-annealing in high pressure oxygen (up to 10 MPa) enhanced T/sub c/ (up to 111 K) and sharpened the superconducting transition. The crystal structures of both Bi-2223 and Bi,Pb-2223 were refined and are reported in this paper. The unit cell of the Pb-free average structure is pseudo-tetragonal with a=5.4210(7), b=5.4133(6) and c=37.009(7) /spl Aring/. An incommensurate structural modulation along the direction of one of the short cell vectors has been defined (q/spl sim/0.21 a/sup */); however, the structure can be conveniently described in a supercell with a 5-fold volume (a=27.105(4) /spl Aring/). One additional O atom per translation unit of the modulation wavelength was found to be inserted into the BiO layers. Approximately 8% of the Ca sites are occupied by Bi atoms, and the refined composition is found to be Bi:Sr:Ca:Cu:O=2.16:2:1.84:3:10.16. Similar features are found in the Pb-doped phase. The magnetization of Pb-free Bi-2223 crystals has been studied. The vortex phase diagram has been traced and the anisotropy of Bi-2223 is found to be /spl gamma/=53 from measurements of the lower critical field, which is much smaller than that of Bi-2212 (/spl gamma/=165). Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, both of which are signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.

Growth and characterization of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10−δ single crystals

Large and high-quality single crystals of both Pb-free and Pb-doped high-temperature superconducting compounds (Bi1−xPbx)2Sr2Ca2Cu3O10−y (x = 0 and 0.3) were grown by means of a newly developed ‘vapour-assisted travelling solvent floating zone’ technique (VA-TSFZ). This modified zone-melting technique was performed in an image furnace and allowed for the first time the growth of large Pb-doped crystals, by compensating for the Pb losses that occur at high temperature. Crystals up to 3 × 2 × 0.1 mm3 were successfully grown. Post-annealing under high pressure of O2 (up to 10 MPa at T = 500 °C) was applied to enhance Tc and improve the homogeneity of the crystals. Structural characterization was performed by single-crystal x-ray diffraction (XRD). Structure refinement confirmed a commensurate modulated superlattice in the Pb-free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 21.829(4) Å, b = 5.4222(9) Å and c = 37.19(1) Å. Superconducting studies were carried out by ac and dc magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔTc ⩽ 1 K). In optimally doped Pb-free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2 × 105 A cm−2 were measured at T = 30 K and μ0H = 0 T. A weak second peak in the magnetization loops was observed in the temperature range 40–50 K, above which the critical current density was found to rapidly decrease as a function of T and H. A comparison between the pinning properties in Pb-free and Pb-doped crystals is reported and discussed.

Investigation of (Bi,Pb)2212 crystals: observation of modulation-free phase

Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi2−xPbxSr2CaCu2O8+δ crystals observed by transmission electron microscopy. Crystals with a nominal lead content of x=0.8, corresponding to an effective lead content of x=0.39, yield the non-modulated phase. The superconducting properties of this modulation-free phase (β phase) have been studied and compared to those of undoped crystals displaying the modulated phase (α phase). Magnetisation measurements reveal that the irreversibility field Hirr(T) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, Hc1, show that the anisotropy factor, ϵ, is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO2 layers which in turn deeply influences the effectiveness of the pinning. These measurements explain the enhanced pinning properties in moderately Pb-doped crystals in which the α phase and β phase coexist. The enhanced pinning is not only due to the α/β interfaces, which act as effective pinning centers: the emergence of modulation-free domains, characterized by a strongly reduced anisotropy, also significantly contribute to this effect.

Modulation-free phase in heavily Pb-doped (Bi,Pb)2212 crystals

Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi 2− x Pb x Sr 2 CaCu 2 O 8+ δ crystals observed by transmission electron microscopy. Crystals with a nominal lead content of x =0.8, corresponding to an effective lead content of x =0.39, yield the non-modulated phase. The superconducting properties of this modulation-free phase (β phase) have been studied and compared to those of undoped crystals displaying the modulated phase (α phase). Magnetisation measurements reveal that the irreversibility field H irr ( T ) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, H c1 , show that the anisotropy factor, ϵ , is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO 2 layers which in turn deeply influences the effectiveness of the pinning. These measurements explain the enhanced pinning properties in moderately Pb-doped crystals in which the α phase and β phase coexist. The enhanced pinning is not only due to the α/β interfaces, which act as effective pinning centers: the emergence of modulation-free domains, characterized by a strongly reduced anisotropy, also significantly contribute to this effect.

Magnetic hysteresis and relaxation in Bi2212 single crystals doped with Fe and Pb

Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviors for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Unlike pure and Fe doped Bi2212 crystals, heavily Pb doped crystal showed strong vortex pinning behavior. We interpret the strong pinning in heavily Pb doped Bi2212 single crystals as arising from the improved Josephson coupling in Bi2212 single crystal after heavy Pb doping. In heavily Pb doped single Bi2212 crystals, H/sub dis/(T) was observed to decrease with increasing T. Here, H/sub dis/(T) is an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. However, in pure and Fe doped Bi2212 single crystals, H/sub dis/(T) was observed to be temperature independent. We also report a significant shift of T/sub CR/, a crossover temperature separating two pinning regimes, toward higher temperatures with heavy Pb doping of Bi2212 single crystals. On the other hand T/sub CR/ did not shift with Fe doping of Bi2212 single crystals. It is argued that the temperature dependence of H/sub dis/(T) and the shift of T/sub CR/ in heavily Pb doped Bi2212 crystals was related to the enhanced c-axis conductivity caused by the Pb situated between the CuO/sub 2/ layers and imposing a 3D characteristic on the vortex lattice.

Study of the peak effect in pure, Pb and Pb+Y doped Bi-2212 single crystals

The peak effect (PE) in pure, Pb and Pb+Y doped Bi2212 single crystals with different oxygen doping levels was studied by measuring M– H loops over a wide temperature range. The PE in pure Bi2212 crystals was obtained only for crystals with optimal oxygen doping and over-doping but not observed for oxygen under-doped crystals. For Pb doped Bi2212 crystals, the PE appeared at a higher field than in the pure crystals and persisted up to T c. For (Bi 1.64Pb 0.36)Sr 2Ca 1− x Y x Cu 2O 8+ y ( x=0, 0.05, 0.11, 0.33) single crystals, results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases ( x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than for heavily Pb doped Bi2212 crystals, indicative of strong pinning due to the co-doping. The formation of Bi 5+ rich clusters, which cause the reduction of the c-axis lattice parameter and ρ c , is proposed to be responsible for the appearance of the PE in the undoped crystals. The co-existence of Pb 4+- and Bi 5+-rich clusters causes the strong PE in Pb doped Bi2212 crystals. Y 3+ is proposed to be an effective dopant at low doping levels for flux pinning at high temperatures. The PE for all the crystals was characterised by plotting ( H max− H min)/ H max vs T/ T c, where H min and H max represent the fields at which the magnetisation starts to increase ( H min) and reaches a maximum ( H max) at the peak position. Results showed that the evolution of the peak effect with temperature in the Pb and Pb+Y doped crystals was similar to that seen in Y123.

Enhancement of vortex pinning by Josephson coupling of two-dimensionalpancake vortices in heavy lead-dopedBi2-xPbxSr2CaCu2Oy

Magnetic hysteresis loops and magnetic relaxation have been measuredfor pure and heavily lead-doped Bi2-xPbxSr2CaCu2Oy (Bi2212)single crystals. From the hysteresis loop measurements, the normalizedcritical current density Jc/Jc0 has been calculated, and from therelaxation measurements the activation energy, current-voltage curves andnormalized relaxation rate have been obtained. The field and temperaturedependences of Jc/Jc0 for Pb-doped crystals were much weaker than forthe pure Bi2212 crystals. A large upward shift in the crossover temperatureTCR, a temperature separating two different pinning regimes, has beenobserved for the first time with heavy lead doping. This was interpreted byimproved inter-layer Josephson coupling of two-dimensional vortices, as aresult of Pb doping. Heavily Pb-doped crystal had TCR = 35 K for the fieldthat was within the secondary peak in its magnetic hysteresis loop. For pureBi2212, TCR = 19 K.

Superconductivity and flux pinning in Y and heavily Pb codoped Bi-2212 single crystals

Studies of superconductivity and flux pinning were carried out on (Bi1.64Pb0.36)Sr2Ca1−xYxCu2O8+y (x=0, 0.05, 0.11, 0.33) single crystals grown by the self-flux method. X-ray diffraction, transport, and magnetic measurements were performed for purposes of characterization. X-ray analysis revealed that the c lattice parameter systemically decreases as the Y doping level increases. The superconducting transition temperature Tc decreases from 80 to 30 K as x increases. A strong annealing effect on Tc and superconducting volume has been observed. Resistance measurements show that x=0.33 samples are semiconductive over a wide temperature range between 4.2 and 300 K for the as-grown state, but become metallic with Tc of 65–70 K after air or oxygen annealing. Flux pinning was studied by measuring the hysteresis loop at different temperatures and different fields. A peak effect was observed in all the codoped samples. Results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases (x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than in heavily Pb doped Bi2212 crystals, indicative of a strong pinning due to the codoping.

Origin and characterisation of peak effect in pure and Pb doped Bi-2212 single crystals

The peak effect in pure and heavily Pb doped Bi2212 single crystals with different oxygen doping levels were studied by measuring M-H loops. The peak effect in pure Bi2212 crystals was obtained only for crystals with optimum oxygen doping and overdoping but not for oxygen underdoping. For Pb-doped Bi2212 crystals, a strong peak effect appeared in higher field than in pure crystals and it persisted up to T c. Formation and decomposition of Bi 5+ and/or Pb 4+ rich clusters, which caused the reduction of c-axis and ϱ c, were proposed to be responsible for the appearance and disappearance of the peak effect in pure and Pb doped crystals. The peak effect was characterised by plotting (H max-H min)/H max vs T/T c, where H max and H min represents the field in which the magnetisation reaches maximum and starts to increase around secondary peak position, respectively. Results showed that the evolution of the peak effect with temperature in the Pb doped crystals was similar to that in Y123.

Large enhancement of peak effect induced by heavy Pb doping in Bi 2Sr 2CaCu 2O 8+ δ single crystals

The peak effect in heavily Pb-doped Bi 2Sr 2CaCu 2O 8+ δ single crystals was studied by measuring the magnetisation hysteresis ( M– H) loop. In contrast to the weak peak effect in pure single crystals, the Pb-doped crystals showed a very strong peak effect in a very wide temperature range, from 20 K up to T c. The peak field, H peak, was as high as 0.4 T. The effect of annealing on the peak effect was also studied. Oxygen overdoping enhances the peak effect more than the underdoping. A comparison study of the peak effect was done for pure, Pb-doped Bi2212 crystals and Y123 samples. A novel method was used to characterise the peak effect. Results showed that the peak effect in the Pb-doped Bi2212 is even stronger than in Y123.

Resistivity Anisotropy and Hall Coefficient of Pb-Doped Bi2212 Single Crystals

In-plane and out-of-plane resistivity and Hall coefficient measurements were carried out on Bi 2.1-x Pb x Sr 2 CaCu 2 O y (x = 0 ∼ 0.6) single crystals with various oxygen concentration. The resistivity anisotropy $$\gamma ^2 ( = \rho _c /(\rho _a {\text{ }}\rho _b )^{1/2} )$$ was decreased with an increase of oxygen concentration and Pb content. Hall coefficient, R H , measurement revealed that the Pb-doping increases carrier concentration in the present system, however, its essential effect on reducing anisotropy was confirmed by comparing R H and γ 2 of the Pb-free and Pb-doped crystals.

Optical and thermally stimulated emission studies on γ-irradiated SrF2: Pb single crystals

Optical absorption (OA), fluorescence, and thermally stimulated emission (TL) studies are made on a Pb-doped (0.3 wt%) SrF2 crystal. Unirradiated crystals show a prominent OA band around 6.05 eV. This band increases after γ-irradiation and also a broad band around 4.77 eV appears which can be attributed to Pb3+ ions. Fluorescence emission is observed at 5.35 eV when the crystal is excited in the Pb2+ absorption region which is around 6.05 eV. Thermoluminescence glow shows a very intense broad peak at 653 K. TL emission shows only the characteristic Pb2+ emission at 5.35 eV. Also the OA of heavily irradiated crystals shows the presence of Pb centres. The probable TL mechanism is that the trapped electron from a Pb centre when thermally released can recombine with Pb3+ ions to give the characteristic Pb2+ emission. All these results are discussed.

Electron microscopy study of Pb-substitution in bulk Bi–Sr–Ca–Cu–O superconductors

We present the results of an electron microscopy study of Pb-substitution in bulk Bi–Sr–Ca–Cu–O superconductors. We observe grains with c-axis lattice parameters of 30.8 Å (2212 phase), 36.7 Å (2223 phase), and 43.2 Å (2234 phase) containing no intergrowth defects. The existence of two separate incommensurate superstructures, Q1 and Q2, which were previously observed in Pb-doped ceramics and single crystals, has been confirmed in this study. It is found that superlattice reflections from the Q2,-modulated structure in the 2223 phase are much weaker than those from the Q2-modulation, whereas the diffraction intensities of the Q1- and Q2-modulations in the 2212 phase are similar. We also show that the periodicity along the b-axis of the Q2-modulation increases with increasing Pb-substitution, contrary to the implications of a previously suggested structural model.

First stage f centre formation in pure and doped KCl crystals by room temperaure X-irradiation

The electrical conductivity and optical absorption of potassium chloride crystals, pure and doped with divalent cation impurities, have been measured before and after X-irradiation at room temperature. The concentration of free positive ion vacancies at room temperature has been calculated from conductivity for each crystal before irradiation and is found to be much less than the first stage F centre concentration. This shows that both free and associated positive ion vacancies are the latent source of F centres in the first stage colouration. Pb+ + ions trap electrons producing Pb+ + and Pb0 centres and making free the associated cation vacancies. Such centres are not produced in Ca-doped crystals where impurity-vacancy complexes trap F centres producing Z centres. The higher ratio of F centres to positive ion vacancies in Pb-doped crystals indicates that free cation vacancies are more effective in producing F centres. However, the concentration of divalent cations is found to decrease in both the crystals after irradiation, the decrease being more in Ca-doped crystals.

Optical Absorption in TlCl: Pb Crystal Induced with Reactor and γ-Ray Irradiation at Low Temperatures

TlCl crystals doped with Pb++ ions were irradiated in a reactor at low temperature and their absorption spectra were measured from 4.2 K to room temperature. Two prominent absorption bands were observed at 429 and 478 nm at 4.2 K which shifted to 426 and 465 nm at 77 K. They were quite similar to the bands which were observed in the undoped TlCl crystals irradiated in the reactor at low temperature. In the Pb–doped crystals more absorption bands were observed at 393 and 750 nm on warming to 77 K. The absorption of the undoped and Pb-doped crystals irradiated by Co60–γ rays at 4.2 or 77 K were also measured and were compared with the results of the reactor-irradiated crystals. Only in the Pb–doped crystals γ-irradiated at 77 K, the 426 and 393 nm bands were observed with the intense 750 nm band. An interpretation of the effects of Pb++ ions on the creation of the defects in the crystal by irradiation is presented.