Hg-1223 Phase Research Articles

Magnetic flux dynamics and structural features in fluorinatedHgBa2CuO4as probed by19FNMR

${}^{19}\mathrm{F}$ NMR spectra and spin-lattice relaxation rates have been measured on two powder ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4}{\mathrm{F}}_{x}$ samples. The results combined with our neutron powder-diffraction data [A. M. Abakumov et al., Phys. Rev. Lett. 80, 385 (1998)] confirm that fluorine atoms can be successfully incorporated in the lattice of superconducting Hg-1201 phase in the nonstoichiometric O3 site. Partial occupation of another oxygen position by fluorine atoms reveals to be possible with the increase of F content. The temperature dependence of the ${}^{19}\mathrm{F}$ NMR linewidth reveals the crossover at 35 K in the magnetic-flux line structure between the low-temperature latticelike regime and high-temperature ``melted'' regime. For the ${1/T}_{1}$ temperature dependence a maximum has been found at 65 K indicating the strong influence of magnetic-flux line motion on the ${}^{19}\mathrm{F}$ spin-lattice relaxation rate.

Phase equilibria in the Hg–Ba–Cu–O system

The already published data of temperatures and enthalpies of decomposition for mercury-based phases were combined with our experimental results from DSC and low-temperature calorimetry and the data for the Ba–Cu–O system taken from the literature. The consistent set of thermodynamic data of all phases in the Hg–Ba–Cu–O system was used for the calculation of the equilibrium phase composition under varying experimental conditions. The selected sections of the p O 2 – p Hg– T stability phase diagram were constructed for the ratio of non-volatile components Ba:Cu=2:1 and the temperatures of 500°C and 800°C. The decomposition temperatures of the Hg-1201 phase were calculated as a function of p O 2 for selected values of p Hg as a parameter.

Mössbauer Spectroscopy and Preparation of Hg-1223, Tl-1223 and Tl-1212 Superconductors

Introduction of the Mossbauer nuclei 57Fe into Hg-1223 phase and that of 57Fe and 151Eu into Tl-1212 and Tl-1223 superconductors were investigated. Samples of high phase purity were obtained. Scanning electron microscopy and optical microscopy in normal and polarized light were employed to study the microstructure of the specimens. Energy dispersive X-ray analysis showed that 57Fe in the Hg-based samples and 57Fe as well as 151Eu in the Tl-based compounds, entered superconducting phases. Incorporation of Eu3+ into the superconducting phase favored the formation of the Tl-1212 phase. Mossbauer spectroscopy showed that Eu3+ entered the Ca-site. Two doublets found in the 57Fe Mossbauer spectra in both the Hg-1223 and the Tl-1223 phase referred to two different micro-environments of Fe3+. The assignment of the 57Fe Mossbauer spectra was made under the assumption that Fe favored the 1223 phase. Fe3+ may replace Cu in both the square pyramidal, five-fold oxygen coordinated Cu sites between the Ca and Ba-(Sr)-O layers, and in the square planar, four-fold oxygen coordinated Cu sites the Ca layers in the superconducting phases in both the Hg- and the Tl-based materials. From the relative areas of the two doublets, we concluded that the Fe3+ preferred the square planar Cu site.

Improvement in Fabrication of Hg-1223 Superconductor by Te-Doping

ABSTRACTThe sealed quartz tube method is one of the usual methods used for sintering mercury-based copper oxide superconductors. By this method, precursor pellets are inserted into the quartz tube together with certain mass ratio to the reagent sample pellets for controlling the mercury vapour pressure. A certain amount of excess HgO is required to ensure the fabricated sample to have the desired composition. In our research, it was found that if a small amount of TeO2 is added and mixed with the pulverized precursor after first calcination, Hg-1223 superconductors can be fabricated without using the precursor pellet and no excess HgO is required. Thus, the fabrication process can be simplified. As mercury and its oxide are toxic, if no excess Hg is involved, it is safe and good for the laboratory-environment. In this research, some amounts of 0.10 and 0.18 wt% (weigh percentage) of TeO2 were added into the precursor, it is shown that the critical temperature of the fabricated samples is comparable to that of the undoped Hg-1223 samples. For 0.10 wt% of TeO2, the Tc of the as-synthesized sample is about 116.7 K whereas the annealed Te-doped Hg-1223 is about 134.2 K. For 0.18 wt% of TeO2, the Tc of the as- synthesized and annealed samples are 115.8 K and 134.6 K, respectively. The phase structure of samples fabricated in this research has been analyzed by the X-ray diffraction technique. It shows that the Hg-1223 phase is dominated.

Crystal Structure of (Hg, Ce, Cu)(Sr, Pr)2(Pr, Sr)Cu 2 O 6+δ and (Hg, $$\square $$ )(Sr, Pr) 2 (Pr, Sr) 2 Cu 3 O 8+δ Phases

This paper reports on the refined compositions and crystal structures of two phases from the homologous series (Hg, M)(Sr, Pr)2(Pr, Sr)k-1CukO2k+2+δ with k = 2 (Hg-1212) — (Hg0.44(2)Ce0.314+Cu0.25)(Sr0.90(3)Pr0.10)2(Pr0.52(3)Sr0.48)Cu2.00O7.00 (a = 3.8634(1), c = 12.2030(8) A, space group P4/mmm) with k = 3 (Hg-1223) — (Hg0.29(1)\(\square _{0.71} \))(Sr0.67(2)Pr0.33)2(Pr0.61(2)Sr0.39)2Cu3.00O9.32(8) or (Hg0.29(1)\(\square _{0.71} \))(Sr0.61(4)Pr0.39)2(Pr0.51(4)Sr0.49)2Cu3.00O9.00 (with a fixed content of superstoichiometric oxygen O(4) in the phase) (\(\square \) is a vacancy; a = 3.8294(9), b = 3.8567(6), c = 15.2763(44) A, space group Pmmm). The implausibly high content of oxygen in the Hg-1223 phase (refinement I) is attributed either to O–O bond formation or to the possible presence of a minor amount of copper in a defect position of Hg; the inclusion of the latter in structure refinement leads to a better reproduction of the real structure but increases the R indices. The crystal-chemical analysis of the title phases and a comparison with the available data for analogous phases indicates that the composition of the crystallographic positions is related to structural features, in particular, to the coordinates of (Hg, M) and superstoichiometric oxygen, whose content depends on the degree of substitution of strontium ions by praseodymium ions. Reasons for the orthorhombic distortion of the Hg-1223 phase are discussed. The absence of superconductivity is explained by the nonoptimal formal charge (FC) of copper, which depends on the oxygen content in the phase.

Synthesis of Hg-1223 superconductors using a cation-exchange process

The newly developed cation-exchange process was applied to convert Tl 2Ba 2Ca 2Cu 3O 10 (Tl-2223) superconductors to HgBa 2Ca 2Cu 3O 8+ δ (Hg-1223) superconductors via Tl–Hg cation-exchange in Hg-vapor at processing temperatures ranging from 700 to 800°C for 6–24 h. Oxygen annealing process at 350°C for 10 h was followed to optimize the content of oxygen in the Hg-1223 samples. The Tl-2223 phase has been successfully converted to Hg-1223 phase with a negligible amount of non-superconducting impurity phase. T c values up to 135 K have been obtained in Hg-1223, which are ∼15 K higher than that of the original Tl-2223 compounds. Since the cation-exchange process is a kinetic process, it is very promising for fabrication of high-quality Hg-1223 superconductor at large scales.

Synthesis and stability of HgRe1212 and HgRe1223 superconductors

Synthesis, phase stability, and superconducting properties of Re-doped Hg1212 and 1223 superconductors are reported. HgRe1212 and HgRe1223 were synthesized using a common synthesis protocol, using commercial BaCaCuO precursor powders with CaHgO/sub 2/ as the external Hg-source. Both starting composition and reaction temperature affect the phase stability and microstructure of the final product. The resulting superconducting phase in both Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/ and Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ compositions was either HgRe1212 or HgRe1223, depending on the reaction temperature. The microstructure and superconducting properties of HgRe1212 and HgRe1223 samples, however, depended to a large extent on the starting composition. Both the HgRe1212 and HgRe1223 samples synthesized from a nominal composition of Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub y/ had larger and aligned colonies of grains whereas the samples synthesized a nominal composition Hg/sub 0.8/Re/sub 0.2/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ had smaller and randomly oriented grains. Measured T/sub c/ values for HgRe1212 and Hg1223 phases were 124 K and 134 K, respectively. Both microstructure and magnetization hysteresis measurements suggest that HgRe1212 is a better candidate for applications.

The magnetic field and temperature dependent transport critical current density of Hg-Pb-Bi-Ca-Cu-O ceramic compounds

Mercury-based ceramic superconductors have been prepared by powder metallurgy, utilizing a technique of direct synthesis from the oxide powders. The tetragonal Hg-1223 phase was prepared and stabilized over a range of Pb and BiPb compositions. The most interesting material had an onset superconducting transition temperature of approximately 133 K. Critical current densities were obtained directly from transport measurements and indirectly from magnetization measurements, The transport critical current densities were orders of magnitude smaller than the intra-grain critical current densities. The data suggest that the transport critical current is limited by weak coupling between the grains, and that the intra-grain critical current densities deteriorate rapidly above 20 K.

Effect of sodium doping on the oxygen distribution of Hg-1223 superconductors

Low-temperature oxygen annealing was performed on Na-doped and undoped Hg-1223 phases. It is argued that Na could perturb the electronic band structure of the material by changing the distribution of oxygen in the Hg–O layer, which then could affect the superconductivity of the material during the annealing process. It has been found that T c can be increased by more than 20 K using an annealing temperature of 300°C for 10 h, while annealing above 350°C leads to loss of superconductivity due to Hg loss. Normal state- ρ tends to decrease with increasing annealing temperatures up to 400°C. In addition, flattening in the normal state- ρ curves of doped samples indicated an increasingly insulating behavior.

Preparation of Hg 0.8Cr 0.2Ba 2CuO 4+ δ under controlled oxygen and mercury partial pressures

Ba 2CuO 3/HgBa 2CuO 4+ δ and Mn 3O 4/Mn 2O 3 couples were employed to control the mercury and oxygen partial pressures, respectively, during the synthesis of chromium doped Hg-1201 phase in the sealed quartz tube. Suitably adjusted conditions allowed us to prepare series of single phase samples of the final composition Hg 0.7Cr 0.2Ba 2CuO 4+ δ determined by weight changes and EMA. The oxygen stoichiometry was modified by the oxygen partial pressure in the range of pO 2=0.05–1.59 atm. The X-ray diffraction and magnetic susceptibility study revealed linear dependence of the structural parameters and parabolic one of T c vs. log pO 2. Multiphase samples, however, form for too high oxygen activity, namely for pO 2=5.16 atm.

Lithium-doping-assisted growth of HgBa 2Ca 2Cu 3O 8+ δ superconducting phase in bulks and thin films

The superconducting HgBa 2Ca 2Cu 3O 8+ δ (Hg-1223) phase has been successfully formed at temperatures as low as 720°C, by the addition of Li at a level of 0.1 to 0.4 per unit cell, thus effectively lowering the processing temperature by more than 100°C. Data indicate that Li most likely forms a flux which catalyzes the reaction of constituents and the formation of superconducting domains at low temperatures. There are indications that the Li may also help improve the onset of superconductivity by locally perturbing the electronic structure as an interstitial. Applying the process to thin-film fabrication has successfully produced c-oriented film with 73% Hg-1223 phase purity at a processing temperature of 800°C and 0.2 Li doping level.

HOMOLOGOUS SERIES OF MERCURY-BASED CUPRATES UNDER DIFFERENT INFLUENCES ON THEIR SUPERCONDUCTIVITY

The superconductivity properties of a homologous series of mercury-based cuprates HgBa 2Ca n−1 Cu n O 2 n+2+ δ with n=1–8 are considered. Experiments under pressure for species with n=1–5 were performed The dependences of both T c max and d T c max/d p versus n values are the cupola-shaped ones up to n=4–5. Experimental data are compared to predictions of the P.W. Anderson model. The T c max( n) values agree for n=3, 4 and disagree for n=5, 6, nevertheless giving a common tendency to become unchanged at large n. The pressure derivative (d T c max/d p) values coincide practically for all n values studied. The results obtained lead to the assumption that the mercury-based cuprates are the `ideal' structures for high T c superconductivity. The T c achieved for the Hg-1223 phase (135 K) presents the highest result for superconductivity at ambient pressure.

Effect of overdoping on the irreversibility field and critical current density of the HgBa 2Ca 2Cu 3O 8+ δ superconductor

For HgBa 2Ca 2Cu 3O 8+ δ which is of the pure Hg-1223 phase without substitutions in the cation sites, it is demonstrated that the larger the amount of oxygen excess ( δ), the better the magnetic irreversibility field ( H irr) vs. normalized temperature (1− T/ T c) characteristics. The best H irr vs. (1− T/ T c) characteristics in the present experiment was obtained for a strongly overdoped sample with T c=107 K which was synthesized at 950°C under a high pressure of 5 GPa using highly-oxidized BaCuO 2+ x ( x=0.13) as one of the starting materials. The relationship between the hole-doping level and the H irr vs. (1− T/ T c) characteristics is discussed.

Superconducting transition temperature in mercury HTSC-cuprates under hydrostatic pressure

A study is made of the properties of the homologous series of mercury HTSC-cuprates HgBa2Can−1CunO2n+2+δ with n=1–8. Experiments are conducted under pressure for samples with n=1–5. The Hg-1223 and Hg-1234 phases were synthesized using a controlled high pressure chamber. The oxygen content of an initial mixture corresponding to the Hg-1234 phase was varied by changing the composition of the initial BaO/BaO2 oxides. The dependence of the superconducting transition temperature Tc on the lattice constant a (and, therefore, on the oxygen content) and of Tcmax and dTcmax/dp on n are convex upward up to n=4, 5. The maximum values always correspond to the Hg-1223 phase. Experimental Tcmax(n) curves for the phases with n=1–6 and dTcmax/dp curves for n=1–5 are compared with Anderson’s theory (the so-called RVB model). A general analysis of these results indicates that the mercury cuprates have an ideal structure for HTSC. The Hg-1223 phase is the “champion” in this ideal structure and the critical temperature corresponding to this phase (Tc=135 K) is the highest at atmospheric pressure.

Chemical thermodynamics of Hg1201 and Hg1223 phases

The decomposition equilibrium of Hg1201 and Hg1223 was studied using a thermomicrobalance. A thermodynamic diagram representing the stability domains of Hg1201 and Hg1223 was constructed. Using the diagram, it was shown that Hg1201 is metastable at 800°C and that Hg1223 is stable at 660°C due to the formation of an atmosphere of mercury vapor and oxygen formed by the decomposition of excess HgO. Chemical analysis revealed that the valence of Hg is +2 regardless of the oxygen content. The dependence of T c on the oxygen content was also studied.

Effect of Fluorination on the Structure and Superconducting Properties of the Hg-1201 Phase

A fluorination of the reduced $\mathrm{HgBa}{}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}(\mathrm{Hg}\ensuremath{-}1201)$ with ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}61\mathrm{K}$ resulted first in an increase in ${T}_{c}$ up to 97 K and then in a decrease and even a suppression of superconductivity due to overdoping. Neutron powder refinements performed on fluorinated $\mathrm{HgBa}{}_{2}{\mathrm{CuO}}_{4}\mathrm{F}{}_{\ensuremath{\delta}}$ showed twice the amount of extra fluorine in comparison with those for the oxygenated $\mathrm{Hg}\ensuremath{-}1201$ phases with close ${T}_{c}$. This supports the ionic model of doping in $\mathrm{Hg}\ensuremath{-}1201$: two holes per oxygen and one hole per fluorine. The apical distances in $\mathrm{Hg}\ensuremath{-}1201$ depend on the number of dopant atoms rather than on their charge. The exchange of extra oxygen for a double amount of fluorine shortens the apical $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ distances, while the in-plane distances and ${T}_{c}$ do not vary.

High Pressure Synthesis and Properties of Hg-1234 and Hg-1223 Phases.

The Hg-1223 and Hg-1234 phases were synthesized using the program-controlled high pressure chamber. The variation of the oxygen content in the starting oxides mixture “HgBa2Ca3Cu4Ox”, performed by changing BaO/BaO2 and Cu2O/CuO ratios influences significantly the phase formation. Dependence of Tc vs. a-parameter for HgBa2Ca2Cu3O8+δ superconductor has a cupola-shaped character, as it was found for the lower members of the homologous series.

Effects of Sn doping in Hg 1− xSn xBa 2Ca 2Cu 2O 8+δ superconductors

Tin-doped samples of Hg 1− x Sn x Ba 2Ca 2Cu 3O 8+ δ (hereafter symbolized as (Hg 1− x Sn x )-1223) with x = 0, 0.05, 0.1, 0.2, 0.4 and 0.5 have been synthesized in sealed silica tubes by the solid state reaction method. The precursor can be prepared in air. The effects of Sn doping on the formation and stability, microstructure of grain growth and critical temperature of the (Hg 1− x Sn x )-1223 superconductor have been investigated by X-ray diffraction, scanning electron microscopy (SEM) and electron probe microanalysis (EDS), thermogravity analysis, resistance measurements and ac susceptibility measurement techniques. Experimental results show that a small amount of Sn doped in Hg-1223 promotes the formation of the triple CuO 2-layer superconducting compound. The stability of the product compounds is also enhanced. Sn doping may play a role as a nucleating centre. The T c for Sn-doped samples (Hg 1− x Sn x )-1223 with x ≤ 0.1 does not notably change as compared with the pure Hg-1223 phase (about 134 K). Sn doping is favourable for synthesizing the Hg-1223 superconductor. However, further increasing of Sn doping into the sample leads to a deterioration on both T c and the fraction of superconducting phase.

New phase of Hg-2435 in the Hg [sbnd]Ba [sbnd]Ca [sbnd]Cu [sbnd]O oxide system

The variety of intergrowth in the Hg Ba Ca Cu O oxide system was found by the high resolution electron microscope(HREM). A new phase of Hg-2435 was synthesized under high pressure for long time. Another longer period structure of -2223- was also found in such prepared sample. However, these complex intergrowth structures only contain the Hg-1212 and Hg-1223 phases.

Intergrowth structures and screw dislocations in Pb-doped Hg-1234 superconductor

The Hg 1−xPb xBa 2Ca 3Cu 4O y superconductor has been studied by transmission electron microscopy (TEM). It is found that intergrowth structures with different combinations of Hg-1212, Hg-1223, Hg-1234, and Hg-1245 phases have been observed clearly in our Pb-doped sample. Moreover, the screw dislocations have been observed in our sample and these typical dislocations are observed on the (100) plane of the Hg-1234 unit cell. The glide plane of the screw dislocation seems to be most dense crystal plane which is (110). The angle between the Burger's vector and the [001] crystal direction of the Hg-1234 unit cell is ∼ 8°, and the magnitude of the Burger's vector is ∼ 19.51Å. Some other microdefects such as edge dislocations and mixed dislocations are also found in our TEM image.