K.A. Lokshin

High pressure synthesis of Hg-1234 and strongly-overdoped Hg-1223 phases

Abstract 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 ‘HgBa 2 Ca 3 Cu 4 O x ’, obtained by changing BaO/BaO 2 and Cu 2 O/CuO ratios, influences significantly the phase formation. By varying the pressure ( P s ), temperature ( T s ), time ( t s ) and oxygen content ( x ), the optimum synthesis conditions of the Hg-1234 phase were determined with P s = 2 GPa, T s = 900°C, t s = 2 h and x = 10.2. Using the initial mixture with the high oxygen content ( x = 10.6) only Hg-1223 with the lattice constants a = 3.8474(9) A , c = 15.79(1) A was obtained. Hg-1223 thus prepared is strongly overdoped and exhibits superconductivity at 118 K, while the treatment in oxygen flow enhanced T c up to 134 K and cell parameters to a = 3.852(1) A , c = 15.84(1) A Dependence of T c vs. a -parameter (and consequently, oxygen content δ) for the HgBa 2 Ca 2 Cu 3 O 8+δ superconductor has a cupola-shaped character, as found previously for the lower members of the homologous series.

High-pressure effects in fluorinated HgBa2Ca2Cu3O8 + δ

We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O8 + δ (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K±1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic a-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the a-axis is one of the keys that controls the Tc of high-temperature superconductors.

Synthesis and characterization of overdoped Hg-1234 and Hg-1245 phases; the universal behavior of Tc variation in the HgBa2Can−1CunO2n+2+δ series

Abstract HgBa 2 Ca 3 Cu 4 O 10+ δ and HgBa 2 Ca 4 Cu 5 O 12+ δ were prepared by high pressure/high temperature technique. The phase composition in the Hg–Ba–Ca–Cu–O system was shown to be very sensitive to synthesis conditions. A variation of the oxygen content in the starting oxide mixture by changing the BaO/BaO 2 ratio allowed the acquisition of Hg-1234 and Hg-1245 in the overdoped state. The heat treatments (nitrogen and oxygen flows, oxygen pressure 100 bar and 2000 bar) resulted in a change of T c s from initial overdoped (116 and 100 K for Hg-1234 and Hg-1245, respectively) to underdoped (95 and 85 K) and, finally, to optimally-doped states (125 and 111 K). The dependencies of T c vs. a -parameter (and, consequently, oxygen content) for these phases exhibit a cupola-shaped behavior, as it was previously found for the lower ( n ≤3) members of the HgBa 2 Ca n −1 Cu n O 2 n +2+ δ series.

Effect of fluorination and high pressure on the structure and properties of the Hg-bearing superconducting Cu mixed oxides

Abstract The T c variation of HgBa 2 CuO 4+ δ (Hg-1201) and HgBa 2 CuO 4 F δ can be achieved by a change in the carrier concentration and by a compression of the structure under high pressure. Oxygenated and fluorinated series exhibit a cupola-shaped behavior for the T c vs. δ dependence, but the curves are shifted away from each other along the δ axis. NPD showed double amount of extra fluorine in comparison with extra oxygen for the oxygenated Hg-1201 phases with close T c s. An exchange of the extra oxygen by a double amount of fluorine causes a significant compression of the apical Cuue5f8O bond distances, while the in-plane ones, as well as T c , do not vary. Fluorination of Hg-1223 resulted in a slight increase in T c in comparison with oxygenated material. The influence of the external pressure on the structure and T c of Hg-1201 strongly depends on the doping level. An increase in the extra oxygen content from underdoped to overdoped state results in the larger compression of the apical Cuue5f8O and Baue5f8O Hg distances while the HgO 2 dumbbell as well as a distance between Ba and O from the (CuO 2 ) layers become practically pressure independent.

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

Abstract A variety of Hg-1223 high- T c superconducting systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples.

Effect of fluorination on the structure and properties of Hg-based superconducting Cu mixed oxides

Abstract HgBa 2 CuO 4+δ (Hg-1201) and HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) were successfully fluorinated by XeF 2 . Fluorination of the reduced Hg-1201 phase causes the increase of T c up to 97 K which is close to that for the optimally doped oxygenated sample. Neutron powder refinements performed on HgBa 2 CuO 4 F δ samples showed that the exchange of extra oxygen for a double amount of fluorine results in shortening of the apical Cuue5f8O bonds while in-plane ones (as well as T c ) do not vary. On the contrary fluorination of Hg-1223 resulted in T c enhancement up to 138 K while oxygenated sample exhibited lower T c = 134 K. From X-ray and neutron powder diffraction study we suppose that the structural factor responsible for the enhancement of T c in fluorinated Hg-1223 is shortening of the in-plane Cuue5f8O distances with (CuO 2 ) layers remaining practically flat.

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.

Synthesis and investigation of (Hg1-xCux)Ba2Ca2Cu3O8+δ

Abstract A reproducible synthesis method for superconducting HgBa 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) has been developed, using sealed silica tubes and a CoO/Co 3 O 4 mixture as oxygen getter to achieve a controlled partial pressure of oxygen. Samples containing more than 95% of Hg-1223 were prepared with this technique. A significant range of substitution of Cu for Hg was revealed by EDS analysis in samples synthesized at 900°C. After post-anneal in oxygen the lattice parameters were calculated from XRD data and T c was determined to be 135 K, by susceptibility methods. We suggest that a high synthesis temperature and a low partial pressure of mercury promote substitution in the Hg-1223 phase.

High-pressure effects in fluorinated

We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O8 + δ (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K±1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic a-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the a-axis is one of the keys that controls the Tc of high-temperature superconductors.

Zeeman splitting of the 63,65Cu NQR line in optimally doped Hg1Ba2Ca2Cu3O8+δ below Tc

Abstract We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer compound Hg 1 Ba 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) with T c =134 K. Above T c two Cu line pairs are observed in the spectrum corresponding to the two inequivalent Cu lattice sites. Below T c the Cu NQR spectrum shows a splitting of these two line pairs into six line pairs. From a multiexponential spin lattice relaxation with a dependency on the initial conditions, we find a formation of magnetic moments in the CuO layers as the origin of the line splitting. Furthermore, the magnetic moments are oriented parallel to the symmetry axis of the electrical field gradient tensor with magnitudes of the order of 1000 G.