S.M. Loureiro

The synthesis and characterization of the HgBa2Ca2Cu3O8+δ and HgBa2Ca3Cu4O10+δ phases

Abstract The third (Hg-1223) and the fourth (Hg-1234) members of the recently-discovered homologous series HgBa2Can−1CunO2n+2+δ have been synthesized by solid state reaction, carried out at 950°C under 50 kbar at different annealing times. These phases have a tetragonal cell with lattice parameters: a = 3.8532 (6) A , c = 15.818(2) A , and a = 3.8540(3) A , c = 19.006 (3) A , respectively. The c parameters are in agreement with the formula c ≌ 9.5 + 3.2 (n − 1). Electron microscopy study showed similar lattice parameters as well as the occurence of different intergrowths and stacking faults. A periodicity of 22 A has also been detected, which may be attributed to the existence of the Hg-1245 phase. EDS analysis data of several grains of Hg-1223 and Hg-1234 are in agreement with the proposed chemical formulae. AC susceptibility measurements show that an increase of the superconducting transition temperature with n in the HgBa2Can−1CunO2n+2+δ series occurs till the third member, after which a saturation seems to be achieved.

Neutron powder diffraction study at room temperature and at 10 K of the crystal structure of the 133 K superconductor HgBa2Ca2Cu3O8+δ

Abstract The crystal structure of the superconductor HgBa 2 Ca 2 Cu 3 O 8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a =3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO 2 )(Ca)(CuO 2 )(BaO)(HgO δ (BaO)(CuO 2 )(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba 2 Ca 2 Cu 3 O x at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ =0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is T c =133 K . No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO 2 ) are coplanar for the Z =0 (CuO 2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.

Synthesis and structural characterization of the 127 K HgBa2CaCu2O6.22 superconductor: Tc variations upon different atmosphere annealings

Abstract The synthesis of the HgBa2CaCu2O6+δ (Hg-1212) superconductor under various experimental conditions in sealed silica tubes is reported. Optimization of synthesis conditions allowed the preparation of bulk superconducting Hg-1212 with Tc onset at 127 K and lattice parameters a=3.858 A , c=12.68 A . The neutron powder diffraction analysis yielded δ=0.22(4). As a result of different atmosphere annealings a relationship between the decrease of Tc and the increase of the lattice parameter a is deduced. A 300°C/10 h treatment in 99% N2/1% O2 reduces Tc to 123 K and increases the a parameter to 3.862 A. An additional 10 h reduction in a 100% N2 flow lowers Tc to 98 K while a increases to 3.865 A. Subsequent oxidation at 300°C for 10 h in O2 flow restores the lattice parameters and Tc onset to their original values. The Tc dependence on annealing conditions is shown to be reversible.

Synthesis and neutron powder diffraction study of the superconductor HgBa2CaCu2O6+δ before and after heat treatment

Abstract The crystal structure of the superconductor HgBa2CaCu2O6+δ has been analyzed by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8526(2), c=12.6367(8) A. The structure is made of the sequence of layers …[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgOδ)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880°C, has an oxygen content corresponding to δ=0.35(2) and a transition temperature of T c ⋍ 104 K . Annealing in O2 (300°C for 20 h) reduces the oxygen content to δ=0.28(2). A sample annealed under the same conditions or in N2 (250°C for 12 h) was found to have a Tc of 123 K. Additional annealing in N2 at 400°C for 18 h lowers Tc to 110 K. Our results indicate that the material with δ=0.35 is overdoped. The oxygen above the O6 stoichiometry is located on the Hg layers of the structure, at the positions 1 2 , 1 2 , 0. No evidence has been found of the substitution of mercury by copper. The copper and oxygen atoms of the (CuO2) layers are almost exactly coplanar in this compound.

Suppression of superconductivity and the overdoped region in HgBa2CuO4+δ

Abstract For the first time an overdoped, non-superconducting Hg-1201 phase was obtained by a high-pressure technique. The formal copper valence determined by iodometric titration was found to be +2.28(3). Superconducting Hg-1201 with T c = 35 K and v Cu = +2.20(3) was obtained by using the same technique and the same precursor, but which had been previously heat-treated. Further heat treatment of the sample increased T c up to 91 K and reduced v Cu to +2.13(3). We show that in the overdoped region of the Gh-1201 system the Cu valence can be changed by varying the precusor heat treatment. The instability of the overdoped phases makes it difficult to achieve the same results by heat-treating the as-prepared samples. A neutron powder diffraction study carried out on the overdoped non-superconducting sample resulted in a value of δ = 0.23(3). The formal copper valence obtained by iodometric titration and in the structural study can be reconciled if the presence of CO 3 2− defects on the Hg layers is assumed, or if a formal valence of −1 is adopted for the doping oxygen atoms.

High-pressure synthesis and heat treatments of the HgBa2Ca4Cu5O12+δ and HgBa2Ca5Cu6O14+δ phases

Abstract The synthesis of the n = 5 and n = 6 members of the HgBa2Can−1CunO2n+2+δ family are reported. Bulk samples containing either Hg-1245 or Hg-1256 as major phases were synthesized using the high-pressure high-temperature technique at 920°C and 18 kbar. The as-synthesized phases ( a = 3.8547(3) A , c = 22.156(5) A for n = 5 and a = 3.8516(1) A , c = 25.232(6) A for n = 6 ) exhibited Tc with onsets of about 100 K. They were characterized by X-ray powder diffraction, transmission electron microscopy and AC susceptibility measurements. Heat treatments increased the Tc onset to 110 K for n = 5 and to 107 K for n = 6. These phases with high n are more inert to thermal annealing under different atmospheres than the members with smaller n. Only strong oxidizing {P(O2) ≥ 500 bar} or reducing {Ar/400 °C} condition can induce significant Tc variations.

The superconducting HgBa2Can-1CunO2n+2+δ homologous series

Abstract Syntheses, heat-treatments, structures and the superconducting properties of the first four members of the homologous series HgBa 2 Ca n -1 Cu n O 2 n +2+δ are presented and discussed. Syntheses under high pressure seem to yield samples which contain less than 5% impurities. The structural characterization has been carried out by X-ray diffraction, electron microscopy and neutron diffraction. Heat-treatments in oxidizing atmospheres usually increase T c . The member with n = 2 does not follow this rule because the as-prepared sample ( T c = 104 K) seems to be overdoped. Heat-treatment in N 2 at 250°C for 10h induces an increase in T c to 123K. Additional heat treatment under N 2 (400°C, 18h) decreases T c to 110K. In Hg-1212 superconductivity exists in underdoped and overdoped regions.

The importance of the precursor in high-pressure synthesis of Hg-based superconductors

Abstract The formation process of high-pressure synthesized Hg-based superconductors is characterized in terms of the average copper valence (〈V Cu 〉) of the precursor. This parameter is varied by oxygen intake of non-stoichiometric phases during the precursor synthesis at specific temperature, time and cooling process treatment. The average copper valence is shown to be a controllable parameter that determines both the overall composition of the precursor and the proportion of the different phases present in it. More important, the average copper valence of the precursor seems to control the formation of a given member n of the HgBa 2 Ca n −1 Cu n O 2 n +2+δ homologous series. We find that higher precursor copper valences favor lower members of the series while higher members are only obtained with lower 〈V Cu 〉.

Structure and superconductivity of the HgBa2Ca3Cu4O10+δ phase

Abstract A bulk superconducting HgBa 2 Ca 3 Cu 4 O 10+δ phase (Hg-1234) was obtained by a high-pressure high-temperature technique in a polyphasic sample containing ≈ 15% of impurities. The lattice parameters of the as-synthesized phase were found to be a = 3.8524(4) A , c = 18.942(5) A , with a T c, onset of 116 K. Subsequent heat treatments showed that only strong reducing or oxidizing conditions allow one to obtain a major variation of lattice parameters and of T c, onset , while heat treatments in 1 bar oxygen do not produce any significant change. The structural model refined by X-ray Rietveld powder diffraction showed a large deficiency of the Hg site [0.79(1)], relatively high thermal parameters for Hg and Ba, and an unusual short HgO ap distance. All these features can be explained by the presence of carbon replacing Hg in the as-prepared Hg-1234 phase.

On the possibility of replacing Hg by Cu in the HgBa2CuO4+δ phase synthesized under high-pressure

Abstract To study the existence of the Hg 1- x Cu x Ba 2 CuO 4+δ solid solution, samples with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.50, were synthesized using a high-pressure and high-temperature technique. These samples have been characterized by X-ray powder diffraction and AC susceptibility measurements. Only the samples with x = 0 and 0.05 were found to be monophase. The sample with x = 0, depending upon the preparation is non-superconducting or has a T c = 50 K. Its lattice parameters show that it is overdoped. All the other samples contain impurities, whose nature and relative quantities depend upon x and the compound used as extra Cu source. From the present results it can be deduced that Hg can in principle be replaced by Cu, but this replacement cannot be more than 10%. The discrepancy between the extra oxygen content determined by iodometric titration (0.13) and that determined by X-ray diffraction (0.20) and the low occupancy factor (or the relatively high thermal parameter) for the Hg cations is explained in terms of the incorporation of CO 3 −2 groups into the structure.