Study of the crystal structure of the high Tc superconductor Hg1-xRe xBa2Ca2Cu3O8+δ by using EXAFS, XANES and XRD

Abstract

Hg based superconducting ceramics present the highest critical temperatures ever reported, particularly the phase 1223 with the highest Tc so far obtained -135 K. However, these compounds are obtained by a complex synthesis route and suffer degradation at ambient atmosphere. This disadvantage may be overcome by doping with an appropriate amount of Re. Samples of nominal composition Hg 0.82 Re 0.18 Ba 2 Ca 2 Cu 3 O 8+δ having different oxygen contents were studied by X ray absorption and diffraction techniques. Using XANES we determined the Re valence = +7 in an octahedral distorted coordination. We determined the distances of Re-O bonds (1.85 Å and 2.10 Å) by EXAFS. XRD Rietveld fits showed segregation of two superconducting phases with distinct lattice parameters. The main phase is associated with Hg, Re-1223 with higher oxygen content, and the secondary superconducting phase is related to undoped Hg-1223.

Development and test of a small resistive fault current limiting device based on Hg, Re-1223 and Sm-123 ceramics

“…Firstly, Ba 2 Ca 2 Cu 3 O x (99.0% PRAXAIR) and ReO 2 (99.0% Aldrich) in powder form were mixed at the molar relationship 1:0.18, respectively 26 . The powder mixture was homogenized in an agate mortar and it was pelletized with an uniaxial pressure of 0.5 GPa.…”

“…The powder mixture was homogenized in an agate mortar and it was pelletized with an uniaxial pressure of 0.5 GPa. Secondly, the pellet was annealed at 850°C in a flow of oxygen for 15 h. The precursor was crushed, homogenized and compacted again before being annealed a second time at 930°C for 12 h under O 2 gas flow 26 . The later procedure is repeated seven more times.…”

“…The ratio between the mercury mass and the gold mass was 0.045 [27,28] . Based on the study of quartz tube filling factor (ff) 26 , we used ff ≅ 1.0 g/cm 3 and ff Hg ≅ 0.010 g/cm 3 [26] . The quartz tubes were sealed in a high vacuum of 3 × 10 -6 Torr.…”

Estudo do composto supercondutor Hg, Re-1223 por difratometria de raios-X, com a aplicação do método de Rietveld

“…O intervalo de 2θ fixado para a medida foi de 2° a 122° com passo de 0,01° com tempo de contagem de aproximadamente 20 segundos. A escolha dessas condições experimentais foram embasadas em trabalhos similares sobre estes materiais [1,2,3,4] . A repetição das condições experimentais teve como finalidade obter condições idênticas às empregadas nesses trabalhos, com o propósito de refazer os refinamentos e confirmar seus resultados.…”

“…A segregação de fases supercondutoras no composto Hg,Re-1223, reportada em trabalhos recentes [1,2,3,4] , é um resultado importante que demanda estudos mais aprofundados sobre a micro-estrutura dessas fases. Assim, o aprimoramento dos refinamentos das estruturas dessas fases e a determinação dos tamanhos de cristalitos das mesmas é uma contribuição deste trabalho ao avanço na compreensão das relações entre a estrutura e micro-estrutura destes compostos e suas propriedades.…”