Tetragonal Triple-perovskite Structure Research Articles

Correlation of Tc with compositions in the (La 1− x− zCa xPr z)(Ba 1.5Sr 0.5)Cu 3O y superconducting series

Fully oxygenated single phase (La 1− x− z Ca x Pr z )(Ba 1.5Sr 0.5)Cu 3O y compounds with tetragonal triple-perovskite structure were prepared by a solid state reaction method. In the Pr series, The T c of (La 1− z Pr z )(Ba 1.5Sr 0.5)Cu 3O y quickly dropped from 54 K for z=0 and the samples became non-superconducting for z>0.30. However, in the Ca series, the T c of (La 1− x Ca x )(Ba 1.5Sr 0.5)Cu 3O y increased from 54 K for x=0 to 73 K for x=0.20. With the amount of Pr ion kept constant, the T c of (La 1− x− z Ca x Pr z )(Ba 1.5Sr 0.5)Cu 3O y was also increased by increasing the amount of Ca ion in each series. Assuming 20% of Pr ions to be present in the tetravalent state, we calculated the hole concentration in the CuO 2 layer, p sh, for all the compounds, which was then correlated with T c. By introducing Ca ions into La sites, T c and the hole concentration in the CuO 2 layer were both also increased. On the other hand, introducing Pr 4+ ions into the La sites by fully oxygenating the samples leads to a decreased hole concentration in the CuO 2 layer. The hole doping and filling effects caused by the Ca 2+ and Pr 4+ ions, respectively, were demonstrated. At constant p sh, T c is decreased with increasing amount of Pr ions, indicating a further deterioration of superconductivity by the Pr ions in the La sites.

Correlation between hole concentration and Tc in (La 2− xY x)Ba 2(Ca yCu 4+ y)O z superconductors

The structural and superconducting properties of series of (La 2− x Y x )Ba 2(Ca y Cu 4+ y )O z (La-2125); 0.0<x<0.5, 0.0<y<1.0 , compounds having tetragonal triple perovskite structure and maximum T c of 78 K for x=0.4 (y=2x) , are investigated using X-ray diffraction, susceptibility, resistivity and oxygen content measurements. It is observed that, both oxygen content and hole concentration ( p) increase as the samples are doped with higher concentration of Ca 2+ and T c increases with increasing y=2 x as p increases upto an optimum level of 0.349 at x=0.4. Thus by adding equal amounts of CaO and CuO alongwith Y 3+ for La 3+ in La 2Ba 2Cu 4O z (La-224) semiconductor, superconductivity occurs. The addition of Ca is turning on a mechanism which affects both hole concentration and T c in a closely related way to that of YBa 2Cu 3O 7− δ (Y-123).

Effect of calcium substitution on superconductivity and hole concentration in La1·5Ba1·5Cu3O z

The superconducting properties of single phase La1·5−x Ca x+y Ba1·5−y Cu3O z , 0·0≤x≤0·60 (LC) and 0·0≤y≤0·70 (CB), compounds with tetragonal triple-perovskite structure are studied, using X-ray diffraction for their resistivity, a.c. susceptibility, and oxygen-content. La1·5−x Ca x Ba1·5Cu3O z (LC) samples, 0·15≤x≤0·60, are superconducting withT R=0c between 40 and 74 K. With the increase inx, the oxygen content, hole concentration in the CuO2 layers as well as theT c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LaCa0·5+y Ba1·5−y Cu3O z (CB) compounds increase with the increase iny, theT R=0c remains nearly constant around 74 K fory=0·0−0·70. A correlation exists between theT c and the hole concentration for LC and CB compounds.

Superconductivity in (La 2− xY x)Ba 2(Ca yCu 4+ y)O z system

The structural and superconducting properties of single phase (La 2− x Y x )Ba 2(Ca y Cu 4+ y )O z (0.0≤ x≤0.5, 0.0≤ y≤1.0) compounds with tetragonal triple perovskite structure are investigated using X-ray diffraction, resistivity and oxygen content measurements. Samples with 0≤ x≤0.5 and 0≤ y≤1.0 are superconducting. Increasing y=2 x, both the hole concentration in the Cu 2O layers and T c R=0 increases to a maximum value of 0.349 and 78 K at x=0.4 and y=0.8, respectively. A correlation between the T c and hole concentration seems to hold.

Superconductivity, hole concentration and peritectic transition of La 1.5Ca xBa 1.5−xCu 3O y compounds

A series of single phase La 1.5Ca xBa 1.5−xCu 3O y(0≤x≤0.50) compounds with tetragonal triple-perovskite structure is prepared. They have an incongruent melting point at 1107±1 °C which is not dependent on the amount of Ca substitution. Oxygen stoichiometry and hole concentration of these materials increase with increasing x. Thermal stability of the oxygen atom in the La 1.5Ca xBa 1.5−xCu 3O y series is independent on the oxygen stoichiometry. For x≥ 0.25, T c increases rapidly from 33 K to 73 K with increasing x. T c of La 1.5Ca xBa 1.5−xCu 3O y (CB) is always lower than that of La 1.5−xCa xBa 1.5Cu 3O y (LC) for a given x, suggesting that an increase in the mole fraction of the trivalent cation in these compounds would decrease its T c.

Rietveld analysis and superconductivity of La 1.5Ca xBa 1.5−xCu 3O y compounds

A series of single phase La 1.5Ca xBa 1.5−xCu 3O y (0 ≤x≤ 0.50) compounds with tetragonal triple-perovskite structure is studied by Rietveld analysis. For x≤ 0.40, all the Ca 2+ ions enter the center site, the Y 3+ site of the tetragonal YBa 2Cu 3O y phase. For x= 0.50, occupation factor of Ca 2+ ion in the Y 3+ site ( OF Ca/Y) is 0.466(2). The maximum allowed OF Ca/Y is close to 0.50 which limits further substitution of Ca 2+ ion to the Ba 2+ ion for the La 1.5Ca xBa 1.5−xCu 3O y series. T c can be explained by the difference between OF Ca/Y and OF La/Ba, ( OF of La 3+ ion in the Ba 2+ site). In other words, superconductivity is determined by the amount of Ca 2+ ions in the Y 3+ site and La 3+ ions in the Ba 2+ site in the unit-cell.

Superconductivity of the La 3.5− x– yR yCa 2 xBa 3.5− xCu 7O z system (R = Gd, Nd and Dy)

The structural and superconducting properties of single phase La 3.5− x– y R y Ca 2 x Ba 3.5− x Cu 7O z (0 ≤ x = y ≤ 0.70) (LRCB with R = Gd, Nd and Dy compounds with tetragonal triple-perovskite structure are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen-content measurements. Increasing Ca and Gd substitution for La and Ca for Ba resulted in a decrease in unit cell axes and an increase in oxygen stoichiometry. Samples with 0.2 ≤ x = y ≤ 0.70 are superconducting and T c R = 0 is between 31 and 79 K. With increasing x = y, both the hole concentration in the Cu2-O layers and T c R = 0 increase to a maximum value of 0.313 and 79 K at x = y = 0.6, respectively. A correlation between the T c and the hole concentration is suggested. Similar superconducting behaviour has been observed for R = Nd and Dy in LRCB compounds.

Rietveld analysis and superconductivity of compounds

A series of single-phase compounds with tetragonal triple-perovskite structure is studied by Rietveld analysis. For , all of the ions enter the centre site: the site of the tetragonal phase. For , a maximum of about 1/2 of the sites are occupied by ions and the extra ions enter the site. Double plateaux are found between and (the difference between the occupancy factor of ions at the site and that of ions at the site). It is suggested that of the La - Ca - Ba - Cu - O compounds containing 50 mol% copper atoms with tetragonal triple-perovskite structure can be predicted from the correlation of versus .

Superconductivity of [formula omitted] compounds

The superconductivity of single phase La 1.5− x 2 Sr x Ba 1.5− x 2 Cu 3 O y, 0 ≤ x ≤ 0.80 (LSB) compounds with tetragonal triple-perovskite structure was studied. Samples with 0.10 ≤ x ≤ 0.80 are superconducting and T c(zero) is found between 28 and 48 K. With increasing x, both the hole concentration in the Cu2O layers (the copper oxygen layers in the center perovskite layer) and T c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LSB series are similar to those of the La 1.5− x 2 Ca x Ba 1.5− x 2 Cu 3 O y (LCB) series, the maximum T c of the LSB series is about 30 K lower than that of the LCB series. The difference in T c is probably a result of difference in locations of the Ca 2+ and Sr 2+ ions in the unit cell. The smaller divalent Ca 2+ ion prefers the smaller trivalent Y 3+ site and acts as a hole dopant for the superconducting Cu2O layers. On the contrary, the larger divalent Sr 2+ ion presumably prefers the larger divalent Ba 2+ site, and thus hole doping on the Cu2O layers is not as prominent as the Ca 2+ ion in the Y 3+ site.

Superconductivity of La1.5−xCaxBa1.5Cu3Oy compounds

A series of single phase La1.5−xCaxBa1.5Cu3Oy (x=0∼0.60) compounds with a tetragonal triple-perovskite structure was prepared. Superconductivity was observed in the samples with x≥0.15. Tc(zero) was increased rapidly from 30 K to Tc≥75 K as x was increased from 0.15 to x≥0.40. It was interesting to find that superconductivity of La1.5−xCaxBa1.5Cu3Oy was dependent on the hole concentration in the Cu2−O layers (psh). With increasing Ca2+ ion substitution, psh was increased. Thus, Ca2+ ion was acted as a hole dopant for the superconducting Cu2−O layers. It was therefore suggested that Ca2+ ions occupied the trivalent center site of the triple-perovskite unit-cell, resulting in an increase of the psh in the nearby Cu2−O layers, and an increase in Tc.

Superconductivity in the triple-perovskite La-Ca-Ba-Cu-O system

Three series of single-phase La-Ca-Ba-Cu-O compounds with tetragonal triple-perovskite structure and respective nominal compositions of La4-xCaxBa3Cu7Oy (LC series, 0.50<or=x<or=1.15), La3CaxBa4-xCu7Oy (CB series, 0<or=x<or=1.25) and La3.5-0.5xCaxBa3.5-0.5xCu7Oy (LCB series, 0<or=x<or=1.25) were prepared by either a solid state reaction method or a polymeric citrate precursor method. All the single-phase materials were superconducting except for samples containing small x in the LCB series. Unit-cell parameters, such as a axes, c axes and cell volume, decreased with increasing Ca substitution. In general, the Tc and hole concentration (p) increased with increasing x from x=0 to x=1.00. In other words, evidence of hole doping was observed during Ca substitution that was the main reason for the increase of the Tc from 0 to 80 K. The optimal Tc was observed in a region close to the formula of La3CaBa3Cu7Oy which had a formula per unit celt of La1.29Ca0.43Ba1.29Cu3Oz where z=7.104, p=0.308, a axis =0.3879 nm and Tc(zero)=80 K.

Superconductivity of the single phase La 4-xCa xBa 3Cu 7O y system

A series of single phase materials with tetragonal triple-perovskite structure and nominal compositions of La 4- x Ca x Ba 3Cu 7O y (0.50⩽ x⩽1.375) was prepared by the solid state reaction method. Due to the differences in the size and valence between tha Ca ions and La ions, increasing the Ca substitution for La resulted in a decrease in unit cell axes, volume and the oxygen stoichiometry. A rapid decrease in the T c was observed when the a-axis was larger than 0.38 nm indicating the importance of the r Cu-O distance in the copper-oxygen planes for the superconductivity. Evidence of hole doping was observed by Ca substitution till x=1.00 which had the highest T c and a maximum hole concentration in the La 4- x Ca x Ba 3Cu 7O y superconductors.