Spectrophotometric determination of oxygen content in calcium

CEJC 3(3) 2005 432–440

Spectrophotometric determination of oxygen content in calcium substituted RBCO (R = Eu, Gd, Er) superconductors Angelina K. Stoyanova-Ivanova1, Tsvetanka Krasteva Nedeltcheva1∗ , Latinka Krumova Vladimirova2 1

Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria 2 Department of Analytical Chemistry, University of Chemical Technology and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria

Received 7 January 2005; accepted 8 April 2005 Abstract: The oxygen stoichiometry in R1−x Cax Ba2 Cu3 Oy (R = Eu, Er, Gd; x = 0, 0.2, 0.25, 0.3) superconducting bulk samples was determined spectrophotometrically. The dependence of the critical temperature on the y-oxygen coefficient and the x-coefficient of the included calcium was studied. c Central European Science Journals. All rights reserved.

Keywords: RBCO superconductors, Ca-substitution, oxygen content, spectrophotometric determination

1

Introduction

Recently RBCO (R=Y, Eu, Er, Gd) superconducting samples were synthesized, in which a part of the rare elements was substituted by Ca2+ [1-3]. The inclusion of Ca2+ in the oxide systems led to an increase both in the quantity of carriers as well as in the critical currents. At the same time, the superconducting temperature decreased to Tc = 80K [3]. It has been determined [4] that there is an existing relationship between the Tc -value and the oxygen content in YBCO superconducting bulk samples. To test the relationship between the Tc -value and the oxygen content in Ca-substituted RBCO samples, the ∗

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A.K. Stoyanova-Ivanova et al. / Central European Journal of Chemistry 3(3) 2005 432–440

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methods, already known in the determination of the oxygen content in YBCO bulk samples [5-8], were applied [1, 2, 3, 9]. The y-values in R1−x Cax Ba2 Cu3 Oy (R = Y, Eu, Er, Gd; x=0 or 0.2) bulk samples were determined by iodometric titrations [1,2]. The oxygen content in Y1−x Cax Ba2 Cu3 Oy (x=0-0.3) was determined [3,9] by the spectrophotometric method [8] that was based on the absorbance measurement of solutions containing the yellow coloured I− 3 complex. This method was preferred because it did not require any calibration and a precise measurement of the sample mass. As the chemical properties of Ca2+ and Ba2+ , as well as those of Y3+ and Gd3+ , Er3+ , Eu3+ are very similar, it was expected that the spectrophotometric method to determine the oxygen content in YBCO [8] could be applied to calcium substituted RCaBCO (R = Eu, Er, Gd) samples. The purpose of this report is to determine the oxygen content in R1−x Cax Ba2 Cu3 Oy (R = Eu; Er; Gd and x=0; 0.2; 0.25; 0.3) bulk samples by the spectrophotometric method and to study the dependence of Tc from the oxygen content (y) and the content of the included calcium (x).

2

Experimental

2.1 Sample preparation R1−x Cax Ba2 Cu3 Oy (x = 0; 0.2; 0.25; 0.3) samples were prepared from R2 O3 (R = Eu; Er; Gd), BaCO3 , CuO and CaCO3 powders with 4N purity. The obtained mixture was ground and sintered three times by standard solid state reaction. The first sintering was at 900 o C in a flowing oxygen for 21 h. The second sintering was at 930 o C for 21 h at the same atmosphere, followed by slow cooling and additional annealing at 450 o C for 2 h. Tablets were pressed at 5-6 MPA, sintered for the third time at 950 o C for 23 hrs and subsequently annealed at 450 o C for 23 hrs.

2.2 Sample analyses 2.2.1 Solutions and apparatus The following reagents were used: 1 mol l−1 hydrochloric acid; 1 mol l−1 sodium hydroxide solution; sodium acetate-acetic acid buffer (CN aAc = 0.08 mol l−1 ; pH = 4.9 ±0.1); 2 mol l−1 glycine (Gly) solution; potassium iodide. The assembly for sample dissolution is illustrated in Fig.1 [8]. To prevent iodine elemination by the action of light, the beaker was covered with dark paper. The absorbances were measured by a Spekol 11 spectrophotometer (Carl Zeis, Jena) using cells with a path length of 1 cm. 2.2.2 Procedure 8-10 mg of a bulk sample, previously powdered and homogenized, was placed in a hemispherical glass container and put on the bottom of one of the beakers. Two measures of 7.5 g KI were placed in both beakers. Two volumes of 10 ml of the hydrochloric acid were


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Fig. 1 Assembly for sample dissolution: (1) bottle of nitrogen gas; (2) beakers; (3) funnels; (4) electromagnetic stirrers; (5) stirrer bars; (6) glass container; (7) hydraulic gates.

introduced in the funnels. The oxygen from the beakers and the solutions was removed by purging with nitrogen gas for 10 min. With continued nitrogen gas flowing, the hydrochloric acid solutions were introduced to the beakers. The sample made contact with the solution when the potassium iodide was dissolved. Two 15 ml volumes of the acetate buffer were placed in the funnels and purged with nitrogen gas for 10 min. At the same time the sample was dissolved by stirring under nitrogen atmosphere. With continued purging and stirring, the acetate buffer was added to the sample and blank solutions.. After this procedure, two 10 ml volumes of sodium hydroxide solution were placed into the funnels. After 5 min, the content of the funnels was added to the beakers and the stirring and purging discontinued. Three dry test-tubes of 10 ml each with stoppers numbered 1, 2 and 3 were prepared. Two volumes of 2.00 ml and one of 5.00 ml were taken from the sample solution and transferred into tubes 1, 2 and 3, respectively. Two blank sample volumes of 3.00 ml each were added to the tubes 1 and 2, which contained 2 ml of the sample solution. Thereafter, 5.00 ml of Gly solution were added to tubes 1 and 3 (tube 3 contains 5.00 ml of the sample solution), and 5.00 ml of distilled water were added to tube 2. The solutions were homogenized and remained in a dark place for 10 min. The absorbances were measured at 430 nm as the absorbance of the solutions in tubes 2 and 3, were read against the solution in tube 1.

2.3 TC –measurement The critical temperature Tc was determined from magnetic AC susceptibility measurements with commercial Lake Shore 7000 Series susceptometer.The value of the AC excitation field was 0.1 Oe at 133 Hz.

3

Results and discussion

According to the procedure, the sample was dissolved in a solution of hydrochloric acid and potassium iodide, under an inert atmosphere. In this medium, Cu (II) and Cu (III) (the last is equivalent to active oxygen), react with iodide ions and the products of the − − reactions are CuI− 2 and I3 complexes (eqs (1) and (2) in Table 1). The CuI2 complex is



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formed, instead of CuI↓ precipitate, as the copper concentration is very low. The triodide complex formed between the released iodine and iodide ions colours the solution yellow. For the absorbance measurement, the pH of the solution was increased to 4.9±0.1 by adding the sodium acetate-acetic acid buffer; and three solutions containing different sample aliquots were prepared. The ligand glycine (Gly) added to two of them, bound copper (II) ions in a stable complexes CuGlyi (i=1;2) [10], and the equillibrium of reaction (1) was drawn to the left (see eq.3 in the Table 1). There was no iodine, released by Cu (II). The sum of eqs (2) and (3) gives eq (4) that describes the total process between Cu (III) and the iodide ions in the presence of Gly. It appears that the quantity of the triodide complex in the solutions, where Gly was present, is equal to 1/2 of the quantity of Cu (III), only (Table 1). The solution without ligand demonstrated that the relationship between the quantities of the iodine and the copper forms was the same as in the solution obtained from the sample dissolution. The liberated iodine was equal to the sum 1/2Cu (II)+Cu (III) (Table 1). To maintain the same ionic strength in all the solutions, the blank sample aliquots were introduced in tubes where the sample volume was less than 5 ml. The total volumes of the blank and sample solutions were a consistent 5 ml. The blank sample solution was prepared simultaneously with the sample solution, passing through all of the procedural stages. In this case, the quantity of the iodine liberated by the harmful oxidation of the iodide ions in the blank and the sample solutions was the same (the absorbance of the blank sample solution measured against water was about 0.01). A differential mode of the absorbance measurement was applied. The solution with ligand and a smaller sample volume (tube 2) was used as a comparison for the measurement of the absorbances of the other two solutions. To denote the volume of the sample solution, the number of the tube, the presence of the ligand and the mode of the absorbance measurment the description is as follows: A53; Gly /A22; Gly – the absorbance of the solution placed in tube 3, that refers to 5 ml of the sample solution and Gly is read against the absorbance of the solution in tube 2 (2 ml of the sample solution and Gly). A21 /A22; Gly – the absorbance of the solution (tube 1) in which the sample solution is 2 ml but Gly is not added, is read against the absorbance of the solution in tube 2 concerning 2 ml of the sample solution and Gly. By the analytical method proposed, the δ-values corresponding to the active oxygen 3+ in R1−x Cax Ba2 Cu2+ 3−z Cuz O6.5−x/2+δ superconductors were determined. The aliquots of the sample solution in the three tubes were properly selected so that the δ-value which was to be calculated directly from the ratio of the two absorbances measured as follows: A53;Gly /A22;Gly δ= A21 /A22;Gly


(1)


3

Gly

= 12 nCu(III)

“dissolution of the sample”

the same as in the stage

iodine is

The quantity of the

reactions

No additional

Table 1 Reactions and quantity of the released I3− both in the presence and in the absence of glycine.

nI − 3

nI − = 12 nCu(II) + nCu(III)

copper forms

the total iodine in the presence of Gly is

3

the total iodine is

3

released by

(4)

(4)

nI − = 12 nCu(III)

3

(3)

nI − = 0

+ 2iGly →

2Cu(Gly)I +I− 3

nI − = nCu(III)

(2)

3

(1)

nI − = 12 nCu(II)

(2)

+3I−

of the iodine

Quantity

reactions

+

− →CuI− 2 +I3

reduction

5I−

2Cu3+

ligand (tube 2)

(tubes 1 and 3)

Cu3+

solutions without

solutions with added ligand

− − I− 3 + 2CuI2 + 2iGly → 2Cu(Gly)I + 7I (3)

absorbance

Preparation of the solutions for the measurement

− 2Cu2+ + 7I− → 2CuI− 2 +I3 (1)

Dissolution of the sample

Oxidation-

Stage of the procedure

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437

The electroneutrality of the molecules (δ=z/2) and the ratio of the quantiities of the copper which formed: nCu(III) /(nCu(II) + nCu(III) ) = z/3 in the molecule formulae, were used to obtain this equation. Then the copper quantities were replaced by the respective quantities of the iodine liberated in the three solutions and Beer‘s law for the absorbance was applied: δ= =

(3/2) nCu(III)

(nCu(II) + nCu(III) )

5 nI −

3

−2 nI −

Gly

3

2 nI − − 2 nI − 3

3

= Gly

5 12 nCu(III) −2 21 nCu(III) 1 2( 2 nCu(II) +nCu(III) )−2 12 nCu(III)

=

= (2)

A53;Gly /A22;Gly A21 /A22;Gly

Gly

Twelve RCaBCO superconducting bulk samples were analysed by the given procedure. The content of the active oxygen expressed by a δ-coefficient value was calculated by eq (1). The mean δ- and y-values obtained from 3 parallel determinations (contidence limit with P=95 % is 0.02), are shown in Table 2-4, where the Tc values are also given. The δ-coefficient values of all the RCaBCO samples where x=0.2 are between 0.46 and 0.47 while δ-values of nonsubstitued Ca samples are little higher. For x=0.25 and 0.3, δvalues are identical in the limits of the random error, but they are strongly different than δ–values at x=0 and x=0.2. x

Theoretical value of the stoichiometric coefficient 6.5 − x/2

Experimental value of the active oxygen coefficient ∆

Value of the common coefficient y = 6.5 − x/2 + δ

Tc [K]

0 0.20 0.25 0.30

6.500 6.400 6.375 6.350

0.48 0.46 0.39 0.40

6.98 6.86 6.77 6.76

92.9 80.1 78.8 79.4

Table 2 Results for the oxygen coefficient in Eu1−x Cax Ba2 Cu3 Oy .

x


Experimental value of the active oxygen coefficient δ


Tc [K]

0 0.20 0.25 0.30

6.500 6.400 6.375 6.350

0.48 0.47 0.42 0.42

6.98 6.86 6.79 6.77

91.5 80.8 79.0 77.9

Table 3 Results for the oxigen coefficient in Gd1−x Cax Ba2 Cu3 Oy .

The results of the temperature dependencies between the oxygen content and the calcium content, for all the samples, are presented in Figures. 2 – 4. For all the samples, where x is lowered to 0.3, the critical temperature gradually decreases with the decreased oxygen content (y) and the increased calcium doping (x).


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x


Experimental value of the active oxygen coefficient δ


Tc [K]

0 0.20 0.25 0.30

6.500 6.400 6.375 6.350

0.49 0.47 0.41 0.41

6.99 6.87 6.77 6.76

90.4 73.6 57.0 66.0

Table 4 Results for the oxigen coefficient in Er1−x Cax Ba2 Cu3 Oy .

94

Eu

92 90 88 86 84 82 80

Critical Temperature T C, K

Critic al Temperature T C, K

94

78 0.00

0.05

0.10

0.15

0.20

0.25

90 88 86 84 82 80 78 7.00

0.30

Eu

92

6.95

6.90

6.85

6.80

6.75

Oxygen Content y

Ca-content x

Fig. 2 The critical temperature (Tc ) dependence from the Ca-content (x) and the oxygen content (y) in Eu1−x Cax Ba2 Cu3 Oy .

92

Gd

90


Critical Temperature TC, K

92

88 86 84 82 80 78 76

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Gd

90 88 86 84 82 80 78 76 7.00

Ca-content x

6.95

6.90

6.85

6.80

6.75

Oxygen Content y

Fig. 3 The critical temperature (Tc ) dependence from the Ca-content (x) and the oxygen content (y) in Gd1−x Cax Ba2 Cu3 Oy .

The dependence between the oxygen (y) and calcium (x) content in RCaBCO samples is presented in Fig.5. A decrease in the oxygen content (y) with an increase in the calcium doping for all series samples is observed. This is consistent with results reported in other existing literature [1; 2]. The y-results for Y1−x Cax Ba2 Cu3 Oy (x = 0 - 0.3) samples [9] are presented in the same figure. It is seen that the substitution of Y with the other


A.K. Stoyanova-Ivanova et al. / Central European Journal of Chemistry 3(3) 2005 432–440 95

95

90

Er

90



439

85 80 75 70 65 60 55 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Er

85 80 75 70 65 60 55 7.00

6.95

Ca-content x

6.90

6.85

6.80

6.75

Oxygen Content y

Fig. 4 The critical temperature (Tc ) dependence from the Ca-content (x) and the oxygen content (y) in Er1−xCax Ba2 Cu3 Oy .

rare-earth elements leads to considerable deviation of y-values only at samples with x= 0.2 calcium doping. 7.00

Gd Er Eu Y[9]

Oxygen content y

6.95

6.90

6.85

6.80

6.75 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Ca-content x Fig. 5 The oxygen content (y) dependence from Ca-content (x) in RCaBCO (R= Eu, Gd, Er or Y).

4

Conclusions • The Tc – temperature decreases with the decrease of the oxygen content y in all samples where x is down to 0.3. • The increase of the calcium content up to 0.25 in all the samples leads to a decrease of the Tc – values. • Regarding the dependence y = f(x), an inverse proportional relationship between y


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and x exists in all R1−x Cax Ba2 Cu3 Oy samples. • The correspondence observed between y- and Tc -values in RCaBCO samples, can be accepted as proof for the validity of the analytical results obtained by the spectrophotometric method, described in this paper.

Acknowledgment The authors are gratefull to K. Nenkov and E. Nazarova for the critical temperature measurements. Financial support from University of Chemical Technology and Metallurgy (Contract 10120) is gratefully acknowledged.

References [1] K. Hatada and H. Shimizu: “Structural and superconducting properties of R1−x Cax Ba2 Cu3 O6+δ (R=Y, Er, Gd, Eu; 0< δ