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Synthesis of monophase HgBa2CuO4+δ under controlled partial oxygen pressure
PHYSICA ELSEVIER
Physica C 255 (1995) 173-179
Synthesis of monophase HgBazCuO4+ under controlled partial oxygen pressure V.A. Alyoshin, D.A. Mikhailova, E.V. Antipov * Department of Chemistry, Moscow State University 119899, Moscow, Russian Federation Received 28 July 1995
Abstract
A reproducible method of synthesis of monophase HgBa2CuO4+ ~ is described. The synthesis is carried out in sealed silica tubes under controlled partial oxygen pressure. The best results were obtained within the [0.012-0.15] bar partial oxygen pressure range at 800°C. Mixtures of transition metal oxides, Co304//CoO or CuO//Cu20 , were used to promote the exact regulation of the partial internal oxygen pressure.
1. Introduction
The high-temperature superconductor HgBa 2CuO4+ B (Hg-1201 phase) with T~ = 94 K was obtained for the first time in an evacuated sealed silica tube (SST) from a stoichiomelric mixture of Ba2CuO3+ 8 and HgO [1]. Due to the low decomposition temperature of HgO, a part of Hg as well as oxygen is removed from the reacting mixture creating an equilibrium gaseous pressure at the synthesis temperature inside of the tube and providing a certain deviation from the initial stoichiometry. This can be the reason why the Hg-1201 samples prepared in sealed tubes usually contain impurity phases like Ba2Cu3Os+ 8 and BaHgO 2. To obtain high-quality Hg-1201 samples it is necessary to optimize different factors, such as the SST volume, weight of the initial mixture, temperature, heating and cooling rates etc. Paranthaman et al. reported the preparation of nearly
* Corresponding author.
monophase Hg-1201 using optimized synthesis conditions described in Ref. [2]. Nearly monophase HgBa2CuO4÷ ~ was also obtained in SST using an additional pellet of a precursor Ba2CuO3+ ~ (33% [3] or 48% [4] of the reactants pellet by weight), to reduce the intemal Hg vapor pressure. It should be noticed that using this technique the partial oxygen pressure is also significantly changed. It was qualitatively shown [5] that the appearance of impurities and their phase composition in the synthesis of HgBa2CuO4+ 8 depends on the partial oxygen pressure in the reaction vessel. It was also found that the increase of the partial oxygen pressure in the SST leads to the appearance of phases which are stable under high oxygen pressure, such as Ba2Cu3Os+ 8 and BaHgO2, and that the decrease of this pressure leads to the appearance of mercury drops. The synthesis of 95% HgBa2CuO4÷ 8 containing samples has been made using Ba2CuO3+ ~ with low extra oxygen content (8). Therefore the oxygen partial pressure plays an important role in the
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V.A. Alyoshin et al./Physica C 255 (1995) 173-179
HgBa2CuO4+ ~ synthesis, and to obtain monophase Hg-1201 compound the internal oxygen pressure needs to be optimized. The reactions that take place in the SST between mercury oxide and Ba2CuO3+ ~ at high temperature are described below. We consider that the interactions occur mainly via vapor (Hg, O2)/solid (precursor) reactions in the sealed vessel as it was shown in Refs. [5] and [6]. Under heating, the mercury oxide decomposes into metallic mercury vapour and oxygen; the saturated vapor pressure under HgO reaches 1 atm at 450°C [7]: HgO ~ Hg(0 ) + 1 / 2 0 2 . The mercury vapor and the oxygen interact with Ba2CuO3+ x in the following way: Ba2CuO3+ x + Hg(0 ) + (1 - x + t ~ ) / 2 0 2
¢* HgBa2CuO4+ ~.
(1)
From this equation it follows that the oxygen partial pressure increase must displace the equilibrium (1) to the formation of HgBa2CuO4+ ~ causing also the decrease of the mercury partial pressure, and thus pure Hg-1201 may be obtained at the correct value of the partial internal oxygen pressure. However, this should be correct only in the case of the interaction between Hg(o), 0 2 and Ba2CuO 3+x for reaction (1). According to Refs. [8] and [9] at high oxygen pressures in the B a - C u - O and B a - H g - O systems, phases such as BaHgO 2, BaCuO2+ ~ or Ba2Cu3Os+ 8 were shown to be stable. These phases may also be stable in the H g - B a C u - O system and can be formed taking into account the following reactions: Ba2CuO3+ x + Hg(o) + (1 - x + y ) / 2 0 2
¢* BaHgO 2 + BaCuO2+y,
(2)
3Ba2CuO3+ x + 4Hg(o ) + (4 - 3x + z ) / 2 0 2 ¢* 4BaHgO 2 + Ba2Cu3Os+ z.
(3)
The formation of BaCuO2+y and Ba2Cu305+ z significantly depends on the oxygen pressure. The latter phase is stable in the B a - C u - O system at 827°C when .p(O 2) >_ 0.35 bar, while the former one exists in the 4 × 10 -5 < p(O 2) < 37.8 bar range [9]. This way the formation of monophase HgBa2CuO 4 + may take place in a limited oxygen pressure interval, outside of which the formation of impurity phases may be more favorable.
The aim of the present work was to study the partial oxygen pressure ranges where the synthesis of monophase HgBa2CuO4+ ~ is possible. For the exact control of the internal partial oxygen pressure an oxygen getter made of transition metal oxides was used to vary this parameter to a definite value.
2. Experimental The synthesis of HgBa2CuO4+ ~ was carded out in two steps from high-purity reactants. First the Ba2CuO3+ a precursor was synthesized through the stoichiometric mixture of BaO 2 and CuO at 900°C for 20 h in oxygen flow, quenched and allowed to c001 in a dry box. The precursor was checked by X-ray powder diffraction which showed that it contained only Ba2CuO3+ a without any additional phases. This precursor was subsequently mixed with the stoichiometrical amount of HgO. All the operations with the synthesized substances were conducted in air, oxygen or argon, dried and purified from CO 2. The weight accuracy was _+0.0002 g. The synthesis of HgBa2CuO4+ a was performed in sealed silica tubes with 15 cm length and 0.8 cm of inner diameter. The tubes were evacuated up to 1 0 - 2 - 1 0 -3 Tort and placed into a two-temperature gradient regulated furnace. For the regulation of the partial oxygen pressure in the SST, a two-phase mixture of oxides C u O / C u 2 0 and C o 3 0 4 / C o O was used as an oxygen getter. The oxygen resulting from the decomposition of HgO under heating is absorbed by the more reduced compound with the consequent formation of the more oxidized one according to the following reactions: Cu20 q- 1 / 2 0 2 ¢* 2CuO,
(4)
3CoO + 1 / 2 0 2 ** Co304.
(5)
Theoretical values of the partial oxygen pressure in equilibria (4) and (5) may be estimated from Eq. (6) [for C u O / C u 2 0 in the 298-1358 K temperature range] and (7) [for C%O4/COO in the 1000-1240 K temperature range], taken from Ref. [7]: log p ( O 2 ) , a t m = - 1 5 0 3 7 / T + 10.865,
(6)
log p ( O 2 ) , a t m = - 1 5 6 0 0 / T + 12.6.
(7)
V.A. Alyoshin et a l . / Physica C 255 (1995) 173-179
A pellet of the reactants mixture was placed in the less heated part of the tube and a pellet of C u O / C u 2 0 or C o 3 0 4 / C o O oxides mixture was placed in the higher-temperature region. Under heating of the SST the mercury oxide decomposes with the release of oxygen, which is absorbed by the getter. The oxygen pressure may be varied by changing the temperature of the getter pellet due to the three phase equilibria (4) or (5). For low oxygen pressures the C u O / C u 2 0 mixture was used, while for middle and high oxygen pressures the C o 3 0 4 / C o O mixture was more effective. The used getter average quantity was 1-1.5 g. This method allowed us to prepare from 0.7 up to 3.5 g of HgBa2CuO4+ 8. The obtained compounds were annealed in purified oxygen or argon flow at the reactor which was placed into a horizontal "Nabertherm" furnace. The furnace temperature accuracy measurement was +5°C. The annealings at higher oxygen pressure were carded out in closed stainless steel reactors. Phase compositions of the initial and synthesized compounds were controlled by X-ray analysis using a Guinier-camera FR-552 (Cu Kcx~ radiation, A = 1.54056 A) with germanium as internal standard. The wet analysis of Hg, Ba and Cu was carded
175
out by the methods described in Ref. [9]. The extra oxygen content 6 in the HgBa2CuO4+ 8 compound was determined by iodometric titration [10]. The AC susceptibility measurements were carried out with an external field of 1 0 e and a frequency of 27 Hz in the 120-12 K temperature range. The superconducting transition temperature was determined from the real part decrease of the magnetic susceptibility.
3. Results and discussion
3.1. Synthesis of HgBa2Cu04+ 8 under controlled partial oxygen pressure The experimental conditions and reproducible obtained results of the synthesis of HgBa2CuO4+ ~ at fixed partial oxygen pressure are given in Table 1. The equilibrium partial oxygen pressures, determined with the use of the oxygen getter, were calculated from Eqs. (6) and (7). The phase % and composition were evaluated by comparison of the main reflections intensities on the X-ray powder diffraction patterns. The amount of prepared samples was 0.7-0.8 g. As it can be seen from Table 1, the appearance of
Table 1 Synthesis conditions, phase composition and cell parameters for HgBa2CuO4+ 8 N
7-1 (oC) a
PO 2 (bar)
Composition
a (A)
c (A)
1
800
7-2 (oC) b 820
0.0013
3.895(2)
9.532(7)
2 3 4
800 810 790
800 890 940
0.012 0.15 0.55
3.8833(4) 3.8823(8) 3.8822(7)
9.53(1) 9.523(3) 9.523(3)
5
810
1000
3.8799(4)
9.520(2)
6
810
3.8819(6)
9.523(3)
7 8
810 810
H g B a 2 C u O 48+(90%), Ba2CuO3+ 8 (10%), Hg H g B a 2 C u O 48,+ Hg HgBa2 CuO4+ 8, Hg (traces) HgBa2CuOo+ 8 (95%), BaCuO2+ 8 (5%), Hg (traces) HgBa2CuO4+ 8 (80%), BaCuO/+ a (20%), Hg (traces) HgBazCuO4+ 8 (95%), Ba2Cu305 + 8 (5%), Hg (traces) HgBa2CuO4+ ~, Hg HgBa2CuO4+ 8 (50%), BaCuOz+ ~ (20%), BaHgO z (30%), Hg
3.8816(8) 3.8839(8)
9.522(3) 9.531(3)
890 890
2.2
0.15 0.15
a T(1) is the temperature of reactants mixture [Ba2CuO3+ 8 and HgO]. b T(2) is the temperature of the oxygen getter (CuO/Cu20, for the first experiment, and C0304/C00, for the rest).
176
V.A. Alyoshin et a l . / Physica C 255 (1995) 173-179
Hg(0) drops is almost a constant feature and when the partial oxygen pressure is decreased, their amount increases (this problem is discussed below). At low oxygen pressure (0.001 bar), Ba2CuO3+ ~ appears as an impurity which can be explained by the equilibrium described in Eq. (1). It should be mentioned that the Hg-1201 phase obtained at low oxygen pressure is strongly reduced, and it exhibits a superconducting transition at 49 K (Table 1, exp. 1). When the oxygen pressure was increased the unreacted Ba2CuO3+ ~ disappeared. In the partial oxygen pressure range 0.012-0.15 bar the Hg-1201 phase was obtained in pure form according to X-ray powder data (Table 1, exp. 2 and 3) and only traces of Hg drops were found. Their appearance can be attributed to the equilibrium partial mercury pressure under the Hg-1201 compound in the SST. Syntheses at these conditions were reproduced several times and the final phase composition of the annealed samples was exactly the same. The cell parameters of the Hg-1201 phase decrease regularly with the increase of the partial oxygen pressure, while T~ increases as well. When the partial oxygen pressure is increased to >_ 0.55 bar, B a C u O 2 + ,~ a n d / o r B a 2 C u 3 0 5 + ~ appear and their amount increases with the increase of the partial oxygen pressure (Table 1, exp. 4, 5). The appearance of BaCuO2+ J B a / C u 3 0 5 + ~ is explainable if the equilibria (2, 3) are taken into consideration. At higher oxygen pressures when BaO 2 was used as oxygen source, HgBa2CuO4+ ~ was not formed, and only phases such as BaHgO 2, Ba2CuO3+ ~ and Ba2Cu3Os+ ~ were detected by X-ray powder diffraction. We could not quantify the partial oxygen pressure inside the SST because the getter was not used. However, the absence of Hg drops after the synthesis showed us that this pressure was in fact high. This means that equilibrium (3) takes place in the SST instead of equilibrium (1). Only when the partial oxygen pressure was in the 0.012 < p(O 2) _< 0.15 bar range impurity phases were not detected by X-ray powder diffraction. In the experiment performed without any additional tablet of oxygen getter, Ba2Cu305 + ~ appeared as an impurity (Table 1, exp. 6) which means that the oxygen pressure in the SST was > 0.15 bar. The pellet of cobalt (or copper) oxides mixture naturally plays a
role of oxygen absorption and decreases the partial oxygen pressure in the SST. These results allowed us to conclude that the partial oxygen pressure exerts a main influence on the phase composition of the as-prepared Hg-1201 samples. In order to study the influence of the internal mercury partial pressure on the phase composition of the prepared Hg-1201 samples we performed some synthesis at fixed partial oxygen pressure (0.15 bar) with extra quantities of mercury oxide (10, 30 and 50 mol.% in addition to the stoichiometric nominal HgBa2CuO4+ ~ composition). The addition of extra amount of HgO has, as a direct consequence, an increase of the partial mercury pressure, and this causes an increase of the amount of metallic mercury found in the SST. It is remarkable that the addition of 10% of extra mercury oxide did not change the phase composition of the sample or the lattice parameters of the Hg-1201 phase (Table 1, exp. 7). However, in a similar experiment without using an oxygen getter described in Ref. [5] such addition of HgO led to the appearance of a large amount of impurity phases, such as Ba2Cu305 + ~ and BaHgO/. In the latter experiment an addition of extra HgO also caused an increase of the intemal oxygen pressure providing thus the appearance of these impurity phases. However, with the addition of 30 and 50% of extra mercury oxide, impurities such as BaCuO2+ 8 and BaHgO 2 appeared and their formation can be explained due to the existence of more complicated phase relations in the quarternary H g - B a - C u - O system which depend on the cation composition, temperature and partial pressures. Our investigations have shown that the possibility of obtaining monophase HgBa2CuO4+ ~ occurs at 800°C and in the partial oxygen pressure interval within 0.012-0.15 bar. Using these conditions, the synthesis of higher quantities of HgBa2CuO4+ 8 even up to 3.7 g was carded out without major difficulty. The experimental conditions are described as experiment 3 in Table 1. The X-ray diffraction pattem of the Hg-1201 sample is shown in Fig. 1. The as-prepared HgBaECUO4+ ~ phase has lattice parameters a = 3.8842(9) A, c = 9.530(4) ,~ and Tc = 85 K. The cell parameters of this compound obtained in a massive sample differ from those determined for phases obtained in samples of smaller weight (Table 1, exp. 3). This fact may be due to the different cooling rate
VA. Alyoshin et al./ Physica C 255 (1995) 173-179
Counts
~ ' ' 1
¸''
. . . . .
~ . . . .
I ' ' ' ' 1
. . . .
L . . . .
I ' ~ '
8250 5500 2750
g -
-
~ ~
-
0
.,.
L
r~,
10
~
~
g~ ~
~
i i , l , l , , i
20
~
. . . .
30
I , , , ~ t * , ~ l , , , , I , , i r
40
50 60 70 80 90 20 Fig. 1. X-ray powderdiffracfionpattemof HgBa2CuO4+~.
of the massive sample in comparison with smallerweight samples. It should be noted that the T~ of the as-synthesized sample (85 K) is smaller than that determined for the as-prepared Hg-1201 samples described in Refs. [1-5] since the synthesis was performed at a lower partial oxygen pressure. The as-synthesized sample was annealed in oxygen flow at 250°C during 24 h. After annealing, T~ increased to 96 K and the cell parameters decreased to a = 3.8794(4) A and c = 9.525(1) ,~. The x(T) curves for the as-synthesized and oxygen-annealed samples are shown in Fig. 2. According to wet-analysis data the amount of Ba, Cu and Hg in the sample is, respectively, 3.34(6), 1.76(4) and 1.48(3) m m o l / g . Assuming that the amount of barium determined by wet analysis corre-
-0.005
-~
•
~
-o.ol ~
-0.015
e
e
e
~
, , , I , , l , I 0 20 40 60 T (K) i
,
r
i
i
, I , 80
, I 100
i
r
i
120
Fig. 2. AC susceptibility vs. T of as-synthesized (1) and annealed in oxygen flow (2) HgBa2CuO4+~ samples synthesized by method 1.
177
sponds to a stoichiometry of 2, then the chemical formula of the compound may be written as Hg0.89(3)Ba2Cul.o6(5)O4+ 8. From this composition we can conclude that there is a significant Hg deficiency in the compound. Earlier the mercury deficiency in HgBa2CuO4+ 8 was found from neutron powder structural refinement [12]. These authors concluded that ,-~ 10% Hg cations are partially replaced by copper ones, and this agrees well with our wet-analysis data. However, the presence of X-ray amorphous impurities, which do not contain Hg, can also provide a lower Hg content in the sample, as determined by wet analysis. It is known, that at T > 650°C HgBa2CuO4+ 8 decomposes in an open system with removal of Hg(0) and oxygen [5]. It has also been shown recently that a large Hg deficiency occurs on the surface of Hg-1201 irradiated samples [13,14]. The decrease of the Hg amount in the as-prepared sample may be caused by a partial loss of mercury in vapor phase which is in equilibrium with the solid state, after which Hg(0) drops condensate in the SST as a result of the cooling process. It is remarkable that according to the wet analysis performed on the sample prepared with an addition of 10% HgO (Table 1, exp. 7) [which created a higher partial Hg pressure], the amount of Ba and Cu was found to be approximately the same as in the previous analysis, being, respectively, 3.34(6) and 1.79(4) m m o l / g . At the same time the Hg content in the sample, which did not contain a noticeable amount of the metallic mercury, increased significantly to 1.63(3) m m o l / g . In this case the chemical formula of the Hg-1201 compound may be written as Hg0.98(3)Ba2Cul.07(6) 04+ ~. This composition corresponds to the ideal chemical formula if the standard deviations are taken into account. It should be noted that there are no differences in lattice constants for this phase and for the phase prepared under the same oxygen partial pressure without addition of HgO (Table 1, exp. 3). From these data we can suppose that the amount of mercury in the HgBa2CuO4+ ~ phase either does not depend or only depends slightly from the partial mercury pressure in the SST. This gives us an indirect proof that the replacement of Hg cations by Cu ones can either occur in a small range or may not even exist at all. The same conclusion was made by Alexandre et al. from the study of the Hg1_xCuxBa2CuO4+ 8 system under high pressure
V.A. Alyoshin et al./ Physica C 255 (1995) 173-179
178
0
[15]. It should be mentioned that a partial substitution of Hg by carbonate group can take place in the Hg-1201 structure [16] providing also a certain Hg deficiency. 3.2. Two-temperature gradient HgBa2CuO 4 + 8 synthesis with an additional pellet o f Ba2CuO 3 + 8
The HgBa2CuO4+ 8 samples prepared according the procedure described above usually contained mercury drops on the SST walls. In order to absorb this mercury, a two-temperature gradient synthesis was carried out using an extra pellet of Ba2CuO 3 + ~. The use of an extra pellet of Ba2CuO3+ ~ in the preparation of HgBa2CuO4+ ~ is well described in Refs. [3] and [4]. Starting pellets of a reactant mixture composed of Ba2CuO3+ ~ and HgO and pure BazCuO 3+ ~ were placed into the opposite sides of a SST. The temperature gradient was 15-20°C with the Ba2CuO3+ ~ pellet on the hotter side to decrease the mercury pressure in the SST and prevent the mercury drops from appearing. The partial oxygen pressure in these experiments was not controlled and the temperature regimes and the proportion between Ba2CuO3+ ~ and HgO weights and the tube's volume were taken from Ref. [4]. According to X-ray powder data the as-prepared HgBa2CuOo4+~ phase had lattice parameters a = 3.8818(5) A, c = 9.525(2) A, T~ = 94 K and extra oxygen content 6 = 0.08(1) (wet-analysis data). This sample was annealed under different conditions, which are listed together with obtained results in Table 2, and the x ( T ) curves are shown in Fig. 3. From Table 2 one can see that the cell parameters of HgBa2CuO4+ ~ decrease with the increase of the oxygen pressure, with the sole exception of sample 4. This might in fact be caused by a punctual non-
i
000
-0.004
g
~ -0.0o6
,~3
"~ -0.008 -0.01 -0.012 0
. . . . . . . . . . 20 40 60
i ...... 80 100
120
T (N) Fig. 3. AC susceptibility vs. T of the as-synthesized (1), p(O2) = 90 bar (2) and argon (3) annealed the HgBa2CuO4+ ~ samples synthesized by method 2.
equilibrium state for sample 4. The variations of T~ versus a parameter or extra oxygen content ( 6 ) are described by a parabolic-like behavior as was shown earlier in Refs. [5,16,17]. The high transition temperatures are reached for the Hg-1201 compounds with a parameter = 3.88 A and copper valence = +2.15; the latter value is a typical one for the optimally doped superconducting Cu mixed oxides. The samples prepared by such a technique usually contained 5-10% of impurity phases such as Ba2Cu305 + ~ and BaCuO2+ ~. However, no mercury drops were observed in these experiments. We suppose that the impurity phases appearance may be due either to a partial oxygen pressure > 0.15 bar in the SST or to the deviation of the cation stoichiometry from the ideal Hg-1201 composition. The second pellet of Ba2CuO3+ ~ according to X-ray powder diffraction analysis contained 5 - 1 0 % of HgBa 2CuO4+ 8.
Table 2 Annealing conditions, cell parameters, Tc and 8 for HgBa2CuO4+ N
P (bar)
T (°C)
t (h)
a (A)
c (A)
Tc (K)
t~
1 2 3 4 5
Ar, t as-prepared 02 , 1 O 2, 125 02 , 90
350
67
250 200 250
24 18 66
3.8887(5) 3.8818(5) 3.8806(6) 3.8801(12) 3.8757(5)
9.540(2) 9.525(2) 9.525(2) 9.521(5) 9.514(2)
68 94 95 97 89
0.01(1) 0.08(1) 0.12(1)
V.A. Alyoshin et al. / P hysica C 255 (1995) 173-179
The two-temperature gradient synthesis of HgBa2CuO4+ ~ with an extra pellet of Ba2CuO3+ 8 has more advantages than the methods described in Refs. [3] and [4] because the mercury concentrates in the low-temperature reactants tablet. However, the disadvantage of this technique is the necessity to control all the synthesis parameters including the reactant's weight, S S T ' s volume, extra oxygen content in Ba2CuO3+ ~ and the temperature region in order to have a good result reproducibility, in contrast to the controlled partial oxygen pressure synthesis described in the previous part.
4. Conclusions A reproducible method of HgBa2CuO4+ ~ synthesis was developed. The synthesis of monophasic Hg-1201 samples was carried out at 800°C under a partial oxygen pressure within the range 0 . 0 1 2 - 0 . 1 5 bar. Pellets of copper or cobalt oxides mixtures were used for oxygen-pressure control. The oxygen pressure inside of sealed silica tubes was regulated by the temperature variation of these oxides mixtures.
Acknowledgements The authors are grateful to G.N. Mazo for having performed the wet analysis of HgBa2CuO4+ ~, to P.E. Kazin for the magnetic measurements and S.M. Loureiro for the help with the manuscript preparation. This work was partially supported by the Russian Scientific Council on Superconductivity (Poisk) and the B M F T Project 13N6401.
179
References [1] S.N. Putilin, E.V. Antipov, O. Chmaissemand M. Marezio, Nature (London) 362 (1993) 226. [2] M. Paranthaman,J.R. Thompson,Y.R. Sun and J. Brynestad, Physica C 213 (1993) 271. [3] Q. Xiong, Y.Y. Xue, F. Chen, Y. Cao, Y.Y. Sun, L.M. Liu, A.J. Jacobson and C.W. Chu, Physica C 231 (1994) 233. [4] O. Chmaissem, L. Wessels and Z.Z. Sheng, Physica C 230 (1994) 231. [5] I.Yu. Torshin, V.A. Alyoshin and E.V. Antipov, Sverhprovodimost: fizika, khimia, tekhnologia (Russian), to be published. [6] R.L. Meng, L. Beauvais,X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue and C.W. Chu, Physica C 216 (1993) 21. [7] I.S. Kulikov, Termodinamika oksidov, Spravochnik (Russian) Moscow (1986). [8] S.N. Putilin, S.M. Kazakov and M. Marezio, J. Solid State Chem. 109 (1994) 406. [9] G.F. Voronin and S.A. Degterev, J. Solid State Chem. 110 (1994) 50. [10] G.N. Mazo, V.M. Ivanov and A.A. Galkin, Vestnik Moskovskogo Universiteta,Seria Khimia(Russian)36 (1995) 288. [11] G.N. Mazo, V.M. Ivanov and A.V. Kumkova, Fresenius J. Anal. Chem. 350 (1994) 718. [12] J.L. Wagner, P.G. Radaelli, D.G. Hinks, J.D. Jorgensen,J.F. Mitchell, B. Dabrowski, G.S. Knapp and M.A. Beno, Physica C 210 (1993) 447. [13] W. Zhou, A. Asab, I. Gameson, D.A. Jefferson and P.P. Edwards, Physica C 248 (1995) 1. [14] H. Chang, Q. Xiong, Y.Y. Xue and C.W. Chu, Physica C 248 (1995) 15. [15] E.T. Alexandre,S.M. Loureiro, E.V. Antipov, P. Bordet, S. de Brion, J.J. Capponi and M. Marezio, Physica C 245 (1995) 207. [16] S.M. Loureiro, E.T. Alexandre, E.V. Antipov, J.J. Capponi, S. de Brion, B. Souletie,J.L. Tolence,M. Marezio, Q. Huang and A. Santoro, Physica C 243 (1995) 1. [17] M.A. Subramanian, ICMAS-93 SuperconductingMaterials, Paris (1993) 167.
Physica C 255 (1995) 173-179
Synthesis of monophase HgBazCuO4+ under controlled partial oxygen pressure V.A. Alyoshin, D.A. Mikhailova, E.V. Antipov * Department of Chemistry, Moscow State University 119899, Moscow, Russian Federation Received 28 July 1995
Abstract
A reproducible method of synthesis of monophase HgBa2CuO4+ ~ is described. The synthesis is carried out in sealed silica tubes under controlled partial oxygen pressure. The best results were obtained within the [0.012-0.15] bar partial oxygen pressure range at 800°C. Mixtures of transition metal oxides, Co304//CoO or CuO//Cu20 , were used to promote the exact regulation of the partial internal oxygen pressure.
1. Introduction
The high-temperature superconductor HgBa 2CuO4+ B (Hg-1201 phase) with T~ = 94 K was obtained for the first time in an evacuated sealed silica tube (SST) from a stoichiomelric mixture of Ba2CuO3+ 8 and HgO [1]. Due to the low decomposition temperature of HgO, a part of Hg as well as oxygen is removed from the reacting mixture creating an equilibrium gaseous pressure at the synthesis temperature inside of the tube and providing a certain deviation from the initial stoichiometry. This can be the reason why the Hg-1201 samples prepared in sealed tubes usually contain impurity phases like Ba2Cu3Os+ 8 and BaHgO 2. To obtain high-quality Hg-1201 samples it is necessary to optimize different factors, such as the SST volume, weight of the initial mixture, temperature, heating and cooling rates etc. Paranthaman et al. reported the preparation of nearly
* Corresponding author.
monophase Hg-1201 using optimized synthesis conditions described in Ref. [2]. Nearly monophase HgBa2CuO4÷ ~ was also obtained in SST using an additional pellet of a precursor Ba2CuO3+ ~ (33% [3] or 48% [4] of the reactants pellet by weight), to reduce the intemal Hg vapor pressure. It should be noticed that using this technique the partial oxygen pressure is also significantly changed. It was qualitatively shown [5] that the appearance of impurities and their phase composition in the synthesis of HgBa2CuO4+ 8 depends on the partial oxygen pressure in the reaction vessel. It was also found that the increase of the partial oxygen pressure in the SST leads to the appearance of phases which are stable under high oxygen pressure, such as Ba2Cu3Os+ 8 and BaHgO2, and that the decrease of this pressure leads to the appearance of mercury drops. The synthesis of 95% HgBa2CuO4÷ 8 containing samples has been made using Ba2CuO3+ ~ with low extra oxygen content (8). Therefore the oxygen partial pressure plays an important role in the
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0 9 2 1 - 4 5 3 4 ( 9 5 ) 0 0 6 3 3 - 8
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V.A. Alyoshin et al./Physica C 255 (1995) 173-179
HgBa2CuO4+ ~ synthesis, and to obtain monophase Hg-1201 compound the internal oxygen pressure needs to be optimized. The reactions that take place in the SST between mercury oxide and Ba2CuO3+ ~ at high temperature are described below. We consider that the interactions occur mainly via vapor (Hg, O2)/solid (precursor) reactions in the sealed vessel as it was shown in Refs. [5] and [6]. Under heating, the mercury oxide decomposes into metallic mercury vapour and oxygen; the saturated vapor pressure under HgO reaches 1 atm at 450°C [7]: HgO ~ Hg(0 ) + 1 / 2 0 2 . The mercury vapor and the oxygen interact with Ba2CuO3+ x in the following way: Ba2CuO3+ x + Hg(0 ) + (1 - x + t ~ ) / 2 0 2
¢* HgBa2CuO4+ ~.
(1)
From this equation it follows that the oxygen partial pressure increase must displace the equilibrium (1) to the formation of HgBa2CuO4+ ~ causing also the decrease of the mercury partial pressure, and thus pure Hg-1201 may be obtained at the correct value of the partial internal oxygen pressure. However, this should be correct only in the case of the interaction between Hg(o), 0 2 and Ba2CuO 3+x for reaction (1). According to Refs. [8] and [9] at high oxygen pressures in the B a - C u - O and B a - H g - O systems, phases such as BaHgO 2, BaCuO2+ ~ or Ba2Cu3Os+ 8 were shown to be stable. These phases may also be stable in the H g - B a C u - O system and can be formed taking into account the following reactions: Ba2CuO3+ x + Hg(o) + (1 - x + y ) / 2 0 2
¢* BaHgO 2 + BaCuO2+y,
(2)
3Ba2CuO3+ x + 4Hg(o ) + (4 - 3x + z ) / 2 0 2 ¢* 4BaHgO 2 + Ba2Cu3Os+ z.
(3)
The formation of BaCuO2+y and Ba2Cu305+ z significantly depends on the oxygen pressure. The latter phase is stable in the B a - C u - O system at 827°C when .p(O 2) >_ 0.35 bar, while the former one exists in the 4 × 10 -5 < p(O 2) < 37.8 bar range [9]. This way the formation of monophase HgBa2CuO 4 + may take place in a limited oxygen pressure interval, outside of which the formation of impurity phases may be more favorable.
The aim of the present work was to study the partial oxygen pressure ranges where the synthesis of monophase HgBa2CuO4+ ~ is possible. For the exact control of the internal partial oxygen pressure an oxygen getter made of transition metal oxides was used to vary this parameter to a definite value.
2. Experimental The synthesis of HgBa2CuO4+ ~ was carded out in two steps from high-purity reactants. First the Ba2CuO3+ a precursor was synthesized through the stoichiometric mixture of BaO 2 and CuO at 900°C for 20 h in oxygen flow, quenched and allowed to c001 in a dry box. The precursor was checked by X-ray powder diffraction which showed that it contained only Ba2CuO3+ a without any additional phases. This precursor was subsequently mixed with the stoichiometrical amount of HgO. All the operations with the synthesized substances were conducted in air, oxygen or argon, dried and purified from CO 2. The weight accuracy was _+0.0002 g. The synthesis of HgBa2CuO4+ a was performed in sealed silica tubes with 15 cm length and 0.8 cm of inner diameter. The tubes were evacuated up to 1 0 - 2 - 1 0 -3 Tort and placed into a two-temperature gradient regulated furnace. For the regulation of the partial oxygen pressure in the SST, a two-phase mixture of oxides C u O / C u 2 0 and C o 3 0 4 / C o O was used as an oxygen getter. The oxygen resulting from the decomposition of HgO under heating is absorbed by the more reduced compound with the consequent formation of the more oxidized one according to the following reactions: Cu20 q- 1 / 2 0 2 ¢* 2CuO,
(4)
3CoO + 1 / 2 0 2 ** Co304.
(5)
Theoretical values of the partial oxygen pressure in equilibria (4) and (5) may be estimated from Eq. (6) [for C u O / C u 2 0 in the 298-1358 K temperature range] and (7) [for C%O4/COO in the 1000-1240 K temperature range], taken from Ref. [7]: log p ( O 2 ) , a t m = - 1 5 0 3 7 / T + 10.865,
(6)
log p ( O 2 ) , a t m = - 1 5 6 0 0 / T + 12.6.
(7)
V.A. Alyoshin et a l . / Physica C 255 (1995) 173-179
A pellet of the reactants mixture was placed in the less heated part of the tube and a pellet of C u O / C u 2 0 or C o 3 0 4 / C o O oxides mixture was placed in the higher-temperature region. Under heating of the SST the mercury oxide decomposes with the release of oxygen, which is absorbed by the getter. The oxygen pressure may be varied by changing the temperature of the getter pellet due to the three phase equilibria (4) or (5). For low oxygen pressures the C u O / C u 2 0 mixture was used, while for middle and high oxygen pressures the C o 3 0 4 / C o O mixture was more effective. The used getter average quantity was 1-1.5 g. This method allowed us to prepare from 0.7 up to 3.5 g of HgBa2CuO4+ 8. The obtained compounds were annealed in purified oxygen or argon flow at the reactor which was placed into a horizontal "Nabertherm" furnace. The furnace temperature accuracy measurement was +5°C. The annealings at higher oxygen pressure were carded out in closed stainless steel reactors. Phase compositions of the initial and synthesized compounds were controlled by X-ray analysis using a Guinier-camera FR-552 (Cu Kcx~ radiation, A = 1.54056 A) with germanium as internal standard. The wet analysis of Hg, Ba and Cu was carded
175
out by the methods described in Ref. [9]. The extra oxygen content 6 in the HgBa2CuO4+ 8 compound was determined by iodometric titration [10]. The AC susceptibility measurements were carried out with an external field of 1 0 e and a frequency of 27 Hz in the 120-12 K temperature range. The superconducting transition temperature was determined from the real part decrease of the magnetic susceptibility.
3. Results and discussion
3.1. Synthesis of HgBa2Cu04+ 8 under controlled partial oxygen pressure The experimental conditions and reproducible obtained results of the synthesis of HgBa2CuO4+ ~ at fixed partial oxygen pressure are given in Table 1. The equilibrium partial oxygen pressures, determined with the use of the oxygen getter, were calculated from Eqs. (6) and (7). The phase % and composition were evaluated by comparison of the main reflections intensities on the X-ray powder diffraction patterns. The amount of prepared samples was 0.7-0.8 g. As it can be seen from Table 1, the appearance of
Table 1 Synthesis conditions, phase composition and cell parameters for HgBa2CuO4+ 8 N
7-1 (oC) a
PO 2 (bar)
Composition
a (A)
c (A)
1
800
7-2 (oC) b 820
0.0013
3.895(2)
9.532(7)
2 3 4
800 810 790
800 890 940
0.012 0.15 0.55
3.8833(4) 3.8823(8) 3.8822(7)
9.53(1) 9.523(3) 9.523(3)
5
810
1000
3.8799(4)
9.520(2)
6
810
3.8819(6)
9.523(3)
7 8
810 810
H g B a 2 C u O 48+(90%), Ba2CuO3+ 8 (10%), Hg H g B a 2 C u O 48,+ Hg HgBa2 CuO4+ 8, Hg (traces) HgBa2CuOo+ 8 (95%), BaCuO2+ 8 (5%), Hg (traces) HgBa2CuO4+ 8 (80%), BaCuO/+ a (20%), Hg (traces) HgBazCuO4+ 8 (95%), Ba2Cu305 + 8 (5%), Hg (traces) HgBa2CuO4+ ~, Hg HgBa2CuO4+ 8 (50%), BaCuOz+ ~ (20%), BaHgO z (30%), Hg
3.8816(8) 3.8839(8)
9.522(3) 9.531(3)
890 890
2.2
0.15 0.15
a T(1) is the temperature of reactants mixture [Ba2CuO3+ 8 and HgO]. b T(2) is the temperature of the oxygen getter (CuO/Cu20, for the first experiment, and C0304/C00, for the rest).
176
V.A. Alyoshin et a l . / Physica C 255 (1995) 173-179
Hg(0) drops is almost a constant feature and when the partial oxygen pressure is decreased, their amount increases (this problem is discussed below). At low oxygen pressure (0.001 bar), Ba2CuO3+ ~ appears as an impurity which can be explained by the equilibrium described in Eq. (1). It should be mentioned that the Hg-1201 phase obtained at low oxygen pressure is strongly reduced, and it exhibits a superconducting transition at 49 K (Table 1, exp. 1). When the oxygen pressure was increased the unreacted Ba2CuO3+ ~ disappeared. In the partial oxygen pressure range 0.012-0.15 bar the Hg-1201 phase was obtained in pure form according to X-ray powder data (Table 1, exp. 2 and 3) and only traces of Hg drops were found. Their appearance can be attributed to the equilibrium partial mercury pressure under the Hg-1201 compound in the SST. Syntheses at these conditions were reproduced several times and the final phase composition of the annealed samples was exactly the same. The cell parameters of the Hg-1201 phase decrease regularly with the increase of the partial oxygen pressure, while T~ increases as well. When the partial oxygen pressure is increased to >_ 0.55 bar, B a C u O 2 + ,~ a n d / o r B a 2 C u 3 0 5 + ~ appear and their amount increases with the increase of the partial oxygen pressure (Table 1, exp. 4, 5). The appearance of BaCuO2+ J B a / C u 3 0 5 + ~ is explainable if the equilibria (2, 3) are taken into consideration. At higher oxygen pressures when BaO 2 was used as oxygen source, HgBa2CuO4+ ~ was not formed, and only phases such as BaHgO 2, Ba2CuO3+ ~ and Ba2Cu3Os+ ~ were detected by X-ray powder diffraction. We could not quantify the partial oxygen pressure inside the SST because the getter was not used. However, the absence of Hg drops after the synthesis showed us that this pressure was in fact high. This means that equilibrium (3) takes place in the SST instead of equilibrium (1). Only when the partial oxygen pressure was in the 0.012 < p(O 2) _< 0.15 bar range impurity phases were not detected by X-ray powder diffraction. In the experiment performed without any additional tablet of oxygen getter, Ba2Cu305 + ~ appeared as an impurity (Table 1, exp. 6) which means that the oxygen pressure in the SST was > 0.15 bar. The pellet of cobalt (or copper) oxides mixture naturally plays a
role of oxygen absorption and decreases the partial oxygen pressure in the SST. These results allowed us to conclude that the partial oxygen pressure exerts a main influence on the phase composition of the as-prepared Hg-1201 samples. In order to study the influence of the internal mercury partial pressure on the phase composition of the prepared Hg-1201 samples we performed some synthesis at fixed partial oxygen pressure (0.15 bar) with extra quantities of mercury oxide (10, 30 and 50 mol.% in addition to the stoichiometric nominal HgBa2CuO4+ ~ composition). The addition of extra amount of HgO has, as a direct consequence, an increase of the partial mercury pressure, and this causes an increase of the amount of metallic mercury found in the SST. It is remarkable that the addition of 10% of extra mercury oxide did not change the phase composition of the sample or the lattice parameters of the Hg-1201 phase (Table 1, exp. 7). However, in a similar experiment without using an oxygen getter described in Ref. [5] such addition of HgO led to the appearance of a large amount of impurity phases, such as Ba2Cu305 + ~ and BaHgO/. In the latter experiment an addition of extra HgO also caused an increase of the intemal oxygen pressure providing thus the appearance of these impurity phases. However, with the addition of 30 and 50% of extra mercury oxide, impurities such as BaCuO2+ 8 and BaHgO 2 appeared and their formation can be explained due to the existence of more complicated phase relations in the quarternary H g - B a - C u - O system which depend on the cation composition, temperature and partial pressures. Our investigations have shown that the possibility of obtaining monophase HgBa2CuO4+ ~ occurs at 800°C and in the partial oxygen pressure interval within 0.012-0.15 bar. Using these conditions, the synthesis of higher quantities of HgBa2CuO4+ 8 even up to 3.7 g was carded out without major difficulty. The experimental conditions are described as experiment 3 in Table 1. The X-ray diffraction pattem of the Hg-1201 sample is shown in Fig. 1. The as-prepared HgBaECUO4+ ~ phase has lattice parameters a = 3.8842(9) A, c = 9.530(4) ,~ and Tc = 85 K. The cell parameters of this compound obtained in a massive sample differ from those determined for phases obtained in samples of smaller weight (Table 1, exp. 3). This fact may be due to the different cooling rate
VA. Alyoshin et al./ Physica C 255 (1995) 173-179
Counts
~ ' ' 1
¸''
. . . . .
~ . . . .
I ' ' ' ' 1
. . . .
L . . . .
I ' ~ '
8250 5500 2750
g -
-
~ ~
-
0
.,.
L
r~,
10
~
~
g~ ~
~
i i , l , l , , i
20
~
. . . .
30
I , , , ~ t * , ~ l , , , , I , , i r
40
50 60 70 80 90 20 Fig. 1. X-ray powderdiffracfionpattemof HgBa2CuO4+~.
of the massive sample in comparison with smallerweight samples. It should be noted that the T~ of the as-synthesized sample (85 K) is smaller than that determined for the as-prepared Hg-1201 samples described in Refs. [1-5] since the synthesis was performed at a lower partial oxygen pressure. The as-synthesized sample was annealed in oxygen flow at 250°C during 24 h. After annealing, T~ increased to 96 K and the cell parameters decreased to a = 3.8794(4) A and c = 9.525(1) ,~. The x(T) curves for the as-synthesized and oxygen-annealed samples are shown in Fig. 2. According to wet-analysis data the amount of Ba, Cu and Hg in the sample is, respectively, 3.34(6), 1.76(4) and 1.48(3) m m o l / g . Assuming that the amount of barium determined by wet analysis corre-
-0.005
-~
•
~
-o.ol ~
-0.015
e
e
e
~
, , , I , , l , I 0 20 40 60 T (K) i
,
r
i
i
, I , 80
, I 100
i
r
i
120
Fig. 2. AC susceptibility vs. T of as-synthesized (1) and annealed in oxygen flow (2) HgBa2CuO4+~ samples synthesized by method 1.
177
sponds to a stoichiometry of 2, then the chemical formula of the compound may be written as Hg0.89(3)Ba2Cul.o6(5)O4+ 8. From this composition we can conclude that there is a significant Hg deficiency in the compound. Earlier the mercury deficiency in HgBa2CuO4+ 8 was found from neutron powder structural refinement [12]. These authors concluded that ,-~ 10% Hg cations are partially replaced by copper ones, and this agrees well with our wet-analysis data. However, the presence of X-ray amorphous impurities, which do not contain Hg, can also provide a lower Hg content in the sample, as determined by wet analysis. It is known, that at T > 650°C HgBa2CuO4+ 8 decomposes in an open system with removal of Hg(0) and oxygen [5]. It has also been shown recently that a large Hg deficiency occurs on the surface of Hg-1201 irradiated samples [13,14]. The decrease of the Hg amount in the as-prepared sample may be caused by a partial loss of mercury in vapor phase which is in equilibrium with the solid state, after which Hg(0) drops condensate in the SST as a result of the cooling process. It is remarkable that according to the wet analysis performed on the sample prepared with an addition of 10% HgO (Table 1, exp. 7) [which created a higher partial Hg pressure], the amount of Ba and Cu was found to be approximately the same as in the previous analysis, being, respectively, 3.34(6) and 1.79(4) m m o l / g . At the same time the Hg content in the sample, which did not contain a noticeable amount of the metallic mercury, increased significantly to 1.63(3) m m o l / g . In this case the chemical formula of the Hg-1201 compound may be written as Hg0.98(3)Ba2Cul.07(6) 04+ ~. This composition corresponds to the ideal chemical formula if the standard deviations are taken into account. It should be noted that there are no differences in lattice constants for this phase and for the phase prepared under the same oxygen partial pressure without addition of HgO (Table 1, exp. 3). From these data we can suppose that the amount of mercury in the HgBa2CuO4+ ~ phase either does not depend or only depends slightly from the partial mercury pressure in the SST. This gives us an indirect proof that the replacement of Hg cations by Cu ones can either occur in a small range or may not even exist at all. The same conclusion was made by Alexandre et al. from the study of the Hg1_xCuxBa2CuO4+ 8 system under high pressure
V.A. Alyoshin et al./ Physica C 255 (1995) 173-179
178
0
[15]. It should be mentioned that a partial substitution of Hg by carbonate group can take place in the Hg-1201 structure [16] providing also a certain Hg deficiency. 3.2. Two-temperature gradient HgBa2CuO 4 + 8 synthesis with an additional pellet o f Ba2CuO 3 + 8
The HgBa2CuO4+ 8 samples prepared according the procedure described above usually contained mercury drops on the SST walls. In order to absorb this mercury, a two-temperature gradient synthesis was carried out using an extra pellet of Ba2CuO 3 + ~. The use of an extra pellet of Ba2CuO3+ ~ in the preparation of HgBa2CuO4+ ~ is well described in Refs. [3] and [4]. Starting pellets of a reactant mixture composed of Ba2CuO3+ ~ and HgO and pure BazCuO 3+ ~ were placed into the opposite sides of a SST. The temperature gradient was 15-20°C with the Ba2CuO3+ ~ pellet on the hotter side to decrease the mercury pressure in the SST and prevent the mercury drops from appearing. The partial oxygen pressure in these experiments was not controlled and the temperature regimes and the proportion between Ba2CuO3+ ~ and HgO weights and the tube's volume were taken from Ref. [4]. According to X-ray powder data the as-prepared HgBa2CuOo4+~ phase had lattice parameters a = 3.8818(5) A, c = 9.525(2) A, T~ = 94 K and extra oxygen content 6 = 0.08(1) (wet-analysis data). This sample was annealed under different conditions, which are listed together with obtained results in Table 2, and the x ( T ) curves are shown in Fig. 3. From Table 2 one can see that the cell parameters of HgBa2CuO4+ ~ decrease with the increase of the oxygen pressure, with the sole exception of sample 4. This might in fact be caused by a punctual non-
i
000
-0.004
g
~ -0.0o6
,~3
"~ -0.008 -0.01 -0.012 0
. . . . . . . . . . 20 40 60
i ...... 80 100
120
T (N) Fig. 3. AC susceptibility vs. T of the as-synthesized (1), p(O2) = 90 bar (2) and argon (3) annealed the HgBa2CuO4+ ~ samples synthesized by method 2.
equilibrium state for sample 4. The variations of T~ versus a parameter or extra oxygen content ( 6 ) are described by a parabolic-like behavior as was shown earlier in Refs. [5,16,17]. The high transition temperatures are reached for the Hg-1201 compounds with a parameter = 3.88 A and copper valence = +2.15; the latter value is a typical one for the optimally doped superconducting Cu mixed oxides. The samples prepared by such a technique usually contained 5-10% of impurity phases such as Ba2Cu305 + ~ and BaCuO2+ ~. However, no mercury drops were observed in these experiments. We suppose that the impurity phases appearance may be due either to a partial oxygen pressure > 0.15 bar in the SST or to the deviation of the cation stoichiometry from the ideal Hg-1201 composition. The second pellet of Ba2CuO3+ ~ according to X-ray powder diffraction analysis contained 5 - 1 0 % of HgBa 2CuO4+ 8.
Table 2 Annealing conditions, cell parameters, Tc and 8 for HgBa2CuO4+ N
P (bar)
T (°C)
t (h)
a (A)
c (A)
Tc (K)
t~
1 2 3 4 5
Ar, t as-prepared 02 , 1 O 2, 125 02 , 90
350
67
250 200 250
24 18 66
3.8887(5) 3.8818(5) 3.8806(6) 3.8801(12) 3.8757(5)
9.540(2) 9.525(2) 9.525(2) 9.521(5) 9.514(2)
68 94 95 97 89
0.01(1) 0.08(1) 0.12(1)
V.A. Alyoshin et al. / P hysica C 255 (1995) 173-179
The two-temperature gradient synthesis of HgBa2CuO4+ ~ with an extra pellet of Ba2CuO3+ 8 has more advantages than the methods described in Refs. [3] and [4] because the mercury concentrates in the low-temperature reactants tablet. However, the disadvantage of this technique is the necessity to control all the synthesis parameters including the reactant's weight, S S T ' s volume, extra oxygen content in Ba2CuO3+ ~ and the temperature region in order to have a good result reproducibility, in contrast to the controlled partial oxygen pressure synthesis described in the previous part.
4. Conclusions A reproducible method of HgBa2CuO4+ ~ synthesis was developed. The synthesis of monophasic Hg-1201 samples was carried out at 800°C under a partial oxygen pressure within the range 0 . 0 1 2 - 0 . 1 5 bar. Pellets of copper or cobalt oxides mixtures were used for oxygen-pressure control. The oxygen pressure inside of sealed silica tubes was regulated by the temperature variation of these oxides mixtures.
Acknowledgements The authors are grateful to G.N. Mazo for having performed the wet analysis of HgBa2CuO4+ ~, to P.E. Kazin for the magnetic measurements and S.M. Loureiro for the help with the manuscript preparation. This work was partially supported by the Russian Scientific Council on Superconductivity (Poisk) and the B M F T Project 13N6401.
179
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