indicates the method is effective in inserting oxygen into La2CuO4. In addition, the ... been performed by heat treatment under a high oxygen pressure at ...
Supercond. Sci. Technol. 13 (2000) 1415–1418. Printed in the UK
PII: S0953-2048(00)13666-8
Hydrothermal oxidation: a new chemical oxidation method to dope oxygen in La2CuO4+δ Y C Lan†, X L Chen†, G C Che‡, Y G Cao†, J Y Li† and Q Y Tu† † Institute of Physics and Centre for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ‡ National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 3 May 2000, in final form 26 July 2000 Abstract. A new oxidation method, chemical oxidation under hydrothermal conditions, is introduced to insert oxygen into La2 CuO4 compounds. X-ray powder diffraction and magnetic susceptibility measurements show that the oxidized La2 CuO4+δ with orthorhombic space group becomes a superconductor with a superconducting transition up to 42 K, which indicates the method is effective in inserting oxygen into La2 CuO4 . In addition, the oxidation mechanism and the unique properties of the closed system are discussed.
1. Introduction
Since the discovery of high-temperature superconductivity in La2−x Srx CuO4 by Bednorz and M¨uller [1], the La-214 family has been extensively studied due to the relative simplicity of its structure for the investigation of the mechanism of high-temperature superconductivity as well as its many interesting and important physical properties such as oxygen ordering, stripe ordering and phase separation. La2 CuO4 is the parent compound of the La-214 family of superconductors and has been used as the host material for the intercalation of anions (O2− , F− and Cl− ) in order to attempt to understand the superconductivity mechanism. Until now, the introduction of excess oxygen has usually been performed by heat treatment under a high oxygen pressure at elevated temperature [2–5], electrochemical oxidation [6–10], chemical oxidation [11, 12], lowtemperature plasma oxidation [13], and ozone oxidation [14]. La2 CuO4 becomes a superconductor with a highest Tc near 45 K after accepting excess oxygen. Recently, we reported a new alternative way to insert the excess oxygen into La2 CuO4 under hydrothermal conditions in the temperature range of 100–140 ◦ C [15]. The insertion is effective due to the unique properties of the experimental assembly. In this paper we report the experimental assembly in detail and discuss the reaction mechanism in the close system. 2. Experimental details
Starting materials were prepared by the conventional solid state reaction route. Stoichiometric amounts of predried La2 O3 and CuO were ground thoroughly, pressed into pellets and calcined in air at 900 ◦ C for 24 h. Subsequently the 0953-2048/00/101415+04$30.00
© 2000 IOP Publishing Ltd
mixture was re-ground, pelletized, sintered at 1100 ◦ C for 6 days in air with several intermediate grindings and then quenched to room temperature in air. A La2 CuO4 single phase was obtained. The chemical oxidation of the materials was performed in autoclaves from 100 ◦ C to 180 ◦ C, using a potassium permanganate aqueous solution. The equipment is illustrated in figure 1. The heart of the system was a modified Tuttletype cold cone seal autoclave which was externally heated. The autoclave consisted of a stainless steel vessel closed by a cone-in-cone seal. The vessel and the cone seat closure were lined with platinum to resist the oxidation. In this way, the oxidation procedure was maintained and contamination avoided. The vessel assembly was kept inside the furnace, the open end and seal were outside the furnace. The temperature was measured by placing a thermocouple near the well of the vessel. The pressure inside the vessel was built up by the water vapour pressure. It is assumed that the pressure created in the autoclave is the saturation vapour pressure at the oxidation temperatures. It is about in the range of 120–199 KPa when the oxidation temperature is 105–120 ◦ C. The vapour pressure in the autoclave can be regarded as a constant when the oxidation temperature is kept constant. A saturated aqueous solution of KMnO4 was prepared using distilled water at room temperature. After filling the autoclave with 3.0 g of powder of the La2 CuO4 powder sample, KMnO4 saturated solution was poured into the reaction vessel at room temperature. The volume of the solution and solid materials was about 18 ml. Then the autoclave was maintained at 105–180 ◦ C for about 2 days and cooled to room temperature naturally. After oxidation treatment, the powder sample was collected, filtered, thoroughly washed with distilled water and dried at room temperature in vacuum as described in [15]. 1415
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Temperature control system
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Figure 1. Schematic diagram of the employed experimental assembly (hydrothermal apparatus). 1, plug; 2, cone seat closure; 3, chamber; 4, hydrothermal vessel; 5, KMnO4 aqueous solution; 6, La2 CuO4 powder; 7, screw cap; 8, platinum liner; 9, furnace; 10, two sets of thermocouples.
X-ray powder diffractions were carried out on a Rigaku D/max-2400 diffractometer using Cu Kα radiation at room temperature. The lattice parameters were calculated using DICVOL91 [16]. Superconducting properties were examined by a SQUID magnetometer under a direct current (dc) magnetic field down to 4.2 K. 3. Results
The x-ray diffraction of the as-prepared La2 CuO4 is shown in figure 2(a). It is La2 CuO4 single phase according to PDF 38–0709 (a = 5.355 Å, b = 5.401 Å, c = 13.14 Å). Figure 2(b) shows typical x-ray diffraction patterns for The contribution of La2 CuO4+δ oxidized at 105 ◦ C. Cu Kα2 is arithmetically subtracted. All peaks can be indexed based on an orthorhombic space group and the sample is still nearly single phase after oxidation. Based on F mmm space group, the lattice parameters calculated from the main peaks are changed greatly from a = 5.353(1) Å, b = 5.401(1) Å, c = 13.142(1) Å of asprepared La2 CuO4 to a = 5.334(1) Å, b = 5.435(1) Å, c = 13.247(1) Å of oxidized La2 CuO4+δ . The 2θ splitting between the (020) and (200) peaks increases after oxidation. So do those between the (024) and (204), (133) and (313) peaks. The orthorhombic distortion, calculated as f = 2 (b − a) / (b + a), varies rapidly from 0.0089 for stoichiometric La2 CuO4 to about 0.019 for oxidized La2 CuO4+δ after oxidation. The increase of 2θ splitting and f indicates the insertion of excess oxygen. The excess oxygen content determined by iodometric titration was δ = 0.21(2). At other treatment temperatures up to 140 ◦ C, 2θ splitting also increases (see figure 2); so does f . 1416
Alcohol and benzene were also used to replace water as the solvent; similar results were observed. Figure 2(c) shows the XRD of the oxidized sample at 115 ◦ C in alcohol solution. If the treatment temperature was higher than 140 ◦ C, the La2 CuO4+δ orthorhombic phase began to decompose. A typical pattern is shown in figure 2(e). We suppose that the ionic oxygen coming from KMnO4 can violently attack La2 CuO4+δ under the hydrothermal conditions, resulting in the decomposition of La2 CuO4+δ . Varying the lattice parameters induces the change of the magnetic properties. Figure 3 shows the magnetic susceptibility data for the oxygenated samples. The 105 ◦ C oxygenated sample is a superconductor with an onset χ temperature Tc = 42 K and χ = 0 at 36 K though the stoichiometric La2 CuO4 shows no superconducting transition down to 4.2 K. The susceptibility data of the sample oxidized at 140 ◦ C also indicate a superconducting transition above 36 K. All of the samples oxidized in the range of 100–140 ◦ C show a high superconducting transition. 4. Discussion
In the unique experimental assembly, KMnO4 decomposes slowly and releases ionized oxygen atoms, 2KMnO4 = K2 MnO4 + MnO2 + 2O
(1)
and an equilibrium can be reached in the closed system. Additionally, relative high-temperature (above 100 ◦ C) can accelerate the decomposition of KMnO4 which can increase the partial pressure of oxygen in the closed autoclave. Thus, a higher pressure of O can be obtained in the closed system, unlike under the conventional chemical oxidation conditions in which the partial pressure of oxygen is low.
Hydrothermal oxidation
Figure 2. Room temperature x-ray diffraction patterns for: (a) the as-prepared La2 CuO4 sample; (b) 105 ◦ C oxidized La2 CuO4+δ sample in aqueous solution; (c) oxidized at 115 ◦ C in alcohol solution; (d) 140 ◦ C; and (e) 180 ◦ C oxidized La2 CuO4+δ sample in aqueous solution, respectively. Table 1. Comparison of different oxidation methods for La1.64 Sr0.36 CuO4 (RT: room temperature).
Oxidation method High oxygen pressure Electrochemical Chemical Plasma Ozone Hydrothermal
Temperature ◦
500–900 C RT–60 ◦ C RT–60 ◦ C 80 ◦ C 150–400 ◦ C 100–140 ◦ C
Time 12–48 h 9–72 h one week 48 h 0.2 h 48 h
δ
Pressure 500 bar–23 Kbar 100 KPa 100 KPa 0.7 Torr 10–4 Torr 100–200 KPa
b
a
0.01 –0.13 0–0.09a,b , 0.18a 0.01–0.22a ? ? 0.21(2)b
Tc (K)
References
40 32, 45 14, 40 38.5∗ 48 K (film) 42 K
[2–5] [6–10] [11, 12] [13] [14] [15], this work
∗
For La1.64 Sr0.36 CuO4 . Estimated from TGA measurements. b Obtained by iodometric titration determinations. a
In the closed system, another equilibrium can also be obtained, (2) La2 CuO4 + δO = La2 CuO4+δ . The high pressure of O would shift the equilibrium in equation (2) to the right, inducing more excess oxygen doping into La2 CuO4+δ . Compared with the conventional chemical oxidation, the chemical oxidation under hydrothermal conditions should be more effective for inserting oxygen into La2 CuO4 from the viewpoint of treatment time and oxygen content, see table 1. The main reason can be explained in terms of the higher temperature and higher partial pressure of oxygen which can only be obtained in the closed autoclave system. (1) From the viewpoint of thermal diffusion, the high temperature above
100 ◦ C favours the excess oxygen insertion into La2 CuO4 . (2) Additionally, in the autoclaves, the rapid decomposition of KMnO4 above 100 ◦ C induces a high pressure of oxygen in the system. In the conventional chemical oxidation methods, the oxidation cannot be carried out above 100 ◦ C due to the boiling point of water and the pressure cannot be higher than 1 atm. The equilibrium between gaseous O (from KMnO4 ) and that in La2 CuO4+δ can be reached in the closed autoclaves. This condition is like the high oxygen pressure method, in which the thermodynamic equilibrium of oxygen exists between La2 CuO4+δ and the oxygen atmosphere. This may be the reason why La2 CuO4+δ with Tc up to 42 K can be obtained after a short time treatment in the closed system. 1417
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to the insertion of excess oxygen into La2 CuO4 due to the high temperature and high oxygen pressure which can only be achieved in the autoclaves. The reaction mechanism and the advantage of the assembly were discussed.
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The authors thank Professor P Z Jiang, Y P Xu and T Xu for the technology support. The work was supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China.
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Temperature (K) Figure 3. DC magnetization measurements for La2 CuO4
(squares), 105 ◦ C oxidized La2 CuO4+δ (circles) and the La2 CuO4+δ oxidized at 140 ◦ C (triangles), respectively, in an applied magnetic field of 10 Oe.
5. Summary
In summary, the chemical oxidation of La2 CuO4 under hydrothermal conditions has been described in which La2 CuO4 was chemically oxidized above 100 ◦ C under thermodynamical equilibrium in a closed system. The oxygen doping induces an increase both of the 2θ splitting and of the orthorhombic distortion. The oxidized samples showed high Tc up to 42 K. This method effectively leads
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