Direct oxidation of La2CuO4 in an aqueous solution of KMnO4

PHYSICA

Physica C 207 ( 1993 ) 97-101 North-Holland

Direct oxidation of E. T a k a y a m a - M u r o m a c h i ,

La2fuO4 in an aqueous solution of KMnO4 T. S a s a k i a n d Y. M a t s u i

National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki, 305 Japan

Received 5 January 1993

This report presents a new method to obtain superconducting La2CuO4+~. The La2CuO4 obtained by the normal solid state reaction was oxidized in an aqueous solution of KMnO4. The product included excess oxygen up to J~0.09 and showed bulk superconductivity below about 40 IC Two different orthorhombic phases appeared upon increasing of J; one had a smaller orthorhombic distortion while the other had a larger one, compared with the stoichiometric La2CuO4. Electron diffraction patterns suggested a complicated manner of ordering of the interstitial oxygen atoms in the sample with J ~ 0.09.

1. Introduction La2CuO4 with a K2NiF4-type structure becomes superconducting b y being d o p e d with carriers. Bednorz a n d Miiller first showed that the substitution by an alkaline earth element for the La site creates holes resulting in the superconductivity with T~ near 30 K [ 1 ]. Subsequently, it has been shown that superconductivity occurs by the excess oxygen i n c o r p o r a t i o n rather than the alkaline earth substitution. The oxidation has usually been carried out by heat treatm e n t o f the La2CuO4 u n d e r high oxygen pressure [ 2,3 ]. Recently, it was reported that La2CuO4 can be oxidized at r o o m t e m p e r a t u r e in an aqueous base using an electrochemical technique [ 4 - 7 ] . The specim e n thus o b t a i n e d showed superconductivity below about 40 K [ 4 - 7 ] . In this short report, we present a new m e t h o d to o b t a i n the superconducting LaECuO4+6, a n d the prel i m i n a r y d a t a o f the structural and superconducting properties. We oxidized as-grown La2CuO4 in an aqueous solution o f KMnO4. The c o m p o u n d was easily oxidized by the K M n 0 4 a n d showed a Tc near 40 K. The a m o u n t o f excess oxygen ~ could be controlled by changing the concentration o f the KMnO4 solution. This m e t h o d seems very convenient to obtain a large a m o u n t o f the superconducting sample.

2. Experimental

The La2CuO4 sample was synthesized using the n o r m a l solid state reaction. La203 (99.9%) d r i e d at 1000°C and C u P (99.9%) d r i e d at 700°C were used as starting reagents. The mixture having the appropriate ratio was calcined at 1100 °C for 4 days with an i n t e r m e d i a t e grinding, then cooled in air. The p r o d u c t thus o b t a i n e d was thoroughly ground in an agate m o r t a r a n d was e x a m i n e d by p o w d e r X-ray diffraction with Cu K a radiation. N o extra peaks were found in the X-ray pattern. A b o u t 2 g o f the sample was put into the KMnO4 solution (ca. 50 cc), then was allowed to react at 50°C for 48 h. Three different concentrations o f the K M n O , solution were tried, i.e., 0.5%, 5%, a n d saturated solutions. After the oxidation, the sample was filtered off, washed with distilled water a n d ethanol, and d r i e d on silica gel at r o o m temperature. The KMnO4-treated samples were e x a m i n e d by means o f p o w d e r X-ray diffraction a n d their lattice constants were d e t e r m i n e d from the X-ray patterns. To see structural changes with the oxidation, electron diffraction patterns o f the oxidized samples were o b t a i n e d by an electron microscope accelerated with 200 kV. The oxygen contents o f the samples were determ i n e d by a t h e r m o g r a v i m e t e r (Perldn Elmer T G A 7). A b o u t 100 mg o f each sample was placed in a

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98

E. Takayama-Muromachi et al. / Direct oxidation of La2CuO~

sat. KMnO4 I~

20

A_ Z_

I

50 60 32 34 40 20 (deg) 20 (deg) Fig. 1. Powder X-ray patterns of as-grown and KMnO4-treated ta2Cl,lO4+ d (Cu Kct radiation). 30

small alumina crucible and weight loss upon heating was measured in an Ar atmosphere. The superconducting transition temperature was determined using a SQUID magnetometer (Quantum Design). About 400 mg of powder sample was examined both in field cooling and zero field cooling under a field of 20 Oe.

I 100.0 ~ ~.

I

I

I

I

%

I KMnO4

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g ~

99.8

3. Results and discussion Figure 1 shows powder X-ray patterns for the asgrown sample and KMnO4-treated samples. In fig. 2, thermogravimetric data are shown for the oxidized samples. The lattice constants and the amount of excess oxygen J determined by the TGA analysis are given in table 1. The J value of the as-grown sample was close to zero (at most ~ 0.004 ~iccording to our previous work [ 8 ] ). The as-grown sample has an orthorhombic structure with the space group Bmab [ 9 ]. The orthorhombic distortion was smaller in the sample oxidized in the 0.5% KMnO4 solution compared with the as-grown sample. When the sample was oxidized in the saturated solution, however, the orthorhombicity became larger; it was even larger than that of the as-grown sample. These observations are compatible with the recent reports on the electrochemically oxidized La2CuO4+a

I

99.6 200

250

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300

I

I

350

400

I

450

500

T (°C) Fig. 2. Thermogravimetric analysis of the KMnO4-treated La2Cu04+6.

Table 1 Excess oxygen content J and lattice constants of La2CuO4+ Synthetic condition

J

Lattice constants (A) a

b

c

asgrown ~0 5.3586(3) 5.4029(3) 13.155(1) 0.5% KMnO4 0 . 0 2 9 5.364(3) 5.382(3) 13.164(7) 5% KMnO4 0.072 mixture of two phases sat. KMnO4 0.094 5.339(1 ) 5.427(1 ) 13.214(4)

E. Takayama-Muromachi et al. / Direct oxidation of La2CuO4

[6,7]. Grenier et al. [7] indicated that the orthorhombic distortion decreased with increasing 3 over the range 0 < 3 < 0 . 0 5 (phase Oi according to their paper) while a second orthorhombic phase (phase OM) appeared in the range 0.05 < 3< 0.09 where the orthorhombicity increased with increasing t5 and was larger than that of the stoichiometric La2CuO4. They suggested that there is a phase boundary at 3~ 0.055 which separates two orthorhombic phases, Oi and

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As shown in fig. 1, we obtained a mixture of the two phases when the sample was oxidized in the KMnO4 solution having the intermediate concentration (50/0); one phase had a smaller orthorhombicity (Ox) while the other had a larger orthorhombicity (O~t). If the Orto-OM transition is of first order, a two-phase coexisting region should exist in the phase diagram which may be the reason of the formation of the mixture. There is, however, another possibility for the mixed sample, i.e., the oxygen diffusion may be sluggish in the present experimental condition and the surface area of a particle might have a larger oxygen content than the inner part. We need further experiments to justify this point. In figs. 3(a) and (b) DC magnetic susceptibility data are shown which indicate the occurrence of bulk superconductivity in the KMnOA-oxidized samples. The superconducting volume fraction increased slightly with increasing 3. In addition, Tco,~t increased from ~ 30 K to ~ 40 K with increasing 3 from 0.03 to 0.09. These observations are also compatible with the experiments for the electrochemically oxidized samples [ 6,7 ]. The samples treated in 5% and saturated KMnO4 appeared to have two-step superconducting transitions near 40 K and near 10 IC This may be caused by the sample inhomogeneity as stated above. The decrease of orthorhombic distortion over the range of excess oxygen 0 < 3< 0.05 seems reasonable from the viewpoint of crystal chemistry. The distortion is caused by the size mismatch between the LaO and the CuO2 layer. By the Oxidation, electrons will be removed from the Cu-O antibonding orbitals and the Cu-O bond length is expected to decrease. This will relax the size mismatch. Indeed, the (La,Sr)2CuO4 system shows this kind of behavior; the system undergoes the orthorhombic to tetragonal transition and the Cu-O bond length decreases

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T (K) Fig. 3. DC susceptibility data of KMnO4-treated La2CuO4+a. (a) Zero field cooled, (b) field cooled.

monotonically with increasing Sr content [ 8 ]. The opposite tendency seen in the higher 5 range (OM phase), i.e., the increase of the orthorhombic distortion, is, therefore, somewhat abnormal and suggests ordering of the doped oxygen atoms. In figs. 4 and 5, electron diffraction patterns projected along various directions are shown for the KMnOa-treated samples. The diffraction patterns of the Oi phase (obtained from the 0.5% solution) are essentially the same as those of the stoichiometric t a 2 C u O 4 reported previously [ 10]. The patterns could not be fully described by the space group Bmb but forbidden reflections such as 010, 030, ... (fig. 4 ( b ) ) or 100, 300, ... (fig. 4 ( c ) ) appeared. Some of them cannot be explained by double reflection. The

100

E. Takayama-Muromachi et al. / Direct oxidation of La2CuO4

Fig. 4. Electron diffraction patterns of La2CuO,+6 oxidized in the 0.5% KMnO4 solution (~~0.03). (a) [001 ]* section, (b)[ 10i ]* section, (c) [010]* section, (d) [ 1i0]* section.

same forbidden spots were observed in the stoichiometric La2CuO4 as well, and it is believed that they are caused by additional small displacements of atoms from the ideal positions [ l 0 ]. In the patterns of the sample with ~~0.09, these reflections disappeared almost completely (see figs. 5(b) and ( c ) ) , but new other weak reflections forbidden in Bmb appeared, such as 001,003 .... in figs. 5 (c) and (d). Moreover, complicated superstructure reflections were observed in the [001 ]* and [ 1i0]* sections. As

shown in figs. 5 (a) and (d), four or more weak extra spots exist around a main spot. These extra spots suggest a complicated manner of ordering of the interstitial oxygen atoms (probably an incommensurate structure) and cannot be explained by a simple model. Lattice image observation for the OM phase is in progress.

E. Takayama-Muromachi et al. / Direct oxidation of La2Cu04

101

Fig. 5. Electron diffraction patterns of La2CuO4+, oxidized in the saturated KMnO4 solution (c~~ 0.09 ). (a) [ 001 ]* section, (b) [ 10 i ]* section, (c) [ 010 ]* section, (d) [ 1101" section.

References [ 1 ] J.G. Bednorz and K. A. Miiller, Z. Phys. B 64 (1986) 189. [2] J.E. Schirber, B. Morosin, R.M. Merrill, P.F. Hiava, E.L. Venturini, J.F. Kwak, P.J. Nigrey, R.J. Baughman and D.S. Ginley, Physica C 152 (1988) 121. [3] J. Zhou, S. Sinha and J.B. Goodenough, Phys. Rev. B 39 (1989) 12231. [4] A. Wattiaux, J.C. Park, J.C. Grenier and M. Pouchard, C. R. Acad. Sci. 310 (1990) 1047. [5]J.C. Grenier, A. Wattiaux, N. Lagueyte, J.C. Park, E. Marquestaut, J. Etourneau and M. Pouchard, Physica C 173 (t991) 139.

[6] F.C. Chou, J.H. Cho and D.C. Johnston, Physica C 197 (1992) 303. [7] J.C. Grenier, N. Lagueyte, A. Wattiaux, J.P. Doumerc, P. Dordor, J. Etourneau, M. Pouchard, J.B. Goodenough and J.S. Zhou, Physica C 202 (1992) 209. [8] E. Takayama-Muromachi and D.E. Rice, Physica C 177 (1991) 195. [9] J.M. Longo and P.M. Raccah, J. Solid State Chem. 6 (1973) 526. [ 10 ] G. Van Tendeloo and S. Amelinckx, Physica C 176 ( 1991 ) 575.