Evidence for superconductivity in fluorinated La 2 CuO 4 at 35 K: Microwave investigations

Transcription

1 PRAMANA ~: Printed in India Vol. 46, No. 4, journal of April 1996 physics pp Evidence for superconductivity in fluorinated La 2 CuO 4 at 35 K: Microwave investigations G M PHATAK, K GANGADHARAN, R M KADAM*, M D SASTRY* and U R K RAO** Chemistry Division, Bhabha Atomic Research Centre, Bombay , India *Radiochemistry Division, Bhabha Atomic Research Centre, Bombay , India **Applied Chemistry Division, Bhabha Atomic Research Centre, Bombay , India MS received 13 July 1995; revised 28 February 1996 Abstract. In the fluorinated La 2 CuO 4_:, prepared using a solid state reaction with NH4 HF2 as a fluorinating agent at 550 K at ambient pressure, superconductivity was detected by microwave and EPR techniques with a T c of 35 K. Keywords. Superconductivity; microwave absorption; EPR; La 2CuO 4. PACS Nos 74.10; 74-60; 74-70; It is widely accepted that the superconductivity in YBa2Cu3OT_ a is related to the mixed valence of copper, which can be varied by varying the oxygen content [1-1. Recently a partial fluorination of non-conducting cuprates was found to be a convenient route to introduce mixed valence of copper. Fluorination resulted in the formation of both n-type and p-type superconductors. Nd z CuO 4_x F~ was found to be an n-type superconductor below 27 K [2] whereas fluorinated Sr2CuO 3 was thought to be a p-type superconductor below 46 K [3, 4]. In the latter case the superconducting phase was believed to be Sr2CuO2F2+ x (x = ). The presence of interstitial fluorines in this structure is expected to produce p-type charge carriers. Therefore, the fluorination of cuprates has the ability to convert insulating cuprates into superconducting products with different possibilities of charge carriers. We have shown that a simple solid state reaction between Sr2CuO 3 and ammonium hydrogen fluoride yields the required superconducting phase. The additional advantage of using ammonium hydrogen fluoride for fluorination of the oxide lies in the fact that different amounts of fluorine can be incorporated into the product. A case in point is fluorination ofsr 2 CuO 3 using this route in which we [5-1 have shown that one can tune T c of the fluorinated product from 0 to 53 K by changing the fluorine content in the product. Slater et ai [6] have shown that Sr2_.~Cax/BaxCuO 3 type of compounds can be fluorinated by NH 4 F to give superconducting products, one of which exhibited a Tc of 64 K. These successes with regard to fluorination led to renewed attempts for fluorinating other perovskites. In the early days of cuprate superconductors, Tissue et al [7-1 reacted La2CuO 4 with F 2 gas and prepared a superconducting compound with T c of 34 K. However, their compound was inhomogeneous. Subsequently no reports 277

2 G M Phatak et al appeared on this compound. In the present investigations, we have shown that fluorination of La 2 CuO 4 using ammonium hydrogen fluoride route, which is a much simpler method than reacting with F 2 gas, does induce superconductivity below 35 K. Microwave technique is best suited for investigating fluorinated cuprates [8]. This method is contactless. Therefore, there is no need to sinter the pellet at high temperature which leads to decomposition of the superconducting compound. In this preliminary note we give microwave evidence for superconducting transition at 35 K in La 2 CuO 4 fluorinated by this route. The starting material La20 3 (99.9%) was preheated in air at 1270K to remove carbonate. The oxides in appropriate stoichiometry were mixed and heated in air at 1150K for 3h followed by heating at 1270K for 80h in air with three intermittent grindings and furnace cooling to room temperature. The resulting La2CuO 4 was mixed with NH4HF 2 in l:l ratio and heated in air at 550 K for one hour. The X-ray diffraction pattern was recorded on a Ni filtered CuK~ radiation on Philips PW! 729 wide angle goniometer. There is no difference observed in X-ray diffraction pattern between parent and the fluorinated compound. If oxygen atom is simply replaced by fluorine atom without change in symmetry of the lattice, no significant change in X-ray pattern is anticipated on fluorination [8]. This is because the anionic sizes of oxygen and fluorine are identical and therefore no change in cell dimensions will take place. Further the atomic numbers of these two atoms are close and hence the scattering powers for X-ray are also very close. Estimation of fluorine in the sample was done by heating the sample with SiO 2 followed by steam distillation of the H E SiF 6 formed into alkali solution and estimating the fluorine content in the resulting solution by ion selective electrode. Based on the analysis, the fluorinated compound may be represented as La2CuO4_~F2~, where 5 = The superconducting transition was monitored using EPR and direct microwave absorption technique. The sample was loaded as a pellet on a sapphire rod. The temperature was varied by using helium closed cycle refrigerator supplied by M/s APD cryogenics. Low field microwave absorption studies were conducted using an X-band Bruker ESP-300 spectrometer, whereas the direct microwave absorption (without field modulation) was monitored using a Varian V-4502 EPR spectrometer having a home built microwave bridge. The direct microwave absorption as a function of temperature was monitored by monitoring the temperature dependent changes in the microwave power reflected from the sample loaded cavity. This is shown in figure 1. Below 35 K, the microwave absorption has shown continuous fall. The changes are not as sharp as those observed in Y-123 and other high temperature superconductors 1-9, 10]. This broad transition is in conformity with that reported by Tissue et al [7]. The onset of superconductivity was further confirmed by monitoring the low field out of phase line which is characteristic of microwave losses at the weak links [11]. This is shown in figure 2. The typical hysteresis loop of the low field signal obtained at 10 K for fluorinated La 2 CuO 4 is shown in figure 3. The field increasing and decreasing cycles are shown by the arrows. It is seen that in the fluorinated La 2CuO 4 sample, below T the signal appears at slightly higher fields during the field decreasing cycle compared to the field increasing cycle. In the present case the extent of hysteresis is less which is probably associated with low superconducting volume fraction and also due to large 278 Pramana - J. Phys., Vol. 46, No. 4, April 1996

3 Superconductivity in fluorinated La 2 CuO 4,< :L 380 IAI IE U 300 u ILl I- ',' 260 Q I I I I I I I I t I Figure 1. La2CuO 4. TEMPERATURE (K) Temperature dependence of the microwave absorption of fluorinated _ ~ ~ ~ 2 5 K 31K P ~ ' ~ 18 K \ 20K ~ 3ZK 35K 7o ~ i z e,.h (GAUSS) ~H(GAUSS) +40 Figure 2. Temperature dependence of the low field signal in the EPR measurement of fluorinated La z CuO 4. Pramana - J. Phys., Vol. 46, No. 4, April

4 G M Phatak et al iok () I0 ' 2() 30 ' H (GAUSS) ~ 40 Figure 3. The hysteresis of the low field signal of fluorinated La 2 CuO 4 at 10 K. The field increasing and decreasing cycles are shown by the arrow. width of superconducting transition. This kind of hysteresis is expected for superconducting compounds [12]. Ji et al[ 13] have shown that at temperatures below the array transition temperatures, the surface resistance during decreasing field is somewhat smaller than that at the same external field during increasing fields. Further they have shown that, when the field is decreased, a minimum resistance point is reached before the field goes to zero. This minimum point in the derivative presentation corresponds to the "EPR line position". Our results are consistent with that expected for the superconductor below the array transition temperature, as this measurement was made at 10 K which is much below the transition temperature. Further it may be noted that a-sharp in phase line also appeared below 20 K. This line was found to be highly sensitive to field cooling and/or exposure to magnetic field in superconducting phase. Similar effects were observed in superconducting phase of Y-123 [14]. This is associated with the trapping of field inside the sample. As the sharp in phase line appears only at zero field, its intensity would decrease as the increase in volume fraction in which the flux got trapped. These results clearly show that the product of fluorination of La 2 CuO,, by NH4HF 2 is superconducting with T~ = 35 K. This gives an alternative and a lot more convenient way of fluorination instead of hazardous F 2 gas route. References [1] R J Cava, A W Hewat, E A Hewat, B Batlogg, M Marezio, K M Rabe, J J Krajewski, W F Peck Jr and L W Rupp Jr, Physica C165, 419 (1990) [2] A C W P James, S M Zahurak and D W Murphy, Nature (London) 338, 240 (1989) [3] M AI-Mamouri, P P Edwards, C Greaves and M Slaski, Nature (London) 369, 382 (1994) [4] R M Kadam, B N Wani, M D Sastry and U R K Rao, Physica C246, 262 (1995) J-5] B N Wani, S J Patwe, U R K Rao, R M Kadam and M D Sastry, Applied Superconductivity (1995) (Communicated) [6] P R Slater, P P Edwards, C G Greaves, I Gamson, M G Francesconi, J P Hodges, M AI-Mamouri and M Slaski, Physica C241, 151 (1995) [7"1 B N Tissue, K M Cirillo, J C Wright, M Daeumling and D C Larbalestier, Solid State Commun. 65, 51 (1988) 280 Pramana - J. Phys., Vol. 46, No. 4, April 1996

5 Superconductivity in fluorinated La 2 CuO 4 [8] U R K Rao, A K Tyagi, S J Patwe, R M Iyer, M D Sastry, R M Kadam, Y Babu and A G I Dalvi, Solid State Commun. 67, 385 (1988) [9] M D Sastry, R M Kadam, Y Babu, A G I Dalvi, I K Gopalakrishnan, P V P S S Sastry, G M Phatak and R M Iyer, J. Phys C21, 1607 (1988) [10] M D Sastry, R M Kadam, Y Babu, A G I Dalvi, I K Gopalakrishnan, P V P S S Sastry and R M Iyer, Physica C1667, 153 (1988) [11] S V Bhatt, P Ganguly and C N Rao, Pramana-J. Phys. 28, L (1987) [12] M D Sastry, K S Ajayakumar, R M Kadam, G M Phatak and R M Iyer, Physica C170, 41 (1990) [13] L Ji, M S Rzchowski, N Anand and M Tinkham, Phys. Rev. 1147, 470 (1993) [14] M D Sastry, K S Ajaykumar, R M Kadam, G M Phatak and R M Iyer, Proc. of DAE Solid State Physics Symposium, Varanasi (1991) Pramana - J. Phys., Vol. 46, No. 4, April