Possible superconductivity at 109 K in YBaCuO materials

P.Udomsamuthirun 1,2,T.Kruaehong 1 ,T. Nilkamjon 1,2 and S. Ratreng 1,2

1 Prasarnmitr Physics Research Unit, Department of Physics, Faculty of Science,

Srinakharinwirot University, Sukumvit 23,Bangkok,10110,Thailand.

E-mail: udomsamut55@yahoo.com

2 Thailand Center of Excellence in Physics(ThEP), Si Ayutthaya Road,

Bangkok,10400 ,Thailand.

Abstract

New YBaCuO superconductors are synthesized by using the standard

solid state reaction method as Y5-6-11, Y7-9-16, Y5-8-13, Y7-11-18, Y156, Y3-8-11,

and Y13-20-23. We find that all material obtained show the Meissner effect at

77 K. The resistivity measurements were made by the four-probe method. The

Y 7-11-18 has the highest Tc onset at 109 K. The XRD spectra showed that

they have the same crystal structure as Y123 with some impurities peaks.

1.Introduction

In 1986 Bednorz and Muller [1] found the first high temperature

superconductor in the compound La214, which showed a transition temperature ( Tc) above 30 K. And in 1987 the transition temperature of YBa2Cu3O7 (Y123) was

increased to around 92 K by Chu and co-workers[2]. The researchers have been carried

out on the YBaCuO-family compound like Y123, YBa2Cu4O8 (Y124), and

Y2Ba4Cu7O15 (Y247) to find a higher Tc in the YBaCuO-family. They found that

Y124 and Y247 became superconductive at 80 K[3] and 40K[4], respectively.

Y247 exhibits a superconductive transition with Tc ranging from 30 to 95 K,

depending on the oxygen content[5,6].

Recently Aliabadi, Farshchi and Akhavan [7] found the new Y-based high

temperature superconductor Y3Ba5Cu8O18 (Y358) became superconductive at

102 K. And they also proposed that in order to have a stronger superconductor with higher Tc in the YbaCuO family one should pump more holes from the chains to the oxygen sites of the planes, tending to diagonal charge order. Y123 has two CuO2 planes and one CuO chain. Y124 has one CuO double chain. Y247 has one CuO2 planes and one CuO chain, and one double chain. Y358 has a crystal structure similar to Y123 with five CuO2 planes and three CuO chains. The increasing number of CuO2 planes and CuO chain have important effect on the Tc of Y358.

The YBaCuO-family have shown the different in their number of CuO2 planes and

CuO chains or double chains that believed to be the carrier reservoirs. However,

Nakajima et al.[8] had proposed the limited increase in the number of the CuO2

planes in all high-Tc cuprate superconductors to three.

We [also] think that there should be a relationship between the superconductors in YBaCuO family. Assumptions about the relationships in these material are made and we synthesized the new superconductors in this family by using our assumptions. We find new YBaCuO superconductors in 7 formulations with a difference in critical temperature in each.

2. Experimental assumptions

We know that the YBaCuO-family are consists of Y123,Y124,Y247 and

Y358 with the Y358 is the highest Tc of this family . The Y123 and Y358 are shown the similar crystal structure[7] . Aliabadi,Farshchi and Akhavan [7] proposed that the lattice parameters, a and b , of Y123 is very close to of Y358 but the lattice parameters, c ,of Y358 is almost 3 time of Y123 . Y123 has two CuO2 planes and one CuO chain and Y358 have five CuO2 planes and three CuO chains. Y358 have five CuO2 planes that 2.5 time of the CuO2 planes of Y123 . We think that three parameters should have some relations as

1.the number of CuO2 planes and number of Ba-atoms.

2.the number of CuO chains and the number of Y-atom.

3.the number of Ba-atom plus Y-atom are equal to the number of Cu-atom.

The relation between the number of CuO2 planes and number of Ba-atom,

and the number of CuO chains and the number of Y-atom can not be proven in this

paper. However, the number of Ba-atom plus Y-atom equal to the number of Cu-atom

can be done experimentally. In Y123, there is 1 Y-atom and 2 Ba-atoms so we

get 3 Cu-atoms. In Y358, there are 3 Y-atom and 5 Ba-atoms so we get 8 Cu-atoms.

So we think that the main ideal to synthesize a new superconductor in this family is the number of Ba-atom plus Y-atom equal to the number of Cu-atom.

To reach the highest Tc, we should pump more holes in this family. As our

relation that the number of CuO2 planes relate to number of Ba-atom . We need

more CuO2 planes so we will not do anything to Ba-atom. To make holes, the

number of Y-atom should be missing. This concept are agreed with the assumption to synthesize Y123 that replacing the La-atom by Y-atom in BaCuO2 perovskite; Y 3+ has an ionic radius smaller than La3+ ; and the Tc is higher. We make the assumption that the number of Y-atom should be missing to create more holes to higher the critical temperature. .

At this point, we can make the assumptions to synthesize new

superconductors in the YBaCuO-family as

1.the number of Ba-atom plus Y-atom are equal to the number of Cu-atom.

2.the number of Y-atom can be missing to reach the higher Tc but the 1st

assumption must be obeyed.

The Y123 and Y358 can be explained by our assumption as Y123 is no Y-

atom missing and Y358 is 1 Y-atom missing every 5 Ba-atom.

According to our assumptions, there are many new superconductors will be

found. Example, In case of 1 Y-atom missing ,the general formula should be

Yx-2 BaxCu2x-2Od . The Y358 is the example of this group which has the percent of Y

atoms missing to the number of Ba-atom as x100 = 20% . The 2 Y-atoms missing, the

general formula should be Yx-3 BaxCu2x-3Od . We can get Y5-8-13 with the percent of

Y-atom missing to number of Ba-atom as x100 = 25% .

3.Experimental details

To prove our assumptions, we synthesize a new group of YBaCuO

superconductors by using the standard solid state reaction method. Appropriate

stoichiometric ratios of powder of Y2O3 , BaCO3 and CuO are mixed, ground, and

react in air at 950 C for 24 hours, and then cooled to 100 C. Calcination is repeated twice

with intermediate grinding. The powders are reground , pressed into pellets 30 mm in

diameter and about 5 mm thickness under 2000 psi pressure. Finally, the samples

obtained are sintering at 950 C for 24 hours and annealed at 500 C for 24 hours in

air. At this point we are not interested in the effect of oxygen-doping. So we obtained all

samples via air annealing.

We first tested the superconductive state by using the Meissner effect at 77 K

and find that all materials obtained show the Meissner effect as Table 1. This

means that all of our samples are superconductors with the critical temperature above

77 K.

Table 1. Shown the new YBaCuO -superconductors synthesized .

The resistivity measurements were made using the four-probe method. All samples

show the fact that with increasing measuring current the onset of resistivity drop are

shifted to lower temperature. The current densities J =2.55x103 A/m2 used are

shown in Figure 1. And the normalized resistivity versus temperature are shown in

Figure 2. The summation of the Tc off-set , Tc middle and Tc onset of our samples

read from Figure 2 are shown in Table 2.

Table 2. The summation of the Tc of our samples.

We find that the highest Tc onset is 109 K that of Y7-11-18.and Y156 is the

lowest Tconset that 95 K. Y3-8-11 is the highest Tcoff-set , 98 K.

We preliminary survey the crystal structure of our samples. By comparing the

XRD spectra from 100 to 900 of our new superconductors with our Y123 spectra,

they shown that the main peaks are the ones which exist in Y123 with some

impurities peak . We find that there is the one of main peak at about 15 0 that do not

included in the Aliabadi,Farshchi and Akhavan [7] s calculation of Y358. We find

that our samples are shown the same crystal structure of Y123 with some impurities

peaks that occur by the missing of Y-atom in some planes agreed with our

assumptions.

4.Result and Discussion

We make the assumptions to synthesize a new superconductor in YBaCuO family as the number of Ba-atom plus Y-atom are equal to the number of Cu-atom

and the number of Y-atom can be missing to reach the higher Tc but the 1st

assumption must be obeyed. The new formula of YBaCuO superconductors are

synthesized by using the standard solid state reaction method as Y5-6-11, Y7-9-16,

Y5-8-13, Y7-11-18, Y156, Y3-8-11, Y13-20-33. The Y 7-11-18 has the highest Tc

onset at 109 K. Our samples are shown the same crystal structure as Y123 with

some impurities peaks that occur by the missing of Y-atom in some planes agreed

with our assumptions.

The effect of oxygen-doping on superconductors is not considered in this

Paper. That may be the one of the main parameters to increase the critical

temperature. We think that the highest Tc superconductor may be found by using our

assumptions and optimize doping. More experimental detail will reveal the

mechanism of superconductivity in this material.

5.Conclusions

We find the assumptions to synthesize a new superconductor in YBaCuO family with the higher Tc. The new formula of YBaCuO superconductors are synthesized by using the standard solid state reaction method as Y5-6-11, Y7-9-16,

Y5-8-13, Y7-11-18, Y156, Y3-8-11, Y13-20-33. The Y 7-11-18 shown highest Tc onset as 109 K. Our samples show the same crystal structure as Y123 with some impurities peaks that occur by the missing of Y-atom in some planes agreed with our assumptions.

Acknowledgement

The authors would like to thank Professor Dr.Suthat Yoksan for the useful

discussion and also thank the Office of Higher Education

Commission, Faculty of Science Srinakharinwirot University,and ThEP Center for the

financial support.

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Figure 1 The resistivity versus temperature are shown.

.(mO-cm)

Figure 2 The normalized resistivity versus temperature are shown.