Synthesis, superconductivity and magnetism of RSr2Cu2NbO7+δ compounds (R = Y, Pr and Gd)

PHYSICA ELSEVIER

Physica C 262 (1996) 266-271

Synthesis, superconductivity and magnetism of R S r 2 C u 2 N b O 7+8 compounds (R = Y, Pr and Gd) V.P.S. Awana *'1 Latika Menon, S.K. Malik Tata Institute of Fundamental Research, Bombay 400 005, India Received 4 May 1995; revised manuscript received 28 December 1995

Abstract Single-phase compounds of the series RSr2Cu2NbO7+ ~ with R = Y, Pr and Gd have been synthesized through a solid-state reaction route. These are found to crystallize in the orthorhombic (space group Imcm) structure which is analogous to that of the superconducting RBa2Cu307_ 8 compounds with complete substitution of Cu-O chains with Nb-O networks. Low-field (50 Oe) magnetic-susceptibility measurements of RSr2Cu2NbOT+ 8 compounds reveal that the Y compound of this series shows a superconducting transition around 13 K with a small Meissner fraction, while the compounds with R = Pr and Gd are non-superconducting down to 1.8 K. The GdSr2Cu2NbO7+ 8 sample shows magnetic ordering, presumably of the Gd moments, with N6el temperature TN ~-2.2 K, which is about the same as the ordering temperature of the Gd moments in GdBaCu307_ ~. However, unlike in PrBa2Cu307_ 8 where Pr moments order antiferromagnetically with TN ~- 17 K, no evidence of magnetic ordering of the Pr moments is observed in the susceptibility of PrSr2Cu2NbOT+ 8 down to 1.8 K. The magnetic susceptibility of the RSr2Cu2NbO7+ 8 compounds follows a Curie-Weiss behavior in the paramagnetic state with effective paramagnetic moments of 7.50p. B, 3.11P'B and 0.728/z B for the Gd, Pr and Y compounds, respectively. The effective Pr moment in PrSr2Cu2NbO7+ 8 is found to be intermediate between that of the Pr 3+ and Pr 4+ free-ion moment as in PrBa2Cu307_ 8.

1. Introduction Ever since superconductivity in the RBa2CuO 7_ 8 (R:123) compounds was discovered [1,2], the Pr based R: 123 compound has attracted a great deal of attention (see for instance Refs. [3-9]). Unlike most other R based 123 compounds, the Pr analog shows a highly semiconducting behavior, an unusually high ordering temperature o f nearly 17 K for the Pr moments along with a large electronic specific-heat

* Corresponding author. E-mail: [email protected]. t Presently at Departamento de F[sica, Universidade Federal de Pernambuco, 50670-901, Recife-PE, Brazil

coefficient y, the latter of which is reminiscent of the heavy-fermion system [3-5]. The origin of the unusually high T N of the Pr moments, the large y and the nonsuperconducting behavior of the Pr: 123 compound is still not well understood. As in the R: 123 compounds, the Pr substitution at the R site in R:214 and 247 compounds also results in an insulating material [10,11]. However, in these compounds the critical Pr concentration suppressing superconductivity is much higher than that observed for the same host rare-earth based R: 123 compounds [10,11]. Moreover, Pr:124 and Pr:247 compounds do not show any magnetic ordering of the Pr moments down to 2 K [12].

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V.P.S. Awana et al./ Physica C 262 (1996) 266-271

Besides the R:123, R: 124 and R:247 compounds, the substitution of Pr has also been investigated in the Bi based euprate systems [13,14]. However, in the latter systems, the effect of Pr as far as suppression of the superconductivity is concerned, does not appear to be very much different than the effect due to the substitution of other rare-earths. In the case of TI based compounds also the situation for this substitution is more or less similar to that in Bi based cuprate systems [15]. Complete substitution of Pr at the Ca site in either the Bi or the TI based high-Tc systems does not result in magnetic ordering of the Pr moments. Although the Pr moments do not show any magnetic ordering in R:124, R:247 or the Bi and TI based cuprate families, in none of these the Pr based compounds exhibit superconductivity. While in the case of Pr:124 and Pr:247, part of the difficulty lies in obtaining a phase-pure sample, in the Bi and TI based cuprates it is the hole filling due to Pr substitution at the Ca 2+ site which leads to suppression of the superconductivity, similar to the substitution by any other rare earth at the same site. Another interesting R based family is the R:1212 family of compounds with nominal formula RSr2Cu2MO7+ 8, where M = Nb, Ta, Ga, Fe, Ti etc. [16]. Most members of this family superconduct either with high pressure oxygenation or with Ca substitution at the host R site [16,17]. In this paper our aim is to study the behavior of the Pr compound with M = Nb in the above series and to compare it with Y and Gd analogs of the same structure. All the three compounds of this series crystallize in the Imcm structure [17,18]. We observed that while the antiferromagnetic ordering temperature of Gd remains the same as is observed in the Gd based 123 compound, the ordering of the Pr moments is not observed down to 2 K. Interestingly the Y analog of this series shows a superconducting transition around 13 K, without high pressure or Ca substitution at the Y site.

2. Experimental The RSr2Cu2NbOT+8 samples with R = Y, Pr and Gd were prepared by the usual ceramic technique by heating stoichiometric amounts of thoroughly mixed powders of Pr6Oll, SrCO 3, Nb205 and CuO at 1020°C for 72 h with several intermedi-

267

ate grindings. The resulting powder was pulverized and re-sintered and this process was repeated at least three times. The final sintering was carried out in flowing oxygen at 1050°C after which the samples were cooled slowly to room temperature. The samples were characterized for their phase purity, and their lattice parameters at room temperature were determined by using a JEOL X-ray diffractometer using Cu Ket radiation. The magnetic susceptibility of these compounds was measured in 50 Oe and 5 kOe fields using a SQUID magnetometer in the temperature range of 1.8-300 K.

3. Results and discussion

3.1. X-ray diffraction It was inferred from X-ray diffractograms for the RSr2Cu2NbOT+8 compounds with R = Y , Pr and Gd, that the samples are single phase in nature and all the lines could be indexed on the basis of the Imcm structure. The lattice parameters obtained from a least-square fit of the observed d values are a = 22.7513(2) A, b = 5.630(3) ,~, and c = 5.534(2) ,~, for R = Y, a = 22.803(2) ~k, b = 5.640(3) A, and c = 5.512(2) ,~, for R = Pr and a = 22.7743(2) A, b = 5.638(3) A, and c = 5.506(2) A for R = G d . We also tried to fit the observed X-ray d values with the P4mmm structure, which resulted in a poor fit. We find that samples of this series crystallize in the 1212 structure of space group Imcm with a few small intensity lines intermixed with the P4mmm structure of the tetragonal R: 123 system.

3.2. Magnetic studies Fig. 1 depicts the low-field (50 Oe) susceptibility as a function of T for YSr2Cu2NbO7+8. This compound shows a superconducting transition near 13 K (as found evidence for by the onset of diamagnetism), though the Meissner fraction is small. Earlier studies have shown that this compound exhibits superconductivity either with high-pressure oxygenation or with Ca substitution at the Y site, both of which provide sufficient mobile holes [17,18]. In the present sample most likely a few mobile holes have been introduced on oxygen annealing without high pressure or without Ca substitution. The Pr and Gd

268

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analogs of this series showed no traces of superconductivity in the susceptibility data. The nonobservability of superconductivity in the Pr and Gd analogs with the same oxygen annealing has probably nothing to do with the magnetic nature of Pr and Gd ions. Because in the present structure also the rare-earth ions are completely isolated from the C u - O planes where the superconductivity is supposed to reside in these systems, like in other superconducting R:123 systems. Fig. 2 shows the magnetic susceptibility, X (measured in a field of 5 kOe), versus temperature, T, for YSr2Cu2NbO7+~. The inset of this figure shows X-1 versus T. The susceptibility of these compounds follows a Curie-Weiss behavior given by X = Xo + C / ( T -

Op),

the carrier concentration, the ordering temperature of the Cu spins decreases and subsequently the system becomes superconducting. In the superconducting regime, initially with a low carder concentration, the Cu ions bear a paramagnetic moment with small T~ values for the system; a further increase in carriers increases the Tc to the optimum value, with no apparent magnetic moment on the Cu ions [20]. The present value of the paramagnetic moment of 0.728/x B per Cu ion, shows that with normal oxygen annealing this system does not acquire sufficient carriers on C u - O to quench the paramagnetic moment of the Cu ions and to show the optimum T~ and a sufficient Meissner fraction for this compound. In an earlier study we observed a magnetic moment of 0.71/x s on the Cu ions for the YBa2Cu206.63 sample having a Tc of 48 K [21]. It appears that the effective copper valence of the sample with a magnetic moment on Cu of 0.728/z B might be 2.07 or so [21]. While the effective paramagnetic moment on Cu is nearly the same for both the Y B a 2 C u 2 0 6 . 6 3 and the Y l r 2 C u 2 N b O 7 + 6 compounds, the former superconducts at 48 K with sufficient Meissner fraction; the latter showed only a filamentary superconductivity around 13 K. The possible reason for this is that while in the former a major portion of effective moment is contributed by the Cu in the C u - O chains which are isolated from the C u - O planes, in the latter as there are no Cu chains, the whole moment is presumably on Cu in the C u - O planes. While it has 0.015

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269

V.P.S. Awana et al./ Physica C 262 (1996) 266-271

been argued earlier [22], that the Tc of high-Tc compounds is mainly controlled by the carder concentration in the C u - O planes, we find that the role of the paramagnetic moments, particularly on Cu in the C u - O planes are equally important. The magnetic susceptibility X (measured in 5 kOe) of PrSr2Cu2NbO7+8 is shown in Fig. 3 as a function of temperature in the low-temperature range. The inset of Fig. 3 shows the inverse susceptibility, corrected for the matrix susceptibility as a function of temperature in the full temperature range of 1.8300 K. As is evident from Fig. 3, neither the X versus T or the X-t versus T plots show any evidence for the ordering of the Pr moments in this sample down to 1.8 K. To clearly see any evidence of ordering of the Pr moments in this compound we have compared the present data with the reported data on the Pr:123 systems. In the Pr:123 compound, the magnetic ordering of Pr manifests itself as a small break in the susceptibility, which shows up best in the plot of d x/dT as a function of temperature (T) [23]. Fig. 4 shows such plots for both the Pr: 123 compound and the present sample. As can be seen from this figure, while there is a distinct break in the plot of the Pr:123 compound around 17 K, 0'07 -

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there is no visible discontinuity (break) in the plot for the PrSr2Cu2NbOT+s. After comparing the two data in Fig. 4 one can assume that probably there is no magnetic ordering of the Pr moments in the present system. To confirm the abovementioned interesting observation, heat-capacity measurements can be fruitful for this system. The magnetic-ordering temperature for the Pr moments in the Pr: 123 structure has been observed to be sensitive for the Cu chain substitutions and the oxygen stoichiometry of the system [23,24]. In case of oxygen-deficient compounds it has been observed earlier [25], that C u - O bond lengths change in Cu-O plane networks with increased P r - O (2) distance. It is believed that the TN of Pr in the Pr:123 compounds is a result of the strong hybridization of the Pr moments with the Cu-O conduction band [25]. Any change in the C u - O plane bond lengths alters the strength of Pr hybridization with the C u - O conduction band and as a result decreases the TN [24,25] for Pr:123 like systems. In the present system it appears that the Pr moments do not hybridize at all with the adjacent C u - O planes; this may be due to the large distance between Pr and C u - O planes. Hence there is no TN for the Pr moments. This assumption can be well checked for this compound through a detailed structural analysis by neutron diffraction.

270

V.P.S. Awana et al./ Physica C 262 (1996) 266-271

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Before fitting the susceptibility of the Pr compound to the Curie-Weiss behavior, we correct it for the matrix susceptibility which is taken to be the same as that of its Y analog. A Curie-Weiss fit to the susceptibility data so corrected yields ~'/'eff~ " 3.11/~ B for the Pr ion and 0p = - 4 K. The effective paramagnetic moment for the Pr ions appears to be intermediate between that of the free-ion Pr 3+ and Pr 4+ moments. The intermediate valence state of Pr can favor the hybridization of Pr with the C u - O conduction band, which in turn assumes a Tn of the same in the present system. Here it is important to mention that the crystal-field calculations have earlier shown that even in Pr: 123, where there is found a strong hybridization between Pr ions and the C u - O conduction band, also the susceptibility results could be well explained on the basis of Pr 3+ ions only [26]. Hence it appears after the crystal-field calculations that the intermediate valence state of Pr in Pr:123 like structures is irrelevant. Only detailed spectroscopic studies can resolve the issue. Fig. 5 shows a plot of the susceptibility versus temperature for GdSrECu2NbOT+ 8 in the low-temperature range. A peak in the susceptibility is observed at 2.2 K which is indicative of the antiferro-

magnetic ordering of the Gd moments in this compound. The N6el temperature, Tn, of 2.2 K in this compound is very close to the TN of the Gd moments in Gd: 123. The inset in Fig. 5 shows a plot of X -1 , corrected for matrix susceptibility, versus temperature in the temperature range of 1.8-300 K. The Curie-Weiss fit to this yields an effective paramagnetic moment of the Gd ions of 7.5/~ B and a paramagnetic Curie temperature of - 2 . 7 4 K. Unlike Pr ions the Gd ions are completely isolated from the C u - O conduction band in R: 123 like structures and hence their ordering temperature depends solely on the interaction between the neighboring Gd ions. Any crystallographic change which takes the Pr ions away from the C u - O conduction band decreases the TN of the Pr ions, while for the Gd ions, it is the G d - G d ion interaction which is important. In the present systems it appears that while the Gd(Pr)Gd(Pr) interaction remains invariant, the hybridization between Pr and the (Cu-O) conduction band might have decreased drastically. Probably that is the cause that there is no observable T N for the Pr sample, while that for the Gd analog is at around 2.2 K. We feel that heat-capacity and neutron-diffraction measurements will be fruitful for these systems to further examine the interesting observations made by us in this paper.

4. Summary and conclusion We have studied the synthesis, superconductivity and magnetism of RSr 2Cu 2 NbO7 + 8 compounds with R = Y, Pr and Gd. These compounds are found to crystallize in the Imcm space group. Susceptibility measurements indicate that while the Y analog shows a filamentary superconductivity at 13 K, the Pr and Gd analogs are nonsuperconducting down to 1.8 K. Paramagnetic-susceptibility measurements indicate that unlike in the Pr" 123 compounds the Pr moments in PrSr2Cu2NbO7+8 do not show any evidence of magnetic ordering down to 1.8 K. The magneticordering temperature of the Gd moments is 2.2 K which is close to that of the Gd moments in Gd" 123. Our magnetic measurements do not show any clear evidence of the ordering for Cu moments in the temperature range of 1.8 K to 300 K in any of the samples of the RSr2Cu2NbO7+ 8 series.

V.P.S. Awana et aL / Physica C 262 (1996) 266-271

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