High-temperature phase changes in RuSr2GdCu2O8 and physical properties

Physica C 387 (2003) 347–358 www.elsevier.com/locate/physc

High-temperature phase changes in RuSr2GdCu2O8 and physical properties N.D. Zhigadlo

a,1

, P. Odier

a,*

, J.Ch. Marty b, P. Bordet a, A. Sulpice

c

a

c

Laboratoire de Cristallographie, CNRS, 25 Avenue des Martyrs, BP 166, F-38042 Grenoble Cedex 09, France b LAIMAN, Universit
e de Savoie, 9 Rue de lÕArc-en-Ciel, BP 240, F-74942 Annecy-le-Vieux Cedex, France Centre de Recherche sur les Tr es Basses Temp
eratures, CNRS, 25 Avenue des Martyrs, BP 166, F-38042 Grenoble Cedex 09, France Received 30 May 2002; received in revised form 30 October 2002; accepted 5 November 2002

Abstract A sol–gel method was successfully applied to synthesize RuSr2 GdCu2 O8 (Ru-1212) as a single phase. The crystallization of Ru-1212 arises at 950 °C under O2 flow with a small amount of secondary phase. The pure Ru-1212 phase is achieved after heating at 1020 °C in O2 flow. The X-ray diffraction (XRD) pattern is refined (Rietveld) in the tetragonal  and c ¼ 11:5678ð4Þ A . In situ high-temperature XRD space group P4/mmm with lattice parameters a ¼ 3:83904ð9Þ A and differential thermal analysis coupled with thermal-weight measurement show a structural decomposition of the phase at Td ¼ 1050 °C followed by a partial melting at Tm ¼ 1118 °C. The decomposition produces crystallized Sr2 GdRuO6 , SrRuO3 phases and a mixture rich in copper containing ‘‘Sr, Gd, Cu, O’’ that does not diffract X-ray. This phase reduces to Cuþ at Tm with an important weight loss and a significant amount of liquid. The grain size and/or inappropriate grain boundaries of the pure phase treated below Tm does not permit to detect in sol–gel samples superconductivity otherwise observed in compounds prepared by solid-state reaction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Rutheno-cuprates; RuSr2 GdCu2 O8 ; Sol–gel; In situ XRD; DTA, magnetic measurements

1. Introduction A new class of rutheno-cuprates, RuSr2 LnCu2 O8 (Ru-1212) and RuSr2 (Ln1þx -Ce1x )Cu2 O10 (Ru-1222) (Ln ¼ Sm, Eu and Gd), belonging to the layered cuprate family was synthesized in 1995 [1].

*

Corresponding author. Tel.: +33-4-76-88-10-45; fax: +33-476-88-10-38. E-mail address: [email protected] (P. Odier). 1 On leave from Institute of Solid State and Semiconductor Physics, P. Brovki 17, Minsk 220072, Belarus; associate researcher at D
epartment des Sciences Chimiques, CNRS, France.

Coexisting magnetism and superconductivity (SC) have been claimed in the R-1222-type (R ¼ Eu and Gd) RuSr2 R1:5 Ce0:5 Cu2 O10y [2] and more recently in the 1212 type (RuSr2 GdCu2 O8 ) [3,4]. However these properties are strongly debated since SC is apparently sample dependent. The properties of Ru-1212 reported in the literature range from non-SC materials with a magnetic temperature ordering ðTAF Þ below 150 K [5,6] to ‘‘bulk SC’’ samples with Tc ¼ 15–46 K and TAF ¼ 132 K depending on sample preparation [3,4,7]. A recent report [8] suggests a correlation between the ordered magnetic state and SC. The ‘‘bulk’’ nature of the SC is also strongly controversial [8–10]. The

0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02306-7

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most common opinion is that CuO2 planes are the SC planes and are homogeneous but this is not obvious to Blackstead et al. [11] who find evidences for magnetically ordered Cu siting either in the CuO2 plane or in the RuO2 magnetic plane. In the former case the SC is not homogeneous, which contradicts Ref. [3] who claim for homogeneous SC (CuO2 ) and magnetism (RuO2 ). The latter case also contradicts Ref. [3] but is supported by Ref. [8] who proves Ru substitutions by Cu to be possible. To reconcile, Blackstead et al. [11] suggests that SrO can be the superconducting plane. The magnetic nature of the RuO6 plane is presently not clear (weak ferromagnetism or AF [12]) and Ru cations are reported to be in an intermediate valency (60% 5þ, 40% 4þ) state according to recent measurements by 99;101 Ru zero-field NMR and EXAFS [12– 14]. Oxygen annealing, normally associated with hole doping in the high-temperature SC, has no clear effects in the case of Ru-1212 compounds. For nonSC samples it has no influence [5] while it progressively enhances the SC on superconducting samples [8,15] but without any detectable modifications of the oxygen stoichiometry [8]. Then ordering of Ru/ Cu atoms, chemical non-stoichiometry or changes in structural distortions can be considered as possible reasons for varying the physical properties [8,16]. Because both Ru and Cu can support mixed valency in this system, modification of the magnetic and SC properties can originate from modified charge distribution between the RuO2 and CuO2 layers in response to local distortions. Recent neutron diffraction experiments showed highly ordered Cu and Ru layers and presence of cooperative rotations of RuO6 octahedra [17]. In summary, the structure itself cannot distinguish between non-SC and SC samples. SC in Ru1212 is often found not in the bulk and may be associated with a very subtle cationic distribution, perhaps in interfaces. Clearly, more work has to be done on the material preparation and on its characterization. Despite several years of intensive work by the scientific community concerning this phase, almost no studies have been published on the formation mechanism leading to Ru-1212 neither on the reaction path involved. This is an important feature

which has to be taken into account for understanding the SC in Ru-1212. In all papers, the Ru1212 phase has been synthesized by the solid-state route which is always associated with SrRuO3 and/or Sr2 GdRuO6 . These phases are difficult to eliminate [1,6] even after prolonged annealing at high temperatures but do not seem to suppress the SC. It is not clear then if the superconducting phase concerns the stoichiometric phase or a substituted or deficient one. Conversely, sol–gel provides advantages such as chemical homogeneity and chemical reactivity, both of which are important requirements in obtaining ceramics of high quality. This technique has been used successfully in numerous cases relevant to electroceramics and superconductors [18]. The basic idea of sol–gel is to start from a solution of cations and to jellify it by polymerization. The gel can then be easily manipulated. Its drying gives a xerogel that transforms to highly reactive nanosize oxides upon heating. In this paper we report for the first time results on the synthesis of pure Ru-1212 phase by a sol– gel method. In addition, in situ high-temperature X-ray diffraction (XRD) and differential thermal analysis (DTA) studies coupled with thermalweight analysis (TGA) have been undertaken in order to identify the phases involved in the Ru1212 synthesis. To our knowledge, none of these properties have been reported so far. Finally, we discuss the implications of these factors on the occurrence of SC in our compounds.

2. Experimental 2.1. Gel formation The formation of RuSr2 GdCu2 O8 gels is carried out by dissolving separately SrCO3 (Aldrich, 99.9%), CuO (Aldrich, 99.9%) and Gd2 O3 (Pechiney-Saint-Gobain, 99.9%) in nitric acid. Ruthenium(III) nitrosyl nitrate solution, Ru(NO)(NO3 )  H2 O (1.5% Ru), was the source of Ru. Its pH is adjusted to 6 with ammoniac addition. Finally, all solutions are mixed in stoichiometric ratio, with addition of water to reach a total volume of approximately 150 ml for obtaining 5 g of Ru-1212 phase. Added to this solution was 10 wt.% of

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acrylamide monomers (Aldrich, 99%), i.e., H2 C@ CHCONH2 ; 1 wt.% of N ,N 0 -methylenebisacrylamide (Aldrich, 99%), i.e., (H2 C@CHCONH)2 CH2 and a few milligrams of a reticulating agent a,a0 azoisobutyronitrile (Fluka, 98%), i.e., C8 H12 N4 to perform the polymerization which is easily achieved by heating to 80 °C. This forms rather hard black–brown gels that are stable with time. Note that the acrylamide monomers react with the copper to form a stable complex with the amino ligand. This impedes the polymerization process unless the equivalent quantity of monomers is added to the system. To prevent this problem, we used EDTA, [CH2 N(CH2 COOH)2 ]2 [CH2 N(CH2 CO2 H)2 ] as an efficient chelating agent, allowing to isolate the copper cation from acrylamide monomers activity. For the purpose, of comparison samples were also formed by solid-state reaction. Dry RuO2 powder, SrCO3 , Gd2 O3 and CuO powders were mixed in stoichiometric ratio (Ru-1212) in a an automatic agate mortar under acetone. The resulting powder (2.7 g), after drying was calcined a first time at 920 °C for 24 h in static air and then at 930 °C (24 h) under Ar flow after intermediate grinding under acetone. After a subsequent dry grinding, the fine powder was then reacted at 1030 °C for 24 h under O2 flow, from what resulted a porous compact, called ‘‘ssr’’. This batch serves to fabricate samples ssrA, ssrB, ssrC and ssrD, according to the procedure recapped in Table 1.

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The phase composition of the powder samples was investigated by XRD (Siemens D-5000 diffractometer in transmission mode and Bruker D8 with a high resolution reflection set up), using CuKa1 radiation. In situ high-temperature XRD were carried out in a high-temperature diffractometer (equipped with a CAS120 Inel detector). The measurements were performed using CoKa1 radiation in a range 10–120° 2h. The temperature was measured with a thermocouple inserted in the sample holder close to the sample. Calibration using melting of Ag and Au permit a rather high degree of confidence on the T measurement (a few degrees) up to 1050 °C. At higher temperature (in the range of 1100 °C), the uncertainty increases with temperature and the sample temperature may be smaller than measured by the thermocouple (10–20 °C). DTA and TGA measurements were carried out in a microbalance (Setaram TAG1500Lyon, France) under O2 flow up to 1200 °C at a rate of 10 °C/min. SEM (Jeol-840) and EDX were used to observe the microstructure and the composition of the compounds. The zero-field-cooled (ZFC) and field-cooled (FC) dc magnetic measurements in the range of 2–300 K were performed in a commercial SQUID (Metronic Ingenierie-France). The resistive measurements were made by standard fourprobe technique on bar-shaped pieces cut out from the pellets with contacts made with silver paint.

Table 1 Samples synthesized, irreversibility between ZFC and FC in the magnetic transition regime, annealing conditions, occurrence of SC Solid-state reaction ssra ssrAa ssrBa ssrC ssrD Sol–gel sga sgA (pur) sgB (pur) sgCb SgD (pur) a b

TAF (K)

DTirr (K)

Annealing

134 136 136

0 2 10

1030 °C–50 h (50 °C/h) 1030 °C–(50 h) þ 160 h (50 °C/h) 1050 °C–160 h (50 °C/h) 1030 °C cumulated 480 h (50 °C/h) ssrA then sintering at 1030 °C–160 h (50 °C/h)

134 134 142 136

0 21 17 34

1020 1030 1040 1050 1030

Impurity: SrRuO3  5 mass%. Impurity: SrRuO3  10 mass%.

°C–(476 h) then 48 h (50 °C/h) °C–160 h (50 °C/h) °C ðÞ (50 °C/h) °C–160 h (50 °C/h) °C 480 h (50 °C/h)

NS S S S S NS NS NS NS

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3. Results To study the Ru-1212 phase formation, the gel samples have been heat treated at different temperatures and then characterized by XRD. At first, the gel was dried in a commercial microwave apparatus for a few minutes. During this process, the dehydration occurs first and is followed by a partial ignition of the organic components. It results an extremely porous brown xerogel that is then easily crushed into an homogeneous powder. It was then heat treated in a ventilated oven (100 °C/h up to 700 °C with a dwell time of 24 h) under low oxygen partial pressure (N2 flow). This thermal treatment removes the remaining organic phases and provides a fine nanoscale powder. Fig. 1(a) shows the XRD pattern for the product after the calcination process at 700 °C, it is composed of SrCO3 , CuO, Gd2 Ru2 O7 , Sr2 GdRuO6 and SrRuO3 . In these synthesis conditions, we do not see any Bragg peaks belonging to the Ru-1212 phase. After treating at 950 °C in air (static) for 18 h the Xray diffractogram is completely changed (Fig. 1(b)) to the typical pattern of Ru-1212 phase with a small amount of SrRuO3 . The contribution of this impurity phase decreases significantly after annealing at 950 °C in O2 flow for 15 h (Fig. 1(c)) and

then for 44 h (Fig. 1(d)). The pure Ru-1212 phase is achieved after a final treatment at 1020 °C in O2 flow for 48 h (Fig. 1(e)). The Ru-1212 product was then analyzed by performing Rietveld refinements using FullProf.2k on high resolution X-ray powder diffracted data. No impurity could be detected in the spectrum. The recorded data are in excellent agreement with the calculated profile, Fig. 2, based on a tetragonal space group P4/mmm with calculated lattice pa and c ¼ 11:5678ð4Þ A  rameters a ¼ 3:83904ð9Þ A and atomic positions not significantly different from those reported previously [17,19]. A distortion typical of this phase arises due to rotations of the RuO6 octahedra amounting 13.2° around the c-axis. We could not exclude possible substitutions on Ru site (by Cu) or on Gd site (by Sr). In situ high-temperature XRD and DTA/TGA studies allow to identify the behavior of the phase at high temperature, in the temperature range where syntheses are generally conducted to provide the SC phase. Both experiments have been done in O2 flow. The XRD spectra were taken for increasing temperatures, each 15 min that is the acquisition time needed for a spectrum. Each 30 min, the temperature was raised by steps of 15–20 °C. Fig. 3 shows an overview of the measurements on SrRuO

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Fig. 1. XRD diffractograms of RuSr2 GdCu2 O8 after treatments at 700 °C in N2 flow for 10 h (a); 950 °C in air for 18 h (b); 950 °C in O2 flow for 15 h (c); 950 °C in O2 flow for 44 h (d) and 1020 °C in O2 flow for 48 h (e).

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Fig. 2. Rietveld refinement profile showing observed (o) and calculated (line) intensities. The markers below the profile correspond to the Bragg peak positions for RuSr2 GdCu2 O8 sample annealed at 1020 °C in O2 flow for 48 h. The difference between observed and calculated intensities is shown at the bottom.

7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

<1150°C 1115°C 1065°C 1050°C 1030°C 1015°C 25°C

Fig. 3. An overview of a high-temperature XRD of the RuSr2 GdCu2 O8 . The intensity has been normalized for an easier comparison. The Ru-1212 phase start to decompose between 1040 and 1050 °C.

Ru-1212 starting on the sol–gel powder annealed at 1020 °C. The diffractogram displayed at the bottom was recorded at room temperature and the next were recorded at increasing temperatures from 1015 °C. A number of details become obvious.

There is no phase transformation of Ru-1212 up to 1050 °C in oxygen flow. However a small change of the main peak can be seen at this temperature, reflecting the onset of the transformation. Above 1050 °C, the Ru-1212 phase decomposes, producing two main crystalline phases: Sr2 GdRuO6 and SrRuO3 , together with a subsequent diffusion bump in the 20–35° 2h range, suggesting a contribution from a liquid phase. From both in situ Xray and magnetic measurements performed on quenched samples we exclude the competitive formation of Gd2 CuO4 . Although a large amount of copper oxides (2 mol of CuO/mol of Ru-1212, i.e. 23 wt.% of Ru-1212) is expected to be rejected in this decomposition, no crystallized CuO, neither any new clearly identified peaks, can be seen by XRD between 1050 and 1150 °C. At 1065 °C three crystalline phases coexist while at 1115 °C only Sr2 GdRuO6 and SrRuO3 are visible. By studies on products formed by solid-state reaction at 1030 °C (similar to sample ssr), we have confirmed that this

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Fig. 4. Comparison of the main Bragg reflection for a sol–gel sample heated 160 h at 1030 °C and 160 h at 1050 °C.

decomposition is intrinsic to the Ru-1212 phase and not a characteristic of the sol–gel phase. The high-temperature phases recombine with a rather high kinetics because the phase is recovered on cooling. Note that long annealing (160 h) at 1050 °C induces SrRuO3 impurity to come up, Fig. 4, due to a chemical shift of the sample composition. This figure compares two pieces of a sample made by sol–gel and reacted under oxygen flow several times (cumulated 476 h) at increasing temperatures up to 1020 °C. This sample has a small content of Sr2 GdRuO6 impurity that was removed after annealing 160 h at 1030 °C and slowly cooled (50 °C/h). One part of this sample was subsequently heated at 1050 °C for 160 h and slowly cooled (50 °C/h). XRD, Fig. 4 shows that this sample has now some SrRuO3 impurities (around 10 wt.%) and a broader Bragg (1 1 0) reflection suggesting either the presence of some other phase or some disordering. Fig. 5 shows TGA and DTA data performed up to 1100 °C under oxygen flow. There is a small oxygen loss (0.1 wt.%) between 400 and 500 °C. We assume that this loss has the same nature than in Cu-1212 (Y) phase (YBa2 Cu3 O7 ) but with a much smaller extension [20]. It would correspond in the case of Ru-1212 to a reduction of the O8 stoichiometry to O7:95 while the change is from O7 to O6:5 for Cu-1212. Obviously the Ru ion, due to its high oxidation state, contributes to stabilize the oxygen stoichiometry in Ru-1212, making the oxygen annealing to have a negligible effect on the

Fig. 5. TGA and DTA of RuSr2 GdCu2 O8 powder in oxygen flow. The powder was processed from sol–gel and annealed at 1020 °C in O2 prior to its analysis.

oxygen stoichiometry [8], except eventually at the grain boundaries [21]. At 1050 °C a sharp but small oxygen loss (0.1 wt.%) is detected occurring at the temperature where an endotherm indicates the onset of the phase transformation noticed in XRD. In a further experiment, DTA have been performed up to 1200 °C under O2 flow (Fig. 6). It shows two endothermic peaks at 1050 °C (maximum) and 1118 °C (onset). None of these thermal events can be assigned to Sr2 GdRuO6 alone because we did not detect any transformation in this compound up to 1450 °C nor to SrRuO3 which melting point is above 2000 °C. The strong endothermic signal recorded at 1118 °C is due to a liquidus. The incongruent melting of this compound has been already mentioned to occur above 1100 °C by Lin et al. [22]. It happens with 1.2 wt.% 20

DTA SG Ru-1212 O2 flow I (onset): 1040 °C, max 1053°C II(onset): 1118 °C, max 1129°C

0

I II

-20

II I -40 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400

Fig. 6. DTA of the same powder as in Fig. 5 up to its melting.

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above this temperature in agreement with a melting phenomenon. On cooling, a first peak appears at the same temperature as for heating, it is attributed to the same transformation which is then non-hysteretic. This reflects a fast recombination process as mentioned earlier. The shape of this

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loss of oxygen which can be attributed to the transformation of almost all Cu2þ into Cuþ . Note that a significant drop of the X-ray diffracted intensity (not shown because the spectra were normalized for comparison in Fig. 3) and an increased diffusion in the range 20–30° ð2hÞ is observed

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Fig. 7. ZFC and FC versus T , measured at 10 Oe, for selected samples to show that DTirr changes with synthesis conditions. (a) sg obtained at 1020 °C; (b) ssr obtained at 1030 °C; (c) ssrB obtained at 1050 °C. The right hand part is a zoom in the range 110–160 K.

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peak suggests an invariant transformation. Its larger intensity is due to a better thermal contact with the crucible after the melting. The second peak is then due to the crystallization of the remaining liquid occurring with a rather large undercooling (80 °C) inherent with the reoxydation of Cuþ into Cu2þ . In most cases the oxygen is not completely recovered on cooling and the properties of the resultant specimens may be drastically changed. To confirm the Ru-1212 phase changes at high temperatures a quench study was performed. Two samples made from a pellet of a sol–gel powder were fired in oxygen at 1060 and 1185 °C, held at these temperatures for 1 h and then quenched on a metal plate in air. The cooling rate was estimated to be 250 °C/min. Even though the XRD patterns of the sample quenched from 1060 °C do not indicate the formation of impurities, secondary Srrich and Cu-rich phases were detected in fractured pieces by EDX and significant amount of liquid phase by SEM. However their weight fraction was

small (<10%) because they were rather insensitive to X-ray. The XRD pattern and EDX analysis of the samples quenched from 1185 °C shows Sr2 GdRuO6 , SrRuO3 as main phases, plus minor phases: Sr3 Ru2 O7 , and Cu2 O (and CuO due to the oxidation of the latter during cooling) and perhaps Gd2 CuO4 that is difficult to distinguish from SrRuO3 . This also point towards a possible invariant nature of the transformation at 1050 °C. ZFC and FC magnetization curves at low fields (10, 4, 2, 0.4 Oe), MðH Þ at 1.6 K have been undertaken to detected any possible signature of SC in the samples prepared. Considering that most samples studied in the literature have been made by solid-state reaction, we have also prepared one batch of powder by solid-state reaction according to the procedure described in part 2 to compare it with our sol–gel samples. The magnetization versus T curves are described first, see Fig. 7. In all samples we observe the typical Gd moments ordering (AF) at 3 K. In some samples, a positive bump is detected at 20 K [5,21]. It is due to a

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Fig. 8. ZFC and FC magnetization measured at 0.4 Oe for (a) ssrA and (b) ssrB. In both cases the curve at left is after deducing the Curie–Weiss contribution. In both samples the MðH Þ had a negative slope below 0.4 Oe.

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contribution of Sr2 GdRuO6 , a frequent impurity of the system, that should not be confused with SC. At TAF ¼ 132–142 K a cusp in the ZFC curve signs an AF ordering attributed to Ru atoms of the Ru-1212 structure [23,24]. In FC experiment a spontaneous magnetization anticipates the AF ordering and shows an high-temperature irreversibility that depends of the sample preparation (Table 1, Fig. 7). We estimate this irreversibility by the difference DTirr between the merging temperature of FC with ZFC curves (of course this depends of the sensitivity of our apparatus) and TAF . Remarkably DTirr depends of sample preparation, and varies from 0 to 34 K. This is not correlated with the presence or not of SrRuO3 which ferro-

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magnetically orders at 165 K, except eventually in the samples called sgC (not shown) that contains a rather large amount of SrRuO3 linked with a degradation of the sample. Systematic exploration of MðH Þ above 1.6 K have been performed in order to detect possible SC. Sample ssr was not superconducting, while ssrA, ssrB, ssrC and ssrD were found to be superconducting. For both former samples the SC is unambiguously a volume effect as evidenced by a negative slope of the first magnetization MðH Þ at low field and low temperature. The first critical field Hc1 is very low, Hc1 < 0:4 Oe at 1.65 K. Below this field, the magnetic shielding represents more than 50% of the sample volume. Furthermore, the ZFC and FC measurements present an irreversible behavior

powder powder 1030°C 1030°C 160 160 h ssrA h ssrA powder powder 1030°C 1030°C 480 480 h ssrC h ssrC

powder powder1030°C 1030°C 160 160 h ssrA h ssrA sintered sintered 1030°C 1030°C 160 160 h ssrD h ssrD 1.2

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Fig. 9. RðT Þ for samples ssrA, ssrB, ssrC and ssrD, sgA and sgC. This figure shows clear trends of annealing and sintering.

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below 12 K that we attribute to the SC (Fig. 8). This Fig. 8(a) and (b) shows on its right part, the magnetization after subtraction of the Curie–Weiss contribution due to the magnetism of the sample. It shows the usual behavior of superconducting materials. We observe a weak deviation to the Curie– Weiss law around 40 K which can be due to the appearance of SC but could be also the sign of a

magnetic anomaly. However, the resistivity starts to decrease very fast below 44 K (Fig. 9). It reinforces the assumption that SC should be responsible for the magnetization anomaly. At lower temperature, for example below 27 K for ssrA, the resistance become vanishingly small ðRlow T =RTc onset < 106 Þ. It reproduces the results reported by Lorentz et al. [25]. The high-temperature part is attributed to

Fig. 10. SEM microstructure of RuSr2 GdCu2 O8 after various annealing under pure O2 flow. (a) ssrA was a portion of ssr after annealing 160 h at 1030 °C (cooled at 50 °C/h); ssrC was a part of ssrA grounded, compressed and sintered 160 h at 1030 °C (cooled at 50 °C/h). (b) sg was the as received powder from synthesis; sgD, the same after annealing under oxygen flow at 1030 °C for 480 h (cumulated time at this temperature, cooling 50 °C/h).

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intra-grain while the low-temperature part is due to the inter-grain counterpart. Samples ssrA, ssrB and ssrC were powders annealed at different time and temperatures with no intermediate grinding to preserve grains integrity as suggested [25], they originate from the same batch ssr. The annealing provides some sintering of the grains of the powder with a possible percolating path. Sample D was sintered after a gentle grinding before (dry) compaction and sintering. Fig. 10(a) shows the microstructure of both with an obvious effect of sintering process.

4. Discussion The results reported in the present study show that Ru-1212 decomposes above 1050 °C and recombines with a fast kinetic on cooling suggesting an invariant transformation. On the other hand, the sample composition may be irreversibly changed if the sample is heated for a too long time in the T range above 1050 °C. It expresses that either some species are removed by vaporization (Ru oxides?) or by some liquid phase expelled from the sample. It rises the question of the exact stoichiometry of the samples after long standing time at 1060 °C where most of the samples reported so far are treated. The high-temperature irreversibility between ZFC and FC is an interesting point because it reflects some intimate changes in the samples. According to previous observations, the Ru moments order antiferromagnetically (G type) at 132 K, but the Ru moments are canted, which results in a net ferromagnetic moment parallel to the ðabÞ plane. This canting is influenced by an external field and causes the irreversibility. Contrarily to the AF ordering, it is very sensitive to the presence of a magnetic rare earth (Gd versus Eu) on the rare earth site and presumably to disorder in the RuO2 layer. Cu is a good candidate to this substitution as shown first by the existence of Ru1x Sr2 GdCu2þx O8 compositions [26] (reminiscent to CuBa2 RECu2 O7 structure) and to the recent observation of magnetically ordered Cu impurities on Ru sites with a locally inhomogeneous magnetization [11]. The results summarized in Table 1 assume that our sol–

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gel procedure or high-temperature heating in oxygen favor such substitutions. In this case, copper might be deficient in the CuO2 planes, which is not good for development of SC. According to the literature most samples are prepared above Td , close to 1060 °C. Although recombination occurs rapidly on cooling it may be not quantitative and superconducting samples may be off-stoichiometry. Our study on solid-state reaction samples shows that SC appears in samples prepared below the decomposition temperature Td (samples ssrA, ssrC and ssrD) or at Td (sample ssrB), but our samples do contains some impurities. Therefore, we cannot conclude whether stoichiometry is essential or not for the onset of SC. After analyzing the effect of heat treatments on SC in our solid-state reaction samples, it becomes clear that the annealing time has a major effect on strengthening the inter-grain superconducting part (Fig. 9(a)). However if the annealing temperature is too high, i.e. above the decomposition T , the reverse is observed (Fig. 9(b)). We thought that sintering could strengthen even more the grain boundaries. The reverse effect is observed (Fig. 9(c)), proving that oxygen diffusion, which is inhibited in the sintered material, plays an important role in the inter-grain SC development. The samples made by sol–gel technique which are pure from a structural point of view, are not superconducting (after magnetization and resistivity studies), although a weak accident is detected by resistivity measurements in sgA sample below 25– 30 K (Fig. 9(d)). Again a too high annealing temperature degrades the resistivity. Perhaps the grain boundaries have not the appropriate composition. By EDX we noticed a loss of Ru at the surface of our sg samples that could be due to RuO2 selective loss after such long heat treating in oxygen. Such effect is enhanced by the small grain size that is shown in Fig. 10(b). On the other hand, the as synthesized powder has a mean grain size of 0.6 lm (mean value) typical of this type of synthesis [27,28] and even after 476 h (cumulated from two consecutive annealing at 1030 °C), the grains do not grow larger that 1.7 lm (mean value). According to the large penetration depth measured in similar samples [25,29], the SC is probably very difficult to be detected by magnetization studies.

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5. Conclusions Pure phase of the rutheno-cuprate RuSr2 GdCu2 O8 has been synthesized by a sol–gel method. High-temperature phase changes have been investigated through in situ XRD and DTA studies. The pure phase is stable up to 1050 °C. Above this temperature, it decomposes into crystallized Sr2 GdRuO6 , SrRuO3 phases and other compositions containing a liquid phase with ‘‘Sr, Gd, Cu, O’’ that is not seen by XRD. At 1120 °C the compound reaches a liquidus and looses oxygen due to reduction of Cu2þ to Cuþ . High-temperature heat treatments, in the temperature range where the decomposition occurs, may induce irreversible changes in the sample structure and composition. We do observe bulk SC in samples prepared in a temperature range where no decomposition has occurred but in a sample where the stoichiometry is not perfect. It is then difficult to conclude as regarding the role of off-stoichiometry on the SC occurrence in Ru-1212 phase. Inter-granular properties have been shown to be sensitive to annealing and to microstructure. In very small grains, intragrain SC cannot be detected due to too large penetration depth and inappropriate grain boundaries assumed to have lost RuO2 .

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