Phase equilibria of the SrO–Yb2O3–CuOx system in air

International Journal of Inorganic Materials 3 (2001) 569–573

Phase equilibria of the SrO–Yb 2 O 3 –CuO x system in air J. Dillingham a , *, W. Wong-Ng b , I. Levin b b

a Geology Department, University of Maryland, College Park, MD 20874, USA Ceramics Division, National Institute of Standards and Technology ( NIST), Gaithersburg, MD 20899, USA

Received 24 August 2000; received in revised form 28 December 2000; accepted 5 January 2001

Abstract The phase diagram for the SrO–Yb 2 O 3 –CuO x system was investigated in air. The solid solution (Sr 142xYbx )Cu 24 O z (0,x#1) (the ‘14–24’ phase) was the only ternary phase formed. This phase exhibits a smaller range of solid solution compared with similar phases containing larger lanthanide ions. Transmission electron microscopy (TEM) studies of (Sr 13 Yb)Cu 24 Oz indicated that the phase was incommensurate, with the modulation wave vector q5(1 / 22z)c*, z¯0.066. The Sr-analogs of the Ba 2 YbCu 3 O 61x high T c superconductor and the associated BaYb 2 CuO 5 ‘green phase’ were not observed in the system. The ionic size of lanthanides ‘R’ in the SrO–R 2 O 3 –CuOx systems affects the number of compounds formed and their crystal chemistry. The phase diagrams exhibit a smaller number of solid-solution phases with decreasing size of R-cations, reflecting the lanthanide contraction.  2001 Published by Elsevier Science Ltd. Keywords: Phase equilibria; SrO–Yb 2 O 3 –CuO x system; Phase diagram; Air

1. Introduction The phase diagrams of the BaO–R 2 O 3 –CuO x systems are important for the processing of high T c Ba 2 RCu 3 O x (213) superconductors [1,2]. Replacing Ba by Sr in the BaO–R 2 O 3 –CuO x systems has significant influence on phase equilibria. For example, it has been reported that the 213 high T c phase Sr 2 RCu 3 O x could not be prepared with R5La, Nd, Y, and Ho [3–9]. Detailed analysis of the phase equilibria in the SrO–R 2 O 3 –CuO x systems is expected to provide further information regarding the crystal chemistry of the alkaline-earth lanthanide cuprates, and possibly reveal new superconductors. For example, the Sr 14 Cu 24 O z (14–24) phase was found to be a superconductor when prepared under high pressure [10]. The (Sr, A) 14 Cu 24 O 41 phase was found to exhibit anomalous microwave absorption (with A5Ca) and spin-gap behavior of the CuO 2 chains (A5Ca and Y) [11–13]. The present study is a part of a continuing phase equilibrium project on high T c and related systems, which includes the SrO–R 2 O 3 –CuO x series of phase diagrams. A long-term goal of the project is to understand the trend of *Corresponding author. Tel.: 11-301-975-5791; fax: 11-301-9755334. E-mail address: [email protected] (J. Dillingham).

phase diagram as a function of the ionic size of R [14]. This paper reports the phase diagram for SrO–Yb 2 O 3 – CuO x , and results of characterization of the Yb-substituted 14–24 phase using both X-ray and electron diffraction. 2. Experimental 1 A total of 26 compositions were prepared in the SrO– R 2 O 3 –CuO x system using conventional solid state sintering. Typically, SrCO 3 , Yb 2 O 3 , and CuO were used as reagents which were mixed with a mortar and pestle for about 15 min, pressed in a die of 1 / 4-in diameter, and placed on a MgO plate for heat treatment. Samples were calcined at 7508C for at least 1 day, repressed and heat treated at least two more times at 9308C. After each heat treatment, samples were allowed to cool in air and then were ground with a mortar and pestle. Heat treatments were performed for the samples until specimens reached

1

Certain trade names and company products are mentioned in the text or identified in illustrations in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by National Institute of Standards and Technology.

1466-6049 / 01 / $ – see front matter  2001 Published by Elsevier Science Ltd. PII: S1466-6049( 01 )00015-0

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Table 1 Heat treatment schedule of samples in the SrO–Yb 2 O 3 –CuO system Sample number

Composition SrO– ]12 (Yb 2 O 3 )–CuO mole ratio

Duration of first heat treatment at 7508C (days)

Total duration of heat treatments at 9308C (days)

Number of heat treatments at 9308C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

5–4–1 2–1–1 1–2–1 13–1–24 12–2–24 11–3–24 10–4–24 9–5–24 8–6–24 4–3–3 1–2–7 1–8–1 12.5–1.5–24 1.9–0.1–1 0.9–0.1–1 37–2–61 35–5–60 34–10–56 26.3–15.8–57.9 20–3–77 23–2–75 38–3–59 47–39–14 55–20–25 25–15–60 60–25–15

3 3 1 2 2 2 2 2 2 5 5 5 6 6 6 3 3 3 3 3 3 3 3 3 3 4

4 4 2 16 16 16 16 16 16 7 7 7 11 5 5 7 7 7 7 7 18 7 10 25 10 22

2 2 2 5 5 5 5 5 5 2 2 2 5 3 3 2 2 2 2 2 5 2 3 6 3 5

equilibrium. Table 1 and Fig. 1 show the compositions prepared. The use of SrCO 3 for the synthesis of Sr-rich specimens resulted in the formation of Sr(OH) 2 and SrCO 3 , and delayed equilibration due to sluggish kinetics. Therefore, samples [24 and [26 using SrO (prepared by decomposing SrCO 3 in a vacuum furnace at 13008C) were

prepared in a dry box, and were kept subsequently in an atmosphere-controlled furnace with continuously running purified air. X-ray powder diffraction was used to identify the phase assemblages, and to confirm solid solution range. A computer-controlled automated diffractometer equipped with a u-compensation slit and Cu Ka radiation was operated at 45 kV and 40 mA. The radiation was detected by a scintillation counter with a solid-state amplifier. The Brucker software package and the reference X-ray diffraction patterns of the ICDD Powder Diffraction File (PDF) [15] were used for phase identification. The specimens for transmission electron microscopy (TEM) were prepared from sintered pellets by conventional grinding, polishing and ion thinning. The specimens were examined using a Phillips 430 TEM operated at 200 kV.

3. Results and discussion

3.1. Phase formation and tie-line relationships

Fig. 1. Phase diagram of the SrO–Yb 2 O 3 –CuO x system in air at 9308C. Compositions prepared for this study are represented in filled circles.

Fig. 1 shows the tie-line compatibilities of phases in the SrO–Yb 2 O 3 –CuO x system. This diagram resembles that of the Y-analog reported by Wu et al. [6], Roth et al. [7], and

J. Dillingham et al. / International Journal of Inorganic Materials 3 (2001) 569 – 573

Deleeuw et al. [8], but differs from the diagrams reported by Ikeda et al. [5], and Chandrachood et al. [16]. Similar to the Y-analog, SrYb 2 O 4 and Yb 2 Cu 2 O 5 are the only phases found in the binary SrO–Yb 2 O 3 and Yb 2 O 3 – CuO x systems, respectively. In the SrO–CuO x system, three phases, Sr 2 CuO 3 , SrCuO 2 and the infinite-layer compound, Sr 14 Cu 24 O z (which is also known as the spinladder phase [17]), were confirmed. The Sr analog of the high T c Ba 2 YbCu 3 O x (213) phase was not observed in the SrO–Yb 2 O 3 –CuO x system; apparently formation of this phase requires high oxygen partial pressure [11], or stabilization by chemical substitution [18–22]. For example, Ga-doped sample Sr 2 YbGa 0.35 Cu 2.65 O 6.7 [18] was reported to be superconducting after annealing under 400 atm oxygen (an onset T c value of about 20 K). The ‘green phase’ type structure such as reported for AR 2 CuO 5 (A5Ba or Sr) [23–25] were not detected in this system. The only ‘ternary phase’ in the SrO–Yb 2 O 3 –CuO system is a solid solution (Sr 142xYb x )Cu 24 O z formed by doping a small amount of Yb 2 O 3 in Sr 14 Cu 24 O z ; tie-lines extended largely from binary compounds of one binary system to the other. For example, four tie-lines originate from SrYb 2 O 4 to Sr 2 CuO 3 , SrCuO 2 , (Sr 142xYb x )Cu 24 O z ,

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and Yb 2 Cu 2 O 5 . The solid solution (Sr 142xYb x )Cu 24 O z was found to be compatible with four phases. In addition to Sr 142xYb x Cu 24 O z –SrYb 2 O 4 , three other sets of tie lines emanate from this phase to Yb 2 Cu 2 O 5 , CuO and SrCuO 2 . The widest tie-line bundle connects the (Sr 142xYb x )Cu 24 O z solid solution to CuO (x50.0–1.0).

3.2. The ( Sr142 x Yb x )Cu24 O41 phase [ the 14 – 24 phase] The crystal chemistry and structure of the binary oxide Sr 14 Cu 24 Oz have been reported in the literature [17,26– 28]. From our X-ray diffraction results, the range of Yb substitution in (Sr 142xYb x )Cu 24 Oz extends from x50 to x51, giving rise to the end member (Sr 13 Yb)Cu 24 O z . Data reported for the (Sr 142x R x )Cu 24 O z phase indicate increase in the range of solid solution with decreasing difference in size of R31 and Sr 21 . For example, the extent of solid solution, x, was found to be 0.7, 0.5, 0.4 and 0.1, for R5Nd, Ho, Dy, and Yb, respectively. The trend of the ‘x’ values correlates with the lanthanide contraction. TEM study of (Sr 142xYb x )Cu 24 O z revealed the incommensurate nature of this structure. A set of selected area electron diffraction patterns obtained from a single grain in the (Sr 13 Yb)Cu 24 O z specimen are shown in Fig. 2. The

Fig. 2. A set of selected area electron diffraction (SAED) patterns from a single grain of the Sr 3 YbCu 3 O 41 compound (nominal composition). The ˚ b¯12.9 A, ˚ and fundamental (strongest) reflections are indexed according to the F-centered orthorhombic unit cell with lattice parameters a¯11.38 A, ˚ The extra reflections correspond to the satellites of the fundamental reflections due to an incommensurate modulation with the wave vector c¯3.91 A. q5(1 / 22z)c*, z60.066; the period of modulation is incommensurate with the basic lattice. As seen in the [001] SAED pattern, the (0kls) satellites (s is the order of the satellites) exhibit k 1 l 1 s52n reflection conditions. The satellite reflections show nearly continuous streaks of diffuse intensity approximately along the [010] direction.

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fundamental (strongest) reflections in these patterns can be indexed according to the F-centered orthorhombic unit cell ˚ b512.9 A, ˚ and c53.91 with lattice parameters a511.38 A, ˚ reported for the (A 12x A x9 ) 14 Cu 24 O z compounds [17,26]. A In addition to the reflections from this basic cell, extra reflections are observed at the incommensurate h, k, q9 positions. These extra reflections can be viewed as satellites of the fundamental reflections due to an incommensurate modulation with the wave vector q5(1 / 22z)c*, z¯ 0.066; satellites up to a third order were observed. The satellite reflections display continuous streaks of diffuse intensity parallel to the b-axis (normal to the direction of modulation). Dark-field imaging (Fig. 3) revealed that these streaks correspond to a high density of planar defects aligned normal to the b-axis, which can be interpreted as antiphase boundaries separating regions with different phase of modulation. The (A 12x A x9 ) 14 Cu 24 O z structures are known to be composed of alternating A 2 Cu 2 O 3 and CuO 2 layers stacked along the b-axis [17,26,27]. The A 2 Cu 2 O 3 and CuO 2 layers feature square planar copper forming infinite sheets and infinite chains, respectively, and have incommensurate periodicities along the c-axis (c sheet ¯3.9 ˚ and c chain ¯2.7 A). ˚ These periodicities become nearly A commensurate at c5mc sheet 5nc chain (m, n-integers) giving rise to [A 2 CuO 3 ] m [CuO 2 ] n commensurate superstructures; three of such superstructures with the m /n combinations of 5 / 7, 7 / 10 and 9 / 13 have been experimentally observed [26,27]. Siegrist et al. found that formation of a superstructure was very sensitive to the preparation conditions which

primarily affected the order in the linear CuO 2 chains. Thus, the incommensurate structures observed in the present study in the Sr 13 YbCu 3 O z compounds can be intuitively attributed to a modulation within the layers of CuO 2 chains; however, the exact origin of this modulation has yet to be determined.

3.3. Comparison with other SrO–R2 O3 –CuOx systems The phase diagram of the Y-analog is shown in Fig. 4 [7]. Despite similar ionic radii of Ho 31 and Y 31 , their corresponding phase diagrams are different. The phase formation and the tie-line relationships of the Y- and Yb-samples are similar, except for the solid solution range of the (Sr 142xY x )Cu 24 O z phase: x has a relatively large value of 4.66 in Fig. 3. A comparison of the SrO–Yb 2 O 3 – CuO x diagrams with other known SrO–R 2 O 3 –CuOx systems (R5La [3], Nd [4], and Ho [9]) reveals that a trend similar to that observed in the Ba-systems [1,2,29,30]: the number of phases increases with increasing size of the lanthanide ion. For example, in the La-system, there are five ternary solid solution series [3], namely, (La,Sr) 2 CuO 42d (d .0), La 82x Sr x Cu 8 O 202d (1.6#x#2.0), La 22x Sr 11x Cu 2 O 61d (0.05#x#0.15), La 11x Sr 22x Cu 2 O 5.51d (0.05#x#0.15), and (Sr 142x La x )Cu 24 O 41 (0#x#4). In contrast, in the Ndsystem, there are three solid solutions and one stoichiometric compound, namely, Sr 22x Nd 11x Cu 2 O y (0#x#0.4), Srx Nd 22x CuOy (1.2#x#1.5), (Sr 142x Nd x )Cu 24 O z (0#x#

Fig. 3. Dark-field image obtained with a streak of diffuse intensity near the [100] zone axis orientation. The image reveals a high density of planar defects aligned nearly normal to the b-axis. These defects can be described as antiphase boundaries separating domains with different phase of modulation. The high density of these boundaries apparently reflects a small (¯1–3 nm) correlation length of the phase of modulation in the b-direction.

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References

Fig. 4. Phase diagram of the SrO– ]12 Y 2 O 3 –CuO x system [7].

7), and SrNd 2 Cu 2 O 6 [4]. In the Ho-system there is only one ternary solid solution series [9], (Sr 142x Ho x )Cu 24 O z , 0#x#5, and a stoichiometric ternary compound SrHo 2 CuO 5 . Finally, in the Yb system, only one ternary solid solution series, (Sr 142xYb x )Cu 24 O z , 0#x#1, is found.

4. Summary The tie-line relationships and phase formation of compounds in the SrO–Yb 2 O 3 –CuO x system were established. Despite similar ionic radii of Ho 31 and Y 31 , their corresponding phase diagrams are different, but that of the Yb 31 and the Y 31 systems are similar. The superconductor phase, Ba 2 YbCu 3 O x and the related green phase, SrHo 2 CuO 5 , were not stable in the Yb system. TEM study revealed the incommensurate structure for the (Sr 142xYb x )Cu 24 O z phase, with the wave vector of incommensurate modulation parallel to the c-axis. A comparison of the Yb system with reported systems of R5La, Nd, Y and Ho indicates a trend due to lanthanide contraction exists: the phase diagrams become less complicated with decreasing size of the lanthanide cation.

Acknowledgements Department of Energy is acknowledged for their partial financial support. We also appreciate the graphic assistance from Niels Swanson of the Ceramics Division, NIST.

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