Phase relations in La2O3–Gd2O3–CuO system at 950 °C

Journal of Alloys and Compounds 416 (2006) 209–213

Phase relations in La2O3–Gd2O3–CuO system at 950 ◦C R. Hory´n ∗ , E. Bukowska, A. Sikora Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław, Ok´olna 2, Poland Received 22 June 2005; received in revised form 12 August 2005; accepted 16 August 2005 Available online 19 September 2005

Abstract Phase equilibria in La2 O3 –Gd2 O3 –CuO system were studied based on samples obtained from appropriate mixtures of the starting components, prepared in air by a conventional solid state reaction. The phase relations prevalent in this system have been deduced based on X-ray diffraction patterns of well-equilibrated samples. An isothermal cross-section diagram through the system at 950 ◦ C is presented. The system is characterised with solid solutions of several well-known binary compounds such as T–La2 CuO4 (orthorhombic at 950 ◦ C), tetragonal T –Gd2 CuO4 , and monoclinic La2 Cu2 O7 (␣-solid solution). The system contains also the T* -type three-component phase of tetragonal symmetry, intermediate to those of T and T . A large solubility domain has been found between La2 O3 and Gd2 O3 (B solid solution of monoclinic structure form typical for RE-sesquioxides), extending deeply towards the ternary system with CuO. Lattice parameters vs. chemical composition are presented for some of the phases found in this system. © 2005 Elsevier B.V. All rights reserved. Keywords: La2 O3 –Gd2 O3 –CuO system; Phase relations; Chemical synthesis and analysis

1. Introduction It is well established that dependent on the level of hole or electron doping of some RE2 CuO4 type binary phases, superconductivity can be realized in these materials with the current carriers being of either p- or n-type. Typical examples of the p-type superconductors are: (T)-La2−x AEx CuO4 and (T* )-(RE1−y 3 REy +4 )2−x (AE)x CuO4 , resulting of doping the binary matrices La2 CuO4 or RE2 CuO4 with either alkaline earth (AE) elements [1] or with appropriate mixture of AE and RE+4 or Th [2,3], respectively. Then several (T )–(RE+3 )2 CuO4 n-type superconductors, resulting of doping the matrices with mentioned above the +4 valent elements exclusively [4]. It is surprising that no matter p- or n-type doping applied, superconductivity can not be realized in (T )-Gd2 CuO4 based matrix. Considering this fact, several effects are proposed to explain lack of superconductivity in, for instance, Ce and/or Th doped solid solutions of this phase. The main are; the orthorhom-

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bic distortion of the structure, caused of CuO4 squares rotation [5], the weak ferromagnetism associated with canting of Cu-ion moments, observed in all the T 214-type phases with the lattice ˚ [6] and/or Cu-deficiency in Cu–O planes parameter a < 3.92 A appearing in course of doping [7]. On the other hand, there are already several reports in literature [8] on superconductivity in free of any doping La2−x REx CuO4 type solid solutions, deposited as thin films on single crystal substrates such as SrTiO3 , KTaO3 , or YAlO3 . These findings indicate on the geometrical factor rather than on any kind of doping, as decisive for the superconductivity in 214 structure type phases. Interesting to note is that, although based on (T)-La2 CuO4 matrix, the thin films reported in [8] exhibited (T )-structure type. The stabilization effect was already observed for x > 0.09 at.% RE. Although plenty of papers on crystallochemistry and superconductivity of Ce doped RE2 CuO4 and AE doped La2 CuO4 compounds have been published to date, there are only few concerning phase relations in appropriate ternary systems [9–11]. Similar situation exists if to take into account free of any doping ternary systems of RE 2 O3 –RE 2 O3 –CuO type. The aim of this work was to recognise phase relations in La2 O3 –Gd2 O3 –CuO system in general, particularly those preva-

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lent in close neighbourhood to domains of solid solutions of (T)-La2−x Gdx CuO4 and (T )-Gd2−x Lax CuO4 type. 2. Experimental The La2 O3 –Gd2 O3 –CuO samples were prepared in air environment by a conventional solid-state reaction of appropriate mixtures of starting components, which were: 3 N purity Gd2 O3 and 4 N purity CuO, both of Johnson–Matthey, and 4 N purity Aldrich La2 O3 . The latter component was each time preheated overnight at 950 ◦ C before being used. The sintering conditions were: 900◦ –1150 ◦ C for 7–21 days, followed by alternative grinding of pelletised powders after each 3 days of sintering. The rule was: the less sample copper content, the higher sintering temperature. Then final grinding, compacting to pellets and final annealing at 950 for 7 days. The annealing was finished with quenching of the resulting products to room temperature. Phase composition of the samples obtained was deduced based on X-ray diffraction patterns, performed in a DRON-3 diffractometer with the use of Fe-filtered Co K␣ radiation. Lattice parameters of individual phases found were calculated and refined with the aid of the DHN/PDF computing programme. The refinements were considered satisfactory when mean values of the 2Θ factor deviations 2(Θcalc. − Θexp. ) ≤ 0.015. This means an error in lattice parameters not higher than several hundredths %. Oxidation state of Cu-ions was determined by iodometric titration, performed on single-phase samples according to the procedure given in [12]. 3. Results and discussion The isothermal cross-section through the system at 950 ◦ C is presented in Fig. 1. The most striking feature of the system is

Fig. 2. X-ray diffraction pattern of the (T* ) phase present in La0.38 Gd0.27 Cu0.35 O3−  sample. Traces of secondary phase CuO contaminate this sample. Preparation conditions applied: 950 ◦ C for 20 days.

presence of tetragonal (T* ) type phase. To our knowledge, this structure type appears exclusively in multi-component cuprates of (214)-type containing RE+4 and AE (alkaline earth) elements as the structure stabilisers [3]. Consequently, general formula of this structure type can be reflected as (RE1−y +3 REy +4 )2−x (AE)x CuO4 . At 950 ◦ C, the (T* ) phase occurs to have very narrow homogeneity domain. This fact appeared to be a serious obstacle in preparation of a single (T* ) phase sample. Indeed, the only sample which, to some degree of certainty, can be considered as presenting only (T* ) phase is sample of the composition La0.38 Gd0.27 Cu0.35 O (see Fig. 1). Nevertheless, its diffraction pattern (shown in Fig. 2) indicates that this sample is slightly contaminated with CuO. If to neglect this impurity phase, then two analytical formulae can be ascribed to this sample namely [La1.0857 Gd0.7714 ]Cu+2.43 O4 and [La1.14 Gd0.81 Cu0.05 ]CuO4 .

Fig. 1. Phase relations in La2 O3 –Gd2 O3 –CuO system at 950 ◦ C (air environment).

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Phase boundaries of the T -Gd

Fig. 3. (2−x) Lax CuO4+ phase, seen through variation of its a-lattice parameter, the latter calculated based on X-ray diffraction of sample series containing 0.25 and 0.333 at.% Cu. The estimated experimental ˚ error do not exceed 0.001 A.

Easy to notice that the first one exhibits certain deficiency on (La + Gd) site and, in order to provide full occupancy of oxygen sublattice, average valence of Cu ions resulting of this deficiency has to be close to +2.45. The second formula reflects free of any deficiency skeleton in which certain fraction of Cu ions shares as well the (La + Gd) site, with an average valence of all Cu ions as corresponding to VCu = 2.047. Of both the formulae considered, the last one seems less probable due to unacceptable presence of low dimension Cu ions on the (La + Gd) site, to date never reported for any system containing (T* )-type phase. Indeed, result of iodometric titration of the sample under discussion (VCu = 2.13) looks to confirm this conclusion. Oxygen deficiency which arises in this sample as the consequence of incomplete oxidation of Cu ions (O ∼ 3.85 instead of 4.0) is quite understandable considering rather low oxygen pressure (air) existing while sample sintering. A considerable difficulty encountered in preparation of the (T* ) phase, as well as a quite high oxidation state of Cu ions the phase exhibits, both indicate on quaternary system of La2 O3 –Gd2 O3 –“Cu2 O3 ”–CuO type to be in fact its origin. Further long term sintering of the (T* ) phase, performed in pure oxygen at 600 ◦ C to get deeper oxidation of Cu ions, has failed. Considering the (T) and (T ) homologies of (T* ) phase, incomplete mutual solubility of these phases has been found. At 950 ◦ C solubility of (T )–Gd2 CuO4 in (T)–La2 CuO4 phase does not exceed ∼15 mol% Gd. Nevertheless, this solubility is much higher than what is reported in [8] for appropriate (T) structure type thin layers. Moreover, with increasing content of Gd, domain of the orthorhombic (T)-La2 CuO4 type solid solution undergo evident deviation towards less Cu content regions. That is why lattice parameters of the (T) ˚ type matrix remain practically constant (a = 5.399–5.405 A, ˚ c = 13.135–13.085 A), ˚ instead to show a b = 5.357–5.349 A, steady decrease. Considering the inverse ability, much higher solubility of (T)La2 CuO4 in tetragonal (T )–Gd2 CuO4 phase can be observed (∼33.3 mol% La). Lattice parameters of the resulting (T ) type solid solution, determined along the line of 33.33 at.% Cu, are presented in Figs. 3 and 4 (see full circles). Therein, lattice

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Fig. 4. Phase boundaries of the T -Gd(2−x) Lax CuO4+ phase, seen through variation of its c-lattice parameter, the latter calculated based on X-ray diffraction of sample series containing 0.25 and 0.333 at.% Cu. The estimated experimental ˚ error do not exceed 0.003 A.

parameters of the same phase, seen in the series of 25.0 at.% Cu (see open circles), are also presented for a comparison. As seen, with increasing contribution of La, these parameters undergo a linear increase up to the overall composition LaGdCuO4+δ . However, in the three-phase domain in which the (T ) type solid solution appears to coexist with the (T* ) and (T) phases (cf. Fig. 1), unexpected behaviour of these parameters has been found. Instead to show composition independent constant values given by the terminal sample 33.33 at.% Cu, they undergo a sharp turn down and then, they decrease steadily as if possessing free level of freedom in this three-phase region. Explanation for such behaviour is difficult. However, the most sensible seems to be following. As soon as the (T ) phase happens to be in equilibrium with the (T* ) one, which is supposed to originate from quaternary system of Cu+3 mentioned, Cu ions of the (T ) phase probably get as well higher oxidation state. With increasing content of the tetragonal (T* ) phase, oxidation of these ions may also increase due to a charge transfer effect, induced by the (T* ) phase and resulting in more stable system of the type (1 − ε)Cu+2 + εCu+3 = Cu2+ε . Great structure similarity of the (T ) and (T* ) phases seems facilitate this process. Consequently, crystal structure of the (T ) phase may get steadily increasing deficiency on cationic site, similar to that reported in [13] for LaMnO3 perovskite, namely of the [La(1−x) Gdx ]2(1−) Cu(1−) O4 type in which (1 − ) = 8/(6 + VCu ). Hence the observed decrease of the lattice parameters, caused of the (T ) structure skeleton shrinkage. By the way, valence of Cu ions (VCu ) found within singlephase domain of the (T ) phase (see 33.33 at.% Cu series), remains practically constant close to 2.067. That such charge transfer effect is highly probable, we refer to the case of 25.0 at.% Cu series (see open spots in Figs. 3 and 4). Note that in this series, nothing unusual is happening to lattice parameters of the (T ) phase—evidently due to absence of the (T* ) phase. That is why, in the composition range under discussion, lattice parameters of the (T ) phase behave as expected. Additional interesting feature found in La2 O3 –Gd2 O3 –CuO system refers to (α) solid solution of monoclinic symmetry, appearing in the system within rather short composition interval. X-ray analysis (see the diffraction diagram in Fig. 5) has allowed

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Fig. 5. X-ray diffraction pattern of the (α) phase present in La0.45 Gd0.05 Cu0.50 O5 sample. Preparation conditions applied: 950 ◦ C for 13 days.

to proof the α phase to be straight derivative of La2 Cu2 O5 reported in [14]. According to [14], La2 Cu2 O5 belongs to a new series of La-cuprates of the La4(1+n) Cu2(4+n) O14+8n type and represents the n = 2 member of the series. As reported in [14], thermal stability of the binary matrix La2 Cu2 O5 is very narrow (from 999 to 1012 ◦ C). Evidently, doping of this matrix with Gd makes the structure stability increased. That is why, in isothermal cross-section through the system at 950 ◦ C presented herein, the (α)-La2 Cu2 O5 type solid solution is seen as an isolated ternary phase of linear in shape domain, comprised in-between 48.0 and 46.5 at.% La, 4.0 and 7.0 at.% Gd, and 48.0 and 46.5 at.% Cu. A sensible formula which can be ascribed to the (α) phase of given above composition is following: [La1−x Gdx ]2 [Cu1−x Gdx ]2 O5 with x = 0.04 and 0.07, respectively. As already mentioned, the phase is monoclinic with the ˚ b = 3.738 A, ˚ c = 27.902 A ˚ and lattice parameters; a = 13.814 A, β = 105.97◦ , and with the β angle not varying significantly with increasing contribution of Gd. Apart from already mentioned the (␣), (T), (T ) and (T* ) phases, La2 O3 –Gd2 O3 –CuO system contains still one more phase, namely three-component solid solution based on the (B)type monoclinic form of RE-sesquioxides. The domain of this solid solution is quite large. Being based on La2 O3 –Gd2 O3 binary subsystem within the range 40.0–80.0 at.% Gd, it quite deeply penetrates the interior of the system up to ∼5.0 at.% Cu (see Fig. 1). The X-ray diffraction lines of this (B) structure type solid solution, present in two-phase sample La0.42 Gd0.48 Cu0.10 O1.5−δ (see Fig. 6), were indexed according to appropriate data reported in [15]. The accompanying phase in this sample being the (T) one. In Fig. 7, we present composition dependence of a-lattice parameter for the (B) type phase seen in two series of samples, namely, in the La1−x Gdx O1.5 binary one, and in the threecomponent series of constant CuO content equal to 25.0 at.%. With this dependence, position of appropriate tie lines connecting domain of the (B) phase with the phases that remain in equilibrium with it, could be precisely justified. Since the composition dependencies of the remaining lattice parameters of this

Fig. 6. X-ray diffraction pattern of the (B) phase present in La0.42 Gd0.48 Cu0.10 O1.5−x sample. The asterisks denote diffraction lines of the accompanying (T) phase. Sample preparation conditions: 1050 ◦ C for 8 days + 950 ◦ C for 13 days.

Fig. 7. Phase boundaries of the (B) phase, determined via its c-lattice parameter variation, the latter calculated based on X-ray diffraction of 0.25 at.% Cu series and of Cu-free La(1−x) Gdx O1.5 type one. The estimated experimental error in ˚ c-parameter does not exceed 0.003 A.

phase appeared to bear identical information that is why they are not presented herein. According to us, introduction of Cu into crystal structure of the (B) phase should result in appearance of oxygen deficiency. For this reason, the resulting solid solution may exhibit an interesting property namely, increasing versus Cu content, ionic conductivity. 4. Conclusions On the basis of the X-ray diffraction analysis of 84 samples, an isothermal (950 ◦ C) section of the ternary system La2 O3 –Gd2 O3 –CuO is presented. A particular attention has been devoted to the (T )-structure type solid solution appearing along the line Gd2 CuO4 –La2 CuO4 . Extension of this solid solution has been established from the change of its lattice parameters versus Gd content. In a close neighbourhood of the (T ) phase, the tetragonal (T* )-(La,Gd)2 CuO4 is found. This phase appears to have very narrow homogeneity domain, and what is striking, it does not need any structure stabilisers such as admixtures of RE+4 + AE (alkaline earth) for its stability. As evidenced by iodometric titration, Cu ions in the (T* ) phase

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exhibit higher than +2 average level of oxidation, appearing due to oxidising environment. In our opinion this has to lead to some vacancies on the (La,Gd) site. Moreover, a stabilisation effect of Gd2 O3 on low thermal stability (999 ◦ C ⇔ 1012 ◦ C) monoclinic binary phase La2 Cu2 O5 has been encountered in this system and evidenced as α-phase. The binary La2 O3 –Gd2 O3 system has been also specified, especially the homogeneity range of the monoclinic (Gd1−x Lax )2 O3 type intermediate phase, evidenced as (B)-phase, which penetrates the ternary system up to 5 mol% CuO. References [1] J.G. Bednorz, K.A. M˝uller, Z. Phys. B 64 (1986) 189. [2] J. Akimitsu, S. Suzuki, M. Watanabe, H. Hawa, Jpn. J. Appl. Phys. 27 (1988) 1859. [3] Y. Tokura, H. Takagi, H. Watabe, H. Matubara, S. Uchida, K. Hiraga, T. Oku, T. Mochiku, H. Osano, Phys. Rev. B 40 (1989) 2568. [4] Y. Tokura, H. Takagi, S. Uchida, Nature 337 (1989) 345. [5] M.B. Maple, N.Y. Ayoub, J. Beille, T. Bjornholm, Y. Dalichaouch, E.A. Early, S. Ghamaty, B.W. Lee, J.T. Markert, J.J. Neumeier, G.

[6]

[7] [8] [9] [10] [11] [12] [13] [14]

[15]

213

Nieva, L.M. Paulius, K. Schuller, C.L. Seaman, P.K. Tsai, Transport Properties of Superconductors, World Scientific, Singapore, 1990, p. 536. T. Schultz, R. Smith, A. Fondado, C. Maley, T. Beacom, P. Tinklenberg, J. Gross, C. Saylor, S. Oseroff, Z. Fisk, S.W. Cheong, T.E. Jones, J. Appl. Phys. 75 (1994) 6723. R. Hory´n, E. Bukowska, A. Sikora, M. Wołcyrz, Acta Phys. Polonica A 106 (2004) 733. A. Tsukada, Y. Krockenberger, H. Yamamoto, M. Naito, Solid State Commun. 133 (2005) 427 (also references therein). J.L. Jorda, M.Th. Saugier Cohen-Adad, J. Less-Common Met. 171 (1991) 127. I. Tanaka, N. Komai, H. Kojima, Physica C 90 (1991) 112. M. Daturi, M. Ferretti, E.A. Franceschi, Physica C 240 (1994) 347. Y. Maeno, H. Teraoka, K. Matsukuma, K. Yoshida, K. Sugiyama, F. Nakamura, T. Fijita, Physica C 185–189 (1991) 587. J.A.M. Van Roosmalen, E.H.P. Cordfunke, R.B. Helmholdt, W.H. Zndbergen, J. Solid State Chem. 110 (1994) 100. R.J. Cava, T. Siegrist, B. Hessen, J.J. Krajewski, W.F. Peck, B. Batlogg, H. Takagi, J.V. Waszczak, L.F. Schneemeyer, H.W. Zandberger, Physica C 177 (1991) 115. O.J. Guentert, R.L. Mozzi, Acta Cryst. 11 (1958) 746.