6 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 1. 01 Grivel J.-C. Chimica Techno Acta. 2018. Vol. 5, No. 1. P. 6–15. ISSN 2409–5613 J.-C. Grivel Department of Energy Conversion and Storage, Technical University of Denmark, 399 Frederiksborgvej, Roskilde, DK-4000, Denmark jean@dtu.dk Subsolidus phase equilibria of the CuO – SrO – ZnO pseudoternary system in air at 900 °C The subsolidus phase equilibria of the CuO – SrO – ZnO system were deter- mined at 900 °C in air. The pseudoternary section does not contain ternary oxide phases but is made of 5 three-phase regions and 2 narrow two-phase regions linked to a Sr 14 Cu 24–x Zn x O 41–y solid solution. The maximum solubility of Zn in this phase is limited to x ≈ 0.1, but this low doping level results in a significant decrease of the electrical resistivity by about one order of magnitude compared to the undoped compound. The other binary oxide phases SrZnO 2 , Sr 2 CuO 3 , and SrCuO 2 do not form solid solutions extending into the ternary system. SrZnO 2 was found to decompose upon contact with ambient air. Keywords: phase equilibria, isothermal section, oxide systems, pseudoternary, CuO, SrO, ZnO, spin-ladder, electrical resistivity Received: 15.11.2017. Accepted: 01.12.2017. Published: 10.05.2018. © Grivel J.-C., 2018 Introduction The discovery of high-tempe- rature superconductivity in the Ba-doped La2CuO4 insulator [1] has initiated a wealth of research aiming at finding new com- pounds with ever higher superconducting critical temperatures, in particular among cuprate compounds. One of the strategies used for this purpose consists in studying the phase equilibria of pseudoternary and higher dimensional systems con taining CuO and an alkaline earth oxide, in par- ticular BaO, SrO or CaO. Nevertheless, whereas ternary sections involving these oxides with rare-earth oxides have been systematically determined (refe rence [2] and references therein), combinations of CuO and (BaO, SrO or CaO) with a tran- sition metal oxide are still poorly investi- gated. In the case of SrO, the exceptions encompass systems in which the transi- tion element is V [3], Nb [4, 5], Fe [6], Ti [7, 8], Zr [8], Hf [8], Ta [9], W [10] and Co [11]. The CuO – SrO – ZnO system does not appear to have been stu died. However, Zn being not magnetic, it rep- resents a favo rable case for high-tem- perature superconductors. Furthermore, the use of ZnO in association with SrO or CuO for various applications [12–19] as well as that of SrZnO2 as a catalyst for PET depolymerization under microwave irradiation [20] or as host material for red or blue emitting phosphors [21, 22] make the study of this system interesting in view of optimizing the processing parameters of samples aimed at various technological 7 applications. The equilibration conditions for the present study (air atmosphere and 900 °C) correspond to conditions similar to those used for the synthesis of various high temperature superconductors [23, 24]. Sur- prisingly, searches in databases such as the ACerS-NIST Phase Equilibria Diagrams database (version 4.0) or Web of Science did not deliver references to CuO – ZnO or SrO – ZnO pseudobinary systems, not even as side-systems in ternary or higher dimensional systems so that even these simple combinations call for investigations. Experimental CuO (Alfa Aesar, 99.995% purity), SrCO3 (Sigma Aldrich, 99.95) and ZnO (Alfa Aesar, 99.9%) powders were used as starting reagents. 33 different nominal compositions were selected and studied. After the powders were weighted in proper amounts, they were thoroughly mixed in an agate mortar and calcined at 900 °C for 60 h. After this first treatment and further grinding, 1 g pellets with 12 mm diameter were pressed under a pressure of 1.8 kbar and sintered at least twice at 900  °C for 100  h with intermediate grinding and re-pressing. The pellets were quenched in air at the end of each heat treatment. All manipulations were performed in air. The phase composition of the samples was determined using X-ray diffraction (XRD) patterns recorded in a Bruker D8 Robot diffractometer using Cu Kα radia- tion (λ  = 1.54  Å). Equilibrium was con- sidered as achieved when no differences were detectable in the XRD patterns after two consecutive heat treatments. Si pow- der was added as an internal standard after the heat treatments for lattice parameter calculations. Low temperature electrical resistivity measurements were conducted in a Dis- plex setup with a DC current of 1 mA by the 4-contact method on samples cut from the pellets into parallelepipeds with 1.5 × 1.5 mm2 cross section and 8 mm length. Gold wires were attached with silver paste cured at 200 °C. Results and discussion As expected, the “ZnO” sample an- nealed at 900 °C consists of the ZnO (zin- cite) phase. Its hexagonal lattice parameters were calculated as a = 3.250(1) Å and c = 5.206(1) Å, in close agreement with the values reported by McMurdie et al. [25]. Including small amounts (1 at.%) of either Sr or Cu in the nominal composition re- sulted in two-phase mixtures, as shown in Fig. 1a, with the appearance of SrZnO2 or CuO peaks, respectively. The lattice pa- rameters of these samples are compared with those of the pure ZnO composition in Table 1. For the Zn0.99Sr0.01O2 nominal composition, the lattice parameters are unchanged within the standard deviation, whereas for Zn0.99Cu0.01O2, a slight decrease of the c-axis length seems to take place. The possibility of introducing Sr into the zincite structure has been explored within various contexts such as modification of the optical and electrical properties of ZnO [12, 13], improving its photocatalytic properties [14, 15], or using SrO as a sintering aid [15, 16]. Conclusions on the actual doping of Sr into ZnO are contradictory, probably due to the widely different preparation routes used for these various applications. Güntürkun and Toplan [16] (solid state reaction at 1000  °C to 1300  °C) found 8 a two-phase equilibrium between ZnO and SrZnO2 even for 1 wt.% SrO in ZnO. Harish et al. [14] (hydrothermal synthesis) reached a similar conclusion. In contrast, Vijaran et al. [13] (thin films made by chemical bath deposition) as well as Srivastava et al. [12] (thin films made by solution spin coating and annealing at 400 °C) claim that Sr enters the ZnO structure. However, the lattice constants of ZnO show a nonmono- tonic behavior versus doping level, with the c-axis parameter first decreasing be- fore increasing. On the other hand, Das et al. [15] (solid-state reaction at 1100 °C) observed a broadening of some ZnO XRD peaks at low Sr doping level and concluded that two distinct ZnO phases with diffe- rent lattice constants coexist. Besides the fact that processing condicitons may in- duce different metastable situations, it is worth mentioning that the relatively low signal-to-noise ratio in the XRD patterns of the thin film samples [12, 13] makes the detection of low intensity impurity peaks difficult, while some high intensity XRD reflections of SrZnO2 have a near overlap with several ZnO reflections. The latter fact may partly be responsible for the reported behavior of the ZnO c-axis parameter upon Sr doping [12, 13, 15]. In any case, there is a clear need for more systematic investiga- tions in order to assess the actual solubi- lity of Sr in ZnO and its dependence on processing parameters. The present study allows concluding that for equilibration at 900 °C in air, the solubility of Sr is clearly lower than 1 at.%. Concerning Cu substitution in ZnO, some recent publications, in which 5 at.% Cu doped samples systematically show a clear CuO XRD impurity peaks [17, 18, 26], claim that a non-negligible amount of Cu actually enters the zincite structure. Again, the very different processing parameters, including hydrothermal systhesis [17], xerogel route [18] or sol-gel [26], all with relatively brief heat treatments at 600 °C or lower temperatures, certainly have an influence on the conclusions. Moreover, no precise studies based on systematic lattice parameters variation were presented. The results of the present equilibration study at 900 °C are in fact more in line with other studies, including TEM characterization showing that CuO nanoparticles segregate on the surface of ZnO crystallites even for 1 at.% Cu doping [19]. The slight decrease of the ZnO c-axis length could be due to very low level Cu doping (ppm range) as observed in ZnO samples exhibiting green luminescence [27]. The presence of CuO in the XRD pat- tern of the sample with Zn0.99Cu0.01O2 nomi- nal stoichiometry indicates that there are no binary oxide compounds between ZnO and CuO. This is further confirmed by the fact that ZnO is detected in the XRD pat- tern of a sample with Zn0.01Cu0.99O2 over- all composition (Fig. 1b). In spite of this two-phase equilibrium, it is clear, as shown in Table 1, that the presence of Zn has re- sulted in changes in the lattice parameters of CuO, with, in particular, a significant Fig. 1. XRD patterns of samples with ZnO (a), Zn0.99Cu0.01O2 (b), Zn0.99Sr0.01O2 (c), CuO2 (d) and Cu0.99Zn0.01O2 (e) nominal compositions: u – ZnO, l – CuO, m – SrZnO2 9 decrease of the b-axis length and increase of the a-axis parameter (as well as an in- crease of the β-angle from 99.5° to 99.7°). The unambiguous detection of low-intensity ZnO reflections in the XRD pattern of the Cu0.99Zn0.01O2 sample, however, indicates that at 900 °C the solubility limit of Zn into CuO is lower than 1 at. %. Similar lattice parameter variations upon Zn doping in CuO had beed reported previously [28]. Prabhakaran and Boothroyd [29] report up to 5 at. % Zn doping, but the pro cessing con- ditions in that case (single crystals grown by the floating-zone method under 4 to 8 atm pressure) mostly provide a hint that the solubility limit of Zn in CuO might in- crease with temperature and may also show some dependence on the oxygen pressure. In the SrO – ZnO binary oxide side system, SrZnO2 was the only binary oxide phase detected in the XRD pat- terns. The lattice parameters of sam- ples with Sr0.98Zn1.02O2, Sr1.00Zn1.00O2 and Sr1.02Zn0.98O2 (Table 1) show little variation that is well inside the confidence interval. This observation, coupled with evidence for two-phase equilibria in the two sam- ples with nominal composition deviating from SrZnO2 (Fig.  2), indicates that the latter phase has a very narrow solid-solu- tion range, if any. It is worth mentioning that the formation on SrO in the Sr-rich Table 1 Phases observed in selected samples after equilibration and crystallographic data for the majority phases. Phases listed between brackets are minority phases. Z = ZnO, C = CuO, SZ = SrZnO2, SOH = Sr(OH)2·H2O, S2C = Sr2CuO3, SC = SrCuO2, S14C = Sr14Cu24O41–y Nominal composition Phases Space group a [Å] b [Å] c [Å] ZnO Z P63mc 3.250(1) – 5.206(1) Zn0.99Cu0.01O Z (C) P63mc 3.251(1) – 5.203(1) Zn0.99Sr0.01O Z (SZ) P63mc 3.251(1) – 5.206(1) Sr1.00Zn1.00O2 SZ Pnam 5.843(9) 3.339(7) 11.33(4) Sr0.98Zn1.02O2 SZ (Z) Pnam 5.841(9) 3.340(8) 11.33(4) Sr1.02Zn0.98O2 SZ (SOH) Pnam 5.843(9) 3.340(7) 11.33(4) Sr1.00Zn0.98Cu0.02O2 SZ (Z, S2C) Pnam 5.843(9) 3.339(7) 11.33(4) Sr0.98Zn1.00Cu0.02O2 SZ (Z, S2C) Pnam 5.841(9) 3.339(7) 11.33(4) CuO C C2/c 4.686(2) 3.424(1) 5.129(1) Cu0.99Zn0.01O C (Z) C2/c 4.702(1) 3.410(1) 5.132(1) Sr2CuO3 S2C Immm 12.70(1) 3.912(1) 3.499(1) Sr2Cu0.95Zn0.05O3 S2C (SZ, SOH) Immm 12.71(1) 3.914(2) 3.501(1) SrCuO2 SC Cmcm 3.572(1) 16.328(3) 3.913(2) SrCu0.95Zn0.05O2 SC (Z, S2C) Cmcm 3.571(1) 16.329(2) 3.912(1) Sr14Cu24O41 S14C Cccm 11.467(3) 13.395(7) 27.649(6) Sr14Cu23.9Zn0.1O41 S14C Cccm 11.468(4) 13.396(10) 27.552(8) Sr14Cu23.75Zn0.25O41 S14C Cccm 11.475(5) 13.391(11) 27.564(14) Sr14Cu23.5Zn0.5O41 S14C (Z, SC) Cccm 11.467(3) 13.394(8) 27.532(7) Sr14Cu23Zn1O41 S14C (Z, SC) Cccm 11.469(3) 13.394(6) 27.532(5) 10 composition is assumed from the detection of Sr(OH)2·H2O [30] in the XRD pattern. This assumption is supported by the fact that a sample with a composition closer to pure SrO (Sr0.99Zn0.01O – not shown), still contained some identifiable SrO re- flections. The Sr(OH)2·H2O compound appears to form quickly upon exposure to the atmosphere. The XRD pattern of the Sr1.02Zn0.98O2 sample (Fig. 2b) was recorded within less than 1 hour after quenching from 900 °C. In view of potential applica- tions of SrZnO2, it is also important to note that this compound degrades at a mode- rate rate when exposed to ambient air. After two weeks, formation of SrCO3 was ascertained in the originally single-phase SrZnO2 sample. In the CuO – SrO pseudobinary side sys- tem, the three binary oxide phases Sr2CuO3, SrCuO2 and Sr14Cu24O41–y were found as expected [31–36]. Replacing 5 at. % of Cu by Zn in the respective compositions of these compounds resulted in samples with equilibrium between three phases. The lat- tice parameters of Sr2CuO3 and SrCuO2 (Table 1) were not affected by the presence of Zn within the confidence interval. In contrast, while the a- and b-axis lengths of Sr14Cu24O41–y were not influenced, it appears that the c-axis lattice parameter became sig- nificantly shorter at a Zn substitution level for Cu of 0.4 at. % only. The XRD patterns of samples with Sr14Cu24–xZnxO41–y (x = 0.00; 0.10; 0.50 and 1.00) nominal compositions are shown in Fig. 3, where it is observed that ZnO and SrCuO2 reflections appear beyond x = 0.10 in agreement with the saturation of the c-axis parameter value (Table 1 and inset in Fig. 4). The effect of Zn substitution on the Sr14Cu24O41–y phase was studied fur- ther by means of electrical conducti- vity measurements. As demonstrated by Fig. 4, the room-temperature resisti- vity of Sr14Cu23.9Zn0.1O41–y is lower than that of Sr14Cu24O41–y, the difference in- creasing at lower temperatures to reach one order of magnitude at 175 K. In the Sr14Cu23.75Zn0.25O41–y sample, the resistivity is even lower. Although the lattice para- meters are not changed as compared with the Sr14Cu23.9Zn0.1O41–y composition, it is likely that this further decrease in resis- tivity is related to a little more Zn doping into the Sr14Cu24O41–y lattice (albeit not all the Zn has entered the structure), instead of being due to minute amounts of impu- Fig. 2. Details of the XRD patterns of samples with Sr0.98Zn1.02O2 (a) and Sr1.02Zn0.98O2 (b) nominal compositions showing 2θ ranges, where the appearance of secondary phases is most evident: m – SrZnO2, u – ZnO, q – Sr(OH)2·H2O. Fig. 3. XRD patterns of Sr14Cu24–xZnxO41–y samples with x = 0.00 (a), 0.10 (b) 0.50 (c) and 0.10 (d): l – Sr14Cu24O41–y, m – SrCuO2, u – ZnO 11 rity phases. This hypothesis is based on the fact that the samples with higher nominal Zn contents, i.e. Sr14Cu23.5Zn0.5O41–y and Sr14Cu23Zn1O41–y, exhibit higher resistivi- ties (not shown in Fig. 4). Nominal compositions situated further within the pseudoternary section resulted in two-phase or three-phase equilibria, without any evidence for the formation of ternary oxide phases. Based on all these observations, the phase equilibria and tie- line compatibilities of phases in the CuO – SrO – ZnO system at 900 °C in air can be summarized as shown in Fig. 5. The sec- tion contains 5 three-phase regions and 2 narrow two-phase fields associated to the Sr14Cu24–xZnxO41–y solid solution. This sys- tem is dominated by ZnO, which is in equi- librium with all other binary oxide phases. The overall structure of this section is rather different from those of the other sys- tems involving CuO, SrO and a transition element oxide [37–51]. The solubility limit of Co in the Sr14Cu24O41–y structure (up to Sr14Cu19Co5O41–y) is significantly larger than that of Zn. In CuO – SrO – RE2O3 systems (RE = rare earth element), RE sub- stitution in Sr14Cu24O41–y also takes place to various extents [37–51], however on the Sr sites rather than on the Cu sites, and the pseudoternary sections are dominated by SrRE2O4 compounds. Conclusions The subsolidus relationship and phase formation of compounds in the CuO  – SrO  – ZnO system were determined at 900  °C in air. The pseudoternary phase diagram is divided into 5 three-phase fields and two narrow two-phase fields are induced by the narrow Sr14Cu24ZnxO41–y (0 ≤ x ≈ 0.1) solid solution. Some Zn sub- stitution in CuO can also be inferred from the variation of the lattice parameters of CuO, but the solubility limit of Zn in CuO under the present equilibration conditions is lower than 1 at. %. Substitution of Zn for Cu in the Sr14Cu24O41–y structure has a very strong effect on its electrical resistivity. The fact that ZnO is in equilibrium with all the other phases save SrO represents a  favorable situation for processing and testing various properties of ZnO-based multiphase nanoparticles and films. Fig. 4. Low-temperature electrical resistivity of samples with nominal compositions belonging to the Sr14Cu24–xZnxO41–y solid solution. Inset: c-axis lattice parameter of the Sr14Cu24O41–y phase in samples with nominal Sr14Cu24–xZnxO41–y (0.0 ≤ x ≤ 1.0) compositions Fig. 5. Phase diagram of the CuO – SrO – ZnO pseudoternary system at 900 °C in air. The studied compositions are marked by symbols. The SrO – SrZnO2 – Sr2CuO3 area was not studied in details due to the fast reaction of SrO with ambient air 12 References 1. Bednorz JG, Muller KA. Possible high-Tc superconductivity in the Ba-La-Cu-O system. Zeitschrift für Physik. 1986; B64:189–93. DOI: 10.1007/BF01303701. 2. Grivel JC. 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