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Topotactic reduction and phase transitions in (T,T′) La1.8Pr0.2CuO4
⁎Corresponding author at: Institut de Chimie Moléculaire et des Matériaux – UMR 5253 – ICG C2M: Chimie et Cristallochimie des Matériaux, Université de Montpellier 2, Case courrier 01504 Place Eugène Bataillon, Bat 15, F-34095 Montpellier cedex 5, France. Tel.: +216 52 250 298; fax: +216 76 220 280. ikb_med@yahoo.fr (M.I. Houchati)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The purpose of this paper is to present the reaction of a cuprate La1.8Pr0.2CuO4 with CaH2 which yields La1.8Pr0.2CuO3.5, a new oxygen-vacancy-ordered arrangement of cooperatively-distorted Cu2O3 planes containing 4-coordinate Cu+ sites. This new compound has been characterized by X-ray diffraction, electron paramagnetic resonance (EPR) and DSC.
The X-ray powder diffraction data have revealed that this cuprate is crystallized with the so-called “pseudo-S” type structure, showing a monoclinic symmetry and space group A2/m. Concerning the EPR measurements, they have shown the presence of Cu2+ cation and a hyperfine structure suggesting a pronounced hybridization of the copper–oxygen bond. Finally, X-ray diffraction has demonstrated that the obtained La1.8Pr0.2CuO3.5, heated in oxygen at 420 °C, turns topotactically into La1.8Pr0.2CuO4 with a T′-type structure (I4/mmm).
Keywords
Non-stoichiometric oxides
Reduction via hydride method
Oxygen mobility
Electron paramagnetic resonance
1 Introduction
Ln2CuO4 with Ln3+ ions smaller than La3+ ions, such as Pr3+, Nd3+, Sm3+, Eu3+, and Gd3+, is crystallized in the T′-structure. Since La2CuO4 is situated near the borderline between T/O (Bmab) and T′-structures (4/mmm), several attempts to synthesize T′-La2CuO4 and T′-(La,Ln)2CuO4 have been reported. T′-La1.8Y0.2CuO4 has been obtained at 600 °C by the co-precipitation method, in which the T′-structure is stabilized due to the low-temperature synthesis and the partial substitution of La3+ by Y3+ with a small ionic radius (Tsukada et al., 2005). T′-La2CuO4 has been prepared by the hydrogen reduction of O-La2CuO4 to obtain the S-phase (Immm) and followed by oxidation at 300–500 °C (Takayama-Muromachi et al., 1990). However, these samples seem not to be well crystallized according to the powder X-ray diffraction patterns. In the same vein, another study has prepared the new phase T′-Ln2CuO4 (Ln = Nd) Chou et al., 1990 by the hydrogen reduction of T′-Nd2CuO4 followed by oxidation, but the former’s hydrogen reduction has shown that the structure is crystallized in another space group, A2/m. Recently, single crystalline films of T′-(La,Ln)2CuO4 have been successfully synthesized at 600 °C using the molecular beam epitaxy (Pederzolli and Attfield, 1998). They have confirmed that T′-(La,Ln)2CuO4 films annealed in a vacuum show superconductivity below Tc = 25 K without nominal carrier-doping. This is very unusual because hole- or electron-doping is usually indispensable for the appearance of superconductivity in copper oxides including CuO2 planes. Hence, although it may be supposed that possible oxygen defects in the films supply the CuO2 planes with electrons; its content is hard to be estimated. Accordingly, the synthesis of the bulk samples of T′-(La,Ln)2CuO4 is essential in clarifying the origin of superconductivity in the non-doped T′-(La,Ln)2CuO4 films. The T′-La2CuO4 phase has been successfully synthesized at a temperature as low as 350 °C by the direct precipitation from the molten KOH/NaOH eutectic mixture (Yoshinori et al., 2007). Because alkaline metal hydroxides have relatively low melting points and dissolve metal oxides, the direct reaction becomes possible at significantly low temperatures. Herein, we report that exposing La1.8Pr0.2CuO4 to metal hydride, CaH2, at a moderate temperature (200 °C) leads to a new La1.8Pr0.2CuO3.5 phase called “pseudo-S”.
It is in the same line of thought that our approach comes to further prove the recent success in the use of hydrides of electropositive metals as powerful reducing agents such as NaH and CaH2 (Hayward and Rosseinsky, 2000; Hayward et al., 2002; Blundred et al., 2004; Hayward, 2006; Poltavets et al., 2006; Hayward, 2005; Hayward et al., 1999; Kageyama et al., 2008; Tsujimoto et al., 2007) . For instance, using NaH yields LaSrCoO3.38 from LaSrCoO4 (Hayward and Rosseinsky, 2000) and YSr2Mn2O5.5 from YSr2Mn2O7 (Hayward, 2006), and using CaH2 yields LaSrCoO3H0.7 from LaSrCoO4 (Hayward et al., 2002), Yb2Ti2O6.43 from Yb2Ti2O7 (Blundred et al., 2004), Sr3Fe2O5 from Sr3Fe2O7 (Kageyama et al., 2008), SrFeO2 from SrFeO2.875 (Tsujimoto et al., 2007) and La3Ni2O6 from La3Ni2O7 (Poltavets et al., 2006). These metal hydrides, normally used as drying agents in organic synthesis, are now regarded as promising reducing agents for nonmolecular compounds to yield unusual frameworks and coordinations in nonmolecular solids because they are more active at considerably lower temperatures than in the case when conventional techniques are used. They also allow solution chemistry to be avoided. However, their potential for the synthesis of new nonmolecular compounds still seems far from realization.
It is in this context that the present paper lies to account for the synthesis of a new well-crystallized cuprate Cu+, La1.8Pr0.2CuO3.5, which is obtained after a topotactic reduction via calcium hydride at a moderate temperature. This new phase is crystallized in the space group A2/m. The oxidation of this phase in oxygen atmosphere below 420 °C leads to the formation of the T′-phase La1.8Pr0.2CuO4.
2 Materials and methods
2.1 Synthesis
Polycrystalline La1.8Pr0.2CuO4 precursor was prepared via a solid-state reaction by rigorously grinding stoichiometric quantities of high purity La2O3 (99.99%), Pr2O3 (99.99%) and CuO (99.99%) in the presence of acetone, thus allowing better intermixture to obtain a total amount of about 5 g. After being dried in ambient atmosphere, this mixture was firstly heated in air at 900 °C for 24 h. After furnace cooling, the resultant powder was manually ground and pressed into pellets of 13 mm in diameter and 1 g in weight, which were again heated in air for 24 h at 900 °C followed by an annealing at 1000 °C for 24 h. This procedure was repeated twice. The obtained product was identified by X-ray diffraction to be stoichiometric La1.8Pr0.2CuO4. All the observed X-ray peaks can be indexed in the orthorhombic space group Bmab, with the following lattice parameters: a = 5.3452(4), b = 5.3957(1) and c = 13.0976(7) (Fig. 1). The synthesis of La1.8Pr0.2CuO3.5 was performed by reacting La1.8Pr0.2CuO4 with CaH2, as a reducing agent. La1.8Pr0.2CuO4 and a two-molar excess of CaH2 were finely mixed in an Ar filled glove box, sealed in an evacuated Pyrex tube, and reacted at 200 °C for two days.
The final product was then washed out with an NH4Cl/methanol solution to remove the residual CaH2 and the CaO byproduct. The T′-La1.8Pr0.2CuO4 phase compound was obtained by heating La1.8Pr0.2CuO3.5 under an oxygen flow at relatively low temperatures (420 °C).
2.2 Characterization
2.2.1 Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) analysis for La1.8Pr0.2CuO3.5 was carried out with a Rigaku DSC8230 instrument. 5.6 mg powdered sample was spread evenly in a large sample holder to avoid mass effects. The DSC run was performed in flowing air with a heating rate of 10 °C/min from 100 °C to 500 °C.
2.2.2 Electron paramagnetic resonance (EPR)
The characteristics of electronic spin interaction in La1.8Pr0.2CuO3.5 were investigated by the electron paramagnetic resonance EPR spectroscopy and the measurements were conducted using a Bruker ER-200D spectrometer having X-band frequencies (9.30 GHz). EPR spectra were recorded at room temperature.
2.2.3 Powder X-ray diffraction
The structural analysis was based on X-ray powder diffraction. Diffraction data were collected in a 2θ range from 10 to 120 with a step interval of 0.014 using a Philips PW 3040 (θ–θ) diffractometer (λCokα). The nuclear structure was refined using the Rietveld method through the FullProf software (Houchati et al., 2012).
3 Results and discussion
3.1 La1.8Pr0.2CuO3.5 compound
3.1.1 Description of the structure
Fig. 2 shows the X-ray diffraction pattern of the new compound La1.8Pr0.2CuO3.5. All the observed X-ray peaks can be indexed in the monoclinic space group A2/m (Houchati et al., 2012), with the following lattice parameters: a = 8.5855(2), b = 3.8446(1), c = 12.9821(3), α = 90, β = 109.5085(3) and γ = 90. The 2nd phase included in the refinement presents La(OH)3 which appears to be a minority fraction (<3%). The values of the refined structural parameters are reported in Table 1. However, the structural model of the “pseudo-S” phase in the monoclinic cell is derived from that of Nd2CuO4 by the ordered removal of one-quarter of the oxygens in the CuO2 layers, as indicated in Fig. 3. This gives rise to seven-coordinate La/Pr sites and CuO4 square planar units with perpendicular conformation alternate along the a-axis. It should be born in mind that La1.8Pr0.2CuO3.5 is an anion-deficient derivative of the T′ phase (Fig. 4), consisting of fluorite-type La2O2 slabs and (CuO1.5□0.5) layers with Cu atoms in dumbbell and strongly-distorted square-planar coordinations. The Cu(1) coordination is a very well aligned square plane with the distances of 2 × 1.92 Å to O(1) and 2 × 2.11 Å to O(2). On the contrary, the regular coordination of Cu2+ in the parent compound La1.8Pr0.2CuO4. Cu(2) has a square plane coordination with the distances of 2 × 2.18 Å to O(2) and 2 × 2.07 Å to O(3). Hence, the Cu2O3 planes are cooperatively distorted. This distortion results in a puckering of the planes, giving birth to a Cu(1)—O(2)—Cu(2) bond angle of 179.80°.
| La1.8Pr0.2CuO3.5 | |||||
|---|---|---|---|---|---|
| Space group: A2/m (No. 1201) | |||||
| Unit-cell parameters: a = 8.5855(2) Å, b = 3.8446(1) Å, c = 12.9821(3) Å, β = 109.5085(3)° | |||||
| The multiplicity of the general position is: 8 | |||||
| Reliability factors: χ2 = 1.48, Rwp = 1.52%, Rp = 1.19% | |||||
| Atom | Wyckoff position | x | y | z | Occ. |
| La1/Pr1 | 4i | 0.1738(2) | 0 | 0.3449(5) | 0.45/0.05 |
| Cu1 | 2a | 0 | 0 | 0 | 0.25 |
| Cu2 | 2d | 0.5 | 0 | 0 | 0.25 |
| La2/Pr2 | 4i | 0.3220(7) | 0.5 | 0.1454(2) | 0.45/0.05 |
| O1 | 2b | 0 | 0.5 | 0 | 0.25 |
| O2 | 4i | 0.2459(1) | 0 | −0.0003(6) | 0.5 |
| O3 | 4i | 0.4635(4) | 0 | 0.8342(8) | 0.5 |
| O4 | 4i | 0.3388(5) | 0 | 0.2580(2) | 0.5 |
| Interatomic distance (Å) | |||||
| Cu(1)–O(1) × 2:1.922(2) | La(2)–O(4) × 2:2.390(1) | ||||
| Cu(1)–O(2) × 2:2.112(1) | La(1)–O(4) × 1:2.085(3) | ||||
| Cu(2)–O(2) × 2:2.180(3) | La(2)–O(3) × 1:2.344(4) | ||||
| Cu(2)–O(3) × 2:2.069(1) | |||||
3.1.2 Electron paramagnetic resonance studies
The Cu2+ 3d9 ion has the electron spin S = 1/2 and the nuclei spin I = 3/2. One can expect that the 3d8 Cu+ ion in fourfold plane coordination is diamagnetic. The room temperature electron paramagnetic resonance (EPR) spectrum of the La1.8Pr0.2CuO3.5 compound is shown in Fig. 5. Analysis of this anisotropic powder-type spectrum has revealed its hyperfine structure signals of the isolated CuO4 centers superimposed with the signal of the Cu2+ ions. This spectrum shows the existence of two types of defects. The first one is located between 2750 and 3250 Gauss relative to the existence of the paramagnetic ion Cu2+. In this section, we note the presence of five lines that have the values of g// = 2.323 and g⊥ = 2.076. Such values are in harmony with the literature (Nakbanpote et al., 2007). In fact, the signal form indicates the symmetry of the site, where the single electron is found. As for the second type, it consists of two peaks located between 3420 and 3490 Gauss having values of g1// = 2.022 and g2// = 2.047. These could be attributed to the defects related to the paramagnetic lanthanum ion La2+ (Cassani et al., 1997).
3.2 Nd2CuO4-type (T′-phase) La1.8Pr0.2CuO4
3.2.1 Calorimetric studies
The transition from T (Bmab) to T′ (I4/mmm) phase undergone by the La1.8Pr0.2CuO4 compound involves a primary transformation via one intermediate phase, stable in a narrow range of temperature (147 K). In order to measure precisely the temperatures of the transitions, Differential Scanning Calorimetry (DSC) experiments were performed. Fig. 6 shows a typical DSC profile for a La1.8Pr0.2CuO3.5 sample heated in an oxygen flow of 20 cm3/min as the temperature increased from 100 to 500 °C at a rate of 10 °C/min. The thermogram reveals two peaks: an exothermal one at 256 °C and an endothermic one at 345 °C. The first peak can be attributed to the oxidation of the La1.8Pr0.2CuO3.5 phase, which implies an increase in oxygen. Therefore, the system will release energy in the form of heat, shown by an exothermal peak after DSC study. The second transition at 345 °C can be accredited to the transition of the “pseudo-S” phase to the T′-La1.8Pr0.2CuO4 phase. Such tentative attributions are confirmed by X-ray diffraction studies.
3.2 2. Description of the structure
T′-phase La1.8Pr0.2CuO4 samples were obtained from the above “pseudo-S” phase La1.8Pr0.2CuO3.5 samples by being heated under an oxygen flow at relatively low temperatures. X-ray diffraction analysis revealed that when the sample was heated in the temperature range of 300 to 500 °C and then cooled to room temperature in air, the sample showed T′ rather than T structure of the La1.8Pr0.2CuO4 starting material. The X-ray diffraction pattern of T′-phase La1.8Pr0.2CuO4 is shown in Fig. 7. All the peaks can be indexed on the basis of the tetragonal lattice (I4/mmm), the experimental profile can be refined with good agreement factors with the structural model of the T′-phase (Table 2). When re-oxidized, the intermediate oxygen deficient phase re-absorbs oxygen atoms, which re-orders the oxygen lattice. According to the experimental results of the calorimetric measurements, the transformation from T to T′ structure for La1.8Pr0.2CuO4 by the red/ox process lowers the crystallinity and induces structural defects. However, from the phenomenological point of view, the structural phase transitions between the different polymorph states, which differ only by oxygen position in the unit-cell of their structures do not only enhance the role of oxygen in the stability of these compounds, but also imply different oxygen sites of close energy, and high oxygen mobility.
| La1.8Pr0.2CuO4 | |||||
|---|---|---|---|---|---|
| Space group: I4/mmm (No. 139) | |||||
| Unit-cell parameters: a = 4.0043(1) Å, c = 12.4825(8) Å | |||||
| The multiplicity of the general position is: 32 | |||||
| Reliability factors: χ2 = 2.21, Rwp = 1.88%, Rp = 1.33% | |||||
| Atom | Wyckoff position | x | y | z | Occ. |
| La/Pr | 4e | 0 | 0 | 0.3525(1) | 0.9/0.1 |
| Cu | 2a | 0 | 0 | 0 | 0.5 |
| O1 | 4c | 0 | 0.5 | 0 | 1 |
| O2 | 4d | 0 | 0.5 | 0.25 | 1 |
| Interatomic distance (Å) | |||||
| La–O(2) × 4:2.376(1) | Cu–O(1) × 4:2.002(3) | ||||
The structure of the type T′-La1.8Pr0.2CuO4 is derived from that of the type T-La1.8Pr0.2CuO4 by a topotactic oxidation–reduction through the intermediate “pseudo S” La1.8Pr0.2CuO3.5 phase. The difference between T and T′ phases is due to the displacement of apical oxygen atoms (4e sites in the space group I4/mmm) in the tetrahedral sites formed by the lanthanide ions (4d sites in the same space group). The sequence of atomic planes along the c-axis thus becomes CuO2 and La–O2–La. As for the Cu2+ cation, it is surrounded by four oxygen atoms, thus forming a square plane and La3+ with a coordination number of 8. Furthermore, while the CuO2 planes in the T′ phase are in extension, the layers around La are in compression, as opposed to the case of the structure of the type T-La1.8Pr0.2CuO4 starting material.
4 Conclusion
During this work, the synthesis conditions of the CaH2 reduction to transform T-La1.8Pr0.2CuO4 into T′-La1.8Pr0.2CuO4 have been optimized by a 2-step topotactic reaction proceeding at low reaction temperatures and yielding bulk T′-La1.8Pr0.2CuO4 in good crystalline quality regarded as a deficient T′-Nd2CuO4. The first step consists in reducing T-La1.8Pr0.2CuO4 via the CaH2-method to La1.8Pr0.2CuO3.5, which has an ordered arrangement of oxygen vacancies, yielding a 4-fold planar coordination for the Cu-atoms. As for the second step, it pertains to the re-oxidization under mild conditions at 420 °C in oxygen atmosphere of La1.8Pr0.2CuO3.5 (which can be regarded as a vacancy T′-Nd2CuO4 phase) to T′-La1.8Pr0.2CuO4. The significance of this reaction relates to its mechanism, as it implies low temperature oxygen mobility realized in a solid oxide, which has technological importance for oxygen membranes of SOFCs.
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