Electron-doped system (La, Nd, Ce)2CuO4 and preparation of T′-type (La, Ln)CuO4 (Ln=Ce, Y)

doi:10.1016/0921-4534(90)90159-C

Physica C: Superconductivity

Volume 165, Issue 2, 15 January 1990, Pages 147-151

https://doi.org/10.1016/0921-4534(90)90159-C Get rights and content

Abstract

Superconductivity of the electron-doped system La_xNd_1.85−xCe_0.15CuO_4−y was studied. The solid solution has a T′-type structure and ranges up to x=1.1. T_c of the system depends not only on the La-content x but also on the oxygen deficiency y. The dependence of T_c on y was obtained for the sample with x=0.8 and compared with the previous results for the sample with x=0. In both compounds, oxygen deficiency is indispensable to realize superconductivity but a very small level of deficiency is enough and more defects suppress T_c.

It was found that T′-type (La, Ln)₂CuO₄ (La=CeandY) can be prepared at lower temperatures (∼600°C) using a coprecipitation technique, while they cannot be obtained at higher temperatures by the usual solid-state reaction. They did not show superconductivity even after reduction.

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Phase diagram and transition temperatures in the system (T-T’) La<inf>2-x</inf>Nd<inf>x</inf>CuO<inf>4</inf> (x ≤ 0.5)
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La_2-xNd_xCuO_3.5 (x ≤ 0.5) compounds have been synthesized by a topotactic reduction reaction with calcium hydride at 280 °C for 48 h. Structural, thermal, vibrational and magnetic properties of La_2-xNd_xCuO_3.5 (x ≤ 0.5) were studied by X-ray powder diffraction (XRD), by differential thermal analysis (DTA) and temperature dependent powder X ray diffraction (TDXD), by Raman spectroscopy and by electronic paramagnetic resonance. The oxidation of the compounds La_2-xNd_xCuO_3.5 (x ≤ 0.5) at 400 °C for 24 h kinetically stabilizes the compounds La_2-xNd_xCuO₄ (x ≤ 0.5) with an I4/mmm type structure. The phase relationship between T’ and T, ensured at high temperature, was followed and discussed by thermal and structural analysis.
Topotactic transformation of (T-T’) La<inf>1.8</inf>Nd<inf>0.2</inf>CuO<inf>4</inf>: Synthesis, structure, electrical properties and oxygen diffusion pathways simulation
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In this context, researchers are pursuing stability conditions to understand whether T’ (La2CuO4) phase exists only as a thin layer. Thus, using the co-precipitation method, Takayama Muromachi et al. [28] stabilizes T′-La1.8Y0.2CuO4 phase at low temperature (600 °C). In addition, using the conventional reduction-oxidation reaction, the T′-La2CuO4 phase is obtained based on the research of Chou et al. [29], where the T′-La2CuO4 phase is obtained following a topotactic reduction under the atmosphere of hydrogen in T-La2CuO4 phase to form S phase followed by oxidation at 400 °C.
The starting sample T-La_1.8Nd_0.2CuO₄ (SG: Bmab) has been synthesized by the conventional solid-state reaction at 900 °C. The T′-La_1.8Nd_0.2CuO₄ (SG: I4/mmm) sample has been obtained via a topotactic reduction reaction of T-La_1.8Nd_0.2CuO₄ (SG: Bmab) using CaH₂ (as reductor) followed by an oxidation at 400 °C in air. The temperature of the phase transition (T-T′) has been determined using the differential thermal analysis (DTA) and the temperature-dependent powder X-ray diffraction (TDXD). The crystal structures of the title compounds have been refined by using the Rietveld method and confirmed by the means of the charge distribution model (CHARDI). The electrical properties of the title compounds have been studied by impedance complex spectroscopy between 200 °C and 650 °C. This study shows two slopes in the Arrhenius plot with experimental activation energies 1.033 eV and 1.657 eV which correspond to the reduced phase and the T′ phase respectively. The simulation of oxygen diffusion in structures by using the Bond Valence Site Energy (BVSE) method shows three-dimensional pathways of oxygen diffusion. The calculated activation energies of the T′ and T structure are 1.619 and 2.369 eV, respectively.
Epitaxial effects in thin films of high-T<inf>c</inf> cuprates with the K<inf>2</inf>NiF<inf>4</inf> structure
2018, Physica C: Superconductivity and its Applications
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There are two problems specific to the growth of T-LCCO films. The first is that Ce tends to segregate out from the T lattice at growth temperatures (Ts) higher than 700°C [70]. Indeed the films grown at Ts>700°C showed no change in the c-axis lattice constant even when the amount of Ce supplied was varied, indicating that Ce is not incorporated into the lattice.
La_2-_xSr_xCuO₄ (LSCO) and La_2-_xBa_xCuO₄ (LBCO) have been recognized as the archetype materials of “hole-doped” high-T_c superconductors. Their crystal structures are relatively simple with a small number of constituent cation elements. In addition, the doping level can be varied by the chemical substitution over a wide range enough to obtain the full spectrum of doping-dependent electronic and magnetic properties. These attractive features have dedicated many researchers to thin-film growth of LSCO and LBCO. The critical temperature (T_c) of LSCO and LBCO is sensitive to strain as manifested by a positive pressure coefficient of T_c in bulk samples. In general, films are strained if they are grown on lattice-mismatched substrates (epitaxial strain). Early attempts (before 1997) at the growth of LSCO and LBCO films resulted in depressed T_c below 30 K as they were grown on a commonly used SrTiO₃ substrate (in-plane lattice parameter a_sub = 3.905 Å): the in-plane lattice parameters of LSCO and LBCO are ≤3.80 Å, and hence tensile epitaxial strain is introduced. The situation was changed by the use of LaSrAlO₄ substrates with a slightly shorter in-plane lattice constant (a_sub = 3.756 Å). On LaSrAlO₄ substrates, the T_c reaches 45 K in La_1.85Sr_0.15CuO₄, 47 K in La_1.85Ba_0.15CuO₄, and 56 K in ozone-oxidized La₂CuO₄₊_δ films, substantially higher than the T_c’s of the bulk compounds. The T_c increase in La_1.85Sr_0.15CuO₄ films on LaSrAlO₄ and decrease on SrTiO₃ are semi-quantitatively in accord with the phenomenological estimations based on the anisotropic strain coefficients of T_c (dT_c/dε_i). In this review article, we describe the growth and properties of films of cuprates having the K₂NiF₄ structure, mainly focusing on the increase/decrease of T_c by epitaxial strain and quasi-stable phase formation by epitaxial stabilization. We further extract the structural and/or physical parameters controlling T_c toward microscopic understanding of the variation of T_c by epitaxial strain.
Topotactic reduction and phase transitions in (T,T′) La<inf>1.8</inf>Pr<inf>0.2</inf>CuO<inf>4</inf>
2017, Arabian Journal of Chemistry
Citation Excerpt :
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.
The purpose of this paper is to present the reaction of a cuprate La_1.8Pr_0.2CuO₄ with CaH₂ which yields La_1.8Pr_0.2CuO_3.5, 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 La_1.8Pr_0.2CuO_3.5, heated in oxygen at 420 °C, turns topotactically into La_1.8Pr_0.2CuO₄ with a T′-type structure (I4/mmm).
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2016, Physica C: Superconductivity and its Applications
Citation Excerpt :
Therefore the lattice constants of T’ cuprates should decrease with the substitution of Ce for RE. As shown in Fig. 12, the c0 decreases with x in accord with the above expectation whereas the a0 increase slightly with x, which is opposite to the expectation [2,36–48]. The increase of a0 with Ce doping is due to the stretching of the Cu-O bond by filling electrons into the antibonding (σ*) orbitals.
The electronic phase diagram of the cuprates remains enigmatic and is still a key ingredient to understand the mechanism of high-T_c superconductivity. It has been believed for a long time that parent compounds of cuprates were universally antiferromagnetic Mott insulators (charge-transfer insulators) and that high-T_c superconductivity would develop upon doping holes or electrons in a Mott–Hubbard insulator (“doped Mott-insulator scenario”). However, our recent discovery of superconductivity in the parent compounds of square-planar cuprates with the Nd₂CuO₄ (T’) structure and the revised electronic phase diagram in T’ cuprates urged a serious reassessment to the above scenario. In this review, we present the main results derived from our synthesis and experiments on T’ cuprates in the undoped or heavily underdoped regime over 20 years, including material issues and basic physics. The key material issue is how to remove excess oxygen ions at the apical site without introducing oxygen vacancies in the CuO₂ planes. In order to put this into practice, the basic knowledge of complex solid-state chemistry in T’ cuprates is required, which is also included in this review.
Metastable Tprime;-phase in bulk la<inf>2-x</inf>Ln<inf>x</inf>CuO <inf>4</inf> (Ln - Sm and Y)
2008, Physica C: Superconductivity and its Applications
We prepared the metastable T′-phase La_2−xLn_xCuO₄ (Ln = Sm and Y) cuprates by a coprecipitation technology, which cannot be acquired by conventional solid-state reaction. The selective stabilization of T′- and T-phase was studied via altering synthesis temperature and pressure. The structural phase diagram for T- and T′-phase is obtained in La_2−xLn_xCuO₄ system. All these samples show semiconducting behavior, and no superconductivity was observed down to 2 K even after reduction. Magnetic measurement shows a Curie–Weiss-like paramagnetic behavior.

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Electron-doped system (La, Nd, Ce)2CuO4 and preparation of T′-type (La, Ln)CuO4 (Ln=Ce, Y)

Abstract

Physica C

Solid State Chem.

Mater. Res. Bull.

Physica C

Z. Phys.

Nature

Electron-doped system (La, Nd, Ce)₂CuO₄ and preparation of T′-type (La, Ln)CuO₄ (Ln=Ce, Y)