M. Hervieu

Oxygen defect K2NiF4-type oxides: The compounds La2−xSrxCuO4−x2+δ

Oxygen defect K2NiF4-type oxides La2−xSrxCuO4−x2+δ have been synthesized for a wide composition range: 0 ≤ x ≤ 1.34. From the X-ray and electron diffraction study three domains have been characterized: orthorhombic compounds with La2CuO4 structure for 0 ≤ x < 0.10, tetragonal oxides similar to LaSrCuO4 for 0.10 ≤ x < 1 and several superstructures derived from the tetragonal cell (a ⋍ n.aLaSrCuO4 with n = 3, 4, 4.5, 5, 6) for 1 ≤ x ≤ 1.34. The compounds corresponding to 0 < x < 1 differ from the other oxides in that they are characterized by the presence of copper with two oxidation states: + 2 and + 3. A model structure for La0.8Sr1.2CuλO3.4, in which copper has only the + 2 oxidation state, and for which the actual cell is tegragonal—a = 18.804 A and c = 12.94 A—has been established. The particular structural evolution of these compounds is discussed in terms of a competition between the capability of Cu(II) to be oxidized to Cu(III) and the ordering of oxygen vacancies.

Crystal Chemistry of High-Tc Superconducting Copper Oxides

This book presents a review of the solid-state chemistry of the materials behind the recent revolution in superconductivity research. Particular attention is paid to the main features that are necessary for the generation of high Tc superconductivity, for example, low dimensionality of the structure, a mixed valence of copper, hole delocalization and the role of lone-pair cations and alkaline earth elements. The relationships between the structures of the various oxide superconductors discovered so far are emphasized. Besides chemical and structural aspects, physical topics are included, such as electron transport properties in connection with oxygen nonstoichiometry and irradiation effects induced by neutrons, electrons and fast heavy ions. The diagrams and photographs in this book aim to simplify visualization of the structures and complement the clarity of the text. This textbook on solid-state chemistry, solid-state physics and materials science is intended for researchers and graduate students.

Superconductivity in the Bi — Sr — Cu — O System

During the investigation of the system Bi—Sr—Cu—O a novel family of superconducting oxides, close to the composition Sr2Bi2Cu207+δhas been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The X-ray diffraction and high-resolution electron microscopy studies have shown its relationships with perovskite and demonstrated its bidimensional character. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.

Extension of colossal magnetoresistance properties to small A site cations by chromium doping in Ln0.5Ca0.5MnO3 manganites

The Mn site substitutions of charge ordered (CO) manganites Ln0.5Ca0.5MnO3 (Ln=La–Tb) have been investigated. Among the different studied doping elements, chromium is shown to be the most efficient one to induce a metal insulator (MI) transition in the CO undoped insulators. A new Tm(〈rA〉) magnetic diagram, where 〈rA〉 is the A site cationic average radius, is established for the oxides Ln0.5Ca0.5Mn1−xCrxO3, which demonstrates that 〈rA〉 is not a rhedibitory parameter for the occurrence of colossal magnetoresistance (CMR) properties. The average size limit 〈rA〉L≈1.19 A below which the MI transition is suppressed in the undoped CO compounds is indeed lowered to 〈rA〉L≈1.14 A in the Cr-doped materials. The electron microscopy study of these Cr doped oxides shows that this spectacular effect corresponds with the disappearance of the structural transition associated with CO.

Crystallographic study of tetragonal, superconducting YBa2(Cu0,93Fe0.05)3O7

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (Tc≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 Å in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.

A 70 K superconductor: The oxycarbonate Tl0.5Pb0.5Sr4Cu2(CO3)O7

Abstract A new thallium copper oxycarbonate Tl 0.5 Pb 0.5 Sr 4 Cu 2 (CO 3 )O 7 has been isolated. This new phase crystallizes in the tetragonal system with a =3.824 A, c =16.516 A. Its study by X-ray diffraction and high-resolution electron microscopy allows a structural model to be proposed which consists of the intergrowth of double rock-salt layers [(TlO) (SrO)] ∞ , with single perovskite layers [SrCuO 3 ] ∞ and layers of carbonate groups according to the general formula [(SrO)(Tl 0.5 Pb 0.5 O)] RS [(SrCuO 3 ) 2 ] P [SrCO 3 ]. In fact this structure is a derivative of a hypothetical structure “Tl 0.5 Pb 0.5 Sr 4 Cu 3 O 10 ” closely related to the 1223 structure Tl 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O 9 . The susceptibility measurements with an AC susceptometer demonstrate that this phase is a superconductor characterized by a sharp transition with a T c of 60 K when synthesized. Moreover, the comparison with other oxycarbonates shows that this phase exhibits a much higher superconducting volume fraction which reaches 70% at 4.2 K. Annealings performed in an argon-hydrogen flow (10%H 2 ) at 290°C allows the T c to be increased up to 70 K, keeping the same superconducting volume fraction; this optimization of T c is explained in terms of hole carrier density governed by an oxygen non-stoichiometry according to the formula Tl 0.5 Pb 0.5 Sr 4 Cu 2 (CO 3 )O 7-δ . The possible synthesis of a series of superconductors with the generic formula (Tl,Pb) n Sr 4 n Cu 3 n -1 (CO 3 )O 10 n -3 is discussed.

Field-Induced Magnetization Steps in Intermetallic Compounds and Manganese Oxides: The Martensitic Scenario

Field-induced magnetization jumps with similar characteristics are observed at low temperature for the intermetallic germanide

Strong shift of the irreversibility line in bismuth and thallium based 2212 HTSC single crystals irradiated by 6.0 GeV Pb ions

{\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}

Introduction of nitrate groups in the “123” structure: The 82 K superconductor YCaBa4Cu5(NO3)0.3(CO3)0.7O11

and the mixed-valent manganite

HREM Study of the Defects in the Orthorhombic Superconductor YBa2Cu3O7±ε. II. - Oxygen Substoichiometry and Cationic Disorder

{\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}{\mathrm{Mn}}_{0.96}{\mathrm{Ga}}_{0.04}{\mathrm{O}}_{3}.