Daniel Groult

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.

V3P5SiO19, a vanadosilicophosphate built up from V2O9 octahedra clusters

Abstract A new vanadium(III) silicophosphate V 3 P 5 SiO 19 has been isolated. Its structure was solved from a single-crystal study in the space group P 6 3 and the parameters a = 14.4671(11) A, c = 7.4605(2) A, Z = 4. The three-dimensional framework of this oxide is built up from corner-sharing PO 4 tetrahedra, disilicate groups Si 2 O 7 and octahedral V 2 O 9 clusters. The vanadium clusters consist of two face-sharing octahedra. The relationships of this structure with that of the Mo(IV) silicophosphates AMo 3 P 5.8 Si 2 O 25 are studied; it is shown that both structures are closely related to each other by the existence of tetrahedral units P 6 Si 2 O 25 . The possibility of nonstoichiometry in the V 3 P 5 SiO 19 oxide is discussed.

Effect of low temperature electron irradiation on the superconductivity of La2−xSrxCuO4

Abstract Two samples of La 2−x Sr x CuO 4 with x = 0.15 and x = 0.2 were irradiated at low temperature (20 K) by 2.5 MeV electrons. A drastic decrease in the critical temperature T c is observed, the damage rate being roughly twelve times large than that in the similarly irradiated superconductor Nb 3 Ge. Under irradiation, T c decreases linearly versus the resistivity (ϱ) increase. Subsequent annealings show that the recovery of T c occurs at temperature as low as 50 K and that the linear relation between T c and ϱ is no longer obeyed.

The 1201 bismuth based cobaltite Bi0.5Cd0.3Sr2Co1.2O5-δ : A new magnetoresistant spin glass like insulator

A new “1201” bismuth based cobaltite, Bi0.5Cd0.3Sr2Co1.2O5−δ, has been synthesized. Its single crystal structure determination shows that this tetragonal phase [a = 3.7765(1) A, c = 8.7812(3) A] exhibits regular single perovskite layers [SrCoO3]∞ characterized by an elongation of the CoO6 octahedra, whereas the rock salt layers [Bi0.5Cd0.3Co0.2SrO2−δ]∞ exhibits disordered phenomena especially concerning the distribution of the metallic species and splitting of the oxygen positions in the intermediate layer [(Bi, Cd, Co)O]. The most important feature of this oxide deals with its spin glass like behavior (Tcusp = 49 K) and negative magnetoresistance properties (MR% = 23% at 70 K). The latter are interpreted in terms of competition between double-exchange (Co3+ue5f8Co4+ ferromagnetic interactions) and superexchange (Co3+ue5f8Co3+ and Co4+ue5f8Co4+ antiferromagnetic interactions).

Structural investigations of two varieties of the quasi-2D conductors (PO2)4(WO3)2m(m=5)

Abstract We present a comparative study of the structural instabilities of the monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2m with m=5 which exists in two crystal varieties corresponding to different stacking sequences of the conducting layers made of m WO 6 octahedra. The variety built with a regular stacking of m=5 (m=5/5) exhibits two charge density wave (CDW) transitions similar to those already found in the regular m=4 and 6 bronzes. On the contrary the variety built with an alternate stacking of m=4 and 6 layers (m=4/6) exhibits an unusual high temperature CDW transition with a non monotonous thermal variation of the satellite intensity and a large hysteresis between the cooling and heating intensity curves. These surprising features could be related to the presence of non equivalent CDW instabilities located in the m=4 and m=6 layers.

Coupled electronic and structural transition in the quasi two-dimensional conductors KxP4W8O32

Abstract The monophosphate tungsten bronzes K x P 4 W 8 O 32 with K in the pseudo-hexagonal tunnels show instabilities with critical temperatures T c depending on x and satellite wave vectors independent of x. T c is found to have a maximum for x~1.30. Possible origins for the alkaline concentration dependence of T c are discussed.

Transport properties of the quasi two-dimensional conductor (PO2)4 (WO3)2m m=5 with 5/5/5 regular structure

Abstract The conductors (PO 2 ) 4 (WO 3 ) 2m show CDW type electronic instabilities. We report electrical properties of the m = 5 crystal variety made of a regular 5/5/5 stacking of WO 3 layers. An anomaly at T~60K is observed in the electrical resistivity, Hall coefficient and thermoelectric power. The origin of the anomaly is discussed.

Irradiation Effects in the High-Tc Superconducting Oxides

As has been discussed elsewhere (Chap.3), the superconducting properties of the new high-Tc copper oxides are strongly affected by chemical and structural disorder. The factors that may generate defects in the oxygen framework are numerous and have several origins, including oxygen content, size of the grain, nature of the precursors and thermal treatments. They can result either in strong inhomogeneities, for the most stoichiometric YBa2Cu3O7−δ superconductors (δ ≃ 0), or in various microstructures, for the reduced ones (δ ≥ 0.3). All these features can be considered a consequence of the disproportionation of Cu(II) into Cu(III) and Cu(I) to take into account the copper coordination states with respect to the oxygen deficiency δ.

Crystal structure and electrical properties of K2P4W12O44, the m=6 member of the series of low-dimensional conductors Kx(PO2)4(WO3)2m

A new member, m=6, of the series K n x n n(PO n 2 n) n 4 n(WO n 3 n) n 2m n has been isolated and studied by single crystal X-ray diffraction and electrical transport measurements. The structure corresponds to that of the MonoPhosphates Tungsten Bronzes with hexagonal tunnels (MPTBhs). It has been solved and refined to conventional R=0.0268 with 2342 independent reflections with I>3σ(I). The unit cell is monoclinic (space group P2 n 1 n/m) with a=6.6736(2), b=5.3543(3), c=11.9005(5) A, β=92.615(3)°. The oxygen surroundings of K, P and W atoms are described and compared with those of other members of the series including Pb-, Na- and K-based analogues. Electrical properties and correlations with structural data are discussed. Large magnetoresistance effects have been observed at low temperature and assigned to possible CDW instabilities.

Neutron diffraction and Mössbauer spectroscopy studies of the mixed valent T1-1212 ferrite (Tl0.5Pb0.5)Sr3Fe2O8

Abstract The mixed valent 1212-ferrite (Tl0.5Pb0.5)Sr3Fe2O8 has been characterized using powder neutron diffraction data collected at 1.5 and 293 K. The structure is confirmed to be close to that of the Tl-1212 superconductor (Tl0.5Pb0.5)Sr2CaCu2O7 but with no deviation from a full occupancy of (Tl,Pb) and oxygen sites. 57Fe-Mossbauer spectroscopy data depend upon the temperature between 4.2 and 293 K. They have been explained on the basis of a mixture of trivalent and tetravalent iron states at 4.2 and 150 K. At temperature higher than 150 K the existence of intermediate iron states Fen+ (3