J. Provost

A series of oxygen-defect perovskites containing CuII and CuIII: The oxides La3−xLnxBa3 [CuII5−2y CuIII1+2y] O14+y

Abstract A series of oxygen-defect perovskites, containing Cu II and Cu III , La 3 Ba 3 [Cu II 5−2 y Cu III 1+2 y ] O 14+ y , has been synthesized at 1000°C, for 0.05 ≤ y ≤ 0.43. The substitution of lanthanum for yttrium and lanthanides has been studied. These oxides are tetragonal: a = a p 2 1 2 and c = 3 a p . The structural study of La 3 Ba 3 Cu 6 O 14.10 shows that oxygen vacancies are ordered, involving for copper three sorts of coordination: square, pyramidal (4 + 1), and distorted octahedral (4 + 2). The distribution of Cu III , as well as the lanthanide ions on the different sites, is discussed.

Superconductivity up to 100 K in lead cuprates: A new superconductor Pb0.5Sr2.5Y0.5Ca0.5Cu2O7−δ

Abstract A new superconductor Pb 0.5 Sr 2.5 Y 0.5 Ca 0.5 Cu 2 O 7−δ has been isolated whose superconducting properties depend upon the thermal treatments, i.e. are closely related to the oxygen stoichiometry. This material exhibits a T on c =100 K whereas the T off c ( R =0) ranges from 45 K to 53 K. The magnetic measurements using a SQUID magnetometer show superconducting volume fractions ranging from 14% to 72%. Anomalies appear in the magnetization curves at low field for some samples. This effect and the broad transition are interpreted in terms of an inhomogeneous distribution of oxygen in the matrix. The structure of this tetragonal phase ( a =3.81 A, c =11.908 A) belongs to the “1212” type observed for thallium cuprates, i.e. consists of an intergrowth of double rock salt-type layers involving lead and strontium and of double copper pyramidal layers interleaved with yttrium and calcium. This oxide differs also from the classical intergrowth structures by the existence of numerous oxygen vacancies in the rock salt type layers (δ≅0.7−0.5); moreoer satellites are observed on E.D. diffraction patterns. These two effects are explained by the presence of the 6s 2 lone pair of Pb(II).

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.

A new member of the thallium superconductive series, the “1212” oxide TlBa2CaCu2O8−y: Importance of oxygen content

Abstract A new superconductor was isolated and characterized by X-ray diffraction and electron microscopy. This oxide crystallizes in tetragonal system ( P4 mmm ) with the following parameters: a = 3.833 A and c = 12.680 A. A structural model was established from X-ray powder data. The structure is a new member of the family (AO)n(A′CuO3−y)n′ (A = Tl, Ba and A′ = Ba, Ca) with n = 2 and n′ = 2. It can be described as the intergrowth of double (AO) layers, rock salt-type, with double oxygen-deficient perovskite layers involving CuO5 pyramids. The optimization of the thermal treatment and the importance of oxygen content are discussed.

Thallium cuprates: The critical temperature is mainly governed by the oxygen nonstoichiometry

Abstract The superconducting properties of the thallium cuprates, and especially their critical temperatures, are strongly affected by annealings in various gaseous atmospheres (nitrogen, oxygen, argon and hydrogen). These annealing effects have been studied for Tl 2 Ba 2 CaCu 2 O 8 , TlBa 2 CaCu 2 O 7 , TlBa 2 Ca 2 Cu 3 O 9 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 as well as for the substituted oxides TlBa 2 Ca 1− x Ln x Cu 2 O 7 and Tl 2 Ba 2 Ca 1− x Ln x Cu 2 O 8 (Ln=Y, Nd). It is demonstrated that a drastic increase of T c can be obtained, due to a very small oxygen deficiency. The most spectacular effect is obtained with a gas mixture H 2 /Ar, which can be performed at a temperature as low as 300°C, leading for example for the 2212 phase to an increase of T c from 97 K to 118 K (to be compared to 125 K for the 2223 phase). Moreover it is shown by electron microscopy that at this annealing temperature the crystal remains unchanged. However, thermal treatments at 400°C or beyond alter the crystals in connection with a slight volatilization of thallium oxide. This dramatic effect of oxygen nonstoichiometry upon T c decreses as the number m of copper layers in the perovskite slabs of the superconductors TlBa 2 Ca m −1 Cu m O 2 m +3 and Tl 2 Ba 2 Ca m −1 Cu m O 2 m +4 increses. These results show that the important factor which governs T c in the thallium cuprates is the hole number per mole of copper in the structure and that this factor seems to be predominant with respect to the number m of copper layers.

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

Abstract Bi-2212 (Bi 2 Sr 2 CaCu 2 O 8 ) and Tl-2212 (Tl 2 Ba 2 CaCu 2 O 8 )-type single crystals have been irradiated by 6.0 GeV Pb ions. The irradiation results in the creation of tubes of amorphized material (latent tracks) which extend throughout the whole thickness of the crystals. Such induced defects act as efficient pinning centers and lead to unusual large changes in the magnetic hysteresis loops. A strong shift of the irreversibility line to high fields and temperatures and a significant charge of its shape are also reported for both compounds, demonstrating that pinning effects have to be taken into account to describe the onset of the FLL motion.

Structural peculiarities of the “1212” superconductor Tl0.5Pb0.5Sr2CaCu2O7

Abstract The superconductor Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 (I) was synthesized and investigated by powder X-ray diffraction and electron microscopy. For comparison the oxide TlSr 2 CaCu 2 O 7 (II) was also prepared. In both oxides a bulk superconductivity was observed (45% to 13% of diamagnetism) with a T c offset ranging from 85 to 20 K. Both compounds are of the “1212” type, i.e. isostructural with TlBa 2 CaCu 2 O 7 and characterized by a tetragonal cell ( a =3.795 (I)–3.794(II) A and c =12.094(I)–12.133(II) A) with space group P4/mmm. However Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 exhibits structural features, different from those of TlSr 2 CaCu 2 O 7 an d TlBa 2 CaCu 2 O 7 . Its electron diffraction investigation shows the existence of numerous extra spots which correspond either to superstructures or oriented domains. The HREM study attests of numerous modulations of the contrast, correlated with short range orders. These structural features and superconductivity in this sample are discussed.

Relationships between composition, oxygen non-stoichiometry, structure modulation and superconductivity in the “2212” bismuth cuprates

The correlations between critical temperature, modulation vector q and oxygen non-stoichiometry have been studied for three series of compositions belonging to the 2212 bismuth cuprates. For this exploration three samples were prepared in different conditions: the pure phases Bi 2 Sr 2 CaCu 2 O 8 (synthesized in argon), Bi 2.1 Sr 1.9 CaCu 2 O 8+δ (synthesized in air), and the mixture Bi 2 Sr 2 CaCu 2 O 8+δ (synthesized in air). Oxygen intercalation and disintercalation were performed and the q vector and critical temperature were measured. In each series of oxides, the q vector decreases as the oxygen content δ increases. The comparison of the different series also shows that the Bi/Sr ratio affects q . It is also demonstrated that, if the oxygen non-stoichiometry can dramatically influence T c s, it is not the only factor, the cationic composition also playing a role. The importance of the purity of the phases for the physical interpretation is discussed, taking into account the fact that the air synthesized “Bi 2 Sr 2 CaCu 2 O 8+δ ” composition is not a single phase and may correspond to various members of the non-stoichiometric phase Bi 2+ x Sr 2− x CaCu 2 O 8+δ .

Superconductivity of La2CuxO4 − y

Abstract Bulk superconductivity is present in La 2 Cu x O 4 − y for 0.98 ⩽ x ⩽ 1.04 and for an y value which is strongly reduced by annealing under high pressure of oxygen. Two superconducting temperatures are observed T 2 and T c ; the first one, T 2 = 37 K, corresponds to the resistivity fall; the second one, T c , corresponds to the appearance of diamagnetism. T 2 and the critical fields of samples stay unchanged whatever the normal state resistivity is, even if it varies by a factor 40000 from a semi conducting to a metallic state. Then a conventional mechanism of pairing is unconceivable. The existence of a bump in the resistivity at T = 220 K of samples which undergo a superconducting transition suggest a possible correlation between coherence effects among copper spins and superconductivity.

The “2201” thallium cuprate: Tc's up to 92 K can be achieved by “hydrogen” annealing

Abstract Polycrystalline tetragonal superconductors Tl 2 Ba 2 CuO 6± δ with critical temperatures ranging from 12 K to 92 K were synthesized by combining two techniques, synthesis in evacuated ampoules and annealing in an H 2 /Ar flow at 290°C. It is demonstrated that several tetragonal forms exhibit bulk superconductivity with diamagnetic volume fractions ranging from 25% to 65%, whereas other tetragonal forms do not superconduct. However, results obtained for the orthorhombic samples suggest that this material does not superconduct. These results show that two factors play an important role in the appearance of superconductivity in the thallium cuprates: the oxygen nonstoichiometry, which governs the hole carrier density, and the homogeneous distribution of the oxygen and vacancies, which should improve superconductivity. Relationships between structure, chemical bonding and superconductivity in Tl 2 Ba 2 CuO 6± δ are also discussed.