J. Gopalakrishnan

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor.

There is now a new series of high-temperature superconductors that may be represented as (AIIIO)2A2IICan-1CunO2+2n where AIII is Bi or Tl, AII is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when AIII is Tl. This compound, Tl2Ba2Ca2Cu3O10, has the highest transition temperature(∼125 K) of any presently known bulk superconductor. The structure of Tl2Ba2Ca2Cu3O10 has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 � and c = 35.9 �. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.

Bulk Superconductivity up to 122 K in the Tl-Pb-Sr-Ca-Cu-O System

New high-temperature superconductors based on oxides of thallium and copper, but not containing barium, have been prepared. A transition temperature (Tc) of about 85 K is found for (Tl0.5Pb0.5) Sr2CaCu2O7 whereas (Tl0.5Pb0.5)Sr2Ca2Cu3O9 has a Tc of about 120 K. Both materials possess tetragonal symmetry with a = 3.80 �, c = 12.05 � for (Tl0.5Pb0.5)Sr2CaCu2O7, and a = 3.81 �, c = 15.23 � for (Tl0.5Pb0.5)Sr2Ca2Cu3O9. A structure refinement of the latter phase has been carried out with single-crystal x-ray diffraction data.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double Pbue5f8O layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Oxygen nonstoichiometry in copper-oxide based superconductors and related systems: Structure of nonsuperconducting Bi2Sr3−xYxCu2O8+y (x≈0.6–1.0)

Abstract Structural refinements of the new compounds Bi 2 Sr 3− x Y x Cu 2 O 8+ y in the range of x that gives insulating rather than superconducting behaviour ( x ≈0.6 and 1.0) have been obtained from single crystal X-ray and neutron powder diffraction data. Results indicate oxygen nonstoichiometry and suggest condensation into pairs or chains of the Biue5f8O atoms along the a axis in the Biue5f8O double layers. Chain-like formation can be found between the copper-oxygen sheets of all the orthorhombic superconductors including La 2 CuO 4 . XANES studies show that the oxidation state of copper (Cu 2+ is not affected by yttrium substitution for strontium. The mechanism of charge compensation appears to involve the insertion of oxygen, over the ideal O 8 composition, in (rather than between) the Biue5f8O sheets. These materials are orthorhombic and the lattice parameters for the subcell of the x ≈0.6 material are a =5.467(3), b =5.4407(6), c =30.434(5) A, space group Amaa, Z =4. The x ≈1.0 phase has a =5.467(2) b =5.426(1), and c =30.183(8) A. The structure is essentially the same as that of superconducting Bi 2 Sr 3− x Ca x Cu 2 O 8+ y . Electron diffraction patterns for the yttrium-containing phases show an incommensurate superlattice along the a axis as seen in the calcium analog. This is due to atomic displacements that are modulated along a with amplitudes in the c direction.

Crystal structure of TlBa2Ca2Cu3O9

Abstract Crystals of the 110 K superconductor TlBa2Ca2Cu3O9 have been grown and the structure refined from single-crystal X-ray diffraction data: a = 3.853(1), c = 15.913(4)A, space group P4 mmm , R = 0.055, R w = 0.057 . The structure contains triple copper-oxygen sheets separated by Ca ions which show ∼5% Tl substitution. These units alternate with single Tlue5f8O layers. The existence of correlated atomic displacements within the thallium-oxygen sheets is again suggested by a disordered Tlue5f8O arrangement in the average structure. The triple Cuue5f8O sheets in TlBa2Ca2Cu3O9 are stacked directly above each other, whereas in Tl2Ba2Ca2Cu3O10, which contains double Tlue5f8O sheets, alternate units are shifted by (a 1 + a 2 ) 2 .

Superconducting Tl2.0Ba2.0CuO6+δ: A high resolution neutron powder and single crystal x-ray diffraction investigation

Abstract The structure of the orthorhombic form of Tl2Ba2CuO6+δ(Tc=90 K), synthesized in sealed gold tubes using BaO2, has been refined using high resolution neutron powder diffraction data collected at 4 K. No significant deficiency in metal stoichiometry could be inferred from the crystal structure analysis. The space group (Fmmm) used in this study gives essentially the same results as the alternative (Abma, Amaa) proposed in earlier work. The large amount of disorder that exists in the Tl2O2-slab is mirrored in the large thermal parameters for this structural unit compared to those for the CuO2 sheet. A site at ( built1 4 built1 4 built1 4 ) is occupied by an amount of oxygen to give overall stoichiometry Tl2Ba2CuO6.10, suggesting that oxygen non-stoichiometry plays an important role in producing high Tc superconductivity in this system. Analysis of single crystal X-ray diffraction data of tetragonal Tl2Ba2CuO6+ δ is totally consistent with the neutron powder diffraction results.

Superconducting and Magnetic Behavior in La2-xNaxCuO4

New phases of the type La2-xAxl+CUO4-y have been prepared where Al+ is sodium or potassium. The sodium phases are superconducting for x values from 0.2 to 0.5 at temperatures up to about 40 K. In addition, there are unusual magnetic properties below about 10 K that may be indicative of spin glass behavior. Phases of the type La2-xKxCuO4-y could only be prepared with x values up to about 0.1, and these phases are not superconducting above 4.2 K

The superconductors (Tl, Pb)Sr2CaCu2O7 and (Tl, Pb)Sr2Ca2Cu3O9: Neutron powder diffraction, high resolution electron microscopy and X-ray absorption studies

Abstract The structures of (Tl, Pb)Sr 2 CaCu 2 O 7 ( T c =85 K) and (Tl, Pb)Sr 2 Ca 2 Cu 3 O 9 ( T c =122 K) have been investigated using neutron powder diffraction and electron microscopy. They are isostructural (14/mmm) with the corresponding thallium/barium end members of compositions (TlBa 2 Ca n −1 Cu n O 2 n +3 ), possessing two or three CuO 2 -sheets separated by (Tl, Pb)O- and Sr 2 O 2 -sheets. X-ray microanalysis and neutron diffraction are consistent with fully occupied strontium sites in both materials. However, the diffraction results suggest considerable site disorder in the (Tl, Pb) and Ca-sites. High resolution electron microscope images indicate occasional defects and, in the case of (Tl, Pb)Sr 2 Ca 2 Cu 3 O 9 , occasional intergrowth of a structure containing 4-layers of copper. X-ray absorption measurements are consistent with formal oxidation states of 4+ for Pb and 3+ for thallium in these materials.

Superconducting Tl2Ba2CuO6: The orthorhombic form

Abstract The structure of an orthorhombic variant of Tl 2 Ba 2 CuO 6 ( Z = 4), with a superconducting onset at 90 K, has been studied using neutron powder diffractometry at 12 K ( a = 5.4834(3), b = 5.4586(3), c = 23.198(1) A), 60 K ( a = 5.4834(3), b = 5.4585(3), c = 23.199(1)A), and 293 K ( a = 5.4967(3), b = 5.4651(3), c = 23.246(1) A). The distortion from I4 mmm symmetry, reported in single-crystal X-ray investigations, to Abma is manifested in the TIue5f8O layer. At the temperatures studied, the coordination of TI to oxygen is (2 + 1 + 2) square pyramidal rather than octahedral. At lower temperatures, distortion decreases but does not disappear. Despite the orthorhombic symmetry the CuO 2 -layers remain flat.

New layered perovskites: ABiNb2O7 and APb2Nb3O10 (A=Rb OR Cs)

Abstract New bismuth- and lead-niobates of formulas ABiNb 2 O 7 and APb 2 Nb 3 O 10 where A = Rb or Cs crystallizing in layered perovskite structures have been synthesized. The interlayer alkali ions in these compounds are easily exchanged for protons. The protonated oxides exhibit proton conductivity in the range 10 −6 ohm −1 cm −1 at room temperature. Formation of ABiNb 2 O 7 compositions in layered perovskite structure in preference to pyrochlore structure is discussed.