H. Nakata

Superconductivity in the new compound (Y1-xCax)0.95Sr2.05Cu2.4(CO3)0.6Oy

Abstract A new Cu-oxide superconductor (Y 1− x Ca x ) 0.95 Sr 2.05 Cu 2.4 (CO 3 ) 0.6 O y has been found by resistivity and magnetization measurements. The transition temperature is about 63 K. The crystal structure consists of the periodic replacements of the Cu(I) site with a Co 3 group in the basic YBa 2 Cu 3 O y structure.

Superconductivity in the new compound Sr2CuO2(CO3)1−x(BO3)x: The new method for carrier doping on layered copper-oxycarbonates

Abstract A new copper oxide superconductor containing carbon and boron Sr2CuO2(CO3)1−x(BO3)x was synthesized under ambient pressure. The superconducting transition was observed for the samples with x = 015 and 0.2. The highest Tc(≈32K) determined by magnetization measurements was obtained for the sample with x = 0.15. The X-ray Rietveld analysis of this compound can be characterized by the tetragonal symmetry (I 4 ) with a = 0.77881 (15) nm and c = 1.49681 (3) nm (x = 0.15). The crystal structure determination suggests that the crystal structure is basically the same as that of Sr2CuO2(CO3).

A new homologous series of oxycarbonate superconductors Sr2(Ca,Sr)n−1Cun(CO3)1−χ(BO3)χOy (n=1, 2 and 3)

Abstract A new homologous series of oxycarbonate superconductors Sr2(Ca,Sr)n−1Cun(CO3)1−χ(BO3)χOy (n=1, 2 and 3) prepared under high pressure have been found by resistivity and magnetization measurements. X-ray powder diffraction and HRTEM studies show that these phases have tetragonal structures with a=3.86 A and c=7.40 A for n=1, a=3.86 A and c=10.62 A for n=2, and a=3.87 A and c=13.98 A for n=3. Resistivity and magnetization measurements revealed that superconducting transition temperatures were 50, 105 and 115 K for n=1–3, respectively. These Tcs are higher than those of the structurally related analogous TlBa2Can−1CunO2n+3 at 100 and 110 K (n=2 and 3).

New Hg based oxycarbonate superconductor HgBa2Sr2Cu2O6+δ(CO3)

Abstract A new Hg based oxycarbonate superconductor HgBa 2 Sr 2 Cu 2 O 6+ δ (CO 3 ) has been found by resistivity and magnetization measurements. The superconductivity has been observed at 66 K by magnetization measurement. The electron-diffraction and HRTEM studies show that the crystal structure basically consisted of [HgBa 2 O 2+ δ ] and [Sr 2 CO 3 ] block layers and has a tetragonal unit cell, with a=3.88 A , c=16.94 A , with an orthorhombic superstructure, with A≈4.5√2a=24.71 A ,B≈√2a=5.49 A , C=c=16.94 A , due to the three times periodic replacements of a CO 3 block for HgO δ in the same c -planes or to a c /2 shift approximately at intervals of three tetragonal cells along the (110) direction. The crystal structure of this compound is described by the following combination: [(Hg 0.5 C 0.5 )(Ba,Sr) 2 O 2.5+ δ ]+CuO 2 +[(Ba,Sr) 2 (C 0.5 Hg 0.5 )O 2.5+ δ ]+CuO 2 .

Incommensurate and commensurate superstructures in the oxycarbonate superconductor TlSr4−xBaxCu2(CO3)Oy (x≈2)

Abstract Superstructures of oxycarbonate superconductor TlSr 4− x Ba x Cu 2 (CO 3 )O y ( x =2; T c =73K) were examined by electron diffraction and HRTEM techniques. Part of the crystals show a commensurate superstructure with a 6 a 0 periodicity due to crystallographic shears by c /2 to form -(C-C-C)-(Tl-Tl-Tl)-(C-C-C-)-(Tl-Tl-Tl)- type of sequences along the (100) direction in the basal planes. Incommensurate superstructures with longer periodicities up to 7.4 a 0 due to mixed intergrowths of -(C-C-C-C)- and -(C-C-C)- sequences were also observed. The average superstructure periodicity of the present specimen is around 7 a 0 , which is relatively shorter than the value of 8 a 0 previously claimed by Goutenoire et al. [Physica C 210 (1993) 359] for a similar superconductor with T c =62 K. This suggests the possibility of a close relationship between the modulation periodicity and the transition temperature in the TlSr 4− x Ba x Cu 2 (CO 3 )O y system with x =1–2.5. Observed structures were compared with an another type of superstructure recently discovered in HgBa 2 Sr 2 Cu 2 (CO 3 )O y by Uehara et al.

(C0.35Cu0.65)Sr2(Y0.73Ce0.27)2Cu2Ox: A new superconductor containing CO3

A new “1222”-type compound containing carbonates, (C0.35Cu0.65)Sr2(Y0.73Ce0.27)2Cu2Ox, was synthesized at 1303 K under an atmosphere of O2 (80%)-CO2 (20%) (partial pressure of CO2: PCO2=0.020 MPa). The sample annealed at 1023 K under the high oxygen pressure of 90 MPa has a superconducting transition temperature, Tc, of 18 K and a superconducting volume fraction of about 10% at 5 K. The structure of the superconductor was analyzed using the Rietveld method for the X-ray powder diffraction pattern. The refined lattice parameters were a=3.8272(1), b=3.8313(1) and c=27.7077(6) A.

Superconductivities in the (Bi,Pb)-oxycarbonate system

Abstract Two (Bi,Pb)-oxycarbonate superconductors have been found by resistivity and magnetization measurements. Transition temperatures are 41 K (54K) for the low- T c phase (high- T c phase). The crystal structures were analyzed by using the HRTEM and X-ray diffraction techniques. The lattice parameters were a 0 =3.821 A , c 0 =39.444 A (a 0 =3.88 A , c 0 =54.52 A ) for low- T c phase (high- T c phase) in the tetragonal unit cell. A crystal structure model is proposed in which the chemical formula for the low- T c phase (high- T c phase) is (Bi,Pb) 2 Sr 4 Cu 2 CO 3 O 8 ((Bi,Pb) 2 Sr 6 Cu 3 (CO 3 ) 2 O 10 ), which can be described as the combination of a single (double) Sr 2 CO 3 block and a single (Bi,Pb) 2 Sr 2 O 4 block.

Structural phase transition of Sr2CuO2(CO3)

Abstract The structural phase transition of Sr2CuO2(CO3) has been studied by X-ray and neutron powder diffraction and phonon Raman scattering. The crystal structure at room temperature is tetragonal with space group of I 4 (S42) and the lattice parameters a = 7.8045(1) A and c = 14.993(1) A . The structural phase-transition temperature determined by X-ray powder diffraction is found at 490 K. The structure above this transition temperature is tetragonal P4212 (D42), and the lattice parameters are a = 5.54364(8) A and c = 7.53823(11) A at 573 K. The neutron-diffraction data are refined by a Rietveld analysis and these structural parameters for below and above the transition temperature are presented. Raman spectra of Sr2CuO2(CO3) have been measured in the temperature region between 5 K and 593 K. In the temperature region between 5 K and 483 K, a new extra mode was not observed, but the peak intensity of the strongest mode at 480 cm−1 was decreased from room temperature to 373 K. Above 490 K, a new peak has been observed at 160 cm−1 and the weak peaks which are observed at room temperature disappear. Judging from the facts that the spectral shape and line width originating from the CO3 slab change at the transition temperature but a soft mode has not been observed, the type of this phase transition is order-disorder due to the change of the CO3 alignments.

A New Family of Superconductors Containing Carbonate Group

Three new oxycarbonate superconductors recently obtained in our laboratory are described. (1) Sr2CuO2(CO3)1−x(BO3)x (Tc = 35 K) was synthesized under ambient pressure, providing a new method for carrier doping on the layered copper oxycarbonate system. (2) Can(Ca, Sr)2Cun+1(CO3)Oy (n = 1, 2, 3) was synthesized under high pressure. Superconductivity was achieved by substituting (BO3)3− for a part of (CO3)2−. (3) A new Hg system HgBa2Sr2Cu2(CO3)O6+δwas synthesized.Tc obtained from the magnetic susceptibility is about 66 K.

The phase diagram of the new superconductor Sr2CuO2(CO3)1−x(BO3)x

Abstract A phase diagram for the boron-doped new superconductor Sr 2 CuO 2 (CO 3 ) 1− x (BO 3 ) x has been systematically investigated. The parent compound Sr 2 CuO 2 (CO 3 ) is an antiferromagnetic (AF) insulator with a Neel temperature of T N ∼285 K. T N of Sr 2 CuO 2 (CO 3 ) 1− x (BO 3 ) x changes drastically with increasing boron content x . In the lightly doped AF state (0≤ x ≤0.04), the strong superexchange interaction within the CuO 2 -layers still yields a long-range AF ordering. The superconductivity is confined to the narrow compositional range 0.04≤ x ≤0.15 and its highest T c is 32 K at x =0.15.