Search

Substitution of both Co and Mo at Ru -site in RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ system takes place iso-structurally in tetragonal structure (space group I4/mmm) with full solubility. The resistance versus temperature (R versus T) behavior of the pristine sample confirmed superconductivity [Formula: see text] around 25 K. The DC magnetization data (M versus T) for the same sample revealed magnetic transition (T mag ) at 140 K followed by superconducting transition [Formula: see text] at 32 K. With progressive Co substitution at Ru -site both [Formula: see text] and [Formula: see text] decrease monotonically and superconductivity is not observed for x ≥ 0.10 (10 at%). The magnetic behavior of materials with Co up to x = 0.2 preserves the magnetic structure of the parent RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ compound. For 0.8 ≥ x ≥ 0.2 a clear antiferromagnetic transition at TN=31 K is observed regardless of Co content. In the case of Mo -doped ruthenocuprates Ru 1-x Mo x Sr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ, it has been found that the magnetic transition temperature decreases with x. The samples of x = 0.8 and 1.0 are found to have no magnetic or superconducting effects. The values of the superconducting transition temperature obtained from the resistivity versus temperature data increase by 4.5 and 7.0 K for x = 0.2 and 0.4 respectively, and then decrease by 17 K for x = 0.6. The observed variation of Tc with x in case of Mo has been explained in terms of a theory which combines the effects of weakening magnetic behavior and reducing carrier concentration in a phenomenological manner.

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) angstroms, b = 2.83387(18) angstroms, and c = 10.6535(7) angstroms (Pnma). Electron diffraction patterns indicated an order of superstructure 3a x b x c, which might be initiated by Li vacancies. The exact composition is estimated at Li(0.92)Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to approximately 0.27 eV from approximately 0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at approximately 10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle.

Polycrystalline MgB2 samples with 0, 2, 4 and 6 percent added nano-Co3O4 being synthesized by vacuum (10-5 Torr) annealing at 750 0C for three hours each are found to be nearly single phase with presence of only a small quantity of Mg/MgO in pristine sample and in addition the Co2O3 in doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in Zero-field-cooled (ZFC) magnetic susceptibility measurements with sizeable signal. The Field cooled (FC) measurements though having sharp transitions, but showed a very small signal, indicating high level of pinning centers in these samples. Further some of the doped samples exhibited Paramagnetic-Meissner-Effect (PME) in applied field of 5 Oe. The critical current density (Jc), being estimated by invoking Bean model for the pristine compound increase by nearly an order of magnitude for 2 and 4 percent nano-Co3O4 doping and later the same decreases sharply for 6 percent sample at nearly all studied temperatures and applied fields. Further the increased Jc (~ 10^8 A/cm2) is coupled with fluxiod jumps (T = 20 K and H = 1 T). Fluxiod Jumps are not seen in relatively low Jc pristine or 6 percent sample. This means the fluxiod-jumps are intrinsic only to the high Jc samples., Comment: 14 pages including TEXT + Figs. Comments Welcome (AWANA.Veer@nims.go.jp) OR awana@mail.nplindia.ernet.in

A new hexagonal perovskite-type oxide Ba8Ta4Ru8/3Co2/3O24 was synthesized by the solid-state method at 1573 K and characterized by electron diffraction (ED), time-of-flight (TOF) neutron powder diffraction, and magnetic susceptibility. Structure parameters of Ba8Ta4Ru8/3Co2/3O24 were refined by the Rietveld method from the TOF neutron powder diffraction data on the basis of space group P63/mcm and lattice parameters a=10.0075(1) A and c=18.9248(2) A as obtained from the ED data (Z=3). The crystal structure of Ba8Ta4Ru8/3Co2/3O24 consists of 8-layered (cchc)2 close-packed stacking of BaO3 layers along the c-axis. Corner-shared octahedra are filled by Ta only and face-shared octahedra are statistically occupied by Ru, Co, and vacancies. Similar compounds Ba8Ta4Ru8/3M2/3O24 with M=Ni and Zn were also prepared. Magnetic susceptibility measurements showed no magnetic ordering down to 5 K.

We have performed specific heat and electric resistivity measurements of Na$_{x}$CoO$_{2}$ ($x=0.70$-0.78). Two anomalies have been observed in the specific heat data for $x=0.78$, corresponding to magnetic transitions at $T_{c}=22$ K and $T_{k}\simeq 9$ K reported previously. In the electrical resistivity, a steep decrease at $T_{c}$ and a bending-like variation at $T_{b}$(=120K for $x=0.78$) have been observed. Moreover, we have investigated the $x$-dependence of these parameters in detail. The physical properties of this system are very sensitive to $x$, and the inconsistent results of previous reports can be explained by a small difference in $x$. Furthermore, for a higher $x$ value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower $x$ value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below $x=0.70$., Comment: 4 pages (3 figures included) and an extra figure (gif), to be published in J. Phys. Soc. Jpn. 73 (9) with possible minor revisions

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate ($y$) contents. Two samples with different Na contents but nearly the same $T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) were investigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting (SC) and normal states is almost the same for the two samples except just above $T_c$. NQR measurements were also performed on different-hydrate-content samples with different $T_c$ values, which were prepared from the same Na-content ($x$ = 0.348) sample. From measurements of $1/T_1$ using the different-hydrate-content samples, it was found that a low-$T_c$ sample with $T_c = 3.9$ K has a larger residual density of states (DOS) in the SC state and a smaller increase of $1/T_1T$ just above $T_c$ than a high-$T_c$ sample with $T_c$ = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between $T_c$ and the increase in DOS just above $T_c$. This increase, which is seemingly associated with the two-dimensionality of the CoO$_2$ plane, is considered to be one of the most important factors for the occurrence of superconductivity.

High resolution X-ray powder diffraction studies have shown SrRhO 3 to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I 4/ mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma – I 4/ mcm transition is first order.

Single crystals of Au0.82Ba2Y0.77Ca0.23Cu2.18O6.85 (Au-1212) have been grown for the first time at 6 GPa and 1380 °C. This phase is the second member of the Au-containing cuprate family of AuBa2(Y,Ca)n−1CunO2n+3 and it exhibited bulk superconductivity with Tc = 84 K. The crystal structure of the Au-1212 phase was determined from single-crystal x-ray data (space group Pmmm, a = 3.8260(2) A, b = 3.8501(2) A, c = 12.075(1) A, R(I > 3σ(I)) = 0.0260, Rw(I > 3σ(I)) = 0.0219). No superstructure was revealed. The main feature of the Au-1212 crystal structure is the partial substitution of Cu1+ for Au, and this results in oxygen vacancies in this plane. Surprisingly, AuO4 planar units form a zigzag chain along the short a-axis.

We have carried out 99/101Ru-NMR, measurements at zero magnetic field in magnetic superconductors RuEu1.4Ce0.6Sr2Cu2O10−δ and RuSr2YCu2O8 to investigate charge and magnetic states of Ru ions in its magnetically ordered state. In RuEu1.4Ce0.6Sr2Cu2O10−δ, only one Ru site corresponding to Ru5+ ions with S=3/2 high spin state was shown to exist. This contrasts sharply to the case of RuSr2YCu2O8 where two kinds of Ru sites of Ru5+ (S=3/2) and Ru4+ (S=1) were observed. These results indicate the magnetic and charge states of the Ru ions in the RuO2 planes are different for both systems.

Critical current densities (Jc) and irreversibility fields (Birr) of high-Tc superconductors AuBa2Can−1CunO2n+3 [Au-12(n−1)n, n=3,4] prepared under high pressure were determined from the hysteresis in DC magnetization loop. Both the Jc and Birr values of the Au-1234 phase were larger than those of Hg- and Bi-based cupurate superconductors, but smaller than those of the Y-123 superconductor. This result suggests that the electronic structure of Au-1234 is less anisotropic compared with the Hg- and Bi-phases. The Au-1223 phase showed very small Jc values corresponding to its small superconducting volume fraction.

We have carried out 99/101 Ru and 63/65 Cu nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr 2 RECu 2 O 8 (RE=Gd, Eu and Y). The two kinds of 99/101 Ru signals were observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru 5+ (S=3/2) and Ru 4+ ( S =1) ions in the RuO 2 layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO 2 and RuO 2 planes. The temperature dependence of nuclear spin-lattice relaxation time T 1 of 63 Cu in RE=Y shows a ‘spin gap’ like behavior, suggesting the system is under-doped.

Results of X-ray fluorescence measurements of MA 2 Ca n-1 Cu n O 2n + 3 compounds with [ M -12(n-1)n] type structures, ( C 0.8 N 0.2 Sr 2 CuO 5.3, [ Cu 0.25 (C,N) 0.75] Sr 2 CaCu 2 O y, Cu 0.5 V 0.5 Sr 2 CaCu 2 O 7.5, Cu 0.5 V 0.5 Sr 2 Ca 2 Cu 3 O 9.4, Cu 0.5 V 0.5 Sr 2( Ca 3.6 Sr 0.4) Cu 5 O 13.5 and Cu 0.5 V 0.5 Sr 2( Ca 4.7 Sr 0.3) Cu 6 O 15.3, are presented. The experiments show that C-atoms form a distorted CO 3 oxyacid group with a partial substitution of copper and nitrogen atoms in the M-sites. We conclude that the CO 32- group in (C,N) Sr 2 Ca n-1 Cu n O y and (Cu,C,N) Sr 2 Ca n-1 Cu n O y is substituted by the NO 2- which induces the creation of the hole carriers in these compounds. We have found that V-atoms in the given compounds form ( VO 4)3- tetrahedrons and have a pentavalent state.

We have performed detailed ac susceptibility measurements to investigate the vortex dynamics in a Ca10(Pt3As8)[(Fe 1-xPtx)2As2]5 single crystal as a function of temperature, frequency, ac amplitude, and dc field. The field dependence of the activation energy U is derived in the framework of thermally activated flux creep theory, yielding a power law dependence of U ∼ Hα with α≈ -1.0 for H above 0.30 T, while below 0.3 T U is independent of the field. The activation energy reaches 104 K at low fields, suggesting strong pinning in the material. The nonlinear function of the activation energy vs. the current density is determined, which shows logarithmic dependence U(J). © 2014 AIP Publishing LLC. ispartof: APPLIED PHYSICS LETTERS vol:104 issue:11 pages:112603-1 status: published

The (Cu 0.5 Cr 0.5 )Sr 2 CuO x phase is the first member of the (Cu 0.5 Cr 0.5 )Sr 2 Ca n −1 Cu n O 2 n +3 homologous series, having a layered structure with a single CuO 2 plane. Numerous samples of this system were prepared under high-pressure/high-temperature conditions while varying the x value. The phases crystallize with tetragonal symmetry when the samples are quenched to room temperature, while they show orthorhombic symmetry when slow cooled (with the exception of the x =5.0 sample). Transmission electron microscopic data collected on several samples have allowed us to suggest that the origin of the orthorhombic distortion in the slowly cooled phases lies in the ordered arrangement of copper, chromium, and oxygen atoms in the (Cu 0.5 Cr 0.5 ) plane. We have also concluded on the absence of superconductivity in the (Cu, Cr)-1201 phases.

Results of X-ray fluorescence measurements of new oxycarbonitrate superconductors M Sr 2 Ca n −1 Cu n O y ( n =1 or 2 and M =Cu, C or N) are presented. The experiments show that C atoms form a distorted CO 3 oxyacid group with a partial substitution of copper and nitrogen atoms in the M sites. Based on the comparison of nitrogen Kα emission spectra and spectra of reference samples it is concluded that the CO 3 2− group in (C,N)Sr 2 Ca n −1 Cu n O y and (Cu,C,N)Sr 2 Ca n −1 Cu n O y is substituted by the NO 2 − instead of the NO 3 − group. This induces the creation of hole carriers in these compounds.

High-pressure synthesis technique was applied for 0212-, 1201- and 1212-type copper oxides, to find several new phases. The (Nd,Ce) 2+ x CaCu 2 O y phase has a shear structure derived from the 0212 structure. It has an orthorhombic superlattice of a s =5.4, b s =27.4 and c s =19.3 A which are related to the tetragonal lattice as a s =√2 a , b s =5√2 b , c s = c . Two step weak ferromagnetic transitions were observed at 150 and 40 K in this Ce-doped phase. The 1201 phase of CCa 2 CuO 5+ δ was prepared for the first time. Superconductivity appeared below ≈11 K in the samples with high excess oxygen contents. The 1212 phases of RuSr 2 LnCu 2 O 8 were isolated for smaller rare earth elements of Ln=Dy, Y, Ho, Er. The Y-analogue undergoes ferromagnetic transition near 150 K followed by superconducting transition at 39 K.

The Cr- and V-based cuprate superconductors were prepared under high pressure conditions. The (Cu,Cr)Sr 2 Ca n−1 Cu n O 2n+3 [(Cu,Cr)-12(n−1)n] phases exists at 6 GPa for a wide range of n, n=1–9, while the (Cu,V)Sr 2 Ca n−1 Cu n O 2n+3 [(Cu,V)-12(n−1)n] phases are stable for n=3–7. It was found that the Sr atoms of the Cr-based series of phases can be substituted partially by Nd and such a substitution affects T c depending on the n value. The variation of T c variation with n of the Cr- V- and Nd-substituted phases are discussed in connection with the hole (carrier) concentration. Critical current densities (J c ) under magnetic field of the (Cu,Cr)-1223 and (Cu,V)-1223 are also discussed. Their J c values under 1 T are extremely high compared with other high T c superconductors.

The low-temperature crystal structure of a one-dimensional chain-ladder composite material ${\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{13.6}{\mathrm{Cu}}_{24+y}{\mathrm{O}}_{41+z}$ was determined by the Rietveld analysis of neutron diffraction data using a superspace group approach. The hole distribution between the chain and ladder planes was estimated by the bond-valence sum (BVS) calculation based on Cu-O interatomic distances. The minimum of the distance between ladder-copper and chain-oxygen atoms [Cu(1)\char21{}O(3)] was revealed to expand with lowering temperature. The BVS calculation indicated that such a structural change corresponds to a redistribution of holes from the ladder to the chain and that almost all of the holes are localized in the chain below or near the N\'eel temperature. By assuming reasonable magnetic interactions between hole-unoccupied Cu sites on the chain plane, we propose a possible magnetic structure model taking into account the distribution of holes and observed magnetic neutron Bragg reflections. The results suggest the presence of spin dimers, spin trimers, and ``lone'' spins in the chain, of which the latter two have effective magnetic moments. These moments may be an origin of staggered antiferromagnetic spin modulation onto the spin-liquid state.

Homologous series of cuprate superconductors (Cu,Cr)Sr 2 Ca n −1 Cu n O 2 n +3 [(Cu,Cr)-12( n −1) n , n =1–9] and (Cu,V)Sr 2 Ca n −1 Cu n O 2 n +3 [(Cu,V)-12( n −1) n , n =3–7] prepared under high pressure were discussed. They have M-12( n −1) n -type structures with the M site occupied by the (Cu,Cr) [or (Cu,V)] mixed atom. The highest T c in each series is 103 K for (Cu,Cr)-1223 and 107 K for (Cu,V)-1234. The n =3 phases of both series show extremely high critical current densities and irreversibility fields which are comparable to or better than those of the YBa 2 Cu 3 O 7 . This seems to be related to the partial occupation of the Cu atom in the charge reservoir layer and the random arrangement of Cu and Cr (or V) there.

Critical current densities ( J c ) and irreversibility fields ( B irr ) of high- T c superconductors (Cu,Cr)Sr 2 Ca n −1 Cu n O 2 n +3 ( n =2,3) prepared under high pressure were determined from the hysteresis in DC magnetization loop. They show higher J c values than other high- T c superconductors such as YBa 2 Cu 3 O 7 , Bi-phases, Hg-phases, (Cu,C)-based phases, etc. for the almost entire range of temperature below T c . Their B irr values are also very high; at 77 K, for instance, B irr of (Cu,Cr)-1223 was over 5 T. In addition to the polycrystalline bulk samples, magnetically grain aligned samples of the (Cu,Cr)-phases were investigated to discuss superconductivity anisotropy. The good J c -related properties of the (Cu,Cr)-phases seem to be caused by their less anisotropic electronic structures and sufficient number of effective pinning centers due to the random arrangement of Cu and Cr in the blocking layer.

Under high pressure, a variety of elements can be accommodated at the cation sites in the charge reservoirs of cuprate superconductors. Furthermore, high pressure allows the insertion of higher number of CuO{sub 2} planes (i.e., the insertion period of a charge reservoir becomes variable). Because of these effects, many new high critical temperature ({Tc}) superconductors have been discovered under high pressure. In this paper, the author will survey their structural and superconducting properties. The role of high pressure and its effect on the size mismatch between the charge reservoir and the CuO{sub 2} plane will also be discussed.

A new series of high- T c superconductors (Cu,Cr)Sr 2 Ca n −1 Cu n O 2 n +3+ δ ( n =1–9) has been synthesized under high pressure at 6 GPa and 1250–1350°C as bulk materials. The range of n from 1 to 9 seems to be the widest among all homologous series ever discovered. The compounds are tetragonal with lattice parameters given by a ≈3.9 A and c ≈8+3.2( n −1) A. The a parameter decreases substantially with n until its minimum at n =3, increasing afterwards until n =9. The superconducting properties correlate well with this variation and change from non-superconducting for n =1 to the highest T c of 103 K for n =3, and then to complete suppression of superconductivity once more for n =8, 9 due to insufficient carrier concentration. The HRTEM study has shown the usual SrO–(Cu,Cr)O–SrO–CuO 2 –(Ca-CuO 2 ) n −1 stacking of planes, common for high- T c superconductors. The copper and chromium atoms in the (Cu,Cr) plane seem to be placed randomly and if any order between them occurs, it should be local and imperfect. Chromium appears to be hexavalent and holes are created either by excess oxygen and/or copper vacancies in the (Cu,Cr)O plane.

We studied structural and electrical properties under high pressure and low temperature for a $S=1/2$ Heisenberg two-leg ladder compound ${\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{13.6}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41+\mathrm{\ensuremath{\delta}}}.$ It showed a superconducting transition above 3 GPa with a maximum ${T}_{c}$ of 13 K. The ${\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{13.6}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41+\mathrm{\ensuremath{\delta}}}$ phase survives at least up to 9 GPa and down to 7 K without decomposition or phase transition, indicating that it is responsible for the observed superconductivity. The most important role of high pressure for realizing superconductivity is enhancement of the interaction between the ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ ladder and the one-dimensional ${\mathrm{CuO}}_{2}$ chain which accelerates redistribution of holes via locally formed ${\mathrm{Cu}}_{\mathrm{ladder}}{\ensuremath{-}\mathrm{O}}_{\mathrm{chain}}$ hybrid orbitals. Electrical property of the normal state is affected by the carrier density, dimensionality, random potential, and spin scattering which are related to structural modification under high pressure.

From X-ray diffraction (XRD) data, AlSr{sub 2}YCu{sub 2}O{sub 7} phase shows a tetragonal unit cell and space group P4/mmm, with lattice parameters a = b = 3.862(5) A and c = 11.095(5) {angstrom}. From selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) studies, a superstructure with related parameters a{sub s} = 2a{sub t} b{sub s} = 4a{sub t}, and c{sub s} = 2c{sub t} was found, which is considered to be due to the existence of two types of symmetry-related zigzag AlO{sub 4} chains along the a-direction. Simultaneously, some interlayer disordering occurs, which can be due to the disordered arrangement of these two AlO{sub 4} chains along the c-axis.

A new oxycarbonate superconductor CCa 3 Cu 2 O 7+δ (CCa 2 CaCu 2 O 7+δ C-1212) with T c = 47 K was synthesized at 6 GPa and 1275°C. It is the first superconductor which contains only Ca as the A-site cation. X-ray powder diffraction and HRTEM studies indicated that this phase has a M-1212 type structure with a = 3.842(1) and c = 10.336(1) A . The metal site in the charge reservoir is occupied only by C without partial replacement of Cu. Carriers are created by excess-oxygen atoms probably introduced into the charge reservoir.

Recently a new superconductor (GezCu1−z)Sr2Ca2−xYxCu3Oy having the highest Tc of 90 K was synthesized under high pressure [A.T. Matveev et al., Physica C 262 (1996) 279]. This compound is thought to be the n = 3 member of a homologous series (GezCu1−z)Sr2Can−1−xYxCunOy. This paper reports two new members of the series with n = 4 and n = 6 which were prepared at 6 GPa. The former phase showed superconductivity below 89 K while the latter below 78 K. They have simple tetragonal cells with a = 3.8411(2), c = 17.733(2) A for n = 4 and a = 3.8496(2), c = 24.183(2) A for n = 6 . HRTEM observations for them indicated M-12(n − 1)n-type structures with GeCu mixed atoms at the M site.

A new series of oxyfluoride compounds, Sr2RCu2O5F (R = Y,La,Nd,Sm,Gd,Dy,Er and Yb), was successfully synthesized under high pressure of 6 GPa. Electron microscopic observation and X-ray Rietveld analysis indicated that they are essentially isostructural to La2SrCu2O6 (tetragonal, 14/mmm) with a half of apical oxygen sites of CuO5 pyramids replaced by F atoms. Superconductivity occurred in the Nd analogue after Ca substitution for x ≥ 0.5 in Sr2Nd1−xCaxCu2O5F. single-phase sample of the Nd analogue was obtained at x = 0.8 having the highest Tc of 85 K with a Meissner fraction of 6.8% and a shielding fraction of 7.4% at 5 K. These small fractions suggest a small grain size, weak intergrain coupling and/or weak pinning force.

A series of compounds Sr2Can−1CunOy (n = 1–4) were prepared under 5–6 GPa and their superconducting properties were examined. The n = 1 member of the series, Sr2CuO3 + δ, has been delivered as a superconductor with Tc of 70 K. However, our experiments suggest strongly that it is not superconducting but the 70 K superconductivity is ascribed to the n = 2 member of the series, Sr2CaCuO2Oy. The n = 3 and 4 members were also prepared and they were confirmed to have Tc's of 109 and 83 K, respectively.

Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) studies were performed on the new superconductor GaSr2Ca3Cu4Oy(Ga-1234),n= 4 member of the homologous series GaSr2Can−1CunO2n+3prepared under high pressure. Although it consists of rather irregular intergrowths of the blocks withn= 3, 4, and 5 [CuO2] layers, the average number of [CuO2] layers is close to 4. In addition, an intralayer ordering between two different types of [GaO4] tetrahedral chains along thebaxis, as well as an interlayer disordering of the chains along thecaxis, are also observed.

Phase search in the GeSrCa/YCuO system under high-pressure of 6 GPa and 1300°C was carried out. There exists a new superconductor, (GezCu1−z)Sr2Ca2−xYxCu3Oy ((Ge, Cu)-1223) having the highest Tc of 90 K. X-ray and electron microscopic investigations showed that it has a M-1223-type structure (e.g. Tl-1223) with the GeCu mixed atoms occupying the M sites. Electron diffraction patterns of (Ge,Cu)-1223 gave a tetragonal lattice without any superlattice spots suggesting random arrangement between Ge and Cu in the (Ge, Cu) plane. Typical cell parameters were a = 3.8299(2), c = 14.582(1) A for a sample with a nominal composition, (Ge0.4Cu0.6)Sr2Ca1.8Y0.2Cu3O9.3.

We prepared new oxyfluoride superconductors, Sr2CaCu2O4+δF2±y and Sr2Ca2Cu3O6+δF2±y, at 1250°C under 5.5 GPa. These phases are the n=2, 3 members of a homologous series, Sr2Can−1CunO2n+δF2±y, respectively, in which the n = 1 phase was reported previously. X-ray and HRTEM studies indicated that they have tetragonal structures with a = 3.843(1), c = 19.88(1) A for n = 2 and a = 3.840(1), c = 26.17(1) A for n = 3 . The n = 2 phase showed a superconducting transition at 99 K, while the n = 3 one at 111 K.

Recently, a new oxyborate superconductor with Tc = 110 K, BSr2Ca3Cu4O11, was prepared under high pressure [E. Takayama-Muromachi et al., Physica C 241 (1995) 137]. This phase is thought to be the n = 4 member of a homologous series, BSr2Can−1CunO2n+3. In this paper, we report new members of the series, BSr2Ca2Cu3O9 (n = 3) and BSr2Ca4Cu5O13 (n = 5), which were prepared under 6 GPa. These members showed Tc's of 75 K and 85 K, respectively. Their X-ray powder diffraction patterns could be indexed assuming tetragonal cells with a = 3.821(1), c = 13.854(6) A for n = 3 and a = 3.837(2), c = 20.2291) A for n = 5 . HRTEM observations indicated that they are esssentially isomorphous with Tl(or Hg)-1223 and 1245.

A new tetragonal high- T c superconductor, AlSr 2 Ca 2− x Y x Cu 3 O 9 was prepared under a high pressure of 5 or 6 GPa. This phase is the n = 3 member of the superconducting family AlSr 2 Ca n −1 Cu n O 2 n +3 . Superconductivity was observed in the range of 0 ⩽ x ⩽ 0.6 with the highest T c of 78 K for the x = 0 phase. T c fell systematically with x resulting in the disappearance of superconductivity near x = 0.65. The n = 2 member of the family, AlSr 2 Ca 1− x Y x Cu 2 O 7 prepared in the present study did not show superconductivity.

The microstructure of the n = 3 member of the homologous series of superconductors GaSr2Can−1CunO2n+3 prepared under high pressure (6 GPa) was investigated by means of selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The GaSr2Ca2Cu3O9+δ phase has an orthorhombic structure with a double periodicity along the b-axis, due to the intralayer ordering between two different types of GaO4 chains. Also, interlayer disordering of the chains along the c-axis was observed.

We estimated critical current densities J/sub c/'s and irreversibility fields B/sub irr/'s of new oxycarbonate superconductors, (Cu/sub 0.5/C/sub 0.5/)Ba/sub 2/Ca/sub n-1/Cu/sub n/O/sub 2n+3/(n=3,4) and (Cu/sub 0.5/C/sub 0.5/)/sub 2/Ba/sub 3/Ca/sub n-1/Cu/sub n/O/sub 2n+3/(n=3,4). (Cu,C)-based oxides, especially single (Cu,C)-layer oxides, showed less excellent flux pinning properties than other high-T/sub c/ oxide superconductors at low temperatures. The slopes of irreversibility lines for the single (Cu,C)-layer oxides were steeper than those of the double (Cu,C)-layer oxides. This result can be understood by the difference of the distance between CuO/sub 2/ superconducting blocks. At 77K, B/sub irr/ of the (Cu,C)-1234 was 3.8 T which was higher than that of Hg-1223 in spite of lower T/sub c/ of (Cu, C)-1234. Neutron irradiation to (Cu,C)-1234 significantly enhanced both J/sub c/ and B/sub irr/. About 6/spl times/10/sup 5/ A/cm/sup 2/ at 77 K in 1 T was obtained after the irradiation, which was one of the highest J/sub c/ ever reported for bulk superconductors. >

The title superconductor with Tc of 110 K was prepared at 1200°C under high pressure of 6 GPa. X-ray powder diffraction and HRTEM observations indicated that the superconducting phase crystallizes in a tetragonal system with the space group, P4/ mmm and lattice parameters, a = 3.8359 (1), c = 17.082 (2) A . It is essentially isomorphous with the Tl (or Hg)-1234 phase.

A new series of high-Tc superconductors AlSr2Can−1CunO2n+3 (Al-12(n-1)n) was prepared at high pressure of 5.5 GPa and 1250°C, whose members are the first superconductors including Al as a main component. Two members with n=4, 5 were obtained in bulk and the former showed superconducting transition at 110 K while the latter at 83 K. They had tetragonal cells with a=3.839(1), c=17.72(1) A for n=4 and a=3.845(1), c=20.87(1) A for n=5. HRTEM observations indicated that they are essentially isomorphous with the Tl (or Hg)-based superconductors, Tl (Hg)-1234 and 1245, respectively. Significant intergrowths of phases with different n values up to n=8 were observed in Al-1245.

The crystal structure of the newly discovered 117-K superconductor, (Cu,C)${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{11+\mathrm{\ensuremath{\delta}}}$, has been refined from time-of-flight neutron-powder-diffraction data. The structure has ``average'' tetragonal symmetry and is similar to that of Tl(or Hg)${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{\mathit{y}}$. C atoms in ${\mathrm{CO}}_{3}$ groups substitute at the Cu site in the (Cu,C)${\mathrm{O}}_{1+\mathrm{\ensuremath{\delta}}}$ layer leading to a chemical composition of (${\mathrm{Cu}}_{0.68}$${\mathrm{C}}_{0.32}$)${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{3}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{11.06}$. This compound has two inequivalent kinds of ${\mathrm{CuO}}_{2}$ layers with pyramidal and square coordination of Cu to oxygen. The inner ${\mathrm{CuO}}_{2}$ layers with Cu in four coordination are less corrugated than the outer ones with Cu in five coordination, and exhibit a structure very similar to those of infinite-layer compounds.

We prepared a new oxycarbonate superconductor, (Cu 0.5 C 0.5 ) 2 Ba 3 Ca 2 Cu 3 O 11 at 1250°C under 5 GPa. This phase is the m =2 and n =3 member of a homologous series, (Cu 0.5 C 0.5 ) m Ba m +1 Ca n -1 Cu n O 2 m +2 n +1 reported previously. Its X-ray pattern could be indexed assuming a tetragonal subcell with a =3.852(1), c=18.61(2) A . Electron microscopic observations indicated ordering between Cu and C in the (Cu, C) plane, resulting in a superlattice, a s =2 a , b s = b and c s =2 c with respect to the tetragonal subcell. This new member showed a superconducting transition at 91 K.

We prepared a new series of oxycarbonate superconductors, (Cu0.5C0.5)2Ba3Can−1CunO2n+5 at 1250°C under 5 GPa. Members of the series with n=4 and n=5 were prepared in bulk. In addition, an intermediate phase in which n=4 and n=5 slabs were stacked alternately was found by electron microscopic observations. X-ray patterns of n=4 and n=5 members could be indexed assuming tetragonal subcells with a=3.855(3), c=21.87(1) A and a=3.857(1), c=25.067(7) A , respectively. Electron microscopic observations indicated that Cu and C are located alternately along the a-axes of the subcells resulting in orthorhombic superstructures with as=2a, bs=b and cs=c. The n=4 compound showed superconducting transition at 113 K. The n=5 compound appeared to be superconducting below ∽110 K, as well.

From the hystereses in DC magnetization curves, we estimated critical current densities J c and irreversibility fields B irr of (Cu 0.5 C 0.5 Ba 2 Ca 2 Cu 3 O 9±δ ((Cu, C)-1223) and (Cu 0.5 C 0.5 )Ba 2 Ca 3 Cu 4 O 11±δ ((Cu, C)-1234) new superconductors, and the data were compared with those values reported for other high- T c oxide superconductors. The temperature dependence of J c in a magnetic field for the (Cu, C)-based oxides is much smaller than that of HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223). At 77 K, B irr of (Cu, C)-1234 is 3.8 T which is higher than that of Hg-1223 in spite of a lower critical temperature of (Cu, C)-1234. The slopes of the irreversibility lines for (Cu, C)-1223 and (Cu, C)-1234 are steeper than that of Hg-1223. The results suggest that the anisotropy in the (Cu, C)-based oxides seems to be smaller than that of Hg-based oxide although the crystal structure is almost the same for both systems.

In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200°C under 5 GPa, we found the new oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3. Two members of the series with n=3 and n=4 were isolated as almost single phases; their structures are closely related to those of Tl (orHg) based 1223 and 1234, respectively. The Tl (Hg) sites are, in the present series, replaced by Cu and C. Disregarding ordering of Cu and C, the present compounds take tetragonal subcells; at=3.859(1), ct=14.766(5) A for n=3 and at=17.930(6) A for n=4. Electron microscopic observation indicated that Cu and C are, in both cases, located alternately along the a-axes of the subcells resulting in superstructures having as=2at, bs=bt and cs=2ct. The n=3 member showed a superconducting transition at 67 K, while the n=4 member at 117 K which is the highest Tc among the oxycarbonate superconductors found thus fur.

High-pressure synthesis at 6 GPa was performed for a series of compounds, Y 1- x Ca x Sr 2 GaCu 2 O 7±δ with 0⩽ x ⩽0. The solubility limit extended up to x =1.0 at 6 GPa in contrast to x ≈0.3 at ambient pressure and the Y-free compound, CaSr 2 GaCu 2 O 7±δ , was prepared successfully. Superconductivity appeared in samples with x ⩾0.4 which showed metallic conductivity above T c . T c increased up to 49 K at x =0.6 with increasing x , while T c decreased to 15 K at x =0.8, probably owing to overdoping of carriers. However, T c of the Y-free sample was as high as 70 K in spite of the fact that it seems to have larger carrier density. Possible reasons for this “abnormal” result are discussed.

The relationship between temperature and the oxygen-defect concentration in Nd 2 − x Ce x CuO 4 was investigated in detail under an oxygen pressure of 10 -3 atm. The effect of the oxygen defects on superconductivity was studied for the range of Ce-content 0.14 ⩽ x ⩽ 0.175. In a sample with x = 0.15 or 0.16, a lesser amount of oxygen defects was needed to realize superconductivity compared with the case x = 0.14. This suggests that introduction of oxyygen defects works to add carriers (electrons) to the system. Oxygen defects, however, play some role in superconductivity besides carrier doping. The a -dimension of the present tetragonal system is well described by a function of the number of electrons doped i.e., oxygen-defect doping has the same effect on a as Ce-doping does.

La 2 CuO 4+δ having various δ values was prepared by the KMnO 4 oxidation method in aqueous solutions at 333 K. Some samples oxidized in concentrated KMnO 4 solutions underwent spinodal decomposition resulting in products having δ∼0.01 and δ∼0.22. The former showed a superconducting transition at ∼ 14 K, the latter at 40 K. The partial molar enthalpy of excess oxygen up to ∼ -57 kJ/mol O was obtained for La 2 CuO 4+δ by high-temperature drop calorimetry. This value suggests that La 2 CuO 4+δ is, disregarding the spinodal decomposition, thermodynamically stable in oxygen fugacities less than 1 atm at room temperature.

High temperature solution calorimetry using a 2PbO B[sub 2]O[sub 3] solvent at 977 K was applied to the high T[sub c]-related compounds, Ln[sub 2]O[sub 3](Ln=Nd-Lu), T[prime]Ln[sub 2]CuO[sub 4](Ln = Nd-Gd), and Ln[sub 2]Cu[sub 2]O[sub 5](Ln = Dy-Lu). The heat of solution, [delta]H[sub s], of Ln[sub 2]O[sub 3] becomes less exothermic with decreasing size of Ln[sup 3+] from [approximately] -130 kJ/mole in La[sub 2]O[sub 3] to [close quotes]-30 kJ/mole in Lu[sub 2]O[sub 3]. The heats of solution of rare earth oxides containing unevenly filled 4f' orbitals are slightly more endothermic than those predicted by a straight line relating [delta]H[sub s] to reciprocal ionic radius, 1/r, for La, Gd, Y, and Lu, presumably reflecting the crystal field stabilization energy of Ln[sup 3+] (CFSE) in the solid. The heat of solution of the T[prime] phase when plotted against 1/r shows a maximum value at Ln=Sm. This may be explained by the CFSE and/or by a nonlinear change in lattice energy of the T[prime] phase as a function of 1/r. The stability relations between T[prime] and Ln[sub 2]Cu[sub 2]O[sub 5] structures are discussed using the thermochemical data obtained. The data generally confirm patterns of stability seen in synthesis experiments.

This report presents a new method to obtain superconducting La 2 CuO 4+δ . The La 2 CuO 4 obtained by the normal solid state reaction was oxidized in an aqueous solution of KMnO 4 . The product included excess oxygen up to δ~0.09 and showed bulk superconductivity below about 40 K. Two different orthorhombic phases appeared upon increasing of δ; one had a smaller orthorhombic distortion while the other had a larger one, compared with the stoichiometric La 2 CuO 4 . Electron diffraction patterns suggested a complicated manner of ordering of the interstitial oxygen atoms in the sample with δ~0.09.