Physica. C, Superconductivity
Superconductivity in the highest-Tc radical-cation [κ-(ET)2Cu[N(CN)2]X,X = Br (Tc = 11.6 K), X = Cl (Tc = 12.8 K, 0.3 kbar)] and radical -anion [KxC60(Tc = 28.6 K)] organic superconductors is discussed.
This LDRD (Laboratory Directed Research and Development) project was funded for two years beginning in October 1992 (FY93) and was designed as a multidisciplinary approach to determining the structural and physical properties of C{sub 60} intercalated with various gases. The purpose of the study was to evaluate the relative permeation and diffusion of various gases with an ultimate goal of finding an effective filter for gas separations. A variety of probes including NMR, X-ray and neutron diffraction; IR spectroscopy, thermogravimetric analysis and mass spectroscopy were employed on C{sub 60} impregnated with a number of gases including O{sub 2}, N{sub 2}, Ar, Ne, H{sub 2}, NO and CH{sub 4}. In order to increase the absorption and decrease the effective time constraints for bulk samples, these gases were intercalated into the C{sub 60} using pressures to several kbar. The results of these measurements which were quite encouraging for separation of O{sub 2} and N{sub 2} and for H{sub 2} from N{sub 2} led to 17 manuscripts which have been published in peer reviewed journals. The abstracts of these manuscripts are shown below along with a complete citation to the full text.
NMR and neutron diffraction measurements reveal that macroscopic phase separation and the tetragonal to orthorhombic (TO) structural phase coincide at two distinct points in the temperature-doping phase plot for oxygen doped La{sub 2}CuO{sub 4+{delta}}. Thus the TO phase line coincides with the phase separation line. This is evidence that the macroscopic phase separation is inhibited in the tetragonal phase. We propose that the interstitial oxygen has higher mobility in the orthorhombic phase and that insufficient mobility suppresses macroscopic phase separation in the tetragonal phase. Neutron diffraction measurements also reveal superlattice peaks which indicate ordering of the interstitial oxygen. Our NMR measurements, have demonstrated a distribution of tilts of the CuO{sub 6} octahedra. We propose a sawtooth modulation of the octahedral tilt in which the sign of the tilt changes when the tilt reaches a maximum value can explain this distribution. The large openings in the La-O layer resulting from the abrupt switch of the sign of the tilt provide an attractive location for the interstitial oxygen. This mechanism would lead to stripe ordering of the interstitial oxygen.
The magnetic and structural phase diagrams of the La{sub 2}CuO{sub 4+{delta}} system and the La{sub 2-x}Sr{sub x}CuO{sub 4+{delta}} are reviewed, with emphasis on recent results obtained from magnetic and structural neutron diffraction, thermogravimetric analysis, iodometric titration, magnetic susceptibility {chi}(T), and {sup 129}La nuclear quadrupole resonance (NQR) measurements.
The authors employ NMR and NQR spectroscopy as probes of local structure and charge environments in metallic La{sub 2}CuO{sub 4+{delta}} ({Tc} = 38 K). They discuss the effect of annealing the sample at various temperatures T{sub a} ({Tc} < T{sub a} < 300K) on the superconducting {Tc}. The dependence of {Tc} on annealing indicates that annealing allows the development of structural order which is important for {Tc}. The {sup 139}La quadrupole frequency, {nu}{sub Q} is smaller than in undoped materials. This is unexpected and may indicate a smaller charge on the apex oxygen in the doped material and thus a different distribution of charge between the La-O layer to the planes. The further, rapid decrease in {nu}{sub Q} just above {Tc} indicates that temperature dependent charge redistribution is occurring. The presence of doped holes induces a distribution of displacements of the apex oxygen off of the vertical La-Cu bond axis. These vary from zero to the value observed in lightly doped (antiferromagnetic) La{sub 2}CuO{sub 4+{delta}}. These measurements demonstrate a striking degree of inhomogeneity in the crystal structure of the La-O layer. Copper NQR spectroscopy shows that there are two distinct copper sites in the CuO{sub 2} planes and thus that either the structure or the charge distribution in the planes is inhomogeneous as well. These inhomogeneities are the intrinsic response of the crystal to doped holes; they are not the result of distortions of the lattice due to the presence of interstitial oxygen atoms.
La{sub 2-x}Sr{sub x}CuO{sub 4+{delta}} with x = 0.01, 0.025, 0.050, 0.10 and 0.16 and excess oxygen {delta} incorporated by high-pressure O{sub 2} anneals. These compounds were examined using time-of-flight neutron diffraction data. Various models were fit by Rietveld least-squares refinement, with the maximum amount of {delta} being only of the order of 10 standard deviations. {delta} is largest for x near 0, is zero for x = 0.10 and is intermediate for x = 0.16. Only the sample with x = 0.01 is found to phase separate distinctly into a nearly stoichiometric phase with {delta} {approx} 0 and an oxygen-rich superconducting phase as the temperature is lowered. Coincidence of phase separation and Neel temperature strongly suggests that the phase separation is driven by free energy provided by long-range antiferromagnetic ordering in the nearly stoichiometric, weakly Sr-doped La{sub 2-x}Sr{sub x}CuO{sub 4}. The excess oxygen stoichiometry shows that at low values of x, hole doping is provided primarily by the excess oxygen, and is enhanced substantially by phase separation. At larger values of x, excess oxygen is no longer incorporated, and hole doping is provided by the substitution of Sr{sup +2} for La{sup +3}.
Journal of Physics and Chemistry of Solids
AC impedance and x-ray diffraction measurements versus temperature and pressure on several ternary fullerene intercalation compounds are reported. The results support our previously established empirical correlation between superconducting onset temperature and 300K fcc lattice constant. Compounds which do not follow this correlation either phase separate or are otherwise unstable at low T and/or high P. © 1993.
The highest {Tc}`s achieved in organic electron-donor-based systems occur in two isostructural ET salts, viz., {kappa}-[(ET){sub 2}Cu][N(CN){sub 2}]X, X = Br ({Tc} = 11.6 K, ambient pressure), X = Cl ({Tc} = 12.8 K, 0.3 kbar) whereas for the electron-acceptor-based systems derived from C{sub 60} they occur in K{sub 3}C{sub 60} ({Tc} = 19 K), Rb{sub 3}C{sub 60} ({Tc} = 29 K), Rb{sub x}Cs{sub y}C{sub 60} ({Tc} 33 K) and Rb{sub x}Tl{sub y}C{sub 60} ({Tc} {approx} 45 K). Research performed at Argonne National Laboratory, and based on the ET and C{sub 60} systems, is reviewed.
The syntheses and physical properties of {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]X (X=Br and Cl) are summarized. The {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br salt is the highest {Tc} radical-cation based ambient pressure organic superconductor ({Tc}=11.6 K), and the {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Cl salt becomes a superconductor at even higher {Tc} under 0.3 kbar hydrostatic pressure ({Tc}=12.8 K). The similarities and differences between {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br and {kappa}-(ET){sub 2}Cu(NCS){sub 2} ({Tc}=10.4 K) are presented. The X-ray structures at 127 K reveal that the the S{hor_ellipsis}S contacts shorten between ET dimers in the former compound while the S{hor_ellipsis}S contacts shorten within dimers in the latter. The difference in their ESR linewidth behavior is also explained in terms of the structural differences. A semiconducting compound, (ET)Cu[N(CN){sub 2}]{sub 2}, isolated during {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Cl synthesis is also reported. The ESR measurements of the {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Cl salt indicate that the phase transition near 40 K is similar to the spin density wave transition in (TMTSF){sub 2}SbF{sub 6}. A new class of organic superconductors, {kappa}-(ET){sub 2}Cu{sub 2}(CN){sub 3} and {kappa}-(ET){sub 2}Cu{sub 2}(CN){sub 3}-{delta}Br{delta}, is reported with {Tc}`s of 2.8 K (1.5 kbar) and 2.6 K (1 kbar), respectively.
NMR and NQR reveal substantial structural changes in the metallic phase of LA{sub 2}CuO{sub 4+}{delta} which occur below 220 K. The oxygen octahedra in the metallic phase are not tilted at phase separation; upon cooling to 40 K considerable tilt has developed. The low temperature structure is highly disordered. 4 refs., 2 figs.
Physical Review B
Superconducting crystals of La2CuO4+x prepared by high-pressure oxygenation have been analyzed by Raman spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+x, like nonsuperconducting La2CuO4.0, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The Ag phonons of La2CuO4.0 and La2CuO4+x occur at essentially the same frequency. At room temperature, La2CuO4+x has no well-defined peak from two-magnon scattering, unlike La2CuO4.0. However, in its phase-separated form, La2CuO4+x exhibits well-defined, two-magnon scattering. This establishes that the La2CuO4.0 phase present in La2CuO4+x at low temperatures is antiferromagnetic. La2CuO4+x samples prepared by slightly different methods are found to have differing amounts of excess oxygen, as indicated by variations in the intensity of the phonon and magnetic scattering. Certain samples of La2CuO4+x had a phonon peak at 630 cm-1 that is absent in La2CuO4.0. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in La2CuO4+x. © 1991 The American Physical Society.