Publications

Abstract not provided.

We have studied the reversible pressure induced structural transition (primitive cubic to polymeric monoclinic) of Na2CsC60 and the superconductivity in both phases. We have established that the single carbon-carbon bonded polymeric phase of Na2CsC60 is superconducting with an ambient pressure transition temperature about 3 K lower than that for the primitive cubic phase. We believe that an analogous lowering of the transition temperature in the corresponding phase of Na2RbC60 either leads to a very low transition temperature or to a total absence of superconductivity. © 2001 Elsevier Science B.V.

In order to understand the hindered rotational and vibrational dynamics of methane trapped in C{sub 60} interstices and to determine the structure around the interstitial site, they have carried out inelastic neutron scattering studies of the methane/C{sub 60} system. At temperatures of 20K and below, they observe inelastic peaks from rotational transitions of the CH{sub 4}. These transitions allow unambiguous assignment of the hindered rotational energy levels and a determination of the interaction potential. The appearance of two peaks for one of the J = 0{r_arrow}3 transitions implies the existence of two distinct kinds of interstitial sites and the measured transition energies suggest a rotational barrier of about 26 and 16 meV for these sites. Time-dependent changes in peak heights indicate slow t{sub 1/2} ({approx} 2.6 hrs) triplet{r_arrow}quintet nuclear spin conversion that necessarily accompanies the J = 1{r_arrow}0 rotational relaxation. They also have observed a sharp inelastic peak at 9.3 meV, which corresponds to a local vibrational mode of CH{sub 4} rattling in its cage at {approximately} 2.2 THz. Other peaks involving higher-energy vibrational excitations in CD{sub 4}/C{sub 60} correspond in energy to assigned peaks in the inelastic neutron scattering spectra of C{sub 60}, albeit sometimes with different intensities.

Neutron powder diffraction at pressures to 6 kbar in gaseous Ne has been used to study the pressure-induced phase transition and compressibilities of Na{sub 2}CsC{sub 60}. The pressure-induced phase can be achieved by compression to about 5 kbar at room temperature. If cooled, this phase can be retained below 200 K upon release of the pressure. The structure is orthorhombic as previously reported (but may differ in its detailed crystal structure) with lattice constants near 80 K and ambient pressure of a=9.385 A, b=10.06 A, and c=14.36 A. Corresponding linear compressibilities are 0.0004, 0014, and 0.0017 kbar{sup -1}, respectively. Identical pressure temperature cycling results in a superconductor with an unexpectedly low pressure dependence for {Tc} while in this phase. Models for the superconducting behavior of this compound are discussed.

We performed neutron powder diffraction on solution-derived lead zirconate titanates (PZT). Three compositions, PZT 45/55, PZT 20/80 and PbTiO{sub 3}, were investigated. The materials were annealed so that the perovskite phase had just begun to grow from the precursor phase. In our materials this precursor phase is the pyrochlore rather than fluorite phase. The results show that in the pyrochlore phase, the (Ti,Zr) and the Pb are ordered in their crystallographic sites while the O are essentially disordered in both of the two usual pyrochlore anion sites.

We have examined several crystals belonging to the Tl-2122 structure type (Tl{sub 2}CaBa{sub 2}Cu{sub 2}O{sub 8} with c = 29{angstrom}), and have shown that cation solid solution occurs. Such cation disorder appears to be responsible for the observed small differences in lattice parameters reported by various investigators and to contribute towards the substantial variation in the superconducting transition temperatures. 10 refs., 2 tabs.