Torben O. Brun

Fast ion transport in the NASICON analog Na3Sc2(PO4)3: Structure and conductivity☆

The room temperature modification of stoichiometric NASICON(Sc) is monoclinic Cc. At 64°C there is a first order transition to a normal-conducting rhombohedral form R3c. Na(1) sites are fully occupied whereas Na(2) sites are partially occupied. At 167°C there is a transition to asuperionic phase, but the structure remains rhombohedral R3c. Vacancies are now shared equally by Na(1) and Na(2) sites. Fast Na-ion motion in stoichiometric Na3Sc2(PO4)3 arises from vacancy motion in a “dogleg” path between Na(1) and Na(2) sites.

Ordering of vacancies in LiAl

Abstract The intermetallic compound β-LiAl contains relatively high concentrations of constitutional vacancies on the Li sublattice for Li deficient compositions. Neutron diffraction measurements on a Li 0.486 Al 0.514 single crystal show that these vacancies order on every tenth (840) plane at 97K. This vacancy ordering is responsible for the “100K” anomaly observed in several physical properties.

The growth of single crystal lithium-aluminum, 7LiAl☆

Abstract Single crystal lithium-aluminum has been prepared as chemically and isotopically pure 7LiAl. Growth has been by the Stockbarger technique in a low-pressure bomb. Pyrolytic boron nitride crucibles have proven to be nonreactive and noncontaminating containers for pyrometallurgial synthesis and for crystal growth from the melt. The chemical and physical quality of the monocrystalline boules have been documented. The lattice constant varies with composition across the β-phase and can be used as a probe to evaluate stoichiometry and phase homogeneity.

Defects and disorder in the fast-ion electrode lithium-aluminum

Abstract Neutron and X-ray diffraction measurements indicate no phase change in β- phase LiAl from 300 to 490 K. The thermal expansion of the lattice constant and the temperature dependence of the Debye-Waller factor for Li are linear and not unusually large. The data are, analyzed in the Bragg-Williams approximation with T c = 1100 K . At elevated temperatures, LiAl approaches an order-disorder transformation. At battery operating temperatures (700 K) there are appreciable concentrations of both types of antisite defects Li Al and Al Li , and there is appreciable Al diffusion.

Lattice dynamics of the mixed-conducting intermetallic compound, β-LiAl

Abstract The intermetallic compound, β-LiAl, that crystallizes in the uncommon Zintl structure is a mixed-conducting electrode and has many unusual properties pointing to the existence of unusual bonding in the semi-metallic compound. In order to elucidate the nature of the bonding in LiAl, we have studied the lattice dynamics of β-LiAl by inelastic neutron scattering. Results for the phonon dispersion curves have been obtained for the principal symmetry directions. A force constant fit to the results indicates that the Al-Al force constants are unusually large. Pair potentials were constructed by conventional pseudopotential calculations. The pair interactions favoring the Zintl structure were used to compute phonon dispersion curves. Good agreement between theory and experiment can be obtained for the acoustic branches.

Quasielastic neutron scattering studies of 7Li diffusion in the mixed conductor, LiAl

Abstract The diffusion of 7 Li in the intermetallic compound, LiAl, has been studied by quasielastic incoherent neutron scattering. The measurements were performed on a single crystal sample of composition, 49% 7 Li and 51% 27 Al. Since the incoherent scattering cross section of 7 Li is small, σ inc = .7 barns, only a limited number of measurements were performed as a function of momentum transfer and temperature. These measurements present the first direct determination of the tracer diffusion, D ∗ , of Li in LiAl. The value of D ∗ Li was determined to be (6±1) × 10 −6 cm 2 /sec at 800 K. This value is in reasonable agreement with the value extrapolated from low temperature NMR measurements as well as with results derived from chemical diffusivity data. The Li diffusion is a result of a very rapid diffusion of constitutional vacancies residing primarily on the Li sublattice. Approximately 5% of the sites on the Li sublattice are vacant in the present sample, Li .49 Al .51 .