The phase transitions and crystal structures of Ba3RM2O7.5 complex oxides (R = rare-earth elements, M = Al, Ga).

The structures of alpha-Ba(3)RAl(2)O(7.5) and beta-Ba(3)RM(2)O(7.5) complex oxides (R = rare-earth elements, M = Al, Ga) have been studied by a combination of X-ray diffraction, electron diffraction (ED) and high-resolution electron microscopy (HREM). The alpha and beta forms have cell parameters related to the perovskite subcell: a = 2a(per), b = a(per)(2)(1/2), c = 3a(per)(2)(1/2), however, the alpha form has an ortho-rhombic unit cell whereas the beta form adopts monoclinic symmetry. The crystal structure of monoclinic Ba(3)ErGa(2)O(7.5) was refined from X-ray powder data (space group P2/c, a = 7.93617(9), b = 5.96390(7), c = 18.4416(2) Å, beta = 91.325(1) degrees, R(I) = 0.023, R(P) = 0.053), the structure of the alpha form (space group Cmc2(1)) was deduced from ED and HREM data. The important feature of the alpha and beta structures is the presence of slabs containing strings of vertex-sharing tetrahedral Al(2)O(7) pairs. Two almost equivalent oxygen positions within the strings can be occupied either in an ordered manner leading to the low-temperature beta phase or randomly resulting in the high-temperature alpha structure. The critical temperature of this order-disorder phase transition was determined by high-temperature X-ray diffraction and by differential thermal analysis (DTA). In situ ED and HREM observations of the second-order phase transition confirmed the symmetry changes and revealed numerous defects (twins and antiphase boundaries) formed during the phase transformation.

A structure model and growth mechanism for multishell carbon nanotubes.

A model that postulates a mixture of scroll-shaped and concentric, cylindrical graphene sheets is proposed to explain the microstructure of graphite multishell nanotubes grown by arc discharge. The model is consistent with the observed occurrence of a relatively small number of different chiral angles within the same tubule. The model explains clustering in a natural way and is consistent with the observation of asymmetric (0002) lattice fringe patterns and with the occurrence of singular fringe spacings larger than c/2 (c is the c parameter of graphite) in such patterns. Anisotropic thermal contraction accounts for the 2 to 3 percent increase in the c parameter of nanotubes, compared with bulk graphite, but is too small to explain the singular fringe spacings. The model also explains the formation of multishell closure domes. Nucleation is attributed to the initial formation of a fullerene "dome."

Structural investigations of recently discovered high Tc superconductors.

A short overview is given of the possibilities of electron microscopy in the determination of the local, atomic scale structure of high Tc superconducting materials. Examples include the detection of weak oxygen ordering, description and characterization of deformation modulations in layered superconductors, and analysis of very long period superstructures. The ordering principles for tetrahedral chains in Ga-, Co-, or Al-substituted YBCO are discussed and their complex defect structures are described. The incommensurate modulation in YBCO-based materials containing SO4-tetrahedra, centered on the Cu(1) sites of the CuO-chain plane, is attributed to the ordering of b-oriented SO4-rich chains in the Cu(1)-S-O layer; the structure is described in terms of an SO4-concentration wave. As examples of the new mercury-based superconducting family we discuss Y0.6Ca0.4Ba2Hg1-xMxCu2O6+y, which crystallizes in the space group P4/mmm with a = 0.3870(1) nm, c = 1.2537(1) nm. This cuprate belongs to the 1212 series; susceptibility measurements show a Tc (onset) of 90K, with a diamagnetic volume fraction of 27% at 4.2K to be reached. A second example is related to the compound Tl2HgBa4Cu2O10+y, in which ordering between single Hg layers and double Tl layers is observed.

A formation mechanism for catalytically grown helix-shaped graphite nanotubes.

The concept of a spatial-velocity hodograph is introduced to describe quantitatively the extrusion of a carbon tubule from a catalytic particle. The conditions under which a continuous tubular surface can be generated are discussed in terms of this hodograph, the shape of which determines the geometry of the initial nanotube. The model is consistent with all observed tubular shapes and explains why the formation process induces stresses that may lead to "spontaneous" plastic deformation of the tubule. This result is due to the violation of the continuity condition, that is, to the mismatch between the extrusion velocity by the catalytic particle, required to generate a continuous tubular surface, and the rate of carbon deposition.

Dynamics of a Second-Order Phase Transition: P macr; to I1 Phase Transition in Anorthite, CaAl2Si2O8.

Electron microscopic study of the reversible P1 to I1 phase transition in anorthite (transition temperature T(c) = 516 Kelvin) shows that the antiphase boundaries (APBs) with the displacement vector R = 1/2[111] become unstable at T(c), and numerous small APB loops are formed. These interfaces are highly mobile, and their vibration frequency increases strongly with temperature. These observations suggest that close to T(c), breathing-motion-type lattice vibrations of the framework cause the two different configurations around the calcium atoms, which are related by a translation of R approximately 1/2[111], to interchange dynamically through an intermediate I1 configuration. The high-temperature I1 structure is interpreted as a statistical-dynamic average of highly mobile antiphase domains of primitive anorthite.

Defect structure of superconducting YBa2Cu3O7-delta.

The superconducting material Y1Ba2Cu3O7-delta has been investigated by electron microscopy. Special attention has been paid to the defects occurring in the material. Twinning on (110) or (110) planes is intrinsically related to the orthorhombicity, and when cooled slowly the twin bands are pseudoperiodic with an average width of approximately 50 nm. The orthorhombic-tetragonal transition is reversible and diffusion controlled. Under particular conditions and in slightly reduced material a 2a0 x b0 superstructure resulting from vacancy ordering is formed. When kept in air under powder form the material seems to be unstable. Planar defects along (001) accompany the degeneration of the material.

High-resolution electron microscopic study of age-hardening in a commercial dental gold alloy.

High-resolution electron microscopy and electron diffraction were applied to elucidate the hardening mechanism in an 18-carat gold commercial dental alloy, Au-31.7 at.%,Cu-8.1 at.%,Pd-5.3 at.%,Ag-54.9 at.%. The interface between the AuCu-I ordered platelets and the surrounding Au3Cu ordered phase was analyzed down to the atomic scale. A geometric model for the interface was deduced directly from the high-resolution electron micrographs. No evidence was found for the presence of a FCC phase at the interface between AuCu-I and Au3Cu.