M. Verwerft

Long period mixed layer polytypes in the Tl2Ba2Can+1O2n+6 superconductor

Abstract By means of high resolution electron microscopy and electron diffraction it is shown that the high- T c superconductors in the homologous series Tl 2 Ba 2 Ca n Cu n +1 O 2 n +6 exist up to at least n =4. It is found furthermore that mixed-layer polytypes with very large unit cells of several tens of nm, built on complicated layer sequences, occur. The possibility of polytype formation is shown to be related to the simultaneous occurrence of perovskite lamellae with different thickness and moreover to the fact that single layer as well as double layer Tl-O lamellae can occur in the same crystal. The most probable formation mechanism is considered to be spiral growth.

Superconductivity and microstructure in Fe-doped Bi-Sr-Ca-Cu-O compounds

Abstract We have studied the influence of Fe impurities in the superconducting compound Bi-Sr-Ca-Cu-O prepared from two basic nominal compositions Bi 2 Sr 2 Ca(Cu 1− x Fe x ) 2 O 8+δ and Bi 4 Sr 3 Ca 3 (Cu 1− x Fe x ) 4 O 16+δ . The superconducting and secondary phases have been analyzed by SEM, EPMA, HREM and X-ray diffraction. The composition of the doped superconducting phase is best described by the formula: Bi 2+ y (Sr, Ca) 3 (Cu 1− x Fe x ) 2− y O 8+δ , where y increases as a function of x ( x Annealing of the samples in a poor oxygen atmosphere improves the T c values of the doped samples. The downward shift of T c for increasing Fe content follows a linear relationship with a slope of 5 K/(Fe/Fe+Cu) at.%. The importance of the Bi-O-Cu-O electron coupling in the superconductivity mechanism is discussed.

A complementary study by electron microscopy of modulated phases in Ba2NaNb5O15

Abstract A complementary study by electron microscopy and electron diffraction of the phase transitions in barium sodium niobate was performed whereby particular attention was paid to the commensurate incommensurate transition at 543 K. The evolution of the discommensuration configurations was followed by in situ thermal treatment and the nucleation stage of the incommensurate phase could be shown to be associated with the appearance of fourfold closed loops of discommensurations. By electron diffraction and satellite dark field imaging the low temperature phase (<105 K) could be characterized as an orthorhombic modulated structure consisting of periodic arrays of discommensurations. Preliminary results are reported of a high resolution electron microscopic study of remarkable defects of the subnanometer size which could serve as pinning centres for discommensurations.

Short range order in high-Tc superconductors investigated by electron diffraction

Observation of modulated diffuse streaks in electron diffraction patterns of Tl2Ba2CuO6 and YBa2Cu3O7−ɛ ceramic superconductors is reported. These streaks are parallel with the c-axis and the intensity modulation has a period in reciprocal space which is larger than the c*-parameter. This behaviour is interpreted in terms of disordered sublattices. It is shown that the fine structure results from short range order in two successive homologous layers forming one bi-layer and that the intensity modulation in the diffuse streaks therefore allows one to determine which of the bi-layers in the material exhibit disorder.

Long period mixed layer polytypes in the Tl2Ba2CanCu1+nO6+2n superconductors

Abstract High Tc superconductors in the homologous series Tl 2−x Ba 2 Ca n Cu 1+n O 6+2n can exist up to at least n=4. It is found furthermore by electron microscopy and electron diffraction that mixed layer polytypes with very large unit cells, up to several hundred Angstrom occur. These large repeat units originate from a periodic mixture of compounds with n=4 and n=3 combined with the fact that both single and double Tl O layers may occur. The critical temperature is slightly affected by these modulations. Susceptibility measurements indicate a Tc of 117K is found; R=0 was obtained at 110K.

Electron microscopy of phase transitions

Abstract The possibilities of electron microscopy for the study of phase transitions are briefly outlined and illustrated by some particular aspects of phase transitions in Pb3V2O8, Ba2NaNb5O15 and Au3+xZn. Lead orthovanadate is used as an example for the application of a procedure to distinguish between the static or dynamic nature of diffuse intensity in diffraction patterns occurring near the transition temperature. For Ba2NaNb5O15 the behaviour of the discommensurations in the vicinity of the ferroelastic transition has been studied and the low-temperature transition around 105 K is proven to be due to a doubling of the baxis and not to a restoration of the fourfold symmetry. The decomposition mechanism of the metastable type long period super-lattices in the binary Au3+xZn alloys is described in terms of the withdrawal of nodes of four domains.