T. Krekels

3-D vacancy ordered superstructures in “homogeneous” YBa2Cu3O7-δ

Abstract High resolution electron microscopy and electron diffraction techniques were applied to study structural aspects of YBa 2 Cu 3 O 7- δ for varying δ, i.e., oxygen contents. Particular attention was paid to the superstructures arising from oxygen vacancy ordering, three types of specimens were investigated: constant stoichiometry, cooled vacuum annealed and microscope heated samples. The most stable vacancy ordered structure appears to be the orthorhombic II or 2 a 0 structure occurring in a wide range of compositions around δ =0.5. Several other superstructures were found in the electron beam heated and rapidly cooled samples which reveal apart from 2 a 0 , also 3 a 0 , 4 a 0 and 5 a 0 structures as well as 2 a 0 ×2 a 0 and 2√2 a 0 ×2√2 a 0 structures. These phases are presumably only transient. Attention is also paid to the interpretation of diffuse intensity lines in the diffraction patterns of beam heated and rapidly cooled samples which could be interpreted in terms of the cluster theory as being due to a transition state which can be considered as a precursor to the orthorhombic II structure. Crystallographic models are proposed for the various superstructures observed in this study as related to the oxygen contents. The order-disorder transition is found to occur in stages with increasing temperature. One observes a loss of correlation of the stacking of the 2-D patterns in successive layers, disordering of the chain separation, followed by disordering within the chains, leading finally to a random distribution of the remaining oxygen atoms in the CuO 1- δ layers.

“Tweed” structure of Fe-doped YBa2Cu3O7−δ

Abstract Fe-doped YBa 2 Cu 3 O 7−δ single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer “tweed” structure of overlapping lenticular domains oriented along [120] and [1 1 0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa 2 Cu7 3 O 7−δ samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.

Ortho II structure in ABa2Cu3O7−δ compounds (A=Er, Nd, Pr, Sm, Yb)

Abstract Oxygen ordering has been investigated in superconducting ABa 2 Cu 3 O 7− δ ceramic materials with A=Er, Nd, Sm and Yb well as in samples of this type with the rate earth A partially substituted by Pr. The critical temperature T c was determined as a function of the oxygen deficiency δ of the compound and the corresponding microstructures were investigated by electron diffraction and electron microscopy. A distinct relationship exists between the width of the 60 K plateau and the ortho II ordering. Our results show that the ortho II ordered phase is the superconducting phase with a characteristic T c of 60 K.

Conical, helically wound, graphite whiskers: a limiting member of the “fullerenes”?

Abstract Conically structured columnar graphite crystals have been studied by electron microscopy and electron diffraction and a formation mechanism is proposed explaining their shape and unusual diffraction effects consisting in periodically interrupted circular ring patterns. A growth mechanism is proposed whereby the initial graphite layer adopts a slitted dome shaped configuration by introducing a fivefold carbon ring in the sixfold carbon network. Successive sheets are rotated with respect to the previous one over a constant angle, thus realizing a helical cone around a “disclination” with a fivefold carbon ring core. This model explains the morphological features and the particular diffraction effects observed on these reproducibly prepared columnar graphite crystals. The growth mechanism has a direct relationship to the formation of the fullerene “bucky ball” molecules.

On the structure responsible for the 22a0×22a0 diffraction pattern in YBa2Cu3O7−δ

Abstract The 2 2 a 0 ×2 2 a 0 -phase appearing in YBa 2 Cu 3 O 7−δ at high δ, was extensively studied by means of electron diffraction and high resolution electron microscopy. The formation conditions and the existency-domains for the phase were determined. Deriving the possible space-groups from the reflection conditions for the superstructure naturally leads to an oxygen-vacancy ordered structure-model, with oxygen in the CuO 1−δ layer and oxygen vacancies in the BaO-layers. However, such a model of ordered point-defects in the YBa 2 Cu 3 O 7−δ -structure does not lead to satisfactory agreement between simulated and observed diffraction patterns. An alternative model, based on the Jahn-Teller type of deformation of the CuO 5 -pyramids in the CuO 2 -layer more satisfactorily agrees with observations. The streaks in the diffraction patterns, due to stacking disorder along the c -direction are also closely examined. Different correlations in the c -direction appear, observed at different stages of a heating process.

123 near equilibrium samples : anomalies of the lattice parameters and some physical properties

Abstract Contrary to the literature, the dependence on nonstoichiometry of the lattice parameters of “nearly” equilibrated 123 samples (particularly the c- and b-axis), shows strong changes of slope in the nonstoichiometric range x = 6.8–6.9. At x = 7.0-6.9 the c-axis decreases with decreasing oxygen concentration. And at x ≅ 6.9 the well known trend sets on:increase of c-axis and decrease of b-axis with decreasing oxygen content. These changes are reflected in the physical properties investigated up to now: • The dependence of Tc on hydrostatic pressure is 6.9, increases strongly in the range x = 6.9-6.8 and shows a change of slope at x = 6.8 where it has reached the value dTc/dP = 0.35 K/kbar. At x = 6.5 is dTc/dP = 0.45 K/kbar. (Also the tc as a function of nonstoichiometry shows the well known decrease at x = 6.8). • The muon spin-depolarization rate as a function of nonstoichiometry shows also an unexpected strong decrease (25%) in the range x = 7.0-6.9. This suggests that the density of the superconducting carriers decreases strongly in this range. It seems possible from the lattice parameter effects that a threshold concentration of oxygen vacancies in the chains is necessary in order that the expansion of the apical bond sets on.

3-D oxygen vacancy ordered superstructures in YBa2Cu3O7t-δ prepared by the constant stoichiometry cooling method

Abstract In very slowly cooled (25°C/h) specimens prepared following the constant stoichiometry method, the 3D structure of the orthorhombic II (2a o ) phase in YBa 2 Cu 3 O 7−δ was determined using electron diffraction and electron microscopy. The unit cell is 2a o x b o x c o ; successive 2D configurations in the CuO 1−δ layers are aligned along the c-direction. Evidence is presented for the occurence of more complicated 2D superstructures: 3a o , 4a o and 5a o . It is suggested that the a o 2√2 × a o 2√2 structure is a deformation modulated phase, rather than a vacancy ordered one. The O II structure is the most and probably the only vacancy ordered stable phase apart from the O I and the tetragonal phases. The more complicated na o phases are probably metastable. The disordering with increasing temperature of the O II structure occurs in different stages.

SO4-chain formation and ordering in [YSrCa]Sr2Cu2.78(SO4)0.22O7−δ

Abstract YBCO-based materials containing SO 4 -tetrahedra centered on the Cu(1)-sites of the CuO-chain plane have been examined by means of electron diffraction and high resolution electron microscopy. An incommensurate modulation is observed and attributed to the ordering of b -oriented SO 4 -rich chains in the Cu(1)-S-O-layer, described in terms of an SO 4 -concentration wave.

A novel type of dislocation in YBa2Cu3+xO7+x

Abstract A novel type of dislocation in YBa2Cu3O7, has been identified and analysed by high resolution electron microscopy. Non-stoichiometry is accommodated by means of Frank type sessile dislocations. In the double CuO layers, resulting from the insertion of an extra CuO layer, sometimes a 90° reorientation of the chains occurs along the layer plane. This orientation change of the double chain can be described as being caused by the propagation of a dislocation with a 1/2 [110] Burgers vector and a (001) glide plane located between the two CuO layers, accompanied by reshuffling of part of the oxygen ions.

Electron diffraction effects of conical, helically wound, graphite whiskers

Abstract Recently a new model for the structure of columnar graphite whiskers has been proposed; this model includes the presence of five-rings of carbon atoms, as in the fullerenes. The whiskers are assumed not to be single crystals but have geometrically well defined textures. Their unusual diffraction patterns exhibit rotation symmetry of high multiplicity. The wide variety of the observed multiplicities is explained by assuming small changes in the conical shape of the whiskers. We describe a series of electron diffraction experiments which have been performed on fracture cones of these graphite whiskers. These experiments confirm unambiguously the proposed microstructure.