Huamin Zou

Structural transition, electrical and magnetic properties of the B-site Co doped Sr(14)Cu(24)O(41) compounds

The effect of Co substitution for Cu on the structure and physical properties of Sr(14)(Cu(1-x)Co(x))(24)O(41) compounds was studied by analyzing the selected-area electron diffraction and convergent-beam electron diffraction patterns, and by measuring the magnetic susceptibility, the electrical resistivity and Raman spectra. It is found that the space group of the CuO(2) chain is changed from Amma to Ammm upon Co doping, but the structure of the Cu(2)O(3) ladder remains unchanged. This indicates that the displacement between two neighboring CuO(2) chains has disappeared due to Co doping. Once a small amount of Co ions are doped into the compound, exceptional changes in the Weiss temperature and in the number of dimers occur. The remarkable increase in the absolute value of the Weiss temperature indicates that the antiferromagnetic interaction in CuO(2) chains becomes very strong due to Co doping. The increase in the Curie coefficient and the number of dimers implies that the Co doping causes Zhang-Rice singlets in the chains to be decoupled into free spins Cu(2+) and holes. Then, the free spins Cu(2+) are coupled into dimers, and the holes transfer from chains to ladders, which causes the resistivity to decrease when the Co dopant concentration is low (x<0.10). When the Co dopant concentration is high (x>0.10), some Co ions are directly substituted for the Cu ions in the ladders, which results in an increase in resistivity with increasing Co dopant content.

A theoretical and experimental analysis of higher-order Laue zone line splittings caused by a dislocation in an icosahedral quasi-crystal

Twelve convergent-beam electron diffraction patterns from an icosahedral phase specimen containing a dislocation along 12 different zone axes, which are equivalent according to icosahedral symmetry, have been photographed and simulated using dynamical theory. The splitting behaviours of higher-order Laue zone lines in these patterns have been analysed in detail. Some deviations between the experimental and theoretical patterns can be explained because the incident probe becomes too close to the dislocation core.

Applicability study of the structure-factor phase method for determining the polarity of binary semiconductors

The structure-factor phase method of convergent-beam electron diffraction (CBED) has been widely applied as an effective tool in determining the polarity of binary compound materials, for example, the typical sphalerite material, GaAs. However, its validity on other polar materials is still unknown. In this paper we extensively investigated its potential applicability onto 11 AB-type semiconductors by dynamical simulations of CBED. Two key factors during the simulation, the difference between A and B atomic numbers and the sample thickness, are discussed in detail. It was found that this method is efficient to determine the polarity for a sphalerite structure under certain conditions, and, reversely, limited to determine the polarity for a wurtzite structure even though it is very similar to the sphalerite structure.

Structural disordering induced by electron irradiation in quasi-one-dimensional blue bronze K0.3MoO3

The structural transformation of the quasi-one-dimensional charge-density-wave (CDW) conductor K0.3MoO3 blue bronze upon electron irradiation is reported. Evidence of the disappearance of Bragg spots corresponding to the C-centre symmetry is clearly demonstrated by the in situ recorded selected area electron diffraction (SAED) patterns in the transmission electron microscope, which acts as the electron irradiation facility as well as the structural analysis and recording instrument. Based on the experimental results and the crystal structure of K0.3MoO3 (Graham and Wadsley 1966 Acta Crystallogr. 20 93), a hypothetical atomic model is proposed to interpret this kind of disordering. The simulated SAED patterns on the basis of the proposed atomic model agree with the experimental ones very well. This strong disorder may break both the coherence of the CDW along the chain direction and the phase order of the CDW in the transverse directions, and furthermore it may suppress the Peierls transition.

Investigation of the surface terminations of icosahedral AlPdMn quasicrystal based on a modified non-spherical model

The atomic positions are obtained from a modified non-spherical model of icosahedral AlPdMn quasicrystal (Fang et al 2003 J.?Phys.:?Condens.?Matter 15 4947) by the cut method. The four-shell pseudo-Mackay clusters?(PMCs) were searched for in a box of 400?? ? 400?? ? 400??. The results show that the number of atoms in the fourth shell, an icosidodecahedron, of the pseudo-Mackay cluster can vary from?15 to?30 because of the cluster overlap, and about 99.96% of the total atoms are included in such incomplete pseudo-Mackay clusters. The characteristics of the atom distribution in the planes perpendicular to a fivefold axis indicate that the planes, which are 1.56???apart from their neighbouring planes, are expected to be the terminal surfaces. If one such a plane and its closest neighbouring plane, between which the spacing is 0.48??, are considered as a thin layer or a corrugated surface, these layers are also the layers with the maximum density. The pair of corrugated surfaces that are 1.56???apart have almost identical chemical composition. These planes form terraces that follow the rule of the Fibonacci sequence with two step heights, 6.60 and 4.08??. On the corrugated surfaces perpendicular to a fivefold axis the pentagonal holes arise from the interspaces of adjacent incomplete PMCs. For the atomic planes normal to a twofold axis, the planes with spacing of 1.48???from their adjacent planes might be expected to be the terminal surfaces, which form terraces with step heights of?6.28 and 3.88???following the rule of the Fibonacci sequence. For the atomic planes normal to a threefold axis, the planes with spacing of 0.86???from their adjacent planes might be expected to be the terminal surfaces. No similar results were found for the atomic layers perpendicular to a pseudo-twofold axis.

Dynamical treatment of the splitting of higher‐order‐Laue‐zone lines induced by dislocations in an icosahedral quasicrystal

The dynamical theory of electron diffraction for quasicrystals (QCs) was used to treat the splitting behaviour of higher-order-Laue-zone (HOLZ) lines induced by dislocations in icosahedral quasicrystals (IQCs). The influences of some parameters on the splitting of HOLZ lines were calculated. On the basis of this calculation, several experimental convergent-beam electron diffraction patterns from the aluminium–copper–iron IQC were simulated. Good agreement between the experiment and the simulation confirms the correctness of the dynamical theory of electron diffraction for QCs.

Microscopic evidence of destroying the order of magnetic sequence in CuO2 chain by Zn, Ni, Co Doping in Sr14Cu24O41 compound

The Zn2+, Ni2+ and Co3+ doped Sr14Cu24O41 compounds were synthesized by standard solid state method. X-ray diffraction results show that the changes in lattice parameters are very small. Selected area electron diffraction patterns (EDPs) show that the diffraction spots corresponding to the CuO2 chain substructure are extended to streaks along a* and b* directions for all the samples, while the diffraction spots produced merely by the Cu2O3 ladder substructure are still very sharp. This means that the periodicities of chains in a* and b* directions are partially destroyed upon doping of Zn, Ni and Co due to that the initial phase of each chain becomes a random variable. The temperature dependence of magnetic properties was measured for every sample. And the number of dimers in CuO2 chain per formula unit (f.u.) and dimer coupling constant are obtained by fitting the temperature dependence of the magnetic susceptibility. It is found that the degree of initial phase disorder is related to the order degree of magnetic sequence in CuO2 chain. For the un-doped sample, the decoupling of dimers is weak, the magnetic sequence is slightly destroyed, and the streaks in EDP are also very weak, which implies the degree of initial phase disorder in CuO2 chain is very low. When Zn2+ and Ni2+ ions are doped, the number of dimers per f.u. decreases, and the intensity of diffraction streaks increases in comparison with the corresponding spots. Furthermore, when the high spin magnetic ions Co3+ are doped, the number of holes in Sr14Cu24O41 decreases, the magnetic sequence is destroyed very seriously, and the spots in EDP are extended to streaks almost completely. The phenomenon that the diffraction spots of CuO2 chain extend to streaks in EDP appears as evidence that the magnetic sequence in the CuO2 chain is destroyed by doping of Zn, Ni, Co.

Experimental study of lattice distortion in ellipsoid-like nano-crystallites of copper

The elastic state of embedded inclusions is of considerable importance to the properties of materials. The non-uniform lattice distortion in the inclusions in which the interfaces are shaped with variable curvature cannot be measured by usual experimental methods. In this paper, the lattice distortions in an ellipsoid-like nano-crystallite of copper were measured by means of the peak finding method in the central part of the HRTEM image. The effects of contrast delocalization are studied by HRTEM image simulations, which show that the measured spacings of peaks in the middle part of the crystallite can be considered approximately equal to the true spacings of columns. With the HRTEM method, our measured results show that the nano-crystallite is expanded in the short axis direction and compressed in the long axis direction. The results calculated with the elasticity theory incorporating interface tension consist with the experimental results of HRTEM.

Experimental measurement of valence electron concentration of icosahedral AlPdMn quasicrystal

The valence charge density distribution for the icosahedral AlPdMn (i-AlPdMn) quasicrystal was obtained with the structure factors of the nine strongest symmetry inequivalent reflections, which were refined by using the quantitative convergent beam electron diffraction (QCBED) technique. It shows that the bonding charge is localized. The enhanced charge density in the middle of the aluminum-transition-metal (Al-TM) bond shown in the valence charge density distribution is the characteristic of covalent bonding. Assuming that the shape of an atom is a sphere with covalent radius, the number of electrons that each atom gains or loses in 55 different pseudo-Mackay clusters (PMCs) was calculated based on the obtained valence charge density distribution. It indicates that almost all the atoms lose electrons except a few Pd atoms that are in some particular shells. It also shows that the atoms of an identified element could have different valences because of chemically and/or structurally different local environments in which the atoms situate. Regardless of the topology and chemical occupancy, the number of valence electrons per atom in a cluster is close to 1.69. This strongly suggests that the pseudo-Mackay clusters are stabilized at a certain electron concentration.

A general procedure for analysing stacking faults and antiphase boundaries in crystals by using diffraction contrast imaging and/or defocus convergent-beam electron diffraction

Abstract A general and systematic procedure is proposed for identifying an arbitrary stacking fault or antiphase boundary in crystals. This procedure is aimed at determining the displacement vector R at the fault plane without knowledge or assumptions about it beforehand and is suitable for both diffraction contrast imaging and defocus convergent-beam electron diffraction techniques. It contains three main points: (i) the extinctive reflection which corresponds to a phase shift of multiples of 2° in a systematic row (g, 2g, 3g,…) is used to determine the fractional part of the dot product g R; (ii) the displacement vector R is determined uniquely from the fractional parts of g R for three reflections g (i = 1, 2, 3), which just form a primitive cell of the reciprocal lattice; (iii) the intrinsic/extrinsic nature of a stacking fault in a face centred cubic crystal is obtained directly from the relationship between R and the foil normal. In addition, based on the second point, all stacking faults and antiph...