Acquisition of high-quality three-dimensional electron diffuse scattering data

The diffraction patterns of crystalline materials with local order contain sharp Bragg reflections as well as highly structured diffuse scattering. The instrumental requirements, experimental parameters and data processing techniques for obtaining high-quality diffuse scattering data have previously been determined for X-ray and neutron diffraction, but not yet for electron diffraction. In this study, we show that the spatial resolution of the diffuse scattering in three-dimensional electron diffraction (3D ED) data depends on various effects, including the convergence of the electron beam, the point spread function of the detector and the crystal mosaicity. In contrast to single-crystal X-ray diffraction, the detector point spread function for 3D ED is broader for a hybrid pixel detector than for a CCD. In our study, we also compare the diffuse scattering in 3D ED data with the diffuse scattering in single-crystal X-ray diffraction data and show that the diffuse scattering in 3D ED data can be obtained with a quality comparable to that from single-crystal X-ray diffraction. As electron diffraction requires much smaller crystal sizes than X-ray diffraction, this opens up the possibility to investigate the local structure of many technologically relevant materials for which no crystals large enough for single-crystal X-ray diffraction are available

\require{mhchem}Misfit phase \ce{(BiSe)_{1.10}NbSe2} as the origin of superconductivity in nobium-doped bismuth selenide

\require{mhchem}Topological superconductivity is of great contemporary interest and has been proposed in doped \ce{Bi2Se3} in which electron-donating atoms such as Cu, Sr or Nb have been intercalated into the \ce{Bi2Se3} structure. For \ce{Nb_{x}Bi2Se3}, with $\text{T}_\text{c} \sim 3 \ \text{K}$, it is assumed in the literature that Nb is inserted in the van der Waals gap. However, in this work an alternative origin for the superconductivity in Nb-doped \ce{Bi2Se3} is established. In contrast to previous reports, it is deduced that Nb intercalation in \ce{Bi2Se3} does not take place. Instead, the superconducting behaviour in samples of nominal composition \ce{Nb_{x}Bi2Se3} results from the \ce{(BiSe)_{1.10}NbSe2} misfit phase that is present in the sample as an impurity phase for small $x$ ($0.01 \leq x \leq 0.10$) and as a main phase for large $x$ ($x = 0.50$). The structure of this misfit phase is studied in detail using a combination of X-ray diffraction and transmission electron microscopy techniques

Spin ladder compound Pb(0.55)Cd(0.45)V(2)O(5): synthesis and investigation

The complex oxide Pb(0.55)Cd(0.45)V(2)O(5) was synthesized and investigated by means of X-ray powder diffraction, electron diffraction, magnetic susceptibility measurements and band structure calculations. Its structure is similar to that of MV(2)O(5) compounds (M = Na, Ca) giving rise to a spin system of coupled S=1/2 two-leg ladders. Magnetic susceptibility measurements reveal a spin gap-like behavior with \Delta ~ 270 K and a spin singlet ground state. Band structure calculations suggest Pb(0.55)Cd(0.45)V(2)O(5) to be a system of weakly coupled dimers in perfect agreement with the experimental data. Pb(0.55)Cd(0.45)V(2)O(5) provides an example of the modification of the spin system in layered vanadium oxides by cation substitution. Simple correlations between the cation size, geometrical parameters and exchange integrals for the MV(2)O(5)-type oxides are established and discussed.Comment: 8 pages, 7 figure

Competition between Anion-Deficient Oxide and Oxyhydride Phases during the Topochemical Reduction of LaSrCoRuO 6

Binary metal hydrides can act as low-temperature reducing agents for complex oxides in the solid state, facilitating the synthesis of anion-deficient oxide or oxyhydride phases. The reaction of LaSrCoRuO6, with CaH2 in a sealed tube yields the face-centered cubic phase LaSrCoRuO3.2H1.9. The reaction with LiH under similar conditions converts LaSrCoRuO6 to a mixture of tetragonal LaSrCoRuO4.8H1.2 and cubic LaSrCoRuO3.3H2.13. The formation of the LaSrCoRuO x H y oxyhydride phases proceeds directly from the parent oxide, with no evidence for anion-deficient LaSrCoRuO6–x intermediates, in contrast with many other topochemically synthesized transition-metal oxyhydrides. However, the reaction between LaSrCoRuO6 and LiH under flowing argon yields a mixture of LaSrCoRuO5 and the infinite layer phase LaSrCoRuO4. The change to all-oxide products when reactions are performed under flowing argon is attributed to the lower hydrogen partial pressure under these conditions. The implications for the reaction mechanism of these topochemical transformations is discussed along with the role of the hydrogen partial pressure in oxyhydride synthesis. Magnetization measurements indicate the LaSrCoRuO x H y phases exhibit local moments on Co and Ru centers, which are coupled antiferromagnetically. In contrast, LaSrCoRuO4 exhibits ferromagnetic behavior with a Curie temperature above 350 K, which can be rationalized on the basis of superexchange coupling between the Co1+ and Ru2+ centers

Energy transfer in Eu3+ doped scheelites : use as thermographic phosphor

In this paper the luminescence of the scheelite-based CaGd2(1-x)Eu2x(WO4)4 solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the 5D0 – 7F2 transition in Eu3+, along with other transitions from the 5D1 and 5D0 excited states. For high Eu3+ concentrations the intensity ratio of the emission originating from the 5D1 and 5D0 levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent