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Acta Cryst. (2014). A70, C135
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In many materials competing interactions of different nature may give rise to incommensurate modulations causing extreme structure complexity. Ab initio solution of the modulated structures even with using high quality synchrotron X-ray and/or neutron powder diffraction data appears to be a very challenging problem due to weakness of the satellite reflections, ambiguity in the determination of the modulation vector(s) and superspace symmetry and difficulties in building the initial model for further Rietveld refinement. These problems can be resolved or, at least, mitigated if the diffraction, imaging and spectroscopic advanced transmission electron microscopy techniques are combined with the analysis of powder diffraction data. Complete reconstruction of the reciprocal space, structure solution using quasi-kinematical electron diffraction data, mapping projected scattering density in the unit cell, visualization of the light atoms, displacive and occupational ordering, mapping chemical composition and coordination number can be utilized to reveal the nature of incommensurate modulations and construct the reliable model for the refinement from powder diffraction data. The benefit of the strategy of combining the powder diffraction data with the reciprocal and real space information obtained using aberration-corrected scanning transmission electron microscopy will be illustrated on the examples of the transition metal oxides: Li3xNd2/3-xTiO3 perovskites with frustrated incommensurately modulated octahedral tilting pattern [1]; perovskites (Bi,Pb)1-xFe1+xO3-y, modulated by crystallographic shear planes [2]; CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y scheelites with incommensurately modulated ordering of cation vacancies [3].

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Acta Cryst. (2014). A70, C177
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Scheelite (CaWO4) related compounds (A',A'')n[(B',B'')O4]m with B', B''=W and/or Mo are promising new materials for red phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. Scheelites can be prepared with a large concentration of vacancies in the A sublattice, giving compositions characterized by a (A'+A''):(B'O4+B''O4) ratio different from 1:1. The creation of cation vacancies in the scheelite-type framework and the ordering of A cations and vacancies are a new factor in controlling the scheelite-type structure and properties. Very often the substitution of Ca2+ by M+ and R3+ (R3+ = rare earth elements) in the scheelite-type structure leads to switching the structure from 3D to (3+n)D (n = 1,2) regime. The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor controlling the scheelite-type structure and luminescent properties of CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y (0≤x≤1, 0≤y≤1) solid solutions. Within this series all complex molybdenum oxides have (3+2)D incommensurately modulated structures with superspace group I41/a(α,β,0)00(-β,α,0)00, while the structures of all tungstates are (3+1)D incommensurately modulated with superspace group I2/b(αβ0)00. In both cases the modulation arises due to cation-vacancy ordering at the A site. The replacement of the smaller Gd3+ by the larger Eu3+ at the A-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo6+ by W6+ switches the modulation from (3+2)D to (3+1)D regime. Acknowledgement. This research was supported by FWO (project G039211N, Flanders Research Foundation) and Russian Foundation for Basic Research (Grants 11-03-01164, and 12-03-00124).

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Acta Cryst. (2014). A70, C227
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"It is known that lone pair cations, such as Bi3+ or Pb2+ have a flexible coordination environment that enables them to operate as ""chemical scissors"". Their flexibility reduces the strain that would otherwise be present at the interfaces separating structure modules. We have found that in complex oxides it allows many variants of interfaces, for example crystallographic shear planes or (non)conservative twin planes in structures, enabling the synthesis of new structural families. A common characteristic for all these new compounds is the presence of magnetical frustration. As a first example, this concept allowed to introduce crystallographic shear planes into the perovskite structure, a feat that was considered highly unlikely before. This allowed to generate a new anion deficient perovskite based homologous series AnBnO3n-2 (n = 4 - 6). There is magnetic frustration at the crystallographic shear plane separating the perovskite blocks, due to competing FM and AFM interactions. Also incommensurately modulated perovskites can be obtained, for example (Pb,Bi)1-xFe1+xO3-y. These arise by replacing Bi3+ with Pb2+, which introduces an oxygen deficiency, which is then accommodated by periodically spaced CS planes to reduce the coordination of the A-cations at the interface. The flexible coordination environment of Bi3+ and Pb2+ makes them ideally suited for these A cation positions. Other possibilities were encountered in BiMnFe2O6 and Bi4Fe5O13F. In BiMnFe2O6 the Bi3+ induces the existence of a non-conservative twin plane. The result is a new structure type with hcp structured modules. In Bi4Fe5O13F, the Bi3+-cations separate layers with magnetically frustrated Cairo lattices."
Keywords: modular; BiFeO3; lone pair.

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Acta Cryst. (2014). A70, C1334
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Nanocrystalline diamond (NCD) is a unique material we produce by direct conversion of glassy carbon into diamond at ca. 20 GPa and 2200 K in a multi anvil press. One of precursor materials we use is commercially available in the form of glassy carbon balls with a diameter of 20 to 50 microns. NCD demonstrates superior mechanical properties (e.g. extremely high yield strength under confining pressure) and has been successfully used for ultra-high static pressure generation (above 600 GPa) in a double-stage diamond anvil cell (DAC) (Ref. 1). To elucidate structure-property relationships in this extremely strong and seemingly inscrutable material we have investigated its microstructure using HRTEM and HAADF-STEM, measured its compressibility by means of synchrotron X-ray diffraction in a DAC, and evaluated its hardness in comparison to that of the hardest known materials - single-crystal diamond and nano-polycrystalline diamond (NPD) (Ref. 2). An additional insight into the volume compressibility was obtained due to X-ray phase contrast micro-imaging using a coherent high-energy synchrotron radiation. The established structure-property relationships will be presented and analyzed.

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Acta Cryst. (2014). A70, C1355
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The hematophanite Pb4Fe3O8Cl crystal structure is built of incomplete perovskite Pb4Fe3O8 blocks separated by layers of chlorine atoms [1,2]. Each perovskite block consists of a corner-sharing FeO6 octahedral layer sandwiched between the sheets of the FeO5 square pyramids. We have proven that the thickness of the perovskite block in the hematophanite structure can be extended to two and even three octahedral layers forming homologous series with the general formula An+1BnO3n-1Cl (where hematophanite is the n=3 member). The n=4 members with composition Pb4BiFe4O11Cl and Pb5Fe3TiO11Cl have been synthesized. We were also able to introduce Aurivillius-type PbBiO2 blocks between the hematophanite blocks forming another new homologous series [PbBiO2]An+1BnO3n-1Cl2. Two successive members with n=3 (Pb5BiFe3O10Cl2) and n=4 (Pb5Bi2Fe4O13Cl2 and isostructural Pb5BiFe3TiO13Cl2) have been obtained. The crystal and magnetic structure has been determined and refined in a wide temperature range (1.5 - 700 K) using a combination of neutron powder diffraction (NPD) and electron microscopy techniques (electron diffraction, high angle annular dark field scanning transmission electron microscopy (STEM), atomic resolution STEM-EDX). Using NPD and STEM-EDX data we demonstrated that Ti4+ cations occupy both octahedral and square-pyramidal sites. This makes these structural types rare examples of Ti4+ in five-fold oxygen coordination environment. Pb4BiFe4O11Cl and Pb5Fe3TiO11Cl are antiferromagnetically (AFM) ordered below 600(10) and 450(10) K, respectively. Pb5BiFe3O10Cl2, Pb5Bi2Fe4O13Cl2 and Pb5BiFe3TiO13Cl2 demonstrate signs of local magnetic ordering below ~600, ~600 and ~400 K, respectively. However, the long range magnetic ordering does not set in and the magnetic reflections appear enormously broadened merging into a halo. Presumably, AFM ordering establishes within the perovskite blocks but is disrupted along the c-axis, because of a high thickness of the non-magnetic modules.

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Acta Cryst. (2014). A70, C1805
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Anion deficiency in perovskite-based ferrites containing lone pair cations Pb2+ and Bi3+ can be accommodated by the formation of crystallographic shear (CS) planes. In this contribution, using (Pb1-zSrz)1-xFe1+xO3-y as a model system, we demonstrate that the lone pair cations are not only responsible for the formation of the CS planes, but also control their orientation. The isovalent substitution of Pb2+ by stereochemically inactive Sr2+ highlights the influence of the A-cation electronic structure owing to very close ionic radii of these cations. Employing a combination of transmission electron microscopy and X-ray and neutron powder diffraction, we have systematically investigated the structure evolution of the compounds with changing P/Sr ratio. Two compositional ranges with distinct orientations of the CS planes have been identified in the system: 0.05 ≤ z ≤ 0.2, where the CS planes orientation systematically varies but stays close to (203)p, and 0.3 ≤ z ≤ 0.45 with the (101)p CS planes. The incommensurate structure of Pb0.792Sr0.168Fe1.040O2.529, a representative compound of the first range, has been refined from the neutron powder diffraction data (S.G. X2/m(α0γ), X = [1/2,1/2,1/2,1/2], a = 3.9512(1)Å, b = 3.9483(1)Å, c = 3.9165(1)Å, β = 93.268(2)0, q = 0.0879(1)a* + 0.1276(1)c*, RF = 0.023, RP = 0.029, T = 900K). Comparing the CS structures in both compositional ranges, we demonstrate that the orientation of the CS planes is primarily governed by a specific bonding configuration of the lone pair cations with the anions.
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