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Chapuis G.,Ecole Polytechnique Federale de Lausanne | Arakcheeva A.,Phase Solutions Ltd.
Rendiconti Lincei | Year: 2013

The discovery of aperiodic crystals is perhaps one of the most important event which has changed our vision on crystalline architectures since the discovery of diffraction 100 years ago. It was the merit of a Dutch crystallographer, P.M. de Wolff, to interpret their diffraction pattern as a three dimensional projection of a higher dimensional reciprocal lattice, idea which led directly to the generalization of the concept of crystal. Aperiodic crystals are currently described as periodic objects in higher-dimensional space, i.e. the superspace and their structures can be described in terms of 3-d cuts. Incommensurate structures, composite structures and quasicrystals all belong to aperiodic structures. Many interesting properties of superspace have been discovered which are also directly applicable to crystals in the conventional sense, i.e. crystals with 3-d periodicity. In particular the concept of structure type can be extended for a better understanding of structure relations. The notion of solid solution has also benefited from superspace considerations. Moreover, superspace is a very powerful tool for a better understanding of structure-property relations in material science, e.g. luminescence properties could be directly associated to the description of structures in superspace. Recently, this concept has been used for the prediction of new structural modifications including polytypes and even polytypic modifications of a well-known pharmaceutical product. © 2013 Accademia Nazionale dei Lincei. Source


Morozov V.,Moscow State University | Arakcheeva A.,Ecole Polytechnique Federale de Lausanne | Arakcheeva A.,Phase Solutions Ltd. | Redkin B.,RAS Institute of Solid State Physics | And 6 more authors.
Inorganic Chemistry | Year: 2012

Scheelite-type compounds with the general formula (A1,A2) n[(B1,B2)O 4] m ( 2/ 3 ≥ n/m ≥ 3/ 2) are the subject of large interest owing to their stability, relatively simple preparation, and optical properties. The creation of cation vacancies () in the scheelite-type framework and the ordering of A cations and vacancies can be a new factor in controlling the scheelite-type structure and properties. For a long time, cation-deficient Nd 3+:M 2/7Gd 4/71/7MoO 4 (M = Li, Na) compounds were considered as potential lasers with diode pumping. They have a defect scheelite-type 3D structure (space group I4 1/a) with a random distribution of Li +(Na +), Gd 3+, and vacancies in the crystal. A Na 2/7Gd 4/7MoO 4 single crystal with scheelite-type structure has been grown by the Czochralski method. Transmission electron microscopy revealed that Na 2/7Gd 4/7MoO 4 has a (3 + 2)D incommensurately modulated structure. The (3 + 2)D incommensurately modulated scheelite-type cation-deficient structure of Na 2/7Gd 4/7MoO 4 [super space group I4̄ (α-β0,βα0)00] has been solved from single-crystal diffraction data. The solution of the (3 + 2)D incommensurately modulated structure revealed the partially disordered distribution of vacancies and Na and Gd cations. High-temperature conductivity measurements performed along the [100] and [001] orientation of the single crystal revealed that the conductivity of Na 2/7Gd 4/7MoO 4 at T = 973 K equals σ = 1.13 × 10 -5 Ω -1 cm -1. © 2012 American Chemical Society. Source


Morozov V.A.,University of Antwerp | Arakcheeva A.V.,Ecole Polytechnique Federale de Lausanne | Arakcheeva A.V.,Phase Solutions Ltd. | Pattison P.,Ecole Polytechnique Federale de Lausanne | And 8 more authors.
Chemistry of Materials | Year: 2015

In this paper, with the example of two different polymorphs of KEu(MoO4)2, the influence of the ordering of the A-cations on the luminescent properties in scheelite related compounds (A′,A″)n[(B′,B″)O4]m is investigated. The polymorphs were synthesized using a solid state method. The study confirmed the existence of only two polymorphic forms at annealing temperature range 923-1203 K and ambient pressure: a low temperature anorthic α-phase and a monoclinic high temperature β-phase with an incommensurately modulated structure. The structures of both polymorphs were solved using transmission electron microscopy and refined from synchrotron powder X-ray diffraction data. The monoclinic β-KEu(MoO4)2 has a (3+1)-dimensional incommensurately modulated structure (superspace group I2/b(αβ0)00, a = 5.52645(4) Å, b = 5.28277(4) Å, c = 11.73797(8) Å, γ = 91.2189(4)o, q = 0.56821(2)a∗-0.12388(3)b∗), whereas the anorthic α-phase is (3+1)-dimensional commensurately modulated (superspace group I1¯(αβγ)0, a = 5.58727(22) Å, b = 5.29188(18)Å, c = 11.7120(4) Å, α = 90.485(3)o, β = 88.074(3)o, γ = 91.0270(23)o, q = 1/2a∗ + 1/2c∗). In both cases the modulation arises due to Eu/K cation ordering at the A site: the formation of a 2-dimensional Eu3+ network is characteristic for the α-phase, while a 3-dimensional Eu3+-framework is observed for the β-phase structure. The luminescent properties of KEu(MoO4)2 samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed. (Figure Presented). © 2015 American Chemical Society. Source


Dahn R.,Paul Scherrer Institute | Arakcheeva A.,Phase Solutions Ltd. | Schaub P.,Paul Scherrer Institute | Pattison P.,European Synchrotron Radiation Facility | And 6 more authors.
Cement and Concrete Research | Year: 2016

Alkali-silica reaction (ASR) is one of the most important deterioration mechanisms in concrete leading to substantial damages of structures worldwide. Synchrotron-based micro-X-ray diffraction (micro-XRD) was employed to characterize the mineral phases formed in micro-cracks of concrete aggregates as a consequence of ASR. This high spatial resolution technique enables to directly gain structural information on ASR products formed in a 40-year old motorway bridge damaged due to ASR. Micro-X-ray-fluorescence was applied on thin sections to locate the reaction products formed in veins within concrete aggregates. Micro-XRD pattern were collected at selected points of interest along a vein by rotating the sample. Rietveld refinement determined the structure of the ASR product consisting of a new layered framework similar to mountainite and rhodesite. It is conceivable that understanding the structure of the ASR product may help developing new technical treatments inhibiting ASR. © 2015 Elsevier Ltd. Source

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