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Ordered Distribution (ordered + distribution)
Selected AbstractsStructure and Dielectric Properties of Pb(Sc2/3W1/3)O3,Pb(Zr/Ti)O3 RelaxorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2004Pavol Juhás The structure and dielectric properties of (1,x)Pb(Sc2/3W1/3)O3,(x)Pb(Zr/Ti)O3 ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x, 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc2/3W1/3)O3 (PSW) can be described by a random-site model with one cation site occupied by Sc3+ and the other by a random distribution of (Sc1/33+W2/36+). The ordering is destabilized in solid solutions of PSW with PbZrO3 (PSW,PZ), but stabilized by PbTiO3 in the (1,x)PSW,(x)PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW,PZ the temperature of the permittivity maximum (T,,max) increases linearly with x; however, for PSW,PT T,,max decreases in the ordered region (up to x= 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW,(0.25)PT; when the order parameter was reduced from ,1.0 to ,0.65, T,,max increased by more than 60°C. [source] Time-dependent evolution of crystal lattice, defects and impurities in CdIn2S4 and GaPPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009Sergei L. Pyshkin Abstract We demonstrate that in III-V and ternary compounds the natural stimuli for ordered distribution of impurities along crystal nodes and interstitials, host atoms and inherent vacancies in right lattice positions prevail over the others which could lead to a heterogenic distribution. The result is obtained in the crystals, prepared in 1963-1977, due to each 10-15 years monitoring of their optical and mechanical properties. Here we have summarized the results of the 2005-2008 monitoring and compare them with the earlier obtained data. We show that the partly inverse spinel CdIn2S4 turns into the perfect normal spinel, while in GaP the impurity ordering leads to the formation of a new type of crystal lattices. Periodically disposed impurities and inherent structural defects modify, improve and stabilize properties of the crystals. New luminescent phenomena of these crystals, including stimulated emission and "hot" luminescence have been discovered. The existing technologies help us to reproduce these naturally ordered structures and to apply them for new generation of optoelectronic devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Revision of pyrrhotite structures within a common superspace modelACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2007Zunbeltz Izaola The structure of pyrrhotite (Fe1,,,xS with 0.05 ,x, 0.125) has been reinvestigated in the framework of the superspace formalism. A common model with a centrosymmetric superspace group is proposed for the whole family. The atomic domains in the internal space representing the Fe atoms are parametrized as crenel functions that fulfil the closeness condition. The proposed model explains the x -dependent space groups observed and the basic features of the structures reported up to now. Our model yields for any x value a well defined ordered distribution of Fe vacancies in contrast to some of the structural models proposed in the literature. A new (3,+,1)-dimensional refinement of Fe0.91S using the deposited dataset [Yamamoto & Nakazawa (1982). Acta Cryst. A38, 79,86] has been performed as a benchmark of the model. The consistency of the proposed superspace symmetry and its validity for other compositions has been further checked by means of ab initio calculations of both atomic forces and equilibrium atomic positions in non-relaxed and relaxed structures, respectively. [source] Transition from the incommensurately modulated structure to the lock-in phase in Co-åkermaniteACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2001Andreas K. Schaper The adaptation of the incommensurate structure modulation in Ca2CoSi2O7 (dicalcium cobalt disilicate) single crystals to decreasing temperature has been examined using in situ high-resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the commensurate lock-in phase of Co-åkermanite exhibits a pronounced hysteresis of a highly strained metastable state with a characteristic microdomain morphology. A network of domain walls surrounding single orientation domains develops out of the room-temperature tartan pattern, the domains increase in size and their alignment changes from crystallographic to random. At 100,K the phase transition becomes almost complete. In parallel, the evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close-packed configuration of overlapping octagons in the commensurate low-temperature lock-in phase. Thereby, the octagon represents the ordered distribution of low-coordinated Ca clusters within a nanodomain extending over 4 × 4 subunits, on average [Riester et al. (2000). Z. Kristallogr.215, 102,109]. The modulation wavevector was found to change from q1,2 = 0.295 (a* ±b*) at 300,K to q1,2 = 0.320 (a* ±b*) at 100,K. [source] Hexagonal high-temperature form of aluminium phosphate tridymite from X-ray powder dataACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2001Heribert A. Graetsch Similar to silica tridymite, AlPO4 tridymite shows a sequence of displacive phase transitions resulting in a dynamically disordered hexagonal high-temperature modification. Rietveld refinement reveals that the thermal motions of the tetrahedra can be described either by strongly anisotropic displacement parameters for oxygen or by split O atoms. Due to the ordered distribution of aluminium and phosphorus over alternating tetrahedra, the space group symmetry of high-temperature AlPO4 tridymite is reduced with respect to SiO2 tridymite from P63/mmc to P63mc. [source] | ||||||||||