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ABSTRACT: The morphology of defect pyrochlore-type, AgSbO3 microparticle/nanoparticles obtained via solid state reaction evolve from irregular to Fullerene-like polyhedra before finally decomposing into metal-organic framework-5 like particles with increase in sintering temperature. The defect pyrochlore-type AgSbO3 particles are slightly Ag deficient while the valence of the antimony ion is shown to be +5 giving rise to a probable stoichiometry of Ag1−xSbVO3−x/2, with x ∼ 0.01–0.04. A highly structured diffuse intensity distribution observed via electron diffraction is interpreted in terms of correlated displacements of one-dimensional (1D) silver ion chains along 〈110〉 directions. A redshifting in the absorption edges in UV-visible absorption spectra is observed for samples prepared at sintering temperatures higher than 1000 °C and attributed to the surface plasma resonance effect associated with small amounts of excess metallic Ag on the Ag1−xSbVO3−x/2 particles. An electrical properties investigation of the silver antimonate samples via dielectric, conductivity, and electric modulus spectroscopy shows a prominent dielectric relaxation associated with grain boundaries. The silver ion conductivity is associated with correlated displacements of 1D silver ion chains along 〈110〉 directions.
Journal of Applied Physics 07/2010; 108(2):024911-024911-7. · 2.17 Impact Factor
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ABSTRACT: A careful phase analysis and TEM investigation of the (Bi1−xCax)FeIIIO3−x/2Vx/2 (V for vacancy), ~0.20 ≤ x ≤ ~0.50, and (Bi1−xSrx)Fe3+O3−x/2Vx/2, 0.2 ≤ x ≤ 0.67, perovskite related solid solution fields has been carried out. Both solid solution fields are, in general, (3+1)-d incommensurately modulated with an incommensurate primary modulation wave-vector magnitude that varies systematically with composition. High Resolution (HR)TEM imaging is used to show the presence of at least 6-fold twinning on a sometimes rather fine scale, the fine scale twinning being more prevalent in the M = Sr than the M = Ca system. The (3+1)-d superspace group symmetry is suggested to be Cmmm (01γ) 0s0 or one of the non-centrosymmetric sub-groups thereof. The magnetic and electrical properties of the systems are briefly discussed and a tentative structural model proposed.
Journal of Physics Conference Series 05/2010; 226(1):012015.
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ABSTRACT: Elastic and anelastic properties of a ceramic sample of elasticoluminescent Sr Al 2 O 4 : Eu have been characterized as a function of temperature by resonant ultrasound spectroscopy. Both the bulk (K) and shear (G) moduli show changes attributable to the influence of the P6 3 22→P6 3 ( 3 A )→P2 1 sequence of structural phase transitions. Softening of K and stiffening of G at the P6 3 22→P6 3 ( 3 A ) transition (T c ≈1135 K ) is consistent with weak strain/order parameter coupling and tricritical character. In marked contrast, the first order P6 3 ( 3 A )→P2 1 transition near 930 K is accompanied by stiffening such that G is ∼50% larger at room temperature than the value for a P6 3 22 structure extrapolated directly from high temperatures and K is ∼15% larger. Softening of G at high temperatures is consistent with the existence of a soft acoustic mode. Both the P6 3 ( 3 A ) and P6 3 22 phases show an increasing acoustic dissipation with increasing temperature and there is an additional peak in the dissipation behavior below ∼650 K </formu-
la>. Landau theory has been used to analyze the overall strain and elastic behavior of Sr Al 2 O 4 : Eu . The stability of the P2 1 structure is considered in terms of coupling between order parameters with Γ 6 and M 2 symmetry, though it could also be stabilized by pairwise coupling of order parameters corresponding to irreps Γ 4 + M 2 , Γ 4 + M 3 , Γ 6 + M 3 , and M 2 + M 3 . Twin walls in this material should have interesting properties, which are quite distinct from those of the bulk material and could contribute differently to the elasticoluminescent properties.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
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Journal of Solid State Chemistry 08/2009; 182:2176-2184. · 2.16 Impact Factor
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ABSTRACT: The structural and associated ferroic properties of the stuffed tridymite-type compounds BaAl2O4 and SrAl2O4 have been investigated by means of ab initio calculations. Structures and energy landscapes have been analyzed in terms of symmetry-adapted distortion modes. Despite their rather different room-temperature symmetries, a triply-degenerate unstable antiferrodistortive rigid unit mode (RUM) of the Al2O4 tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave (1q) distortion mode competes with a hexagonal configuration resulting from three superposed such waves (3q). The very small energy difference between these two configurations in the case of BaAl2O4 would explain the recent electron microscope observation of orthorhombic symmetry on the nanoscale. A second unstable mode of polar character (also a RUM for the tetrahedral framework) is also present. While this second instability is too weak to condense in BaAl2O4, in the case of SrAl2O4 it is fundamental to make the 1q configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in SrAl2O4 to monoclinic. In contrast with previous literature, SrAl2O4 is a proper ferroelectric and a pseudoproper ferroelastic, with an uncommon bilinear coupling between its spontaneous polarization and shear monoclinic strain. One expects direct switching of spontaneous polarization through a shear stress, and conversely switching of the ferroelastic spontaneous shear strain through an electric field. This property must be related to the elasticoluminescent and electroluminiscent properties reported in Eu-doped SrAl2O4. The calculated energy maps have been systematically compared with variations in the so-called global instability index, introduced within the empirical bond valence model. The similarity of the variation in both quantities (for the most unstable distortion modes) reported in other systems is also observed here.
Phys. Rev. B. 02/2009; 79(6).
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ABSTRACT: Three techniques of transmission electron microscopy (satellite dark field imaging, high resolution imaging and convergent beam diffraction) and their use in the study of charge density wave materials are discussed. Several recent results on 1T-VSe2, 1T-TaS2, TaTe4, NbTe4, (TaSe4)2I and 2H-NbSe2 are presented.
01/2006: pages 23-32;
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ABSTRACT: The recently determined structure of the commensurate superlattice state of 2H-TaSe2 is analysed in terms of the phenomenological free energy expansion of Jacobs and Walker. It is shown that the magnitude of the inter-layer term is considerably larger than has been previously assumed. Some consequences of this are discussed.
01/2006: pages 84-87;
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ABSTRACT: The room temperature structures as well as the temperature-dependent conductivity and dielectric properties of the A(3)CoNb(2)O(9) (A = Ca2+, Sr2+ and Ba2+) triple perovskites have been carefully investigated. A constrained modulation wave approach-to Rietveld structure refinement is used to determine their room temperature crystal structures. Correlations between these crystal structures and their physical properties are found. All three compounds undergo insulator to semiconductor phase transitions as a function of increasing temperature. The hexagonal Ba3CoNb2O9 compound acts as an insulator at room temperature, while the monoclinic Ca3CoNb2O9 compound is already a semiconductor at room temperature. The measured dielectric frequency characteristics of the A = Ba compound are excellent. (C) 2004 Elsevier Inc. All rights reserved.
Journal of Solid State Chemistry. 01/2004; 177(12):4428-4442.
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ABSTRACT: structural relaxation in two members of the cubic BZN pyrochlore (Bi1.5-aZn0.5-b)(Zn0.5-gNb1.5-d)O(7-1.5a-b-g-2.5d) solid solution. An average neutron powder diffraction structure refinement is carried out for one of these. Electron probe micro-analysis suggests that the primary mechanism for non-stoichiometry in cubic BZN is the removal of ZnO from the nominally fully occupied (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 end-member. A detailed bond valence sum analysis of a recently reported average crystal structure is used
to suggest possible local cation ordering schemes along with the induced displacive relaxation that is likely to accompany such local cation ordering. The observed diffuse distribution is qualitatively interpreted via Monte Carlo modelling.
Journal of Solid State Chemistry 01/2004; 177:231-244. · 2.16 Impact Factor
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ABSTRACT: Bond valence sum calculations are used to investigate the crystal chemistry of the elpasolite-related oxyfluoride K3MoO3F3 in order to obtain insight into the type/s of structural distortion (away from an ideal,high symmetry, elpasolite type parent structure) responsible for a characteristic,highly structured, three-dimensional diffuse intensity distribution. The first required type of local structural distortion corresponds to large amplitude MoO3F3 octahedral rotations while the second is associated with O/F ordering and associated induced Mo ion shifts. Monte Carlo modelling is used to show how the latter when coupled with an appropriate local crystal chemical constraint can give rise to the observed structured diffuse scattering. The study is part of a wider search for diffraction evidence of oxygen/fluorine ordering in metal oxyfluoride systems.
Journal of Solid State Chemistry 01/2003; 170:211-230. · 2.16 Impact Factor
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ABSTRACT: A detailed TEM and XRD study has been carried out on B8 related ternary solid solution phases in the Ni1+x My Te2 (M=Ag, Cu, In) systems. Despite previous reports of a large B8 ternary compositional homogeneity range in the Ni1+x My Te2 (M=Ag) system, no evidence for incorporation of Ag into Ni Te could be found. In the case of M=Cu, several Ni1+ My Te (M=Cu) compositions were found to be single-phase and part of a genuine ternary solid solution, but only if the Cu content was at much lower concentrations than previously suggested. In the case of M=In, two roughly determined solid solution fields were observed, an expected one based on the Ni1+xTe2 solid solution (SS1) and a second unexpected, quite distinct and compositionally broad solid solution (SS2) centred on the nominal composition Ni2.80 In0.80 Te2 . The latter SS2 solid solution was shown to be incommensurately modulated with a composition dependent primary modulation wavevector. The superspace group symmetry of the latter is determined and the observed B8 parent cell dimensions used to discuss possible substitution mechanisms. The origin of the incommensurate modulation in the case of the SS2 solid solution is attributed to the regular alternation of In-rich and Te-rich layers along the c direction.
Journal of Alloys and Compounds 01/2003; 353:133-142. · 2.29 Impact Factor
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ABSTRACT: Tl3MoO3F3, a previously unreported member of the A2BMVIO3F3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of TlF with MoO3 at 655�C. DSC shows two major polymorphic phase transitions at 42�C and 130�C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by a = 3ap + cp; b = 3bp; c = 1/2(-ap + 3cp); when expressed with respect to the underlying ideal elpasolite-type parent structure. This superstructure, while related, is not isomorphous to that recently reported for K3MoO3F3. The existence of a shared subset of strong Gp+/-J/5(204)* type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl3MoO3F3 and Rb2KMoO3F3. This suggests that a particular pattern of local O/F ordering and associated Mo ion shifts, recently shown to be responsible for the existence of this diffuse distribution in the case of K3MoO3F3, may
be common to the entire family of elpasolite-related A2BMVIO3F3 compounds.
Journal of Solid State Chemistry 01/2003; 174:44-51. · 2.16 Impact Factor
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ABSTRACT: A detailed electron diffraction and XRPD study has been made of the room temperature polymorph of K3MoO3F3. It is shown that the true symmetry of this polymorph is neither tetragonal, trigonal or triclinic as previously reported but rather monoclinic I1a1, a = 2ap-cp, b = 4bp, c = ap+2cp when expressed in terms of the underlying elpasolite (ordered perovskite) parent structure type. A highly structured, three-dimensional, continuous diffuse intensity distribution (presumably arising from local O/F ordering and associated structural relaxation) is shown to co–exist with the sharp satellite reflections characteristic of the monoclinic supercell.
Journal of Solid State Chemistry 01/2002; 163:267-274. · 2.16 Impact Factor
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ABSTRACT: The Landau theory of charge density wave states in 2H-TaSe2 is re-examined in the light of the commensurate superlattice structure determination by Bird, McKernan and Steeds (1985). It is shown that interlayer interactions play a far more important role in determining the phasing of the CDW/PSD than has been previously assumed. Some consequences of this for the nature of incommensurate states and the interpretation of previous experimental data are also discussed.
Journal of Physics C Solid State Physics 11/2000; 18(3):519.
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ABSTRACT: The theory of high-energy electron diffraction from modulated structures is discussed. A perturbation theory is used in which the change of potential caused by the modulation is treated as being weak in comparison with that of the average structure. An expression for the single scattering diffraction amplitude is derived and analysed in terms of its symmetry. The large thickness limit of this expression is also considered. It is shown that it takes a simple form, enabling information about the nature of a modulated structure to be obtained from diffraction experiments. Double diffraction is also briefly discussed to show that such effects are experimentally distinguishable from single diffraction.
Journal of Physics C Solid State Physics 11/2000; 19(19):3497.
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ABSTRACT: The lattice distortions of NbTe4 and TaTe4 are investigated by electron microscopy and electron diffraction. Evidence is presented for a high-symmetry distortion, with a space group of P4/mcc for sublattices and commensurate superlattices in both materials. Satellite dark-field images show discommensuration arrays within the incommensurate state at NbTe4 which are replaced in a fascinating microstructural display during a slow, strongly first-order, lock-in transition by a network of commensurate domains. Similar commensurate domain networks are also imaged in room-temperature TaTe4. The distortions are discussed from the basis of crystal symmetry and simple landau free-energy arguments.
Journal of Physics C Solid State Physics 11/2000; 18(1):1.
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ABSTRACT: For pt.I see ibid., vol.19, p.3497 (1986). The extraction of quantitative information from large angle, satellite dark field, convergent beam diffraction patterns taken from the incommensurately modulated ternary phase NiGeP is described. Expressions for the incommensurate satellite structure factors are derived in terms of an initially unconstrained phonon eigenvector. The little-co-group of the modulation wave-vector is used to derive the normal modes of the unmodulated lattice and the appropriate eigenvectors for two of the modulations observed in experiment. By fitting the variation of satellite diffraction intensity with incident orientation to that observed experimentally, the eigenvector components of one of these are determined.
Journal of Physics C Solid State Physics 11/2000; 19(19):3507.
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ABSTRACT: An electron diffraction and imaging study of charge-density-wave phases in 1T-VSe2 is presented. Two phase transitions are found. At 110K a 3q-state is formed which at 80K transforms into an irregular pattern of 2q-domains. A Landau theory of these transitions is presented. In order to explain the 3q-2q transition sixth-order terms must be included in the free-energy expansion.
Journal of Physics C Solid State Physics 11/2000; 19(3):359.
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ABSTRACT: It has long been recognised that the very characteristic, two-dimensional diffuse intensity distributions observed at 'high' temperatures in the octahedral CDW/PLD-bearing transition-metal dichalcogenides must be closely related to the subsequent development of longer-range order at lower temperatures. An interpretation of the 'high-temperature' diffuse intensity distributions in terms of thermally excited nearly soft phonon modes is found to provide a very useful means of interpreting the experimentally observed behavior in the lower-temperature, longer-range-ordered CDW/PLD states.
Journal of Physics C Solid State Physics 11/2000; 20(35):5975.
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ABSTRACT: New results are presented on 1T-TaS2. They were obtained by combining convergent-beam electron diffraction with high-quality satellite dark-field imaging. In the nearly commensurate 1T2 state linear defects were analysed as displacement faults and the fault planes and vectors were determined. The visibility of these faults was found to be strongly temperature dependent. Temperature-dependent sequences were obtained in chosen areas to follow the dynamic changes which occurred on cooling. Direct dislocation interaction between crystal dislocations and the 1T1/1T2 interface was observed. Radiation damage effects were detected and investigated using a 300 kV electron microscope with variable operating voltage. The first detailed TEM investigation has been carried out on the T phase often obtained on warming the material from its low-temperature locked-in (1T3) state. Detailed comparison of the results obtained has been made with previously reported X-ray results and although there is considerable agreement some differences were detected. A simple Landau theory argument is developed to account for the out-of-plane components of the satellite vectors in the T phase. Some further investigation of the 1T3 phase was carried out and the results discussed in relation to earlier TEM and X-ray investigations.
Journal of Physics C Solid State Physics 11/2000; 20(26):4019.
