Ray Withers | The Australian National University (original) (raw)
Papers by Ray Withers
Acta Crystallographica Section A Foundations of Crystallography
Journal of Solid State Chemistry, 2001
The high-temperature Ni6±xSe5 phase has been carefully re-investigated via electron diffraction a... more The high-temperature Ni6±xSe5 phase has been carefully re-investigated via electron diffraction and TEM imaging. Electron diffraction reveals an (in general) incommensurately modulated, continuously variable (within narrow limits) reciprocal lattice, indirectly confirming the notion of a continuously variable (within narrow limits) Ni6±xSe5 solid solution phase. The superspace group symmetry of the incommensurate interface modulated structure is determined and used to predict the allowed conventional three-dimensional space group symmetries when the primary modulation wavevector locks in to {1}/{2}a*. The mechanism for accommodating nonstoichiometry is suggested to be the introduction into a 2×1×1 superstructure of variably spaced, nonconservative (100) APB's characterized by the displacement vector R=[{1}/{4}, {1}/{2}, 0].
J Solid State Chem, 2005
A wide range of barium titanate hollandites of the form Baâ{sup 2+}Mâ{sup 2+}Ti{sub 8-x}{sup 4+}O... more A wide range of barium titanate hollandites of the form Baâ{sup 2+}Mâ{sup 2+}Ti{sub 8-x}{sup 4+}Oââ (M=Zn, Co, Mg, Fe and Mn) and Baâ{sup 2+}M{sub 2x}{sup 3+}Ti{sub 8-2x}{sup 4+}Oââ (M=Fe) with nominal x ranging from 1.0 to 1.4 have been synthesized and examined to investigate the solid solution range and the nature of the ordering of the Ba ions. Electron diffraction
Scientific reports, 2016
Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for... more Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field depend...
Advances in Condensed Matter Physics, Dec 22, 2013
We characterized the temperature dependent (∼25-200 ∘ C) electromechanical properties and crystal... more We characterized the temperature dependent (∼25-200 ∘ C) electromechanical properties and crystal structure of Pb(In 1/2 Nb 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 single crystals using in situ electrical measurement and neutron diffraction techniques. The results show that the poled crystal experiences an addition phase transition around 120 ∘ C whereas such a transition is absent in the unpoled crystal. It is also found that the polar order persists above the maximum dielectric permittivity temperature at which the crystal shows a well-defined antiferroelectric behavior. The changes in the electrical properties and underlying crystal structure are discussed in the paper.
Journal of Solid State Chemistry, 1995
The crystal structure of Ca 5Y 4S 11 [a = 6.942(1) Å, α = 33.380(5)°, space group R3 m, No. 166, ... more The crystal structure of Ca 5Y 4S 11 [a = 6.942(1) Å, α = 33.380(5)°, space group R3 m, No. 166, Z = {2}/{11}, Dx = 3.042 g cm -3] was determined using Rietveld refinement of X-ray powder diffraction data collected with Cu Kα 1 radiation using a Guiner-Hägg camera. Ca 5Y 4S 11 is one end-member composition ( x = {2}/{7}) of the solid-solution (1- x)CaS. xY 2S 3, which can be considered as a modulated NaCl-type structure with modulation wave-vector q = {1}/{2}(111)∗. The sulfur sublattice is fully occupied but the metal sublattice contains {2}/{11} vacancies on average. The refinement showed that the additional observed satellite reflections were almost entirely due to metal atom/vacancy ordering rather than sulfur atom displacement. Large anisotropic thermal parameters on both the sulfur and metal atoms were consistent with disordered displacement of these atoms by 0.2-0.4 Å normal to [111]. The chemical plausibility of the refined structure and the implication of this result for other substoichiometric NaCl-type solid solutions are discussed.
Chemistry of Materials, Jul 13, 2010
ABSTRACT We present, for the first time, the ionic conductivity properties of two different, but ... more ABSTRACT We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi38Mo7O78 and Bi37.5Mo7.5O78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi38Mo7O78 is more complex than previously reported, and that this compound is a 5 × 3 × 6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi37.5Mo7.5O78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements.
Ferroelectrics, 2004
ABSTRACT The strongest crystal chemical constraints are usually local and fix local parameters su... more ABSTRACT The strongest crystal chemical constraints are usually local and fix local parameters such as bond lengths, angles and even stoichiometries (although not necessarily orientations) in local co-ordination polyhedra. Surprisingly often, however, constraints of this sort are insufficiently severe or long range enough to lead to well-defined, conventional 3-d crystalline order. Instead quite distinct new types of “modulated structure,” commonly associated with the existence of sharp, highly structured, characteristic diffuse intensity distributions, can result.
Bulletin of the American Physical Society, Mar 5, 2015
Rigid Unit Modes; their energy and temperature dependence with aluminium phosphate ADAM BERLIE 1 ... more Rigid Unit Modes; their energy and temperature dependence with aluminium phosphate ADAM BERLIE 1 , The Bragg Institute, ANSTO, YUN LIU, Australian National University, DEHONG YU, GORDEN KEARLEY, The Bragg Institute, ANSTO, CHRIS LING, University of Sydney, RAY WITH-ERS, Australian National University -One of the problems within crystallography is the concept of the average structure where due to dynamical translations or librations the structure is not truly static. This type of behaviour is common within polyhedral based compounds such as AlPO 4 where, in this case, the rigid tetrahedra can move or tilt with respect to each other as a consequence of the instability of the 180 • Al-O-P bond. We explore the energy and temperature dependence of these modes using inelastic neutron scattering and heat capacity measurements as well as using computational modelling to assign the observed behaviour to these rigid unit modes.
Phil Mag Lett, 1987
The structural analysis by convergent-beam electron diffraction is described for a weak orthorhom... more The structural analysis by convergent-beam electron diffraction is described for a weak orthorhombic superlattice observed in both mineral and synthetic samples of NiAs. The three equivalent directions for the commensurate wavevector, q = ⅓(a* + b*), defined a domain structure within the hexagonal subcell. Within a single domain, the symmetry around the c axis was reduced from 6mm to 2mm.
Acta crystallographica Section B, Structural science, crystal engineering and materials, 2015
The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modul... more The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modulated δ-Bi2O3-related structure, in which the modulation vector ℇ `locks in' to a commensurate value of 1/3. The structure was refined in a 3 × 3 × 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 - xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 `tetrahedra of octahedra' and chains of corner-sharing WO6 octahedra along 〈110〉F directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with pre...
Upon heating, the 1:2 triple perovskites A 3 CoNb 2 O 9 (A ¼ Ca, Sr or Ba) each undergo well-defi... more Upon heating, the 1:2 triple perovskites A 3 CoNb 2 O 9 (A ¼ Ca, Sr or Ba) each undergo well-defined insulator to conductor phase transitions at 8,8, 8,126 and $325 1C, respectively. As the trend in the transition temperatures for these materials does not correlate with the size of the reported band gaps, neutron powder diffraction has been used to investigate if this change in electrical behaviour of the materials was due to a structural phase change. It was found that in the regions of the suspected phase transitions there were only slight perturbations of the structures, namely thermal expansion of the lattices and an apparent muting of the amplitude of the octahedral rotations in the A ¼ Ca and Sr compound at higher temperatures. r
J Solid State Chem, 2000
Cu/Co ordering in TlCo2-xCuxSe2, (x~1) has been investigated using electron and X-ray powder diff... more Cu/Co ordering in TlCo2-xCuxSe2, (x~1) has been investigated using electron and X-ray powder diffraction. The average structure of the x=1 compound can be indexed on an F-centered orthorhombic cell, a=5.5518(6) Å, b=5.4755(5) Å, and c=13.7387(17) Å. Additional weak satellite reflections characterized by the primary modulation wave vector qi~0.184a* and indicative of an incommensurate modulated structure are revealed by electron diffraction. Characteristic extinction conditions in conjunction with mmm Laue symmetry and known crystal chemical constraints imply a (3+1)-dimensional superspace group symmetry of Fmmm(alpha01)00s. The incommensurate modulation is attributed to occupational ordering of the two transition metal atoms and associated structural relaxation. The real space interpretation of the Cu/Co ordering pattern is that copper and cobalt atoms form separate layers in the structure at z={1}/{4} and {3}/{4}, respectively. These layers are periodically interchanged as one moves along the x direction. Two periods of this square-wave-like modulation is close to 11a in length but is non-rational compared with the average structure.
J Solid State Chem, 1999
The extent of the pyrochlore solid solution field (at 1500°C) in the (1−ε)ZrO2·εSmO1.5 system has... more The extent of the pyrochlore solid solution field (at 1500°C) in the (1−ε)ZrO2·εSmO1.5 system has been determined as running from 38.5 to 55 mol% SmO1.5. Subtle changes in crystal structure have been investigated as a function of composition across this solid solution field using a wide-angle convergent beam electron diffraction technique. Systematic changes in the one unknown oxygen atom fractional coordinate x are found as a function of composition and investigated from a crystal chemical point of view using the bond length – bond valence approach. The parameter x is found to be ∼0.352 for ε=0.40 and 0.348 for ε=0.45 and 0.55. Both values of x are significantly larger than the value (x=0.342) found for the stoichiometric 50 mol% SmO1.5 specimen by the same technique. The increased x value found for nonstoichiometric compositions on either side of the ideal pyrochlore composition (ε=0.5) is consistent with a systematic shift toward the average value for this parameter in the so-called “defect fluorite” structure (x=0.375), the phase field for which appears on either side of the relevant pyrochlore solid solution field.
Progress in Solid State Chemistry, 1998
J Solid State Chem, 2007
The (3+1)(3+1)-d incommensurately modulated structures of four members of the NiGe1-xPxNiGe1-xPx ... more The (3+1)(3+1)-d incommensurately modulated structures of four members of the NiGe1-xPxNiGe1-xPx solid solution field have been successfully refined from X-ray powder diffraction data (Rw(all)/Rwp=2.13/3.92Rw(all)/Rwp=2.13/3.92; 1.52/4.25; 1.27/3.44 and 2.00/4.03 for x=0.4x=0.4, 0.5, 0.6 and 0.7, respectively). The 4-d superspace group symmetry is Amam(00γ)s00 (Z=4Z=4; a=5.0468(2)a=5.0468(2), 5.0188(2), 4.9796(2) and 4.9651(1)Å; b=6.0636(3)b=6.0636(3), 6.0576(2), 6.0183(2) and 6.0031(1)Å; c=3.4877(2)c=3.4877(2), 3.4812(2), 3.4593(1) and 3.45442(7)Å; γ=0.7769(2)γ=0.7769(2), 0.7467(1), 0.7241(1) and 0.7046(1) for x=0.4x=0.4, 0.5, 0.6 and 0.7, respectively). The underlying average structure is of NiAs type while the (in general) incommensurate primary modulation wave-vector, γc*γc*, varies continuously and smoothly with composition. The two largest amplitude displacive atomic modulation functions (AMFs), for all samples, were the Ni displacement along b AMF and the Ge/P displacement along a AMF. The refined amplitude of the former was found to systematically increase with P content from 0.215Å for NiGe0.6P0.4NiGe0.6P0.4 to 0.294Å for NiGe0.3P0.7NiGe0.3P0.7 while the magnitude of the latter was found to increase with P content from 0.177Å for NiGe0.6P0.4NiGe0.6P0.4 to 0.253Å for NiGe0.3P0.7NiGe0.3P0.7. These displacive shifts significantly modulate the local crystal chemistry i.e. the local interatomic distances and co-ordination polyhedra. This continuously variable, incommensurately modulated, intermediate structure type is shown to provide a natural link or bridge between the two extreme end-member structures i.e. NiGe (of MnP structure type) and NiP by simply choosing the commensurate options with γ=1γ=1 and 12 respectively.The NiP (top) and NiGe (bottom) crystal structures. The structural relationship is emphasised by indicating the xsin[Ge/P] modulation wave in the 4-d space group setting Amam(00γ)s00; γ=1γ=1 for NiGe, γ=1/2γ=1/2 for NiP and 0.78<γ<0.700.78<γ<0.70 for NiGe1-xPx with 0.4>x>0.70.4>x>0.7.
Phys Rev B, 1982
The experimentally observed Raman scattering data on 2H-TaSe2 has been reanalyzed in the light of... more The experimentally observed Raman scattering data on 2H-TaSe2 has been reanalyzed in the light of the recent discovery that the commensurate superlattice has orthorhombic and not hexagonal symmetry. A recent model for the orthorhombic, commensurate phase can be consistent with the observed Raman spectra only if the interlayer interactions are weak. Similarly, the weakly interacting charge-density-wave picture is shown to be incorrect and, consequently, amplitude and phase modes are, in general, coupled.
J Solid State Chem, 2005
The structure of the pyrochlore-type oxide Bi 2InNbO 7 has been investigated between room tempera... more The structure of the pyrochlore-type oxide Bi 2InNbO 7 has been investigated between room temperature and 700 °C using electron and synchrotron X-ray powder diffraction and at room temperature and 10 K using neutron diffraction methods. Bi 2InNbO 7 exhibits an A 2B 2O 7 cubic pyrochlore-type average structure at all temperatures that is characterized by an apparently random mixing of the In 3+ and Nb 5+ cations on the octahedral B sites. The Bi cations on the eight-coordinate pyrochlore A sites are displacively disordered, presumably as a consequence of their lone pair electron configuration. Heating the sample does not alter this disorder.
Solid State Phenomena, 1989
Acta Crystallographica Section A Foundations of Crystallography
Journal of Solid State Chemistry, 2001
The high-temperature Ni6±xSe5 phase has been carefully re-investigated via electron diffraction a... more The high-temperature Ni6±xSe5 phase has been carefully re-investigated via electron diffraction and TEM imaging. Electron diffraction reveals an (in general) incommensurately modulated, continuously variable (within narrow limits) reciprocal lattice, indirectly confirming the notion of a continuously variable (within narrow limits) Ni6±xSe5 solid solution phase. The superspace group symmetry of the incommensurate interface modulated structure is determined and used to predict the allowed conventional three-dimensional space group symmetries when the primary modulation wavevector locks in to {1}/{2}a*. The mechanism for accommodating nonstoichiometry is suggested to be the introduction into a 2×1×1 superstructure of variably spaced, nonconservative (100) APB's characterized by the displacement vector R=[{1}/{4}, {1}/{2}, 0].
J Solid State Chem, 2005
A wide range of barium titanate hollandites of the form Baâ{sup 2+}Mâ{sup 2+}Ti{sub 8-x}{sup 4+}O... more A wide range of barium titanate hollandites of the form Baâ{sup 2+}Mâ{sup 2+}Ti{sub 8-x}{sup 4+}Oââ (M=Zn, Co, Mg, Fe and Mn) and Baâ{sup 2+}M{sub 2x}{sup 3+}Ti{sub 8-2x}{sup 4+}Oââ (M=Fe) with nominal x ranging from 1.0 to 1.4 have been synthesized and examined to investigate the solid solution range and the nature of the ordering of the Ba ions. Electron diffraction
Scientific reports, 2016
Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for... more Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field depend...
Advances in Condensed Matter Physics, Dec 22, 2013
We characterized the temperature dependent (∼25-200 ∘ C) electromechanical properties and crystal... more We characterized the temperature dependent (∼25-200 ∘ C) electromechanical properties and crystal structure of Pb(In 1/2 Nb 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 single crystals using in situ electrical measurement and neutron diffraction techniques. The results show that the poled crystal experiences an addition phase transition around 120 ∘ C whereas such a transition is absent in the unpoled crystal. It is also found that the polar order persists above the maximum dielectric permittivity temperature at which the crystal shows a well-defined antiferroelectric behavior. The changes in the electrical properties and underlying crystal structure are discussed in the paper.
Journal of Solid State Chemistry, 1995
The crystal structure of Ca 5Y 4S 11 [a = 6.942(1) Å, α = 33.380(5)°, space group R3 m, No. 166, ... more The crystal structure of Ca 5Y 4S 11 [a = 6.942(1) Å, α = 33.380(5)°, space group R3 m, No. 166, Z = {2}/{11}, Dx = 3.042 g cm -3] was determined using Rietveld refinement of X-ray powder diffraction data collected with Cu Kα 1 radiation using a Guiner-Hägg camera. Ca 5Y 4S 11 is one end-member composition ( x = {2}/{7}) of the solid-solution (1- x)CaS. xY 2S 3, which can be considered as a modulated NaCl-type structure with modulation wave-vector q = {1}/{2}(111)∗. The sulfur sublattice is fully occupied but the metal sublattice contains {2}/{11} vacancies on average. The refinement showed that the additional observed satellite reflections were almost entirely due to metal atom/vacancy ordering rather than sulfur atom displacement. Large anisotropic thermal parameters on both the sulfur and metal atoms were consistent with disordered displacement of these atoms by 0.2-0.4 Å normal to [111]. The chemical plausibility of the refined structure and the implication of this result for other substoichiometric NaCl-type solid solutions are discussed.
Chemistry of Materials, Jul 13, 2010
ABSTRACT We present, for the first time, the ionic conductivity properties of two different, but ... more ABSTRACT We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi38Mo7O78 and Bi37.5Mo7.5O78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi38Mo7O78 is more complex than previously reported, and that this compound is a 5 × 3 × 6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi37.5Mo7.5O78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements.
Ferroelectrics, 2004
ABSTRACT The strongest crystal chemical constraints are usually local and fix local parameters su... more ABSTRACT The strongest crystal chemical constraints are usually local and fix local parameters such as bond lengths, angles and even stoichiometries (although not necessarily orientations) in local co-ordination polyhedra. Surprisingly often, however, constraints of this sort are insufficiently severe or long range enough to lead to well-defined, conventional 3-d crystalline order. Instead quite distinct new types of “modulated structure,” commonly associated with the existence of sharp, highly structured, characteristic diffuse intensity distributions, can result.
Bulletin of the American Physical Society, Mar 5, 2015
Rigid Unit Modes; their energy and temperature dependence with aluminium phosphate ADAM BERLIE 1 ... more Rigid Unit Modes; their energy and temperature dependence with aluminium phosphate ADAM BERLIE 1 , The Bragg Institute, ANSTO, YUN LIU, Australian National University, DEHONG YU, GORDEN KEARLEY, The Bragg Institute, ANSTO, CHRIS LING, University of Sydney, RAY WITH-ERS, Australian National University -One of the problems within crystallography is the concept of the average structure where due to dynamical translations or librations the structure is not truly static. This type of behaviour is common within polyhedral based compounds such as AlPO 4 where, in this case, the rigid tetrahedra can move or tilt with respect to each other as a consequence of the instability of the 180 • Al-O-P bond. We explore the energy and temperature dependence of these modes using inelastic neutron scattering and heat capacity measurements as well as using computational modelling to assign the observed behaviour to these rigid unit modes.
Phil Mag Lett, 1987
The structural analysis by convergent-beam electron diffraction is described for a weak orthorhom... more The structural analysis by convergent-beam electron diffraction is described for a weak orthorhombic superlattice observed in both mineral and synthetic samples of NiAs. The three equivalent directions for the commensurate wavevector, q = ⅓(a* + b*), defined a domain structure within the hexagonal subcell. Within a single domain, the symmetry around the c axis was reduced from 6mm to 2mm.
Acta crystallographica Section B, Structural science, crystal engineering and materials, 2015
The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modul... more The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modulated δ-Bi2O3-related structure, in which the modulation vector ℇ `locks in' to a commensurate value of 1/3. The structure was refined in a 3 × 3 × 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 - xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 `tetrahedra of octahedra' and chains of corner-sharing WO6 octahedra along 〈110〉F directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with pre...
Upon heating, the 1:2 triple perovskites A 3 CoNb 2 O 9 (A ¼ Ca, Sr or Ba) each undergo well-defi... more Upon heating, the 1:2 triple perovskites A 3 CoNb 2 O 9 (A ¼ Ca, Sr or Ba) each undergo well-defined insulator to conductor phase transitions at 8,8, 8,126 and $325 1C, respectively. As the trend in the transition temperatures for these materials does not correlate with the size of the reported band gaps, neutron powder diffraction has been used to investigate if this change in electrical behaviour of the materials was due to a structural phase change. It was found that in the regions of the suspected phase transitions there were only slight perturbations of the structures, namely thermal expansion of the lattices and an apparent muting of the amplitude of the octahedral rotations in the A ¼ Ca and Sr compound at higher temperatures. r
J Solid State Chem, 2000
Cu/Co ordering in TlCo2-xCuxSe2, (x~1) has been investigated using electron and X-ray powder diff... more Cu/Co ordering in TlCo2-xCuxSe2, (x~1) has been investigated using electron and X-ray powder diffraction. The average structure of the x=1 compound can be indexed on an F-centered orthorhombic cell, a=5.5518(6) Å, b=5.4755(5) Å, and c=13.7387(17) Å. Additional weak satellite reflections characterized by the primary modulation wave vector qi~0.184a* and indicative of an incommensurate modulated structure are revealed by electron diffraction. Characteristic extinction conditions in conjunction with mmm Laue symmetry and known crystal chemical constraints imply a (3+1)-dimensional superspace group symmetry of Fmmm(alpha01)00s. The incommensurate modulation is attributed to occupational ordering of the two transition metal atoms and associated structural relaxation. The real space interpretation of the Cu/Co ordering pattern is that copper and cobalt atoms form separate layers in the structure at z={1}/{4} and {3}/{4}, respectively. These layers are periodically interchanged as one moves along the x direction. Two periods of this square-wave-like modulation is close to 11a in length but is non-rational compared with the average structure.
J Solid State Chem, 1999
The extent of the pyrochlore solid solution field (at 1500°C) in the (1−ε)ZrO2·εSmO1.5 system has... more The extent of the pyrochlore solid solution field (at 1500°C) in the (1−ε)ZrO2·εSmO1.5 system has been determined as running from 38.5 to 55 mol% SmO1.5. Subtle changes in crystal structure have been investigated as a function of composition across this solid solution field using a wide-angle convergent beam electron diffraction technique. Systematic changes in the one unknown oxygen atom fractional coordinate x are found as a function of composition and investigated from a crystal chemical point of view using the bond length – bond valence approach. The parameter x is found to be ∼0.352 for ε=0.40 and 0.348 for ε=0.45 and 0.55. Both values of x are significantly larger than the value (x=0.342) found for the stoichiometric 50 mol% SmO1.5 specimen by the same technique. The increased x value found for nonstoichiometric compositions on either side of the ideal pyrochlore composition (ε=0.5) is consistent with a systematic shift toward the average value for this parameter in the so-called “defect fluorite” structure (x=0.375), the phase field for which appears on either side of the relevant pyrochlore solid solution field.
Progress in Solid State Chemistry, 1998
J Solid State Chem, 2007
The (3+1)(3+1)-d incommensurately modulated structures of four members of the NiGe1-xPxNiGe1-xPx ... more The (3+1)(3+1)-d incommensurately modulated structures of four members of the NiGe1-xPxNiGe1-xPx solid solution field have been successfully refined from X-ray powder diffraction data (Rw(all)/Rwp=2.13/3.92Rw(all)/Rwp=2.13/3.92; 1.52/4.25; 1.27/3.44 and 2.00/4.03 for x=0.4x=0.4, 0.5, 0.6 and 0.7, respectively). The 4-d superspace group symmetry is Amam(00γ)s00 (Z=4Z=4; a=5.0468(2)a=5.0468(2), 5.0188(2), 4.9796(2) and 4.9651(1)Å; b=6.0636(3)b=6.0636(3), 6.0576(2), 6.0183(2) and 6.0031(1)Å; c=3.4877(2)c=3.4877(2), 3.4812(2), 3.4593(1) and 3.45442(7)Å; γ=0.7769(2)γ=0.7769(2), 0.7467(1), 0.7241(1) and 0.7046(1) for x=0.4x=0.4, 0.5, 0.6 and 0.7, respectively). The underlying average structure is of NiAs type while the (in general) incommensurate primary modulation wave-vector, γc*γc*, varies continuously and smoothly with composition. The two largest amplitude displacive atomic modulation functions (AMFs), for all samples, were the Ni displacement along b AMF and the Ge/P displacement along a AMF. The refined amplitude of the former was found to systematically increase with P content from 0.215Å for NiGe0.6P0.4NiGe0.6P0.4 to 0.294Å for NiGe0.3P0.7NiGe0.3P0.7 while the magnitude of the latter was found to increase with P content from 0.177Å for NiGe0.6P0.4NiGe0.6P0.4 to 0.253Å for NiGe0.3P0.7NiGe0.3P0.7. These displacive shifts significantly modulate the local crystal chemistry i.e. the local interatomic distances and co-ordination polyhedra. This continuously variable, incommensurately modulated, intermediate structure type is shown to provide a natural link or bridge between the two extreme end-member structures i.e. NiGe (of MnP structure type) and NiP by simply choosing the commensurate options with γ=1γ=1 and 12 respectively.The NiP (top) and NiGe (bottom) crystal structures. The structural relationship is emphasised by indicating the xsin[Ge/P] modulation wave in the 4-d space group setting Amam(00γ)s00; γ=1γ=1 for NiGe, γ=1/2γ=1/2 for NiP and 0.78<γ<0.700.78<γ<0.70 for NiGe1-xPx with 0.4>x>0.70.4>x>0.7.
Phys Rev B, 1982
The experimentally observed Raman scattering data on 2H-TaSe2 has been reanalyzed in the light of... more The experimentally observed Raman scattering data on 2H-TaSe2 has been reanalyzed in the light of the recent discovery that the commensurate superlattice has orthorhombic and not hexagonal symmetry. A recent model for the orthorhombic, commensurate phase can be consistent with the observed Raman spectra only if the interlayer interactions are weak. Similarly, the weakly interacting charge-density-wave picture is shown to be incorrect and, consequently, amplitude and phase modes are, in general, coupled.
J Solid State Chem, 2005
The structure of the pyrochlore-type oxide Bi 2InNbO 7 has been investigated between room tempera... more The structure of the pyrochlore-type oxide Bi 2InNbO 7 has been investigated between room temperature and 700 °C using electron and synchrotron X-ray powder diffraction and at room temperature and 10 K using neutron diffraction methods. Bi 2InNbO 7 exhibits an A 2B 2O 7 cubic pyrochlore-type average structure at all temperatures that is characterized by an apparently random mixing of the In 3+ and Nb 5+ cations on the octahedral B sites. The Bi cations on the eight-coordinate pyrochlore A sites are displacively disordered, presumably as a consequence of their lone pair electron configuration. Heating the sample does not alter this disorder.
Solid State Phenomena, 1989