Showing papers by "Brendan J. Kennedy published in 1999"
TL;DR: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods as mentioned in this paper, and two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K, which is a remarkable feature of the results.
Abstract: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods. From inspection of the diffraction data two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K. A remarkable feature of the results is the regular variation in the out-of-phase octahedral tilt angle over the entire temperature range.
277 citations
TL;DR: In this paper, a modified SERS (surface-enhanced Raman scattering) substrate is used for detecting aromatic compounds, based on self-assembled monolayers (SAMs) formed from compounds such as alkanethiols.
Abstract: Modified SERS (surface-enhanced Raman scattering) substrates are based on self-assembled monolayers (SAMs) formed from compounds such as alkanethiols. Chain lengths ranging from ethanethiol to octadecanethiol were used to investigate properties of modified SERS substrates. These properties include determining the magnitude of the SERS electromagnetic enhancement, developing a sensitivity factor for detecting aromatic compounds, and evaluating SERS substrates for performance characteristics such as stability and solvent effects. A SERS electromagnetic enhancement was determined to be 2.1 × 103 for detecting aromatic compounds. The effects of experimental conditions on the SERS detection process were addressed. SERS sensitivity was shown to be highly dependent upon a correlation between the SERS interfacial distance dependence and a hydrophobic effect exhibited by the alkyl chain of the thiol. The distance dependence was more significant than the hydrophobic effect for detecting aromatic compounds. For comp...
247 citations
TL;DR: The high-temperature phase transitions of cubic perovskites have been studied using powder neutron diffraction and the Rietveld method as discussed by the authors, and the transition from the tetragonal to cubic phase is consistent with a continuous phase transition.
Abstract: The high-temperature phase transitions of ${\mathrm{SrZrO}}_{3}$ have been studied using powder neutron diffraction and the Rietveld method. The material is tetragonal $(I4/mcm)$ between \ensuremath{\sim}1020 and 1360 K. At higher temperatures (g1360 K) the structure was found to be the ideal cubic perovskite $(Pm3\ifmmode\bar\else\textasciimacron\fi{}m).$ From the neutron-diffraction data and the space-group assignments, the transition from the tetragonal to cubic phase is consistent with a continuous phase transition. The angle of tilt of the oxygen octahedron in the tetragonal phase is taken to be the order parameter and its temperature variation is typical of a tricritical phase transition.
232 citations
TL;DR: In this paper, the crystal structure of SrHfO{sub 3} has been studied at high temperatures using powder neutron diffraction and the Rietveld method and it has been shown that at higher temperatures the structure is the ideal cubic perovskite (Pm3{sup -}m).
Abstract: The crystal structure of SrHfO{sub 3} has been studied at high temperatures using powder neutron diffraction and the Rietveld method. From 300 K to approximately 670 K the structure of SrHfO{sub 3} is orthorhombic (Pnma). By 870 K the material adopts a second orthorhombic structure (Cmcm). The material then undergoes a further phase transition and is tetragonal (/4/mcm) from {approx}1000 to 1353 K. At higher temperatures (>1360 K) the structure is the ideal cubic perovskite (Pm3{sup -}m). The angle of rotation of the oxygen octahedron in the tetragonal phase is taken as the order parameter and its temperature variation is consistent with a second order phase transition.
135 citations
TL;DR: Veterinary anti-inflammatory Cu(II) complexes of indomethacin (1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid = IndoH) and isostructural H(2)O complexes were synthesized from different methods yet displayed different variable temperature magnetic susceptibity data.
Abstract: Veterinary anti-inflammatory Cu(II) complexes of indomethacin (1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid = IndoH), of the general formula [Cu2(Indo)4L2] (L = N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), and water), were studied by zero-field and X-band EPR spectroscopies, electronic spectroscopy, magnetic measurements, and X-ray powder diffraction. The complexes are similar to Cu(II) acetate monohydrate, with a strong antiferromagnetic exchange interaction, J, ranging from −141 to −152 cm-1. Variable temperature magnetic susceptibility data for all of the complexes are similar, with the exception of a [Cu2(Indo)4(H2O)2] complex, which displays an unusual increase in magnetic moment with decreasing temperature from 50 to 10 K. The X-ray powder diffraction patterns of the DMF and DMA dimers show that they are isostructural. Two isostructural H2O complexes were synthesized from different methods yet displayed different variable temperature magnetic...
128 citations
TL;DR: In this paper, the structural phase transitions in perovskite-like NaTaO3, from room temperature to 933 K, have been examined, and the structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes.
Abstract: Neutron powder diffraction has been used to examine the structural phase transitions in the perovskite-like NaTaO3, from room temperature to 933 K. The room temperature orthorhombic structure (Pbnm, a = 5.4768(1), b = 5.5212(1), c = 7.7890(2)) transforms to orthorhombic Cmcm at around 700 K, then to tetragonal P4/mbm at 835 K, and finally to cubic Pmm above 890 K. The structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes, the tilting of successive octahedra being out of phase along the b-axis, and in phase along the c-axis. The two tilt angles are comparable just above 700 K, but the out-of-phase tilt angle falls smoothly to zero as the transition to tetragonal is approached, in the manner suggestive of a tricritical transition. This results in an unusual variation of lattice parameters with temperature in the orthorhombic Cmcm phase. In the tetragonal phase the lattice parameters vary smoothly; however near the transition to cubic the in-phase tilt angle changes more rapidly with temperature than might be expected in a continuous phase transition.
118 citations
TL;DR: The influence of thermal annealing on cation disorder in ABi============ 2============Nb======€ 2======672€ 1======€ 9======€ 1.1€ 1€ 0.
Abstract: The influence of thermal annealing on cation disorder in ABi
2
Nb
2
O
9
(A=Sr, Ba) has been studied using synchrotron powder X-ray diffraction methods. BaBi
2
Nb
2
O
9
adopts a tetragonal (I4
1
/mmm) structure, whilst SrBi
2
Nb
2
O
9
is orthorhombic, space group A2
1
am, irrespective of the annealing conditions. In both complexes the A-type cations, Sr and Ba, are disordered over the perovskite and Bi
2
O
2
layers. Quenching the samples from high temperature results in a slight increase in both the cell volume and the degree of disorder, the extent of disorder being greater in the Ba complex. Variable temperature diffraction studies are also reported.
101 citations
TL;DR: In this paper, the perovskite crystal structure at room temperature and at elevated temperatures to 1673 K was examined and the orthorhombic structure was confirmed at both temperatures.
Abstract: Neutron powder diffraction has been used to examine the crystal structure of the perovskite , at room temperature, and then at elevated temperatures to 1673 K. The orthorhombic structure at room temperature (Pbnm, a = 5.5245(1) A, b = 5.4922(1) A, c = 7.7740(2) A) was confirmed. At temperatures from about 420 K the structure is rhombohedral, in space group . The rhombohedral structure is characterized by rotation of the oxygen-atom octahedra about the threefold axis, and compression of these octahedra parallel to the same axis. The rotation angle varies from 11.0 to , and the octahedral strain parameter (a measure of the compression) from 0.969 to 0.983 over the temperature range of this study. The question as to whether the two effects are coupled is considered but not yet resolved.
79 citations
TL;DR: Sr3MO4F has been synthesized forM=Al and Ga and shown by Rietveld refinement of neutron and synchrotron X-ray powder diffraction data to adopt a layered structure in which the oxygen and fluorine are ordered and located in different layers.
65 citations
TL;DR: The structure of defect pyrochlores has been investigated in this article, where it was shown that there is a contraction of the BiO 8 polyhedron in the metallic oxides due to the decreased stereochemical activity of Bi 6s electrons, as a consequence of their mixing with B d orbitals.
26 citations
TL;DR: The structure of cis-bis( l -alaninato)copper (II) {cis-[Cu( l-ala)2]} was determined using single crystal neutron diffraction techniques at both low (7 K) and ambient temperature as mentioned in this paper.
TL;DR: In this paper, the structure of spinel-type oxide CuRh2O4 has been determined between 25 and 650°C using powder neutron diffraction methods, and a first-order phase transition from a tetragonal to a cubic structure occurred at ca. 575°C.
TL;DR: In this article, the electron and nuclear density distributions of the stoichiometric pyrochlore, Y2Sn2O7 are obtained by the Maximum Entropy Method (MEM) using synchrotron and neutron powder diffraction data.
Abstract: Electron and nuclear density distributions of the stoichiometric pyrochlore, Y2Sn2O7 are obtained by the Maximum Entropy Method (MEM) using synchrotron and neutron powder diffraction data. It is recognized from the MEM electron density distribution that there exist relatively strong covalency between Sn-O(2) atoms to form Sn2O6 sublattice and weak covalency between Y-O(1) atoms to form Y2O sublattice. There is no covalent bond linkage between these two sublattices. From the nuclear densities, it is found that the thermal vibrations of Y and Sn atoms parallel to both the Y-O(1) and Sn-O(2) bond directions are rather restricted.
TL;DR: The structure of the Bi-doped pyrochlore, Bi 0.5 Nd 1.5 Ru 2 O 7 has been studied between 20 and 300 K using powder synchrotron diffraction methods as mentioned in this paper.