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John S O Evans

Publications and source records attributed to John S O Evans.

13 recordsLinked to original sources
PubMed2006

The 136-atom structure of ZrP2O7 and HfP2O7 from powder diffraction data.

There has been considerable debate in the literature about the true room-temperature structure of ZrP2O7 and related materials. In this article we describe how a combination of information from solid-state 31P NMR and powder diffraction data can be used to determine the structure of this 136 unique-atom material. The structure has been solved using a combination of simulated annealing and Rietveld refinement performed simultaneously on X-ray and neutron diffraction data. Despite the close to cubic metric symmetry of the material, we show how its true orthorhombic structure (space group Pbca) can be refined to a high degree of precision.

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PubMed2005

Negative thermal expansion in ZrW2O8: mechanisms, rigid unit modes, and neutron total scattering.

The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material has been investigated by reverse Monte Carlo (RMC) modeling of neutron total scattering data. We obtain, for the first time, quantitative measurements of the extent to which the and polyhedra move as rigid units, and we show that these values are consistent with the predictions of rigid unit mode theory. We suggest that rigid unit modes are associated with the NTE. Our results do not support a recent interpretation of x-ray-absorption fine structure spectroscopy data in terms of a larger rigid structural component involving the Zr-O-W linkage.

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PubMed2005

Characterization of oxygen dynamics in ZrW2O8.

The dynamics of oxygen motion in ZrW(2)O(8) have been characterized using (17)O solid-state NMR. Rates of dynamic exchange have been extracted from magnetization transfer experiments over a temperature range of 40 to 226 degrees C, and distinct values for the associated activation barrier have been observed on either side of the order/disorder phase transition at approximately 175 degrees C. A detailed model for the dynamical process is proposed, which reconciles the observation of continuing oxygen dynamics in the low-temperature phase with the static order implied by earlier X-ray diffraction studies.

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PubMed2005

A synthetic route to size-controlled fcc and fct FePt nanoparticles.

We report here a new synthetic route to FePt nanoparticles using a stoichiometric mixture of Na2Fe(CO)4 and Pt(acac)2. The structure of FePt nanoparticles, their size, chemical composition, and magnetic property can be controlled by various synthetic parameters, such as the solvent type, nature, and molar ratio of surfactants and stabilizers, synthesis temperature, and purification process. Partially ordered fct (L10) nanoparticles with room temperature magnetic coercivity can be synthesized directly in tetracosane solution at 389 degrees C. The fcc FePt synthesized in nonadecane can be transformed into the magnetically important fct phase at 430 degrees C without significant particle sintering.

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PubMed2005

Structural chemistry of A3CuBO6 (A = Ca, Sr; B = Mn, Ru, Ir) as a function of temperature.

Variable temperature X-ray and neutron powder diffraction techniques have been used to identify structural phase transitions in Cu-rich A(3)A'BO(6) phases. A transition from monoclinic to rhombohedral symmetry was observed by X-ray diffraction between 700 and 500 K in Sr(3)Cu(1-x)M(x)IrO(6) (M = Ni, Zn; 0 < or = x < or = 0.5). The temperature of the phase change decreased in a linear manner with Cu-content and was essentially independent of the nature of M. Ca(3.1)Cu(0.9)MnO(6) was shown to pass from a rhombohedral phase to a triclinic phase on cooling below 290 K; the structure of the triclinic phase was refined against neutron diffraction data collected at 2 K. Ca(3.1)Cu(0.9)RuO(6) undergoes a transition between a disordered rhombohedral phase and an ordered monoclinic phase when cooled below 623 K. Neutron diffraction has been used to determine the structure as a function of temperature in the range 523 < or =T/K < or = 723 and hence to determine an order parameter for the low temperature phase; the second-order transition is shown to be incomplete 100 K below the critical temperature.

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PubMed2004

Beyond classical applications of powder diffraction.

This article provides a short tutorial review of how laboratory powder diffraction methods can be used to establish a variety of "non-traditional" pieces of information about solid state materials-information beyond simple phase identification or structure determination at a specific temperature. The examples show how unique insights into synthetic pathways, reaction mechanisms, reaction kinetics, polymorphism, phase transitions and physical properties can be obtained for a variety of different types of materials.

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PubMed2004

The nature of oxygen exchange in ZrW2O8 revealed by two-dimensional solid-state 17O NMR.

17O magic angle spinning (MAS) NMR has been used to determine the nature of oxygen exchange in ZrW(2)O(8). A highly effective isotopic labelling technique has been developed and 1D NMR and 2D exchange spectroscopy (EXSY) experiments have revealed that mutual exchange occurs between all oxygen sites, even at temperatures considerably below the alpha to beta order-disorder phase transition.

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PubMed2004

Structures and phase transitions of trigonal ZrMo2O8 and HfMo2O8.

This paper describes the structures, thermal-expansion properties and phase transitions of the trigonal forms of ZrMo(2)O(8) and HfMo(2)O(8). Both phases adopt a P(-)3m structure at room temperature and show positive thermal expansion. Both phases also undergo a displacive phase transition at high temperature (ZrMo(2)O(8) at 487 K and HfMo(2)O(8) at 463 K) to a higher-symmetry structure that has lower thermal expansion. The structure of the high-temperature alpha'-AMo(2)O(8) form (A = Zr and Hf) has been refined from powder diffraction data in space group P(-)3m1.

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PubMed2003

Synthesis and characterisation of a new high pressure polymorph of Cu2WS4.

In this communication we report the synthesis and structural characterisation of a new body centred polymorph of Cu2WS4 prepared using hydrothermal methods. I-Cu2WS4 crystallises in space group I42m with cell parameters a = b = 5.44427(8), c = 10.0687(2) A and has a new structure type containing layers of edge-sharing CuS4 and WS4 tetrahedra.

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PubMed2003

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite.

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.

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