Michael J. McKelvy

Characterization of Pt microcrystals using high resolution electron microscopy

Abstract Small colloidal particles of platinum have been produced with and without a protective agent (polyvinylpyrrolidone), and have been studied by high resolution electron microscopy. Initially, the protected particles have been found to be single crystals in the size range 5–30 A. After continued beam exposure, the protective coatings effect is lost and the particles coalesce, forming both larger single crystals and multiply-twinned crystals. In contrast, a sample of the unprotected particles shows extensive initial linking and twinning, but no further beam-induced agglomeration.

In-situ nanoscale observations of the Mg(OH)2 dehydroxylation and rehydroxylation mechanisms

Environmental transmission electron microscopy has been used to probe the mechanisms that govern Mg(OH)2 dehydroxylation and rehydroxylation processes at the near-atomic level. Dehydroxylation and rehydroxylation rates for these in-situ observations were controlled by regulating the water vapour pressure over the sample. Generally, the dehydroxylation proceeded via the nucleation and growth of an oxide lamella, resulting in the formation of oxide and/or oxyhydroxide regions within the reaction matrix. Competition between rapid-nucleation–slow-growth and slow-nucleation–rapid-growth mechanisms can dramatically impact the nanostructure formed during dehydroxylation. Steps, both parallel and perpendicular to the {0001} planes, were observed to form during dehydroxylation. The nanocrystalline MgO formed was highly reactive and readily rehydroxylated with increasing water vapour pressure. Rehydroxylation proceeded via the nucleation and growth of Mg(OH)2 crystals in the heavily dehydroxylated matrix. The partial edge dislocations formed (both parallel and perpendicular to {0001}brucite) as the result of Mg(OH)2 nanocrystal intergrowth and anneal out with time, resulting in the formation of relatively large single crystals of Mg(OH)2. Such high mobility of Mg-containing species during rehydroxylation can be directly associated with the high chemical reactivity observed during rehydroxylation, which can facilitate key reaction processes, such as CO2 mineral sequestration.

Conditions variation and procedural improvement for hydrothermal crystal growth of oxides under highly oxidizing conditions

Abstract The hydrothermal technique for the growth of small single crystals of the higher oxides of the rare earths has been investigated with a view toward increasing crystal size and perfection. Conditions such as the substrate: mineralizer ratio, pressure cycling, capsule size and charge concentration, depressurization rate and temperature, thermal gradient and initial and final growth pressure were investigated.

Externally controlled pressure and temperature microreactor for in situ x-ray diffraction, visual and spectroscopic reaction investigations under supercritical and subcritical conditions

A microreactor has been developed for in situ, spectroscopic investigations of materials and reaction processes with full external pressure and temperature control from ambient conditions to 400 °C and 310 bar. The sample chamber is in direct contact with an external manifold, whereby gases, liquids or fluids can be injected and their activities controlled prior to and under investigation conditions. The microreactor employs high strength, single crystal moissanite windows which allow direct probe beam interaction with a sample to investigate in situ reaction processes and other materials properties. The relatively large volume of the cell, along with full optical accessibility and external temperature and pressure control, make this reaction cell well suited for experimental investigations involving any combination of gas, fluid, and solid interactions. The microreactor’s capabilities are demonstrated through an in situ x-ray diffraction study of the conversion of a meta-serpentine sample to magnesite un...

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Abstract Ionic conductivity of KBrxI1−x(0⩽x⩽1) mixed crystals, σx, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σx, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBrxI1−x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σx/σ1 (σ1 refers to KBr) have been suggested.

Cooperative Jahn-Teller distortion in PrO2

We report neutron diffraction data on single crystal PrO2 which reveal a cooperative Jahn-Teller distortion at TD=120±2 K. Below this temperature an internal distortion of the oxygen sublattice causes the unit cell of the crystallographic structure to become doubled along one crystal axis. We discuss several possible models for this structure. The antiferromagnetic structure below TN=13.5 K is found to consist of two components, one of which shares the same doubled unit cell as the distorted crystallographic structure. We also present measurements of the magnetic susceptibility, the specific heat capacity, and the electrical conductivity of PrO2. The susceptibility data show an anomaly at a temperature close to TD. From the specific heat capacity data we deduce that the ground state is doubly degenerate, consistent with a distortion of the cubic local symmetry. We discuss possible mechanisms for this. The conductivity shows an activated behavior with an activation energy Ea=0.262±0.003 eV.

High-resolution electron microscope analysis of {100} twinning in β-rhombohedral boron

Abstract {100} twinning in β-rhombohedral boron formed during rapid cooling of a multiphase mixture was reexamined by high-resolution electron microscopy. Comparison of observed and calculated images shows that the twin boundaries (which pass through the center of the B 84 units in the structure) occur in pairs, separated by one to five unit cells of the twinned orientation. Twinned regions are typically separated by 100 to 500 A. The twin boundary pairs may also be described as stacking faults in the pseudocubic close-packed arrangement formed by the B 84 units, which are pseudospherical in symmetry.

Ionic conductivity of KBr-KI mixed crystals

Abstract Ionic conductivity, σ x , of single crystals of KBr x I 1− x solid solutions (0 ⩽ x ⩽ 1) has been measured at temperatures from 558 K to close to their melting points. There is a good correlation between the phase diagram and the variation in σ x . The composition x exhibiting the minimum in solidus temperature corresponds to the maximum in ionic conductivity. The increase in σ x as a function of x has been calculated in the intrinsic region and agrees well with the experimental results. The effect of homovalent substitution in mixed crystals of alkali halides is discussed.

Influence of static Jahn-Teller distortion on the magnetic excitation spectrum of PrO2: A synchrotron x-ray and neutron inelastic scattering study

A synchrotron x-ray diffraction study of the crystallographic structure of PrO{sub 2} in the Jahn-Teller distorted phase is reported. The distortion of the oxygen sublattice, which was previously ambiguous, is shown to be a chiral structure in which neighboring oxygen chains have opposite chiralities. A temperature dependent study of the magnetic excitation spectrum, probed by neutron inelastic scattering, is also reported. Changes in the energies and relative intensities of the crystal field transitions provide an insight into the interplay between the static and dynamic Jahn-Teller effects.

Mercury sublattice melting transition in the misfit intercalation compound H1.24TiS2

Abstract Mercury can be intercalated into TiS 2 to form a compound, Hg 1.24 TiS 2 , which exhibits novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer, and the formation of incommensurate Hg chains in the guest galleries. Herein, X-ray powder diffraction (XPD) and differential scanning calorimetry (DSC) have been used to determine the effects of temperature on the Hg 1.24 TiS 2 structure from ambient temperature to 500 K. DSC studies reveal the presence of a reversible thermal transition near 473 K. The XPD patterns taken below the transition temperature are all characteristic of the ambient temperature structure together with modest sublattice thermal expansion with increasing temperature. However, above the transition temperature, all of the reflections uniquely associated with the Hg sublattice disappear, while the positions and intensities of the (001) reflections confirm the Hg remains intercalated. Thus, above the 473 K transition the in-plane Hg-sublattice structure and the associated intercalant Hg chains have melted to form guest layers with liquid-like disorder. The evolution of the host and Hg sublattice cell parameters as a function of temperature exhibits the expected discontinuous behavior associated with such a first-order transition.