G. Neville Greaves

Zeolite collapse and polyamorphism

The phenomenology of zeolite collapse is outlined, drawing on recent synchrotron x-ray diffraction experiments and computer simulations of low density cage structures like zeolite A and zeolite Y. Attention is drawn to the importance of polyamorphism in destabilizing this type of microporous crystal and its role in order-disorder as well as amorphous-amorphous transitions, together with associated differences in entropy and density between polyamorphic phases and the precursor zeolite. Magic angle spinning NMR and inelastic x-ray scattering are used to highlight changes in structural order and mechanical rigidity between the starting zeolite and the final high density polyamorph. In conclusion, two-level systems detected with inelastic neutron scattering are described and their involvement in dictating the dynamics of the collapse of zeolitic cage structures.

Following the formation of nanometer-sized clusters by time-resolved SAXS and EXAFS techniques

Time-resolved in situ SAXS and XAS measurements were carried out to monitor the formation of nanoparticles of the sulfides of cadmium and zinc, from solutions containing he corresponding acetate, and thioacetamide under solvothermal conditions. Analysis of the SAXS data shows that particles of ca 5 nm in radius form within the first few minutes of the reaction and then grow uniformly to ca 20 nm over a period of two hours resulting in a highly mono-dispersed particle distribution. EXAFS data of the CdS particles also prepared by solvothermal methods and recorded at 20 K, support the formation of nano-meter sized particles.

Atomic and vibrational origins of mechanical toughness in bioactive cement during setting

Bioactive glass ionomer cements (GICs) have been in widespread use for ∼40 years in dentistry and medicine. However, these composites fall short of the toughness needed for permanent implants. Significant impediment to improvement has been the requisite use of conventional destructive mechanical testing, which is necessarily retrospective. Here we show quantitatively, through the novel use of calorimetry, terahertz (THz) spectroscopy and neutron scattering, how GICs developing fracture toughness during setting is related to interfacial THz dynamics, changing atomic cohesion and fluctuating interfacial configurations. Contrary to convention, we find setting is non-monotonic, characterized by abrupt features not previously detected, including a glass–polymer coupling point, an early setting point, where decreasing toughness unexpectedly recovers, followed by stress-induced weakening of interfaces. Subsequently, toughness declines asymptotically to long-term fracture test values. We expect the insight afforded by these in situ non-destructive techniques will assist in raising understanding of the setting mechanisms and associated dynamics of cementitious materials.

WAXS and NMR studies of intermediate and short range order in K2O-SiO2 glasses

Abstract Wide-angle X-ray scattering and 29 Si NMR have been employed to investigate the medium-range structure of xK2O–(1−x)SiO2 glasses, with x varying in the limits 5% 1.4 A −1 A −1 range. The peaks broaden below x=20%, and at the lowest K2O fraction, a bimodal line shape is found. This broadening is interpreted in terms of phase separation at low K2O fraction. The NMR spectra consist of several (usually three) Gaussian components assigned to the different Q species (SiO4 tetrahedra with different connectivity) present. All three components are uniformly deshielded as K2O is incorporated into the structure. The fraction of non-bridging oxygens (NBOs) derived from the distribution of Q species matches the value obtained from the overall composition, except for the x=17.41% sample, again indicating phase-separation at x

A metal-organic framework with ultrahigh glass-forming ability

We have discovered and clarified the ultrahigh glass-forming ability of the metal-organic frameworks—ZIF-62 [Zn(Im2−xbImx)]. Glass-forming ability (GFA) is the ability of a liquid to avoid crystallization during cooling. Metal-organic frameworks (MOFs) are a new class of glass formers (1–3), with hitherto unknown dynamic and thermodynamic properties. We report the discovery of a new series of tetrahedral glass systems, zeolitic imidazolate framework–62 (ZIF-62) [Zn(Im2−xbImx)], which have ultrahigh GFA, superior to any other known glass formers. This ultrahigh GFA is evidenced by a high viscosity η (105 Pa·s) at the melting temperature Tm, a large crystal-glass network density deficit (Δρ/ρg)network, no crystallization in supercooled region on laboratory time scales, a low fragility (m = 23), an extremely high Poisson’s ratio (ν = 0.45), and the highest Tg/Tm ratio (0.84) ever reported. Tm and Tg both increase with benzimidazolate (bIm) content but retain the same ultrahigh Tg/Tm ratio, owing to high steric hindrance and frustrated network dynamics and also to the unusually low enthalpy and entropy typical of the soft and flexible nature of MOFs. On the basis of these versatile properties, we explain the exceptional GFA of the ZIF-62 system.

Kinetics of Decelerated Melting

Abstract Melting presents one of the most prominent phenomena in condensed matter science. Its microscopic understanding, however, is still fragmented, ranging from simplistic theory to the observation of melting point depressions. Here, a multimethod experimental approach is combined with computational simulation to study the microscopic mechanism of melting between these two extremes. Crystalline structures are exploited in which melting occurs into a metastable liquid close to its glass transition temperature. The associated sluggish dynamics concur with real‐time observation of homogeneous melting. In‐depth information on the structural signature is obtained from various independent spectroscopic and scattering methods, revealing a step‐wise nature of the transition before reaching the liquid state. A kinetic model is derived in which the first reaction step is promoted by local instability events, and the second is driven by diffusive mobility. Computational simulation provides further confirmation for the sequential reaction steps and for the details of the associated structural dynamics. The successful quantitative modeling of the low‐temperature decelerated melting of zeolite crystals, reconciling homogeneous with heterogeneous processes, should serve as a platform for understanding the inherent instability of other zeolitic structures, as well as the prolific and more complex nanoporous metal–organic frameworks.

Pt nanoparticles decorated rose-like Bi2O2CO3 configurations for efficient photocatalytic removal of water organic pollutants

Pt nanoparticles decorated with rose-like Bi2O2CO3 configurations were synthesized via a simple photoreduction method at room temperature. The structure, morphology, optical and electronic properties, and photocatalytic performance of the as-prepared materials were characterized. Compared to pure Bi2O2CO3, the Pt/Bi2O2CO3 photocatalysts show better performance in decomposing RhB, BPA and OTC under visible light (λ > 420 nm). The enhanced photocatalytic activity of Pt/Bi2O2CO3 could be attributed to the modification in light absorption (λ > 420 nm) charge migration and the separation of photo-generated electrons (e−) and holes (h+). Free radical trapping experiments demonstrated that the main active species of the catalytic reaction are different in decomposing RhB and BPA.

Thermal collapse and hierarchy of polymorphs in a faujasite-type zeolite and its analogous melt-quenched glass

We examine the route of structural collapse and re-crystallization of faujasite-type (Na,K)-LSX zeolite. As the first step, a rather stable amorphous high density phase HDAcollapse is generated through an order-disorder transition from the original zeolite via a low density phase LDAcollapse, at around 790u2009°C. We find that the overall amorphization is driven by an increase in the bond angle distribution within T-O-T and a change in ring statistics to 6-membered TO4 (T = Si(4+), Al(3+)) rings at the expense of 4-membered rings. The HDAamorph transforms into crystalline nepheline, though, through an intermediate metastable carnegieite phase. In comparison, the melt-derived glass of similar composition, HDAMQ, crystallizes directly into the nepheline phase without the occurrence of intermediate carnegieite. This is attributed to the higher structural order of the faujasite-derived HDAcollapse which prefers the re-crystallization into the highly symmetric carnegieite phase before transformation into nepheline with lower symmetry.

Characterising density fluctuations in liquid yttria aluminates with small angle x-ray scattering

Small angle x‐ray scattering (SAXS) has been measured in the wavevector range 0.01<Q<1u2009A−1 for supercooled yttria‐alumina melts using a laser‐heated aerodynamic furnace. SAXS intensity rises gradually with temperature reflecting density fluctuations deriving from isothermal compressibility. With decreasing Q a minimum is located close to 0.1u2009A−1 at the foot of the inter‐atomic structure factor, below which SAXS rises, suggesting scatter from longer range fluctuating volumes.

Structure factor changes in supercooled yttria-alumina

Changes in the structure factor of yttria‐alumina liquids have been identified in the supercooled range. Different inter‐polyhedral configurations between AlO4 and YO6 groups distinguish low density and high density liquid phases. The coexistence of phases at high temperatures have been identified in simultaneous measurements of small angle x‐ray scattering.