The field of viscousliquid and glassysolid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg,and a fourth dealing with "short time processes. " The first part tackles the high temperature regime T>Tg, in which the system is ergodic and the evolution of the viscousliquid toward the condition at Tg is in focus. The second part deals with the regime T∼Tg, where the system is nonergodic except for very long annealing times, hence has time-dependent properties (aging and annealing). The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for "superionic" conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Qmechanical response. The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, T g , and a fourth dealing with ''short time processes.'' The first part tackles the high temperature regime TϾT g , in which the system is ergodic and the evolution of the viscous liquid toward the condition at T g is in focus. The second part deals with the regime TϳT g , where the system is nonergodic except for very long annealing times, hence has time-dependent properties ͑aging and annealing͒. The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for ''superionic'' conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response.

The mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41-1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H 2 O and OH environments. The main mid-IR bands occur at 950-1100, 810-830, 660-670, and 440-450 cm −1 , consistent with single silicate chain structures. For the C-S-H samples, the mid-IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite-like structural environments of C/S ratios >1.2. The 950-1100 cm −1 group of bands due to Si-O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite-like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite-like domains cannot be determined from these data. A shoulder at ∼1200 cm −1 due to Si-O stretching vibrations in Q 3 sites occurs only at C/S ≤ 0.7. The 660-670 cm −1 band due to Si-O-Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near-IR region, the combination band at 4567 cm −1 due to Si-OH stretching plus O-H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si-OH linkages at C/S ratios greater than this. The primary Si-OH band at 3740 cm −1 decreases in a similar way. In the far-IR region, C-S-H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm −1 , indicating the presence of CaOH environments, even though portlandite cannot be detected by X-ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C-S-H, which is described.

Graphitic carbon nitride compounds were prepared by thermal treatment of C−N−H precursor mixtures (melamine C 3 N 6 H 9 , dicyandiamide C 2 N 4 H 4 ). This led to solids based on polymerized heptazine or triazine ring units linked by −N or −NH− groups. The H content decreased, and the C/ N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 °C due to increased layer condensation. The UV−vis spectra exhibited a strong π−π* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600−650 °C showed an additional absorption near 500 nm that is assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H 2 evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h −1 H 2 evolution with UV−vis irradiation, but this dropped to ∼0.23 μmol h −1 when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H 2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H 2 production rates (31.5 μmol h −1 ) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O 2 evolution was determined for RuO 2 -coated samples using the 300 W lamp with aqueous AgNO 3 solution as the sacrificial agent. The materials all showed production rates ∼9 μmol h −1 . A highly crystalline compound containing polytriazine structural units ((C 3 N 3 ) 2 (NH) 3 •LiCl) prepared in this study did not show measurable photocatalytic activity.

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High-pressure synthesis on an industrial scale is applied to obtain synthetic diamonds and cubic boron nitride (c-BN), which are the superhard abrasives of choice for cutting and shaping hard metals and ceramics. Recently, high-pressure science has undergone a renaissance, with novel techniques and instrumentation permitting entirely new classes of high-pressure experiments. For example, superconducting behaviour was previously known for only a few elements and compounds. Under high-pressure conditions, the 'superconducting periodic table' now extends to all classes of the elements, including condensed rare gases, and ionic compounds such as CsI. Another surprising result is the newly discovered solid-state chemistry of light-element 'gas' molecules such as CO2, N2 and N2O. These react to give polymerized covalently bonded or ionic mineral structures under conditions of high pressure and temperature: the new solids are potentially recoverable to ambient conditions. Here we examine innovations in high-pressure research that might be harnessed to develop new materials for technological applications.

Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material. At pressures larger than 10 GPa we observed pressure-induced amorphization. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous-amorphous transition is remarkably similar to that reported previously for water, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination approximately 5 (ref. 16).

The structure and electronic properties of single-layered carbon nitride graphenes are examined computationally with hybrid-exchange functionals in periodic density functional theory calculations. Unlike pure carbon graphene that provides a metallic nanomaterial, carbon nitride graphenes form semiconductors with band gaps ranging up to 5 eV. The band gap is sensitive to external perturbations that can be introduced chemically by adatom adsorption or physically by constraining the lattice parameter. Carbon nitride graphenes could possibly pave the way for a new range of smaller and faster transistors, as well as have useful sensing and actuating properties.