The catalytic activity and reactivity of the charge-transfer complexes and the ion radical salts have been investigated by means of the ortho-parahydrogen conversion, the deuterized reaction and the hydrogen-deuterium equilibration. The predominant reaction on the aromatics-alkali metal ion salt occurred through a chemisorption mechanism. These findings were confirmed by the electrical conductivity and also the photoemission measurements of unexposed and hydrogen-exposed ion salts. Hydrogenase activity in the dry state is also discussed.
A series of measurements of Hall effect, thermoelectric power and electrical conductivity have been performed on the vitreous CdGexAs2 having a Ge content of 0.2, 0.3, 0.4, 0.6 and 1 mol respectively. The electrical conductivity has an exponential behaviour in the temperature range 185–500 K only for the 0.6 and 1 mol composition. It deviates from the exponential behaviour at lower temperatures for the 0.2, 0.3 and 0.4 compositions. The thermoelectric power is positive for CdGe0.2As2, negative for CdGe0.6As2 and CdGe1.0As2, whereas it changes sign for CdGe0.3As2 and CdGe0.4As2. The Hall coefficient is negative for alll compositions except for the CdGe1.0As2. The Hall mobilities differ widely for the different compositions and have the values of 1.5 × 10–2 to 1.3 × 10–1 cm2 V–1 s–1 at room temperature.
The hole mobility in rubrene, in the c-direction, has been determined using time-of-flight measurements to be 2 cm2 V–1 s–1 at 473 K. The transport mechanism may be explained in terms of a band model with shallow trapping. No interpretable electron transport data were obtained.
Frequency spectra and normal modes of vibration have been computed for vitreous silica. They have been calculated from atomic arrangements in physical models based on the random network theory. The positions of bands in the computed spectra agree well with observed features in the experimental infra-red and Raman spectra of the glass. Detailed analysis of the normal modes indicates that the bands at 1050, 750 and 400 cm–1 are associated with bond-stretching, bending and rocking motions, respectively, of the oxygen atoms. Atomic vibrations in the glass are, on the whole, less extended in space than the plane wave-like modes which prevail in perfect crystals. The spatial localization tends to be greatest at high frequencies and near band edges. If non-bridging oxygen atoms are present in the structure, the frequenncy spectrum exhibits an additional band of very intense localization, associated with bond-stretching vibrations of the non-bridging atoms.
The conductivities of single crystals and compressed samples of isocytosine, pentaerythritol, tetrolic acid and cetyl alcohol, all of which, on the basis of their structural properties, are likely to be protonic semi-conductors, have been measured under a wide range of conditions including inter alia variations in field strength, crystallographic direction, crystal thickness and ambient gas. The bulk conductivity of ultra-dry isocytosine is extremely low even at high temperatures: at 473 K σ[010] is 5.6 × 10–14 and σ[001] less than 8.3 × 10–14 ohm–1 cm–1. The surface conductivity of the ultra-dry solid is, however, significant; for compressed discs the measured conductivity at 473 K is ca. 10–11 ohm–1 cm–1, and there is a reproducible activation energy for conduction of 1.40±0.04 eV. In the presence of water vapour, the conductivity of isocytosine increases markedly, non-ohmic effects being evident and there are strong indications of protonic conductivity.Pentaerythritol, which displays anisotropy as between (101) and (001) faces in its surface conductivity, does not appear to be a protonic conductor even in the vicinity of its first-order transition temperature. Tetrolic acid shows no signs of protonic migration, and neither does cetyl alcohol at room temperature. The conductivity of cetyl alcohol rises sharply (a thousand-fold increase over a few degrees range) beyond 312 K, when rotation within the (001) sheets of hydroxyl group occurs, and the process is almost certainly protonic.
Inelastic cold neutron scattering experiments have been carried out on polycrystalline (hexagonal) and vitreous GeO2. Coherence effects are observed for both materials for ω [graphic omitted] 300 cm–1 and it has been possible to estimate the approximate form of two average acoustic branches of the spectrum for the polycrystal. For the glass the coherence effects may be interpreted in terms of wave vector conservation conditions operating in a similar manner to that in polycrystals. The approximate shape of two average acoustic branches has been estimated together with the frequency of a further branch near the equivalent of the Brillouin zone boundary. The data have also been examined on the basis of the incoherent approximation to determine the main features of the frequency distribution G(ω) for the glass at ω < 600 cm–1, in which region the most prominent peak is near 300 cm–1.The low frequency peaks in G(ω) account extremely welll for the low temperature heat capacity. The neutron scattering results have been compared with the infra-red and Raman spectra of vitreous GeO2 showing that the relation between G(ω) and the spectra is different in each case but that the infra-red absorption coefficient probably approximates reasonably well to G(ω), except below about 200 cm–1. A comparison has also been made with a theoretical frequency distribution for a model of vitreous GeO2. The neutron spectra for the glass show a peak at 13 cm–1, estimated to contain about 0.05 % of the total modes and attributed to resonance modes associated with some particular, but so far unidentified, structural defects.
The photo-induced electrical conductivity of a number of arylidene-1,3-indandiones was investigated in order to determine the effect of charge carrier generation on the intramolecular electron shifts. The variation in the polarity of the compounds was achieved by placing electron donor and electron acceptor substituents on the arylidene group. The comparison of dark and photoconductivity of 15 arylidene-1,3-indandiones was obtained on sandwich cells of polycrystalline layers. More detailed experiments with the better photoconductors were carried out with sublimed films and pressed pellets.The photoconductivity of benzylidene derivatives indicated that photoconductivity increases with polarity of the molecule and achieves a maximum value in p-dimethylaminophenyl compound (DMAPID). Photovoltaic cells, giving 1.17–1.24 V open circuit photovoltage, were prepared from DMAPID sublimed film sandwich cells. The spectral response, voltage dependence and temperature dependence of several photoconductors were investigated. Although the generation of light-induced charge carriers is facilitated by increasing the dipole moment, the dark conductivities remained unaffected by such structural modifications.
In the system Ge + As + Te, the heavy Te atom facilitates the formation of mesomeric pσ-bonding systems. The glass forming regions are thus small and depend strongly on the quenching conditions. As in the systems Ge + Sb + Se and Ge + As + Se all metallically conducting melts in the system Ge + As + Te solidify to a crystallline structure even when quenched in water. However, not all semiconducting melts belonging to this system become glassy under these conditions. As expected, the transition from melts solidifying to a glass structure to those solidifying to a crystalline structure is more gradual, as is also the transition from metallic to semiconducting melts. All the semiconducting melts become more or less metallically conducting at temperatures between 900 and 1000 °C. The transition can be described by a parabolical or a log log dependence on temperature.
The regions of glass formation and liquid-liquid immiscibility have been determined in the soda-baria-silica system. Immiscibility occurs in the high silica corner of the system, and extends over a continuous region from the BaO—SiO2 to the Na2O—SiO2 limiting binaries. In an extensive region close to the glass-forming boundary and including mostly the barium disilicate field, crystallization occurred by homogeneous nucleation to produce a glass ceramic. Some of these compositions also showed metastable immiscibility. Nucleation, growth and coarsening of the crystals were studied; first, a metastable high-temperature polymorph of barium disilicate appears growing as spherulites, but on prolonged heating a lathe-like structure appears.
The vibrating condenser method was applied for measurement of the contact potential difference with organic crystals. It gave work functions of reasonable values for aromatic hydrocarbon crystals. From the work function, combined with other observable parameters, an energy diagram for the crystals can be constructed, which indicates that the location of the Fermi level is slightly different from that of the intrinsic level. Supplementary experiments on the photoelectromotive force due to the Dember effect and the photovoltaic effect in violanthrene crystal, gave results consistent with the above conclusion, indicating the hole conduction.
Hydroxyl groups on the surface of metal oxides are amphoteric in character. From experiments with anatase, rutile, η-alumina, α-Fe2O3,CeO2, and SnO2, half the OH groups are acidic in character, the other half are mainly basic and may be exchanged for other anions. This behaviour is explained by the structure of the hydroxylated surface. Acidity and basicity depend on the nature of the oxide. Several reactions are described for the determination of total OH content as well as the quantities of acidic and basic OH groups.
Adsorption and catalysis at the surface of metal oxides is directly related to the electronic constitution in the vicinity of the surface not merely in terms of the predominant electronic character but more importantly the complex assembly of trapping electronic states. Experimental techniques have been developed using both thermally stimulated electron current measurements and thermal glow curves to establish in detail the complex distribution of electronic states. They can lead to quantitative information on both the position and occupancy of trapping states. Preliminary correlation has also been established between trapping states and simple adsorption phenomena. Detailed characteristics have been established for various samples of very high purity zinc oxide and TiO2 including a study of the effect of controlled impurities at a level of a few p.p.m. and variations due to particle size. Direct correlation is made with the adsorption of gases, simple catalytic reactions, the interaction with binders used in electron photography and general characteristics of electron photographic coatings.
It is frequently stated that amorphous semiconductors have, in contrast to crystals, a value of the conductivity which is relatively insensitive to composition. This is explained by assuming that each atom in a glass has as many nearest neighbours as the number of bonds it can from (Ge, 4; As, 3; Te, 2), so that there are no free electrons available to carry a current. The validity of this concept will be examined; it is not true for some amorphous films (Mg-Bi) which are not strongly bonded. Also some glasses, when heated above the softening point, seem to change their coordination numbers and become metallic.The theoretical models necessary to describe these results are outlined. In liquid metals and most amorphous metal films, the Ziman theory should be applicable, giving a conductivity equal to Se2L/12π3ħ, where S is the Fermi surface area and L the mean free path. When this is about 3000 Ω–1 cm–1, L is comparable with the distance between atoms and it cannot be smaller. For materials such as liquid Te for which the conductivity is lower, a "pseudogap" affects the conductivity. The lowest possible metallic conductivity is about 200 Ω–1 cm–1. For materials (liquids or non-crystalline solids) with lower conductivity, the current is due either to electrons excited to the "mobility shoulder" or to hopping conduction of the kind familiar in impurity conduction. A real gap (as contrasted with a pseudogap) must exist in transparent materials, and can be understood in terms of the tight-binding approximation.
Field-induced switching between two impedance states has been observed in both pure and doped organic thin films. Aromatic hydrocarbon films display this phenomenon reproducibly when a mobile electrode material such as gallium-indium alloy is employed. Controlled doping of the hydrocarbons with electron acceptors leads to reproducible switching characteristics which are not electrode dependent. For the doped films, activation energies of conduction typical of bulk charge-transfer complexes characterize the low impedance states whereas the activation energies of the high impedance states are typical of organic insulating films. It is suggested that the switching mechanism involves the formation of conducting filaments.
Electrical and magnetic properties of poly(phenyleneaminochloranils, (PPhCh) obtained from different aromatic diamines (donors) and chloranil or quinone (acceptors) have been investigated. For each PPhCh, the current-voltage characteristics at different temperatures were recorded and Ea, σ293 and σ0 values were determined. Some of the (log10I,ƒ(1/T)), curves showed a kink and lead to two Ea values. For high temperatures the compensation law was fulfilled and a single value of the parameters α and β was calculated for all the PPhCh investigated. The characteristic temperature Tc does not correspond to the Tk value, nor do the Ea values correspond to the 1E1 value of the repeating unit. It is probable that the Ea values are modified by trap levels distributed throughout the sample. In support of this view, the dependence of Ea on voltage was investigated, and the trap density for unit energy was found. The magnetic properties, i.e., the susceptibility χ0 related to free carriers, depend on polymer structure. The χ0 values decrease with decrease of donor polarity and increase when the electron affinity of acceptors increases, thus enhancing the electrical conductivity.
A technique for the measurement of the Hall effect Faraday-type rotation at microwave frequencies is described. The essential features of this electrodeless technique are the employment of a bimodal resonating cavity and the fact that intra- and inter-crystalline defects can be ignored. A new approach of possible significance for measurements on hydrated biological materials is proposed, by which the perturbations of a resonating cavity on insertion of a thin, small dielectric specimen may be formulated to include perturbations higher than the first order. The theory of the Hall effect Faraday-type rotation relevant to biological materials, where the effect of displacement currents associated with loosely bound water must be included, is outlined. Hall mobility results are given for some biomacromolecules, rat liver mitochondria and spinach chloroplasts.
The surfaces of fine powder samples of MgO, CaO and SrO have been characterized by the diffuse reflectance technique over the wavelength range 200–2000 nm. Spectra due to F+s centres (electron in surface anion vacancy), SH centres (F+s centre with nearby —OH group) and adsorbed O–2 species have been identified. In powders which have been freed from surface contaminants fluorescences exist which can be quenched by O2, H2O or CO2. These fluorescences are interpreted as due to the presence of surface states which create an “effective” band gap at the oxide surfaces at significantly lower energies than the band gaps typical of the bulk oxides.
Since Eugène Houdry discovered some clays to be excellent catalysts for the "cracking" of heavy oils, the surface properties of silica-alumina combinations have been studied in various places and from various angles. The surface properties of silica-alumina-combinations differ in many ways from those of silica or of alumina, as such. Three different procedures for preparing the silica-aluminas may be mentioned, viz., (a) adsorption (chemisorption) of aluminium hydroxide on a wet silica surface, (b) precipitation of aluminium hydroxide on a wet silica gel, (c) co-gelling of a silica acid-and an aluminium salt solution. The influence of the alumina adsorption on the specific surface area, as measured with the B.E.T.-method and with the lauric acid method will be discussed. An attempt is made to give a picture of the binding of aluminium hydroxide on the silica, which leads to three possibilities, depending on the relative amount of both constituents. One of these possibilities leads to preparations with an acid character.
It is proposed that photoionization of molecules may be studied by observing the photo-conductivity induced by them in matrices of simple molecular crystals with wide band-gaps. Photoconductivity has been observed in the near ultra-violet in dilute solid solutions of iodine in carbon dioxide at 77 K and in a series of substitued anilines in xenon at 40 K. Both the wave-length and light intensity dependence of the photoconductivity differ considerably from those of either the pure solutes or the pure matrices.
