The structure and dynamics of molecular crystals

Structural and dynamical properties of a number of molecular crystals have been investigated by coherent inelastic neutron scattering,Raman scattering and neutron powder diffraction. Measurements of the phonon dispersion relations in perdeuteronaphthalene have been assigned with the aid of a rigid molecule lattice dynamics calculation. The intermolecular forces were derived from semi - empirical potential functions which represented the interactions between non -bonded atoms. An attempt was made to improve the model by altering the potential function parameters to get a closer correspondence between observed and calculated frequencies. The method of total profile refinement has been used in the analysis of neutron powder diffraction data from perfluorodiphenyl, octafluoro- naphthalene and para- .diiodo and para -dibromo -tetrafluorobenzene. It has been shown that it is possible, by means of reasonable constraints, to refine a molecular structure involving a large number of atoms. The Rietveld program for structure refinement was used for perfluorodiphenyl but the constraint facilities of this program have been found to be inadequate for the lower symmetry systems of interest. The development and use of a new refinement program, embodying more versatile constraint facilities is described. The Raman spectrum of perfluorodiphenyl has been interpreted in terms of a simple polarisability model. The low frequency totally symmetric spectra are explained by the coupling of a lattice vibration with a torsional mode of the molecule. The phase transition in octafluoronaphthalene has been investigated by Raman scattering and neutron powder diffraction. The weight of experimental evidence points to a unit cell doubling with no change in space group symmetry. Lattice dynamics calculations support this, and indicate that an instability in a zone boundary acoustic mode, may be associated with the transition

Measures for diffusion of solar PV in selected African countries

This paper investigates how African governments are considering supporting and promoting the diffusion of solar PV. This issue is explored by examining so-called ‘technology action plans (TAPs)’, which were main outputs of the Technology Needs Assessment project implemented in 10 African countries from 2010 to 2013. The paper provides a review of three distinct but characteristic trajectories for PV market development in Kenya (private-led market for solar home systems), Morocco (utility-led fee-for service model) and Rwanda (donor-led market for institutional systems). The paper finds that governments’ strategies to promoting solar PV are moving from isolated projects towards frameworks for market development and that there are high expectations to upgrading in the PV value chain through local assembly of panels and local production of other system elements. Commonly identified measures include support to: local production; financing schemes; tax exemptions; establishment and reinforcement of standards; technical training; and research and development