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Abstract: Chameleon diamonds typically change color from greyish-green to yellow when they are heated or cooled (thermochromic behaviour) or kept in the dark (photochromic behaviour). Those diamonds are type Ia with A aggregates largely dominating the nitrogen speciation. They contain moderate to high amounts of hydrogen, in addition to some isolated nitrogen and traces of nickel. Their UV Visible spectrum comprises the continuum typical of type Ib material, the 480 nm band and a broad band centred around 800 nm. It is mainly modified in the red part of the visible spectrum when the color change occurs. We here propose an electronic model consistent with all observed color behaviours, based on the premise that yellow is the stable color whereas green is the metastable one. The chameleon effect is possibly linked to a nitrogen–hydrogen complex. [Copyright &y& Elsevier]

Abstract: Various compositions of solid solutions K3P(Mo1-xWx)12O40 (0x1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren–Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap Eg was found to increase exponentially from ∼2.5 to ∼3.30eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between Eg and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo1-xWx)12O40]3- anions on the basis of tight-binding electronic structure calculations. [Copyright &y& Elsevier]

The cerium borates o-CeBO3, m-CeBO3 and CeB3O6 have been shown to be isostructural to their lanthanum derivatives. From diffuse reflectance, electron energy loss spectroscopy (EELS) and band structure calculations, it has been evidenced that a Ce3+ 4f–5d transition is responsible for weak absorption peaks around 3.5 eV while the O2p–Ce5d charge transfer gives rise to a strong absorption around 7 eV. Starting from self-consistent full potential LAPW calculations, the dielectric tensors of the three compounds were computed and compared to experimental data. It results in a satisfactory fit between the observed and the calculated extinction coefficient k and the index of refraction n. [Copyright &y& Elsevier]