Raman scattering from fractals. Simulation on large structures by the method of moments

We have employed the method of spectral moments to study the density of vibrational states and the Raman coupling coefficient of large 2- and 3- dimensional percolators at threshold and at higher concentration. We first discuss the over-and under-flow problems of the procedure which arise when -like in the present case- it is necessary to calculate a few thousand moments. Then we report on the numerical results; these show that different scattering mechanisms, all {\it a priori} equally probable in real systems, produce largely different coupling coefficients with different frequency dependence. Our results are compared with existing scaling theories of Raman scattering. The situation that emerges is complex; on the one hand, there is indication that the existing theory is not satisfactory; on the other hand, the simulations above threshold show that in this case the coupling coefficients have very little resemblance, if any, with the same quantities at threshold.Comment: 26 pages, RevTex, 8 figures available on reques

Lithium ion conducting boron-oxynitride amorphous thin films : synthesis and molecular structure by infrared spectroscopy and density functional theory modeling

Li ion containing oxynitride amorphous thin films are promising materials for electrochemical applications due to their high ionic conductivity, mechanical stability and chemical durability. Here we report on the preparation of Li boron-oxynitride (LiBON) amorphous thin films by rf sputtering of Li-diborate and Li-pyroborate targets in nitrogen atmosphere. The materials produced were subsequently studied by infrared transmittance spectroscopy assisted by density functional theory calculations using representative Li boron-oxide and boron-oxynitride clusters. The combination of experiments and calculations allows us to propose accurate vibrational assignments and to clarify the complex infrared activity of the LiBON films. Both experimental and calculated spectra show that nitrogen incorporation induces significant structural rearrangements, manifested mainly by a change in boron coordination number from four to three, and by the formation of boron-nitrogen-boron bridges. The nature of boron-nitrogen bonding depends on the composition of the sputtering target, with an exponential relationship adequately describing the dependence of B-N stretching frequency on bond length. Besides bonding to two boron atoms by covalent bonds, the nitrogen atoms interact also with Li ions by participating in their coordination sphere together with oxygen atoms. Likely, boron-nitrogen bonding in LiBON films facilitates Li ion transport due to induced charge delocalization within the boron-nitrogen-boron bridges and reduced electrostatic interaction with the Li ions