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of Mg-containing crystals, suggests a six-coordination of Mg2+ ions. Both Na+and Mg2+-motion bands were found to upshift in frequency and grow in intensity upon increasing the cation content in binary glasses. In ternary glass systems with constant amount of total modifier the frequency of the Mg2+ band was found to exhibit a nonlinear behavior.The cation dependence of the far-IR frequency maxima was successfully modeled via a Born-Mayer ionic potential. Thus, an effective ionic charge of the cation-site interactions defined by <7cff = {qcq.\)]/2 was calculated from the experimental cation-motion frequencies. The same quantity, qei!, squared was demonstrated to scale with the integrated cation-motion band intensity (A). Thus, the latter becomes another very sensitive probe for the ionic cation site interactions. The slope of the plot of {A) vs. v2 appears the same for all binary and ternary systems studied so far and may be of universal value.The composition dependence of qett was taken to indicate changes in the ionic charge density relevant to the cation-site interactions. In the case of Na+ such changes can only be viewed within the frame of purely ionic interactions. However, the observed minima in qti{, related to Mg2+-oxygen site interactions, may as well originate from covalent contributions. Even if these observed reductions in qti{ are solely due to partial covalency, between magnesium and oxygen, the character of the magnesium-oxygen site interactions was found in all cases to be primarily ionic.Understanding the changes in qtñ, on the basis of the nature of the oxygen sites, requires structural knowledge of the boronoxygen network which will be addressed in the following paper.Acknowledgment. The authors are grateful to Professor C. A. Nicolaides of the National Hellenic Research Foundation for his support throughout this work. G.D.C. thanks Professors W. M. Risen of Brown University and C. A. Nicolaides for making his collaboration to this work possible. This project has been financially supported by the National Hellenic Research Foundation.

This study reports Raman and infrared spectra of hybrid organic-inorganic MAPbX3 perovskites (MA = CH3NH3, X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were recorded at three laser wavelengths (514, 785 and 1064 nm) under on-and offresonance conditions, as well as at room temperature and 100 K. Use of different excitation wavelengths allowed the unambiguous acquisition of 'true' Raman spectra from the perovskites, without degradation or photo-induced structural changes. Low frequency PbX vibrational modes were thoroughly identified by comparison of Raman and far-IR results. Red Raman frequency shifts for almost all MA vibrations from 200-3200 cm -1 , particularly intense for the torsional mode, were observed towards heavy halide derivatives, indicative of strengthening the interaction between halides and the organic cation inside the inorganic cage. Different MA-X bonding schemes are evidenced by torsional mode pairs emerging at the orthorhombic phase.MAPbBr3 was further characterized by variable temperature Raman measurements (100-295 K).Broadening of the MA rocking mode slightly above the tetragonal I to II phase transition is connected with disorder of the MA cation. Our results help understanding perovksite materials properties (ferroelectric domain formation, anomalous hysteresis) and their use as efficient light absorbers in solar cells. information is available free of charge via the internet at at

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The infrared-reflectance spectra of silica glasses were measured and analyzed with a Kramers-Kronig transformation for a better understanding of the response of silica, especially in the high-frequency region (1000 -1300 cm '). The silica samples were prepared by the sol-gel technique and subsequently subjected to heat treatments at selected temperatures to induce the formation of structures with a variable connectivity of the silicate network. The infrared reAectivity was found to increase with heattreatment temperature. This was attributed to glass densification, resulting from condensation of Si-OH groups into Si-0-Si network bridging units. This densification mechanism was also confirmed by Raman measurements.The profiles of the high-frequency parts of the transverse-optic (TO) and longitudinaloptic (LO) spectra were found consistent with the presence of two coupled modes (AS& and AS&) for the asymmetric stretching of Si-0-Si bridges, as proposed in previous works. Along these lines, the shoulder at -1200 cm in the infrared-absorption spectrum of silica, which is particularly enhanced in the spectra of silica gels, can be understood as arising from the TO response of the AS2 mode. The LO-TO splitting of the various infrared-active modes was found to depend on heat-treatment temperature. This efFect was attributed to variations of silicate network connectivity with temperature, which is presurnably a8'ecting the range of the Coulombic forces inducing the LO-TO splitting.