A theoretical and experimental investigation of hetero- vs. homo-connectivity in barium silicates

Abstract

Barium silicates may be found in contact aureoles and are used in several important technologies (e.g. LEDs). The BaO-SiO 2 system stabilizes 13 crystalline phases with different silicate tetrahedral and connectivity profiles. Aside from phases composed of a single structural unit (isolated or homo-connected tetrahedra) one encounters the relatively rare case of heteroconnected tetrahedra in which varying proportions of several Q n species are linked together. Here, we analyze the 29 Si MAS NMR and Raman spectroscopic manifestations of the connectivities in seven barium silicates: Ba 2 SiO 4 , high-BaSiO 3 , Ba 4 Si 6 O 16 , Ba 5 Si 8 O 21 , Ba 6 Si 10 O 26 , high-BaSi 2 O 5 and sanbornite (low-BaSi 2 O 5 ). The structures and purity of these phases were confirmed by Rietveld refinement. From the Raman spectroscopic database on 144predominantly homo-connected crystalline silicates, the mean Q n mode frequencies (ν Qn ± 1σ) are found at 827.7 (±13.8) cm -1 for Q 0 , 905.3 (±22.1) cm -1 for Q 1 , 993.5 (±25.9) cm -1 for Q 2 , and 1068.4 (±17.6) cm -1 for Q 3 units. Experimentally, homo-connected barium silicates show good agreement with these values whereas the hetero-connected phases show a wider range of ν Q2 than of ν Q3 frequencies. While the 29 Si NMR chemical shifts of the barium silicates are in agreement with known structural trends, those measured for the Q 2 resonances remain essentially constant, which may be caused by the lattice distortion around the large Ba 2+ cations. To complement and rationalize experimental measurements, first-principles calculations, at the This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Vibrational Raman modes of alamosite (PbSiO3): Combining density functional theory calculations and in situ temperature experiments

“…In silicates, the vibrational normal modes in the highwavenumber region are primarily attributed to the symmetric stretching of Si-O bonds (A g ). [32][33][34] Our theoretical calculations of the monoclinic alamosite agree with the general assignment, further indicating the presence of antisymmetric vibrational modes lying at the same spectral region (Table 1). The existence of these B g components with low spectral intensities reveals the difficulty for assigning the vibrational modes in this region due to the complex arrangement of silicon tetrahedra within the crystal.…”

“…In silicates, the vibrational normal modes in the high‐wavenumber region are primarily attributed to the symmetric stretching of Si–O bonds ( ) 32–34 . Our theoretical calculations of the monoclinic alamosite agree with the general assignment, further indicating the presence of antisymmetric vibrational modes lying at the same spectral region (Table 1).…”