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-SiO2 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 hetero-connected tetrahedra in which varying proportions of several Qn species are linked together. Here, we analyze the 29Si MAS NMR and Raman spectroscopic manifestations of the connectivities in seven barium silicates: Ba2SiO4, high-BaSiO3, Ba4Si6O16, Ba5Si8O21, Ba6Si10O26, high-BaSi2O5, and sanbornite (low-BaSi2O5). The structures and purity of these phases were confirmed by Rietveld refinement. From a Raman spectroscopic database of 144 predominantly homo-connected crystalline silicates, the mean Qn mode frequencies vQn(±1σ) are found at 828 (±14) cm-1 for Q0, 905 (±22) cm-1 for Q1, 994 (±26) cm-1 for Q2, and 1068 (±18) cm-1 for Q3 units. Experimentally, homo-connected barium silicates show good agreement with these values, whereas the hetero-connected phases show a wider range of νQ2 than of νQ3 frequencies. While the 29Si NMR chemical shifts of the barium silicates are in agreement with known structural trends, those measured for the Q2 resonances remain essentially constant, which may be caused by the lattice distortion around the large Ba2+ cations. To complement and rationalize experimental measurements, first-principles calculations at the density functional theory level have reproduced measured frequencies within a mean absolute deviation of <7 cm-1. Our work highlights how the results provided by 29Si NMR and Raman spectroscopies and ab initio calculations can be combined to rationalize the structure of complex systems. The present findings also shed light on the vibrational modes that may be used to track bond lengths in situ at extreme conditions and the behavior of homo-vs. hetero-connectivity, revealing clear implications for evaluating silicate glasses and melts where hetero-connectivity is the rule rather than the exception.