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

Author(s)

Moulton, Benjamin J. A. ; Gomes, Eduardo O. ; Cunha, Thiago R. ; Doerenkamp, Carsten ; Gracia, Lourdes ; Eckert, Hellmut ; Andres, Juan ; Pizani, Paulo S.

• Document type: Journal article
• Source: AMERICAN MINERALOGIST; v. 107, n. 4, p. 13-pg., 2022-04-26.

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 Si-29 MAS NMR and Raman spectroscopic manifestations of the connectivities in seven barium silicates: Ba2SiO4, high-BaSiO3 , Ba4Si6O16, Ba5Si8O21, Ba-6 Si10O26, high-BaSi2O5, and sanbomite (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 Q(n) mode frequencies nu(Qn) (+/- 1 sigma) are found at 828 (+/- 14) cm(-1) for Q(0), 905 (+/- 22) cm(-1) for Q(1), 994 (+/- 26) cm(-1) for Q(2), and 1068 (+/- 18) 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 nu(Q2) than of nu(Q3) frequencies. While the Si-29 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 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 Si-29 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. (AU)

• FAPESP's process: 16/18567-5 - Nuclei entrapment: the pre-nucleation structural ordering of homogeneous nucleating BaO-2SiO2 and CaO-SiO2 glasses
• Grantee: Benjamin John Albert Moulton
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 17/06649-0 - New Optical Materials based on Rare-Earth Doped Oxyfluoride Glasses and Ceramics: Structural Studies by Electron Paramagnetic Resonance Methods
• Grantee: Carsten Doerenkamp
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 13/07793-6 - CEPIV - Center for Teaching, Research and Innovation in Glass
• Grantee: Edgar Dutra Zanotto
• Support Opportunities: Research Grants - Research, Innovation and Dissemination Centers - RIDC
• FAPESP's process: 19/12383-8 - Study of structural and vibrational properties of glass and glass-ceramic materials via Raman scattering as a function of temperature and high hydrostatic pressures.
• Grantee: Thiago Rodrigues da Cunha
• Support Opportunities: Scholarships in Brazil - Post-Doctoral