Structure of diopside, enstatite, and magnesium aluminosilicate glasses: A joint approach using neutron and x-ray...

Neutron diffraction with magnesium isotope substitution, high energy x-ray diffraction, and ²⁹Si, ²⁷Al, and ²⁵Mg solid-state nuclear magnetic resonance (NMR) spectroscopy were used to measure the structure of glassy diopside (CaMgSi₂O₆), enstatite (MgSiO₃), and four (MgO)_x(Al₂O₃)_y(SiO₂)_1-x-y glasses, with x = 0.375 or 0.25 along the 50 mol. % silica tie-line (1 - x - y = 0.5) or with x = 0.3 or 0.2 along the 60 mol. % silica tie-line (1 - x - y = 0.6). The bound coherent neutron scattering length of the isotope ²⁵Mg was remeasured, and the value of 3.720(12) fm was obtained from a Rietveld refinement of the powder diffraction patterns measured for crystalline ²⁵MgO. The diffraction results for the glasses show a broad asymmetric distribution of Mg-O nearest-neighbors with a coordination number of 4.40(4) and 4.46(4) for the diopside and enstatite glasses, respectively. As magnesia is replaced by alumina along a tie-line with 50 or 60 mol. % silica, the Mg-O coordination number increases with the weighted bond distance as less Mg²⁺ ions adopt a network-modifying role and more of these ions adopt a predominantly charge-compensating role. ²⁵Mg magic angle spinning (MAS) NMR results could not resolve the different coordination environments of Mg²⁺ under the employed field strength (14.1 T) and spinning rate (20 kHz). The results emphasize the power of neutron diffraction with isotope substitution to provide unambiguous site-specific information on the coordination environment of magnesium in disordered materials.