Epsilon sodium disilicate: a high-pressure layer structure [Na 2Si 2O 5

Abstract

Epsilon sodium disilicate (ε-Na 2Si 2O 5) synthesized at 7 GPa, 1100°C, for a 12-hr run time (M6/8 superpress, Edmonton) is orthorhombic with a = 5.580(1), b = 9.441(4), c = 8.356(3) Å, Pbc2 1, Z = 4, D x = 2.749 g · cm −3. The structure ( R = 4.0%) is based on a disilicate sheet of alternating six-membered rings of UUUUDD and DDDDUU SiO 4 tetrahedra in the (100) plane, with linking Na(1) five-fold coordinated to 2.57 Å and Na(2) four-fold coordinated to 2.55 Å: 〈Si(1)O〉 = 1.633 Å, Si(1) O nbr = 1.580 Å; 〈Si(2)O〉 = 1.623 Å, Si(2)O nbr = 1.571 Å. The structure is similar to that of β-Na 2Si 2O 5, the 1 bar, 610–700°C polymorph, that has rings of UDUDUD tetrahedra. Densification is accommodated largely by a decrease in dihedral (SiOSi) bond angles that range from 127.0 to 129.3°, compared with 135.1° to 137.1° for β-Na 2Si 2O 5 ( D x = 2.57 g · cm −3) and 138.9° to 160.0° for α-Na 2Si 2O 5 ( D x = 2.50 g · cm −3). The ε phase structure is consistent with 29Si MAS-NMR and Raman spectra, and supports our earlier suggestion that densification of silicate melts to moderate pressures is accommodated predominantly by a decrease in dihedral bond angle through crimping of ring structures and a decrease in the number of SiO 4 tetrahedra per ring.