Direct evidence of rigidity loss and self-organization in silicate glasses

The Brillouin elastic free energy change between thermally annealed and quenched (Na2O)x(SiO2)1−x glasses is found to increase linearly for x > 0.23 (floppy phase), and to nearly vanish at x < 0.18 (stressed-rigid phase). The observed (x) variation closely parallels the mean-field floppy mode fraction f (x) in random networks, and fixes the two (floppy, stressed-rigid) elastic phases. In calorimetric measurements, the non-reversing enthalpy near the glass transition temperature Tg is found to be large at x < 0.18 and at x > 0.23, but to nearly vanish in the 0.18 < x < 0.23 range, suggesting the existence of an intermediate phase between the floppy and stressed-rigid phases. Silicate melts and glasses are important geophysical [1], optoelectronic [2] and microelectronic materials [3], and they find applications as window glass materials, optical fibres and thin-film gate dielectrics [3]. The functionality of materials often derives from their structures at different length scales. At a basic level, the molecular structure of sodium silicate (Na2O)x(SiO2)1−x glasses consists of a network of Si(O1/2)4 tetrahedra in which the addition of sodium oxide produces Si(O1/2)4−mOmNam (or Q 4−m in NMR notation) local units having m = 0, 1, 2 and 3 non-bridging oxygen (NBO) sites attached to Na+ ions. The addition of a few (10) mole per cent of Na2O lowers [4] the glass transition temperature, Tg, of the base (SiO2) material (1200 ◦C) sharply (to 600 ◦C) because of a loss in global connectivity as some Q3 units (m = 1) emerge [5] at the expense of Q4 (m = 0) ones. The sharp reduction of Tg destroys the mechanical equilibrium that prevailed [5, 6] in the pristine glass (x = 0), and drives alloyed glasses to become stressed-rigid (i.e., hyperstatic in the language of mechanical trusses). This is largely the case because the bond-bending constraint of bridging oxygen atoms that were intrinsically broken [6, 7] at 1200 ◦C becomes restored in the weakly alloyed glass as the Tg-values plummet to 600 ◦C. However, upon continued addition of Na2O the 0953-8984/05/324889+08$30.00 © 2005 IOP Publishing Ltd Printed in the UK 4889