We examine the physical behavior of dry (Li2O)x(B2O3)100-x glasses (LBG) and (Na2O)x(B2O3)100-x glasses (SBG) in Raman Scattering, Modulated-Differential Scanning Calorimetry, FTIR and Impedance Spectroscopy. Calorimetric data show the existence of a reversibility window in the 14% < x < 27% range for LBG and in the 20% < x < 40% range for SBG. In analogy to previous work we identify these with the rigid but unstressed Intermediate phase (IP).
Analysis of Raman scattering by mixed-ring vibrational modes permits accessing the Medium Range Structure (MRS) of glasses. These modes consist of a triad (705 cm-1, 740 cm-1, 770 cm-1) close to the Boroxyl ring mode (808 cm-1), and their scattering strengths are found to display maxima at specific x. Results show that Pentaborate (16%) and Tetraborate (20%) species form from Boroxyl rings as Li2O is alloyed in B2O3, and these MRSs contribute to the formation of the IP in LBGs. In SBGs, Raman scattering shows that Tetraborate (20%), Triborate (25%), Diborate (33%) and Tripentaborate (37%) species each contribute to the formation of the much wider IP.
In sodium-borate glasses, Electrical conductivity σ(x) increases by two orders of magnitude across the range of soda (0 < x < 45%) additive, displaying a bimodal behavior: a jump near the stress transition followed by a plateau in the IP. These observations are consistent with carrier mobility increasing pronouncedly as networks elastically soften with increasing modifier content.
Complex Cp data on LBG reported by S. Kojima, show that the stretched exponent, β(x), equals 1/3 at x= 0 (B2O3), but increases to 0.60 near x = 20%. The trap model of J.C. Phillips predicts stretched exponents to be given by β = d/(d+ 2), which for d = 2 (2D networks) give β = 0.50, and for d = 3 (3D networks) give 0.60, provided short- and long-range forces contribute to network relaxation. A β = 1/3 is expected, if only short range (covalent) forces contribute in a 2D network, as would be the case in pristine B2O3. The increase of β = 0.60 near x = 20% fits well to the case of a 3D network relaxing by both short range (covalent) and long range (ionic) forces. These data suggest that a network dimensionality crossover from a quasi 2D at x = 0 to 3D at x> 20% occurs in these glasses. Such dimensionality considerations dovetail well with constraint counting to understand the nature of the three elastic phases and the boson mode scattering strength. The three elastic phases in SBG are as follows: at x < 20% glasses are in the stressed-rigid phase, in the 20% < x < 40% range in the rigid but unstressed IP, while at x > 40% in the elastically flexible phase. These findings, to the best of our knowledge, are the first to identify the three elastic phases in LBG and SBG, and their molecular origin in terms of network medium range structures.