Structure of (Na₂O)_<i>x</i>(P₂O₅)_{100–<i>x</i>} Glasses Probed in IR Reflectance and Raman Scattering

Modulated differential scanning calorimetry (MDSC) experiments on titled glasses reveal the enthalpy of relaxation at T_g to show a local minimum in the 32.5% < x < 42.5% range of soda, providing evidence of a reversibility window (or intermediate phase). Furthermore, trends in fragility index m(x) of melts deduced from complex C_p measurements using MDSC show a local minimum in the reversibility window. The molecular structure of these glasses is now probed in Raman scattering and IR reflectance. At the stoichiometric composition (x = 50%), the glass and crystal show the symmetric stretching (SS) and asymmetric stretching of P–O_t ( O_t = terminal O) in long chains to reveal complementary behaviors in Raman and IR Transverse Optic response. In the glass, the SS mode of P–O_t bond in chains (1166(1) cm^-1), characteristic of Q² species, and the SS mode characteristic of Q³ species (near 1302 cm^-1) are observed and systematically tracked with x in the 0 < x < 50% range. This has permitted extracting the fraction of the Qⁿ (n = 2,3) species as a function of x, which provides a powerful means to elucidate glass structure as in NMR¹.

¹ R.K Brow, JNCS 263 1 (2000).