Local and Extended range molecular structures of (Na₂O)_x(P₂O₅)_100-x glasses probed by Raman Scattering and Infrared reflectance

Raman scattering and IR reflectance reveal striking similarities between crystalline (c-) and glassy (g-) NaPO₃. The c- and g- are each composed largely of chains of Q² species, a 4-fold P having 2 bridging (O_b) and 2 terminal (O_t) Oxygen neighbors, and display three vibrational features; an asymmetric stretch (as) of P-O_t (1300 cm^-1), a symmetric stretch (ss) of P-O_t (1172 cm^-1) and a ss of P-O_b (685 cm^-1). There are glaring differences as well, and these come from the fact that ss of P-O_t and P-O_b although quite narrow in the c- but are rather broad in the g-, and can be deconvoluted in terms of a triad of modes due to long chains (LC), large rings (LR) and small rings (SR) in the latter. These triad of modes are also observed in IR displaying complementary behavior, with modes of SR and LR, weakly excited in Raman but strongly in IR. These vibrational features are also observed at non-stoichiometric (x ≠ 50%) compositions ranging from 20%<x<61% and have permitted to elucidate the role of glass structure in stabilizing the three Topological phases. In the Intermediate Phase range¹ (37.5% < x < 46%), LC far exceed the count of LR and SR thus promoting the configurational entropy of that phase.
1. C. Mohanty et al., Front. Mater. 2019, 6, 1-20.