Structural investigations of yNa2S + (1 − y)PS5/2 glasses using Raman and infrared spectroscopies

Abstract

yNa2S+(1−y)PS5/2 glasses have been prepared by melt quenching and two transparent glass forming regions exist, at 0.33≤y≤0.55 and at y=0.65. Further, stoichiometric polycrystalline compounds have been prepared by melting and slow cooling at compositions of y=0.33, 0.5, and 0.6. The short range order structures of the glasses and the polycrystals have been examined using both Raman and infrared (IR) spectroscopies. As expected from the meta-thiophosphate composition, y=0.33, NaPS3 consists primarily of chains of corner shared (NaS)PSS2/2, P2, units, but there is evidence of a small fraction of edge-shared tetrahedra that form dimers of composition Na2P2S6. For the y=0.5 pyrothiophosphate composition, Na4P2S7, as seen for other pyrophosphate stoichiometries, two P1 units, (NaS)2PSS1/2, are corner shared to form the overall Na4P2S7 composition. However, the Raman spectroscopy of this phase also shows that a small fraction of the Na4P2S7 groups lose the bridging sulfur between the two (NaS)2PSS1/2 units to create a small number of Na4P2S6 groups, identified as P1P groups, thereby creating small numbers of homoatomic PP (in the Na4P2S6 units) and SS (presumably in loose S8 units) bonds. At y=0.6, the orthothiophosphate structure, Na3PS4, is formed where all of the sulfurs are non-bridging and are terminated by Na+ ions. For all of these compositions, spectral evidence suggests that a limited fraction of the P is involved in PP bonds and that a small amount of P4Sx molecular cages which may contain P3, PSS3/2, and P:3, PS3/2, units may exist at all compositions. It is further found that each stoichiometric composition, y=0.33, 0.5, and 0.6, exhibits structural polymorphs, leading to a broader distribution of phosphorus sites and a tentative description of these polymorphs is presented. From these spectral assignments for the polycrystalline stoichiometric compositions, a simple structural model of the glasses has been developed using normal vibrational mode assignments to interpret the Raman and IR spectra of the glasses. As y increases from 0.33 to 0.65, the progressive formation and substitution of chain forming P2 groups, P1 dimers, and depolymerized P0s can account for the majority of P structures, where Pi denotes a P group with i number of bridging S atoms, across both the low and high Na2S glass forming ranges.