Experimental Evidence for an Inorganic Molecular Glass

Abstract

Inelastic neutron-scattering experiments have been performed for the first time on glassy phosphorusselenium alloys. The effective density of vibrational states so obtained is highly structured in the case of glassy Po.67Seo.33 and provides evidence for the existence of molecular clusters in the structure based predominantly on P4Se3 units, i.e., a "zero-dimensional" glass. PACS numbers: 61.40.+b, 61.12.Ex, 63.50.+XThe atomic-scale structure of glasses, particularly concerning medium-(or intermediate-) range order, is still a very contentious issue. One useful way of approaching this problem for the case of inorganic glasses is in terms of the local network dimensionality. 1 Thus, a "zerodimensional" (OD) structure corresponds to a molecular glass whose structure is (ideally) composed entirely of discrete molecular species. 2 The interest in such materials lies in the fact that for these the local short-range structure is well defined and so its influence on the medium-range order can be assessed. However, until now there has been no direct experimental evidence for the existence of such stable OD structures in the bulk glassy phase for inorganic materials. There is evidence, e.g., from photoemission, 3 that, for example, evaporated thin films of As x Six are composed primarily of AS4S4 molecular clusters; however, these films are extremely unstable and, upon annealing or exposure to light, polymerize readily to form a cross-linked network very similar to that of the bulk glass. In addition, sharp peaks have been observed in the Raman spectra of certain inorganic bulk glasses, for example B2O3 (Ref. 4) and GeSe2 (Ref. 5), perhaps indicative of decoupled molecular modes; however, the vibrational densities of states (VDOS) for B2O3 (Ref. 6) and GeSe 2 (Ref. 7) do not exhibit correspondingly sharp intense features, indicating that molecular characteristics, if responsible, are not a dominant structural motif.It is interesting, therefore, that recently Price and coworkers 8,9 have suggested, on the basis of (elastic) neutron-scattering, extended x-ray absorption fine structure, and Raman scattering results on selenium-rich phosphorus selenide glasses, that the system P x Seix might be a possible candidate for such OD amorphous structures. With this in mind, we have undertaken the first inelastic neutron-scattering studies of P x Seix glasses, in particular, investigating the previously unstudied phosphorus-rich compositions. (In addition, elastic neutron diffraction experiments have been carried out on the same compositions. 10 ) The motivation behind such studies has been to obtain information on the VDOS, unaffected by the matrix-element effects which occur in Raman scattering. A signature of vibrationally decoupled molecular clusters comprising a molecular (OD) glass is expected to be a vibrational spectrum exhibiting characteristically narrow peaks corresponding to the individual vibrational modes of the clusters, unbroadened by coupling of these modes to the rest of the structure.The PxSei-* system forms glasses...

GeAs sulfide glasses with unusually low network connectivity

“…As these molecular species are not cross-linked, materials such as glassy Ge 3 As 52 S 45 are characterized by extremely low network connectivity and, as such, are good examples of zero dimensional network glasses. Other non-cross-linked molecular species, including S 8 , P 4 S 10 , P 4 S 9 , P 4 S 3 , P 4 Se 3 and As 4 S 4 , have previously been identified in chalcogenide glasses [11][12][13][14][15][16][17]. However, the only other examples of zero dimensional networks in vitreous chalcogenides, i.e.…”

“…glassy As 10 S 90 [18] and P-rich selenide glasses, e.g. P 67 Se 33 , which is comprised in part by cagelike P 4 Se 3 molecules that are analogues of the present As 4 S 3 species [15]. It might be argued that, for the former compositions, low network dimensionality is expected in view of their low nominal hri value that approaches 2.0.…”

Molecular Dynamics in Supercooled P−Se Liquids near the Glass Transition: Results from 31P NMR Spectroscopy

“…7,8 Subsequently and more recently we have shown the existence of molecular pseudobinary As-S glasses doped with Ge in the Ge-As-S system with compositions near Ge 3 As 52 S 45 that are composed predominantly of As 4 S 3 molecules, held together by van der Waals forces. 9 The As 4 S 3 molecules are topologically similar to P 4 Se 3 molecules with a cagelike structure and a high degree of sphericity.…”

Molecular structure, glass transition temperature variation, agglomeration theory, and network connectivity of binary P-Se glasses

“…The molecular structure of the Se-rich (0ϽxϽ0.52) and P-rich (0.60ϽxϽ0.80) glasses has been the subject of previous investigations by differential scanning calorimetry, 6 diffraction methods, [7][8][9] Raman scattering, 10 and 31 P and 77 Se solid-state NMR. 11,12 31 P NMR chemical shifts of these building blocks have been established.…”