Characterization of Metamict and Glassy Lead Phosphates
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Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA ABSTRACT Single crystals of Pb 2P 20 7 (beam stable under 200 kV TEM electrons to a fluence > 1027 e/m 2 and an ionizing dose > 1014 Gy) were rendered metamict (amorphized) with ion-implantation (100 kV P+ ions with several fluences between 5 x 1017/m 2 - 2 x 1021/m 2). Pb 2P 20 7 and
PbO.P 20 5 glasses were also ion implanted at identical fluences. Radial distribution functions for metamict and glassy phosphates generated using energy filtered electron diffraction (EFED) data collected on 100 kV
field emission STEM (VG HB-5) indicate significant alterations in the medium range order. INTRODUCTION Electrons interact with matter far more strongly than X-rays or neutrons which enables them to be used as powerful characterization tools for the structural analysis of very small volumes of solids, such as those involved with the metamict regions generated using ion-beam irradiation. However, an electron diffraction pattern from an amorphous region contains information from both elastically scattered and inelastically scattered electrons. As the scattering intensity information from the elastically scattered electrons alone can be related to structure of the specimen, filtering out the inelastically scattered electrons becomes necessary. Using such a technique, Qin and Hobbs [1] have successfully generated the radial distribution functions from metamict silicas. Lead phosphate glasses have been investigated for decades with respect to fundamental glass properties. In these glasses, [P0 4] tetrahedra, (like [Si0 4] tetrahedra in silicates) can link up together by corner sharing oxygen atoms to form polymerized structures up to and including three-dimensional glassy networks. Radial distribution functions have been generated for lime phosphate glasses [2] and amorphous aluminum phosphates [3] using X-ray diffraction techniques. More recently, Wright 25 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
et al. [4] generated RDF for pure vitreous phosphorous pentoxide. Such techniques, though useful in generating RDFs from bulk amorphous materials, cannot prove useful in deriving such information from very small volumes; for example, ion-implanted metamict lead phosphate layers are 200 nm deep for pyrophosphate compositions and 225 nm for metaphosphate compositions [5, 6]. Sales et al. [7] have examined medium-range order in near-surfaces of single crystals of lead pyrophosphate (Pb 2P 207), rendered aperiodic (metamict) by ion implantation as well as lead pyrophosphate glass, using high-performance liquid chromatography flow injection analysis (HPLC-FIA). In this study, we report the RDFs generated for ion-irradiated lead pyrophosphate single crystals and for both ion irradiated and non-irradiated lead phosphate glasses using energy filtered electron diffraction (EFED) data collected on the 100 kV field emission STEM (VG HB-5).
EXPERIMENTAL PROCEDURE Sample preparation techniques for single crystals of lead pyrophosphate (Pb 2P
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