31 P Dipolar NMR Approaches Towards the Development of Structural Principles in Non-Oxidic Glasses.
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DIPOLAR NMR APPROACHES TOWARDS THE DEVELOPMENT OF STRUCTURAL PRINCIPLES INNON-OXIDIC GLASSES.
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HELLMUT ECKERT, DEANNA FRANKE, DAVID LATHROP, ROBERT MAXWELL, AND MICHAEL TULLIUS, Department of Chemistry, University of California, Santa Barbara, Goleta, CA 93106. ABSTRACT While the utility of solid state NMR to provide structural information in amorphous systems is well established in silicate and borate glasses, the systematic application of modern NMR techniques to non-oxidic systems has just begun. One of the central issues in these types of glasses is the question of the chemical bond distribution and the existence of short range order. The present study utilizes the selective measurement of homonuclear 31p.31P dipole-dipole couplings via spin echo decay to address this issue in the glass systems P-Se, PSe-Ge and CdGeAs2-xPx. The results indicate that the first two systems are characterized by a high degree of chemical ordering. The chemical bond distribution in CdGeAs2-xPx glasses is found to deviate considerably from that in the stoichiometry-analog crystalline materials. INTRODUCTION In spite of the. long-standing significance of non-oxidic glasses in infrared optics and semiconductor technology, the structural organization of these systems is not well understood. While the past few years have witnessed substantial progress in this area [1-3] there is still a definite need for new experimental probes that can test the existing structural hypotheses in a more stringent manner than presently possible. Solid state NMR is an element-selective, inherently quantitative method ideally suited for this objective. Since NMR is also spatially very selective,' the spectroscopic information remains largely unaffected by the lack of periodicity in non-crystalline systems. During 25 years of intense research, solid state NMR has proven very instrumental for developing structural concepts for oxide-based glasses [4]. In contrast, the unique power of this technique for providing structural information in non-oxidic glass systems is just emerging. Most applications of solid state NMR to glasses have traditionally focused on chemical shift information. In contrast, we have recently shown at the example of the P-Se glass system that measurements of internuclear 31p_31P dipole-dipole couplings can provide unique insights into the atomic distribution in these glasses, hence allowing one to evaluate the statistics of homopolar (P-P) versus heteropolar bonding Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
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[5]. This issue is central in the study of non-oxidic glasses, as it relates directly to the existence of short-range order. In the following, we will discuss new results obtained on the P-Se glass system, the ternary chalcogenide glass system PSexGey, as well as the chalcopyrite glass
system CdGeAS2-XPX. NMR METHODOLOGY The magnitude of the internuclear magnetic spin-spin coupling is commonly expressed in terms of the second moment M2, the average squared local field present at the nuclei due to
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