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A Simple Model of 19F Spin-Echo Formation in Polycrystalline Fluorapatite

Abstract 19 F NMR spin echoes, following a 90°-τ-180°- t ′ pulse sequence, have been observed from a sample of polycrystalline geological fluorapatite [Ca 10 -(PO 4 ) 6 F 2 ]. Model calculations of the echo shapes, the free-induction decay, and the homonuclear coupling part of the FID were carried out. These calculations utilized a theoretical FID for a single chain of dipolar-coupled 19 F spins determined previously by Chao and Engelsberg. The observed echo envelope closely matches the calculated homonuclear FID. In order to match the model FID to the observed FID, it was necessary to introduce an exponential decay factor with time constant T e = 340 ± 20 μs. This exponential decay factor probably accounts for the effects of magnetic impurities. Model echo curves were calculated using T e = 340 μs; these curves follow, for the most part, the shape of the observed echoes for τ ≤ 200 μs, particularly in matching the position of the maximum amplitude of each echo.