Nuclear magnetic resonance in Al-rich quasiperiodic crystals
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T. E. Phillips and K. Moorjani The Johns Hopkins University, APL, Laurel, Maryland 20707
L. H. Bennett National Bureau of Standards, Gaithersburg, Maryland 20899 (Received 24 October 1985; accepted 21 January 1986) The 27A1 and the 55 Mn nuclear magnetic resonance powder pattern lineshapes have been obtained in icosahedral and decagonal (Tphase) Al-Mn quasiperiodic crystals, and are compared to that of orthorhombic Al 6 Mn. The quasiperiodic crystals yield much broader spectra with little resolved structure. The quadrupole and Knight shift parameters for the 55Mn resonance in orthorhombic Al6Mn have been determined as \vQ j = 0.76 MHz, Ka% = — 2.7 X 10~4, Kiso = + 5 X 1CP3. The results imply that Al 6 Mn and the quasiperiodic crystals have similar electronic and magnetic properties.
I. INTRODUCTION 1
Recently, Shechtman et al. observed long-range icosahedral (fivefold) symmetry in rapidly solidified Al-rich Al-Mn samples. Another related quasiperiodic crystal, the T phase with a unique tenfold axis, has been reported by Bendersky.2 These two phases form with between 8 and 22 at. % Mn. We report here preliminary results on the nuclear magnetic resonance (NMR) lineshapes of these phases as well as the Al 6 Mn orthorhombic phase. We utilize these results to draw some tentative conclusions about the structure of the icosahedral phase. Nuclear magnetic resonance is a probe of the local environment of the nucleus, and both the 55 Mn and 27 Al nuclei are good candidates for observation, having high natural abundances and large gyromagnetic ratios. II. DISCUSSION Crystalline Al 6 Mn is a centered orthorhombic crystal with 28 atoms per unit cell (Pearson symbol = oC28, space group Cmcm, strukturbericht symbol D2h). The unit cell contains one type of Mn site and three types of Al sites.3 One of our icosahedral samples has the nominal composition Al 5 5 Mn, another Al 6 Mn. The x-ray diffraction patterns of these samples reveal an excess of metallic Al. A third icosahedral sample has the nominal composition Al3 7 Si0 3 Mn, where the Si is incorporated to stabilize the icosahedral phase. This sample has much less aluminum metal than the binary samples. The final sample studied has the T-phase quasiperiodic crystalline structure with a nominal composition of 21 at. % Mn. All these samples were prepared by rapid solidification using melt spinning. The orthorhombic sample received a subsequent anneal. In each sample both the 27A1 and 55 Mn resonances were examined at room temperature. The NMR powder J. Mater. Res. 1 (2), Mar/Apr 1986 http://journals.cambridge.org
spectra were obtained with wideline cw spectrometers and with pulsed spectrometers using coherent boxcar echo detection. Various frequencies (12,19.8, and 99.34 MHz) were employed. The 55Mn powder pattern of orthorhombic Al6Mn at 99.34 MHz, Fig. 1(A), shows a two-peaked structure, characteristic of second-order quadrupole effects, with a separation of 80 Oe between peaks, and with tails persisting about 40 Oe beyond the peaks. At lower frequencies a hint of this stru
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