Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4 |
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Authors: | Jean Senegas Michel Pezat Jean Pierre Darnaudery Bernard Darriet |
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Institution: | Laboratoire de Chimie du Solide du CNRS, Université de Bordeaux I, 351 Cours de la Libération, 33405, Talence, Cedex, France |
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Abstract: | Wide line and pulsed NMR studies are reported for MgH2; and Mg2NiH4 hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH2, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgHx, (x<0.04).Mg2NiH4 shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T1 and T2 relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10?8 < D < 4.6 × 10?8 cm2/s. |
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