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INTRODUCTION

THE

system Mg-H has several important aspects. The exothermically formed hydride MgH2, which is a tetragonal, ionic compound with an appreciable covalent contribution ~has been considered as a convenient storage medium for hydrogen for automotive and heat storage applications. 2-7 In this context numerous studies were undertaken concerning reaction kinetics, 8-~s thermodynamics ~9and catalytic surface effects3 ~ On the other hand, there exist also dilute, endothermic solutions of H in Mg (with H occupying tetrahedral sites) with well defined solubility limitsY -26 This work presents a TEM study of the precipitation of the hydride phase MgH 2 in high-purity Mg, its structure and morphology. Emphasis is also placed on the question whether support could be obtained for recent models of the hydriding process in MgJ 4,Is The structure of MgH2 is tetragonal, with a = 0.45025 nm, c = 0.30123 nmY The space group is P 4 J m n m . The atomic positions are 2Mg in (000) (1/2 1/2 1/2) and 4H in _+_ (xx0) (1/2 + x, 1/2 - x, 1/2) with x = 0.306 __+0.003, 27 a schematic drawing of MgH 2 is presented in Fig. 1. Further high-pressure phase transitions of MgH2 were recently reported. 28 In the course of the study it was found that MgH~ reflections may have to be separated from reflections of the following extraneous phases: Mg (OH)s, hexogonal (trigonal), S.G. P3M1; MgO, cubic, S.G. FM3M. Magnesium hydroxide may arise here from the reaction of the hygroscopic MgH2 with water vapor from the air.

Solution A

Solution B

5 pct HCI04 in CH3OH 70 V, 200 mA -10 ~ Flow Rate 10 (Tenupol polishing apparatus)

20 g LiCI 100 ml glycerine in 1 1 CH3OH 75 V, 220 mA Flow Rate 2.5, -10 ~

(Tenupol polishingapparatus)

In the work relating to the formation of the MgH2 phase, hydrogen charging was performed after the final electropolishing step. The reverse procedure (hydriding and subsequent preparation for TEM) would lead to insurmountable difficulties. Also, inspection of the aspolished foils reveals no second phases or gas bubbles. Occasional round features (as on Fig. 2) are artifacts from polishing and not H2-bubbles. The 3 mm discs were exposed in an all-metal, high pressure reactor to a high-purity (99.999 pct) hydrogen atmosphere at ~ 5 9 106 Pa and 270 ~ for about 16 h. These conditions ensured the formation of large hydride patches in the thinned portions of the Mg samples. Due to the ~C

EXPERIMENTAL High purity Mg (99.99 pct) was used in this study. 3 mm discs were punched from annealed 50 ~m ribbons and electropolished for TEM using either one of the following solutions A and B. The first one was specially developed for this study. 29 T. SCHOBER is staff scientist at the Institut ftir Festkrrperforschung, Kernforschungsanlage Jlllich, 5170 Jttlich, Germany. He is temporarily at Materials Science Division, Bldg. 212, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted June 26, 1980. M E T A L L U R G I C A L TRANSACTIONS A

O

2,,

(~ Mg H-

( r = 0064 nrn) ( r = 0.135 rim)

Fig. l--The tetragonal structu