Ultrafine particles of TiH
2 are formed by the H
2 gas evaporation, the typical habit of which shows a dodecahedron. On the analysis of the habit, the growth of the particles is quite likely to be resulting from the martensitic transformation from the bcc structure to CaF
2 structure owing to the hydrogen absorption. The desorption of the H
2 from the TiH
2 particles by a heat treatment in a vacuum brings about the formation of hcp Ti particle, while the absorption of H
2 gas of Ti particles, in a hydrogen atmosphere, the formation of TiH
2 particles with CaF
2 structure. The formation of twins in a particle is always necessary for the transformations between TiH
2 and Ti.
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