One-, Two- and Three-Dimensional Disorder of D-Atoms in Non-Stoichiometric TaD_x

Abstract

The bcc tantalum metal can absorb H₂ or D₂ gas to a ratio H(D)/Ta ∿ 1. Tetrahedral sites are occupied randomly at temperatures ∿ 300 K [1,2]. Below this temperature the H or D atoms order in β-TaD_0.5 and δ-TaD crystal structures with the intermediate non-stoichiometric solid solutions β-TaD_0.5+x and δ-TaD_1-x. The phase boundary between the two structures is either a second order phase transition at the approximate composition TaD_0.75 [1] or a miscibility gap with a first order phase transition [2]. Upon further”cooling to ∿208 K a new ordered ζ phase is formed at composition TaD_0.75–0.80 [1,2]. In this investigation the phase transitions from a to ζ phase at approximate composition TaD_0.75 will be considered. Based on the close relationship between the three ordered structures and on the large disorder of D atoms in β and δ phase,one can deduct an atomistic diffusion mechanism of D atoms between (0,4) fourth and (0,6) sixth nearest neighbor positions with translations T₄(a/2, a/2, 0) and T₆(a/2, a/2, a/2) of bcc lattice with lattice constant a. This mechanism should also be valid for α-phase at high D content, if thermodynamic data are considered. The calculated configurational entropy agrees with experimental values, if a blocking of first, second and third nearest neighbor positions is considered [1,3]. A group theoretical analysis of the crystal structures shows that second order phase transitions are possible for β → δ, β → ζ and δ → ζ phase transitions. It will be argued that the mechanisms of all phase transitions relay on the diffusion process of deuterium.