Deuteron Magnetic Resonance Studies of Structures and Phase Transitions in the Tantalum-Deuterium System

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The deuteron magnetic resonance in the tantalum-deuterium system has been measured as a function of the composition (0.45 to 0.198 D/Ta) and of the temperature (90 to −179°C), and the results has been discussed in relation to the neutron diffraction and the heat capacity data. The most probable arrangement of the deuteriums in the polymorphic varieties has been proposed. For the low temperature form β1 form), it has an ordered structure which belongs to the space group A222, with the four tantalums at the 4K positions and the two deuteriums at the 2a positions. Since the deuterium positions are described by a single Z′ principal axis direction, which coincides with the c axis, the quadrupole coupling constant is not averaged by the rapid diffusion of the deuteriums. For the intermediate temperature form (β2 form), a structure constructed with four types of domains has been proposed. In this structure, the space group is the same as that of the β1 form, and the deuterium positions in the domains are 2a, 2b, 2c, and 2d respectively. For the high temperature form (α form), the observed quadrupole coupling constant, which is averaged to zero, can be accounted for if the deuteriums are randomly distributed over all the available tetrahedral sites. However, the calculated configurational entropy based on this assumption is larger than the measured value.
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