Neutron Diffraction Data for Ta2D and the Probable Arrangement of the Deuteriums in Two of Its Polymorphic Varieties

Abstract

Neutron diffraction data are presented for Ta₂D at 4°K, 78°K, room temperature, and 53°C. The patterns are indexed assuming a pseudocubic unit cell, the cell edge being 6.74 A at the three lowest temperatures and 3.37 A at 53°C. The enlarged unit cell in the low temperature (β₁) form is attributed to ordering of the deuteriums in the tetrahedral interstices. Attempts are made to establish the arrangement of the deuteriums (D's) in the β₁ and β₂ forms. For the latter, the stable form at 53°C, the intensities can be accounted for in three ways—by random distribution over ⅔ (L group), over ⅓ (S group) or over all of the tetrahedral interstices. From general considerations the first of these is thought to represent the actual situation in β₂—Ta₂D. In this the D's are distributed over the sites at ½ ¼ 0, ½ ¾ 0, ¼ ½ 0, ¾ ½ 0+000 and +½ ½ ½. Analysis of the β₁ pattern shows that despite the fact that this form has a vanishing residual entropy there is no unique configuration of the deuteriums. Instead, to account for the observed intensities one must accept a structure in which each deuterium is statistically occupying more than one site. Assuming each D to be distributed over two sites three satisfactory structures are obtained. These, designated A, B, and C, belong to space groups I4, I$\text{4̄}$ , and I4₁, respectively. Structure A seems the most reasonable of the three and hence is accepted as the one which is probably correct. In it the 8 D's are statistically occupying the 16 tetrahedral sites at ⅛ ¼ 0, ¼ ⅞ 0, ⅞ ¾ 0, ¾ ⅛ 0, ⅝ 0 ¼, ⅜ 0 ¼, 0 ⅜ ¼, 0 ⅝ ¼+000, and +½ ½ ½. There is an apparent conflict between conclusions based on thermodynamic work and those developed from analysis of the diffraction data for the β₂ form as well as for the β₁ form. The observed configurational entropies of both forms are much less than those expected for the accepted structures. The discrepancies are thought to be due to local order.