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>Neutron Diffraction Data for Ta2D and the Probable Arrangement of the Deuteriums in Two of Its Polymorphic Varieties
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Neutron Diffraction Data for Ta2D and the Probable Arrangement of the Deuteriums in Two of Its Polymorphic Varieties
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机译：Neutron Diffraction Data for Ta2D and the Probable Arrangement of the Deuteriums in Two of Its Polymorphic Varieties
Neutron diffraction data are presented for Ta2D at 4deg;K, 78deg;K, room temperature, and 53deg;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 53deg;C. The enlarged unit cell in the low temperature (bgr;1) 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 bgr;1and bgr;2forms. For the latter, the stable form at 53deg;C, the intensities can be accounted for in three waysmdash;by random distribution over frac23; (Lgroup), over frac13; (Sgroup) or over all of the tetrahedral interstices. From general considerations the first of these is thought to represent the actual situation in bgr;2mdash;Ta2D. In this the D's are distributed over the sites at frac12; frac14; 0, frac12; frac34; 0, frac14; frac12; 0, frac34; frac12; 0+000 and +frac12; frac12; frac12;. Analysis of the bgr;1pattern 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, designatedA, B,andC,belong to space groupsI4,I4macr;, andI41, respectively. StructureAseems 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 frac18; frac14; 0, frac14; frac78; 0, frac78; frac34; 0, frac34; frac18; 0, frac58; 0 frac14;, frac38; 0 frac14;, 0 frac38; frac14;, 0 frac58; frac14;+000, and +frac12; frac12; frac12;.There is an apparent conflict between conclusions based on thermodynamic work and those developed from analysis of the diffraction data for the bgr;2form as well as for the bgr;1form. 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.
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