Heat Capacities and the Residual Entropy of Ta2H

Heat capacities of Ta2H have been determined over a range of temperature extending from 12° to 555°K. Three very sharp λ points were observed, at 306, 332, and 333.5°K, suggesting that Ta2H exists in four forms—β1 below 306°K, β2 between 306 and 332, β3 at about 333, and α above about 334°K. The heat capacity data for Ta2H are used together with similar data obtained earlier for elemental tantalum and the entropy of formation of Ta2H, evaluated from vapor pressure work by Kofstad, Wallace, and Hyvönen, to compute the residual entropy of β1—Ta2H. This is found to be —0.39±0.30 cal—1 deg—1 mole—1. The data also are used to estimate configurational entropies of the β2 and β3 forms. These are 0.70 and 3.3 cal—1 deg—1 mole—1, respectively. The corresponding quantity for the α form is 4.9. The λ points are attributed to rearrangements of the hydrogens in the 12-fold more abundant tetrahedral sites. At low temperatures the solute hydrogen atoms are presumed to populate the sites in an orderly fashion. With increasing temperature the ordered arrangement is altered and finally, in the α phase, destroyed. It is inferred from entropy calculations, however, that local order, probably pairing, persists in the α form even to 555°K, the highest temperature studied. Consideration is given to the possibility that the upper two λ points, which nearly coincide, should really be one, the splitting being due to concentration gradients or strains. Differential thermal analysis of samples containing from 27.5 to 44.0 atomic % hydrogen suggests that the splitting is a true property of Ta2H, but the results are not completely conclusive.