Experimental

Rates of dissolution of Pr₆O₁₁ in 8 N acetic acid solution were found to follow the first-order rate law with an energy of activation of 4 to 5 kcal. This value of the energy of activation could be reproduced although the values of the rate constants varied from sample to sample. For one sample, the rate constants were 1×20 x 10^-3 and 1×58 x 10^-3min^-1at 29 and 40^°C respectively, while for another sample the values were 2×9 x 10^-4 and 3×5 x 10^-4 min^-1 at 40^o and 50^oC respectively. The low value of the energy of activation is significant and together with the first-order dependence of the reaction, it indicates that the process is akin to a simple dissolution. An attempt to study the rate of dissolution of a 1:4 mixture of Pr₂O₃ and PrO₂(corresponds to Pr₆O₁₁) was unsuccessful since the dissolution of Pr₂O₃ was complete in about 5 min at 20^oC. Further, X-ray patterns of Pr₂O₃ and PrO₂ mixed in the proper proportions to form PrO_1.71 and PrO_1.833 showed only lines belonging to Pr₂O₃ and PrO₂.

In order to examine the nature of the intermediate products, samples of residues from solvolysis at 40^oC were separated after intervals of 15 min and 1 hr and their powder patterns were taken. The pattern of the 1 hr sample turned out to be that of PrO₂(a= 5× 400 ± 0× 005Å) with several additional lines which belonged neither to a fluoride type lattice nor to Pr(OH)₃. The pattern of the specimen after 15 and 25 min showed larger d-spacing and the fluoride lines could be accounted for by cells with lattice constants of 5×460±0×005 and 5×435±0×003Å respectively. While even these patterns did not show any Pr₆O₁₁ lines, they showed the same additional lines as the 1-hr specimen. The absence of Pr₆O₁₁ lines after only 15 min of solvolysis is surprising. A fluoride cell with a=5×435Å which is between the size of the Pr₆O₁₁and PrO₂ cells could mean a solid solution. Using a smooth curve obtained by plotting the unit cell lengths of all the cubic and rhombohedral phases of praseodymium oxides from the literature vs. the composition( pseudo cell dimensions were used where necessary), it was found that a= 5×435Å corresponds to a composition of PrO₁_×₉₂ . A similar composition with about the same lattice constant was also obtained by solvolysis of Pr₆O₁₁ with 16 N acetic acid. A miscibility gap is known to exist between Pr₆O₁₁ and PrO₂ and how such a uniphasic composition in this region could result is not clear; but it is possible that in the medium of a solution of praseodymium ions, such solid solutions could form. This would also explain the absence of Pr₆O₁₁ lines and is a strong argument in favour of solid solution formation.

<== попередня сторінка	\|	наступна сторінка ==>
Texts from scientific articles	\|	Practice

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.002 сек.