2352
G, G, LIBOWITZ AND J, G,PACK
ing polymer radical of 4-VP would be much more easily transferred to Ni" in comparison wit'h that of MVP in which the vinyl group is in the meta position to nitrogen. H
I
-CH,-C*
(I -Ni-
I I1
l
I
-
---CH,-CH
I
0 N
-Ni-
I
chemistry, and this is explained by energy transfer to sulfur atoms from other parts of the molecule (or energy localization on the sulfur atoms) or some radical reactions. An increase of radical concentration could be explained by the latter mechanism. Although little is known for the coordination compounds containing sulfur, it is very probable that the thiocyanate ion is more polarizable than the chloride anion and that the former anion is converted to the sulfur radical as a result of electron transfer oxidation via coordination bonds. Certainly such participation of anions in radical production depends on the kind of ligand. When vinylpyridines are the ligands, the sulfur radical is no longer observed and the propagating radical alone is detected independent of the anions employed. This specificity of radical formation should be interpreted in respect to the reactivity of the complexed ligands.
111
l
The zinc complexes of pyridine and picolines with
SCK- produced the sulfur radicals at higher temperature but those with C1- produced the carbon radicals on the pyridine ring. Conversion of radical sites to sulfur has been reported for sulfur compounds in radiation
Acknowledgment. The authors wish to express their thanks to Dr. H.Yoshida and Mr. T. Kawamura for their helpful discussions. Thanks are also due to Mr. K. Matsuda and Mr. T. Takagaki for carrying out some irradiations with a Van de Graaff accelerator at the Osaka Laboratories, Japanese Association for Radiation Research on Polymers.
The Gadolinium-Hydrogen System at Elevated Temperatures. Vacancy Interactions in Gadolinium Dihydride by G . G . Libowitz and J. G. Pack Ledgemont Laboratory, Kennecott Copper Corporation, Lexhglon, Massachusetts OW175 (Received December 9, 1.968)
Pressure-composition-temperature measurements were performed on the gadolinium-hydrogen system at 800, 850, and gooo. The equilibrium hydrogen pressures and phase boundaries obtained were in agreement with extrapolationsfrom the previous work of Sturdy and Mulford below 800'. The metal-rich phase boundary of gadolinium dihydride was found to occur at H/Gd values of 1.71, 1,56, and 1.4 at 800, 850, and goo', respectively, The large number of experimental points in the hydrogen-deficient single-phase region of gadolinium dihydride permitted further verification of previously derived thermodynamic activity relationships assuming interaction between defects in nonstoichiometric compounds. The existence of hydrogen vacancies in gadolinium dihydride was corroborated, and the free energy of interaction between hydrogen vacancies was found to be 2 kcal/mol.
Introduction A previous pressure-composition-temperature study of the gadolinium-hydrogen system in the range 600800" by Sturdy and Mulfordl established that there are two hydride phases' a cubic dihydride and a trihydride. Both hydrides are nonstoichiometric, and The Journal of Physical Chemistry
the dihydride exhibits a very wide homogeneity range. The purpose of the present investigation was twofold: (1) to extend the Phasediagram and thermodynamic data on the gadolifium-hYdrogen system to higher (1) G. E. Sturdy and R. N. R. Mulford, J . Amer. Chem. Soc., 78, 1083 (1956).
2353
THEGADOLINIUM-HYDROGEN SYSTEMAT ELEVATED TEMPERATURES temperatures, and (2) to obtain precise activity data in the single-phase dihydride region in order to further examine the applicability of previously derived2 thermodynamic activity relationships which permit ident'ification of the predominant defects present and evaluation of the defect interaction energies.
Experimental Section Materials. The gadolinium metal (nominal purity 99.9%) used in this investigation was obtained from the Lunex Company. The sample, however, was found to contain 0.1 to 0.4% holmium and about 450 ppm of oxygen. The total of all other impurities (
