2996
P. Chieux, M. J. Sienko, and F. DeBaecker
A Neutron Diffraction Study and Phase Diagram Investigation of the Solid Lithium-Ammonia Compound P. Chleux, lnstltut Laue Langevln, Qrenoble, France
M. J. Sienko," Cornell Unlverstiy, Ithaca, New York 14853
and F. DeBaecker Laboratolre de Chlmle Physlque, Lliie, France (RecelvedJuly 30, 1975) Publlcatlon costs asslsted by the Natlonal Sclence Foundation
Low-temperature neutron-diffraction experiments have been carried out on the lithium-deuterated ammonia system in the vicinity of Li(NDd4. In all the samples, there was observed a strong background scattering similar to that displayed by the liquid, suggesting either highly strained regions or coexistence of an amorphous phase with the solid compound. The structure of the compound a t 3 K was indexed as bccub with a0 = 14.80 f 0.01 A; at 85 K, a0 = 15.03 A. No phase change was observed in the interval 60-85 K. Partial results are also preserited for the Li-NH3 compound, where significant changes were observed in the neutron diffraction patterns between 60 and 85 K. Low temperature DTA on a 20 mol % Li-NDZ sample showed a thermal effect a t 27 f 5 K, which may be related to appearance of a few additional neutron diffraction lines in all the Li-ND3 samples below 30 K.
Ammonia forms compounds with the alkali element lithium, the alkaline earth elements Ca, Sr, Ba, and the rare earth elements Eu and Yb.l The compounds are of special interest, because they are metallic, are apparently composed of only ammoniated cations at the lattice points, and are relatively low in electron density compared to normal metals. The last point may be especially important as these materials may be on the verge of the metal-nonmetal transition. The lithium compound, Li(NH3)4, has been the one most extensively studied. From x-ray powder studies a t 77 K, Mammano and Sienko2 suggested that the solid compound exists in two phases: a cubic form with a0 = 9.55 A, stable between 82.2 and 88.8 I
