In order to study intercalates formed by the complete reaction of ammonia with TiS2 and NbS2, very long reaction times are required. These intercalates attain equilibrium ammonium concentrations of 25 mole% according to the formula (NH+4)y' (NH3)y' TSy'-2, where T=Ti of Nb, only after reaction periods of months. A value of y'=0.25 also is consistent with the charge transfer of 0.25 e-/TaS2 observed for ammoniated TaS2. The above ammonium concentration is in good agreement with a proposed 2a×2a ammonium superstructure for the above stage I intercalates, suggesting that the extent of ammonium formation is not primarily governed by the host band structure, which is substantially different for the above hosts, but by Coulombic intercalant repulsions. This superstructure is consistent with the presence of planar ammonia in (ND+4)0.22(ND3)0.34TiS0.22- 2, since the ammonia positions are always midway between nearest-neighbor ammonium positions, with the ammonia plane perpendicular to the corresponding ammonium-ammonium vector. The sensitivity of the ammonia structure to guest-guest interactions is evidenced by the observation of a non-planar ammonia structure for Li+0.23(ND3)0.63TiS0.23-2, where ammonia cannot be symmetrically and linearly coordinated by Li+.
Bibliographical noteFunding Information:
We are grateful to the National Science Foundation for support through grants DMR-841364102 and DMR-8605937, Arizona State University for the necessary computer time, and the Center for Solid State Science for the use of its Materials Preparation Facility. We are also grateful to Professor John Cow-ley and Dr. Robert B. Von Dreele for helpful con- versations. In addition, this study has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory, which is funded by the U.S. Department of Energy, BES-Materials Science, under contract W-31-109-Eng-38.