The hydrogen--doped stage 1 KHg--GIC's were prepared in a
two--step process, analogous with the preparation of
C8KHx.[152] First, several batches of
stage 1 KHg--GIC's were prepared by the usual two--zone
method, as described in Section 
.
These specimens where characterized by (00l) x-ray
diffraction and their zero-field superconducting transition
was measured, as described in Section .
Raman scattering spectra were also recorded on some samples,
as described in Section .
In order to perform the hydrogen doping, the same samples that were characterized without hydrogen were transferred under a vacuum of about 10-5 torr to a new ampoule. Hydrogen gas was purified through diffusion through a Pd-Ag tube inside a Resource Systems Model RSD-1 hydrogen purifier. (200±2) torr of this gas was admitted to the sample tube through a glass break-seal. After about 5 minutes of exposure, 200 torr of He gas was also bled into the tube for thermal contact during the low-temperature measurements. The sample tube was then sealed off from the vacuum system with a gas torch.
After the hydrogen had been admitted to the sample tube, the pressure of the gas was monitored using a Setra Model 300D capacitive manometer, although in the early experiments a mercury-column manometer was used. No change in hydrogen pressure was determined within experimental error; that is, the time rate of change of pressure was about the same whether or not the glass tube contained a KHg-GIC sample. Thus the any slight change in the pressure during exposure is attributed to thermal drift of the sensor.
The apparently small hydrogen uptake of the HOPG-based KHg-GIC's is consistent with the very slow hydrogen sorption that occurs in HOPG-based K-GIC's.[97] For example, the C8KH0.19 specimens used in the superconductivity studies of Kaneiwa et al. were reacted with hydrogen gas for 105 days.[125] The slow speed of hydrogen diffusion into HOPG-based GIC's is attributed to the low defect density of HOPG. If C4KHg has more defects than C8K, it might absorb hydrogen more quickly. Nonetheless, the hydrogen uptake of C4KHg must be quite small.
It would be interesting to try longer hydrogen exposure times or higher hydrogen pressures to see if more hydrogen is absorbed. High hydrogen pressures were avoided in these experiments for safety reasons, but experiments with higher hydrogen pressures are planned at the Tokyo Institute of Technology in the near future.[77] Hydrogen exposure times longer than 5 minutes were not used because of fear that air leaks into the apparatus could degrade the sample quality. Samples attached to the vacuum system without hydrogen gas addition tend to begin to lose their surface luster after about 5 minutes. Making KHg-GIC's out of a less-ordered host material (such as Grafoil) might also permit larger hydrogen absorption.[97]
Despite the fact that bulk absorption of hydrogen must be
quite small, the effect of hydrogen on the C4KHg
samples was easily visible. Isothermally prepared GIC's,
which were initially pink, became blue during hydrogen
exposure, and then turned a dark violet. Gold GIC's became
blue--violet indefinitely afterward. The samples' color did
not change noticeably after a period of about 12 hours. A
C8KHg sample exposed to hydrogen showed no color
change at all. These color changes may well occur only on the
surface, and so should not be taken too seriously. As
discussed in Section , post-hydrogenation
(00l ) scans showed that the samples' repeat distances
were unchanged upon hydrogen doping. No change in the lattice
constant was anticipated due to the small amount of hydrogen
that was absorbed. Hydrogen absorption also seemed to have
little impact on the Raman spectrum. However, the effect of
hydrogenation on the superconductivity was more dramatic, as
will be described in the next section.