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Chemical Analysis of KHg- and CsBi-GIC's


The determination of the stoichiometry of graphite intercalation compounds is an important aspect of their characterization. The reason is that intercalation is a slow chemical process, particularly for higher-stage and ternary compounds. Even if an intercalation reaction proceeds for several weeks at an elevated temperature, the final product (a GIC) may not have reached its equilibrium state. The hypothetical metastable final state of the GIC might be a mixed-stage compound, which wouldn't be so bad, since mixed-stage samples are easily detectable with (00l) x-rays. Mixed-stage samples can be separated out from the well-staged ones and discarded.

A much more insidious situation arises if the compound is well-staged but is off the equilibrium stoichiometry. For one thing, there is the question of what the equilibrium stoichiometry is, a longstanding problem with acceptor GIC's.[74] In principle, stoichiometries can be determined from the x-ray diffraction pattern, as was done with the neutron scattering data. However, to have real confidence in the stoichiometries found from fits to (00l) integrated intensities, one needs either a large number of peaks, or extensive knowledge of the structure of the material in question. Especially in the case of multiphase systems like the KHg- and CsBi-GIC's, an independent check of the stoichiometries determined from diffraction data is highly desirable.

One type of non-destructive chemical analysis that has been used with great success on GIC's is Rutherford backscattering spectrometry (RBS). RBS provides information on composition only in about the top one micron of a specimen,[212] but it has the advantage of doing so in a structure-independent way. Also, since RBS gives depth resolution as well as species identification, any alloy adsorbed on the surface can readily be differentiated from the bulk. RBS measurements were performed on KHg-GIC's by Salamanca-Riba et al., who found stoichiometries of C3.0 ± 1.5KHg0.77 ± 0.08 for stage 1, C7 ± 1.4KHg0.72 ± 0.07 for stage 2, and C13 ± 1.3KHg0.62 ± 0.06 for stage 3. The low ratio of carbon atoms to potassium atoms found in this study is somewhat surprising, but the margin of error is so large that no definite conclusions can be drawn. However, the tentative C/K ratio of 3.0 supports the existence of the ( sqrt3 × sqrt3)R30° phase, for which 3.0 is the stoichiometric C/K ratio. For the (2 × 2)R0° and (sqrt3 × 2)R(30°, 0°) phases, a ratio of four is anticipated. Once again, the ( sqrt3 × sqrt3)R30° phase seems to be turning up in a surface-sensitive experiment; this phase had previously been seen only in Raman scattering and TEM studies.[247] Because of the nature of the measurements in which the ( sqrt3 × sqrt3)R30° phase has been seen, one might wonder if its appearance is related to deintercalation due to sample heating.

The better-bracketed parameter found by RBS is the Hg/K ratio, which appears to be about 0.7. The stoichiometries found for the KHg-GIC's in several different experiments are summarized in Table gif. As is evident from the Table, a Hg/K ratio of 0.7 is less than any of the values established from the x-ray or neutron diffraction experiments. This Hg/K ratio was only determined on one GIC of each stage,[212] so its magnitude may be affected by sample dependence. However, the agreement for all three samples of different stage is impressive. In fact, one might wonder if the constant Hg/K ratio with such a widely varying C/K ratio is not a sign that another property of the surface rather than the bulk is being measured. This possibility was put forward by the authors of Ref. [212], who noted that surface depletion of mercury from C4KHg under heating had been observed using x-rays.[82] The explanation of the tendency toward surface depletion of mercury is that since it intercalates after the potassium does,[82,70] it must also deintercalate first. This idea is believable also because mercury has a high vapor pressure and is rather chemically inert, with the result that its gradual surface depletion is a common phenomenon in many types of materials. For example, surface mercury depletion is a problem in HgCdTe wafers used in infrared detectors.[143]

RBS measurements have also been performed on the CsBi-GIC's in collaboration with Dr. J. Steinbeck.[36] The experimental conditions were similar to those of Ref. [212], except that the GIC's were cleaved before measurement to remove the alloy adhered on the surface. For an alpha-phase stage 1 CsBi-GIC sample, the result of this experiment was a stoichiometry C(2.5 ± 0.65)CsBi0.59 ± 0.03, in excellent agreement with the previously reported Bi/Cs ratio of C4CsBi0.55.[145,271] The accord between the Cs/Bi ratio found here and that determined by x-rays[145,271] increases one's confidence in the Hg/K ratio found from RBS, although there is not a problem with volatility in the CsBi-GIC's the way that there is in mercury-containing materials. The problem in synthesizing the CsBi-GIC's is signalled by the unexpectedly low C/Cs ratio, which is an indication of the tendency to have macroscopic alloy inclusions in the samples, even when the surfaces are cleaned of any adhered metal. These inclusions are discussed further in Section gif.

Table: Stoichiometries of the KHg-GIC's as determined from various experiments. ND = not determined by this experiment; NR = not reported by the authors of the cited reference.

Another non-destructive measurement that can be performed on air-stable samples is weight uptake. If the mass of the host graphite is determined before intercalation, and the stoichiometric ratio of the two intercalant metals is known from another measurement, such as RBS, then after intercalation the extent to which the reaction was completed can be estimated. For C4CsBi0.55, a simple calculation shows that the expected weight uptake is 5.2 times the graphite mass. Measured weight uptakes were usually on the order of 5.5 to 5.6 times the graphite mass, even after careful cleaning of the surface. This excess weight is another indication of the inclusions indicated by the RBS and TEM studies. (TEM observations of the inclusions are discussed in Section gif.)

While it would be highly desirable to perform weight-uptake measurements on the KHg-GIC's, this has not been done because of the compounds' high reactivity in air. An ideal set-up would be to have a microbalance inside a dry-box for routine weight-uptake measurements after intercalation, a practice carried out at the University of Kentucky.[63] Weight uptake data on C4KHg would be particularly interesting to compare to measurements of the areal fraction of superconductivity from inductive studies of the zero-field transition. (See Section gif for an explanation of how the areal fraction of superconductivity is determined.)

The next-best-thing to weight uptake and RBS for the determination of stoichiometry is wet chemical analysis. Wet chemical analysis was performed on C4KHg GIC's by Dr. W. Correia of the Center for Materials Science Chemical Analysis Facility. The basic procedure is to dissolve the specimen in a solution and precipitate out the various constituents separately for weighing. The contribution of each of the elements in the sample can be determined this way. While these measurements can be fairly accurate, and have the advantage of giving information about the bulk of the specimen, they also have the distinct disadvantage of destroying the sample. Therefore, while wet chemical analysis is useful, it has been performed on only a few specimens. The results are included in Table gif.

Drawing conclusions from the data in the Table is difficult. There does not seem to be a strong connection between Hg content and Tc, contrary to much speculation that Hg-deficient samples have lower transition temperatures.[207] The lowest Tc sample actually has the highest Hg/K ratio of 1.2. The observation that gold lower- Tc specimens have more beta phase than higher- Tc specimens is therefore consistent with the work of Yang et al.,[272,270] who found the beta phase of C4KHg has a higher Hg content (of 1.3). The Tc = 1.3 K sample has a composition that agrees well with the RBS measurements, while the Tc = 1.54 K sample agrees in composition with the neutron scattering measurements. All the specimens appear to be close to the expected C/Hg ratio of 4. A systematic study of any possible relation between stoichiometry and superconductivity still needs to be performed on fully characterized samples. The available evidence does not make a strong case for the importance of variable stoichiometry since most of the samples have chemical formulae close to that originally reported by El Makrini.[70]

Of course the most important characterization that needs to be done on superconducting materials is the measurement of the zero-field superconducting transition temperature, called Tc here. Zero-field Tc experiments are discussed in the next section.

next up previous contents
Next: Zero-Field Tc Measurements Up: Normal-State Characterization of Previous: Neutron Diffraction Studies (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995