| 1. |
- Sundqvist, Bertil, et al.
(author)
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Pressure-temperature phase boundaries in KC24: Evidence for a kinetically-hindered low-temperature staging transition
- 1986
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 34:5, s. 3532-3535
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Journal article (peer-reviewed)abstract
- The pressure-temperature phase diagram of KC24 was mapped out using in-plane resistivity anomalies to identify phase boundaries. Surprisingly, the 300-K staging transitions at 3.4 and 6.4 kbar are found to connect to the 1-bar ordering-stacking transitions at Tu=123 K and Tl=96 K, respectively, whereas the loci of P, T anomalies cannot be equilibrium phase boundaries since the low-P, low-T transitions do not involve stage transformations. We suggest that this unusual behavior is due to kinetic hindering of the low-T staging transitions, predicted by simple equilibrium models and observed in the analogous Li compound. A possible origin of the slow staging kinetics at low T in KC24 is the pinning of discommensurations at Daumas-Herold boundaries.
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| 2. |
- Sundqvist, Bertil, et al.
(author)
-
Pressure-temperature phase diagrams of stage-2-4 potassium-graphite intercalation compounds deduced from anomalies in the basal plane resistivity
- 1987
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 35:15, s. 8231-8242
-
Journal article (peer-reviewed)abstract
- The basal-plane resistivities ρ of stage-2–4 potassium-graphite intercalation compounds (K-GIC’s) have been measured as functions of temperature T and pressure p using a contactless method. The pressure range investigated was 0–1.4 GPa. For stage-2 K-GIC (KC24) measurements were made in the temperature range 90–300 K, while for the higher-stage materials the range was mainly limited to 245–300 K. In all cases ρ increases with p through the anomalous regime near the phase transitions, whereas dρ/dp is always negative in the stable high-p (high-stage) phases. Approximate p-T phase diagrams are deduced from anomalies in ρ corresponding to the staging and in-plane order-disorder transitions, respectively. In all cases the slopes dp/dT of the phase boundaries for the high-pressure staging transitions are such that the transitions extrapolate to positive T at atmospheric p, although no such transitions have been reported. The low-p staging-ordering phase boundary of KC24 is particularly complex, involving segments with both dp/dT→0 (125<200 K) and dp/dT→∞ (T≊125 K). The p-T diagrams are compared with recent structural experiments and models. We suggest that the complexity of the KC24 phase diagram is attributable to the weakly incommensurate in-plane structure at low p and T.
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