Pressure-temperature phase diagrams of stage-2-4 potassium-graphite intercalation compounds deduced from anomalies in the basal plane resistivity

• 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.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

Nyckelord

Graphite intercalation compounds

GIC

potassium

K

stage 2

stage 3

stage 4

electrical resistivity

contactless measurements

basal plane resistivity

anisotropy

high pressure

phase diagram

phase transformations

order-disorder transitions

Condensed matter physics

Kondenserade materiens fysik

fysik

Physics

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