Structural phase transitions in stage 1 potassium-graphite intercalation compounds at high temperatures

Abstract

X-ray diffraction studies of melting and stacking transitions of K-layers in stage 1 K-GIC have been investigated as a function of temperature at a constant P K, the vapor pressure of K. At high p K, two successive phase transitions have been observed; the low-T αβγδ staking solid — αβ stacking solid - liquid phases. The melting transition shows some continuous natures at T m, the melting temperature; no discontinuous change of the first peak wave-number, q o, of K-liquid halo, the existence of correlated liquid having the same symmetry as solid, a remarkable growth of intra- and interlayer correlations just above T m, and a continuous growth of ordering of solid below T m. However a discontinuity in the correlation lengths at T m indicates that the melting is weakly first order. At low p K, the αβγδ solid melts directly into liquid without appearance of αβ phase. The analysis of Debye-Waller factor has revealed an anisotropic thermal displacement of K-atoms in solid at high temperatures.