X-ray photographic study of the phase transitions in KC24 single crystals

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Abstract
An X-ray photographic study of the temperature dependence of the in-plane ordering of potassium in stage 2 KC24 single crystals was performed between 300 K and 10 K. In the disordered high temperature phase, one observes a modulation of the diffuse halo reflecting strong third-nearest neighbour correlations which develop as T is reduced. At 124 K ± 2 K long range ordering occurs and our diffraction data confirm previous studies. The present work shows, for the first time, that the diffraction patterns change drastically at about 90 K, thus revealing a change in the in-plane potassium order at low temperature