X-RAY DIFFUSE-SCATTERING IN CUBIC ADAMANTANE (C10H16)

X-ray diffuse scatterings are observed in cubic adamantane (C10H16) ar ound X-points and Z-lines in the Brillouin zone boundary. The intensit y of the diffuse maximum at the X-point increases with decreasing temp erature. Diffuse intensity distribution is analyzed with the molecular field approximation in the Ising system. Cubic-tetragonal phase trans ition is explained by the instability of the orientational fluctuation with a wave vector q=(0, 0, 2pi/a):X-point. Short-range interaction b etween the first neighbouring molecules plays a dominant role in this phase transition.