A structural phase transition of an intercalation compound Mn1/4NbS2 has be
en investigated by X-ray diffraction at high temperatures. The lattice para
meter c exhibited a discontinuous change at 640K. The superlattice reflecti
ons observed below 640K disappeared suddenly above 640K. The phase transiti
on at 640K took an aspect of the first-order phase transition. The precise
structure analyses were performed at various temperatures above and below t
he phase-transition temperature. It was revealed that Mn atoms were arrange
d in disorder in the high-temperature phase, while the Mn atoms were ordere
d forming the 2a(0) x 2a(0) x c(0) superlattice in the low-temperature phas
e. The Nh and S atoms around the ordered Mn atoms slightly shifted from the
high-symmetry position in the low-temperature phase. The order parameters
were the degree of order of the Mn atoms and the degree of displacement of
the Nb and S atoms.