CRITICAL PHENOMENON IN A MOLECULAR-IONIC CRYSTAL (CD3ND3)(2)[SNCL6]

The phase transition in (CD3ND3)(2) [SnCl6], for which the fluctuation of the order parameter is exceptionally strong among molecular system s, was studied by high-resolution neutron diffraction. The transition was accompanied by contraction of the a and expansion of the c axes as T-c = 154K was traversed from above. The square root of the c expansi on was proportional to the occupancy of one of the deuterium sites whi ch is the order parameter of the transition. The mechanism of this cou pling is discussed.