Ordering process in sodium nitrite observed by using neutron diffractometry

From neutron diffraction data, the nuclear density of sodium nitrite is estimated directly by using a maximum entropy method and is compared with the results of usual least-squares calculations. The obtained nuclear density shows that the disordered NO₂ molecule becomes partially ordered in the ferroelectric phase at temperatures just below the transition temperature. However, the density distribution cannot be explained by using a two-Gaussian function, which suggests that the ordering is not a classical stochastic process but a quantum tunneling in a double minima local potential.