The phase transition in sodium nitrite

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Abstract
Neutron diffraction data were collected on Sodium Nitrite at 162 ± 1°C just below the transition and at 145 and 150°C. Data were also collected at room temperature to provide a complete set of amplitudes of motion of individual atoms. It was found that the quasi-mirror plane that relates the two unevenly occupied possible nitrite (and sodium) sites is about 0.03 A closer to the oxygen y parameter than is the center of mass.The amplitudes of motion correlated satisfactorily with dynamical, spectroscopic and elastic data.The amplitude data is in agreement with mechanical data that the a direction is 'softer' than the b or c direction. Evidence is presented of probable static disorder about average atomic positions over small regions along crowded b axis in the vicinity of the transition.Evidence of rotational motion about the c axis is also presented.