Sodium nitrate - a difficult discontinuous phase transition

Capacitance dilatometry was used to measure the relative thermal expansivity, \u394L/L\u2080, of single crystals of sodium nitrate (NaNO\u2083) as a function of temperature. From these measurements, the phase transition at Tf = 550 K shows characteristics of a discontinuous transition. A theory based on the competition between long-range interactions and the entropy gained by rotating NO3\u207b\u2083 groups at high temperatures predicts a discontinuous transition at T = Tf with \u394L/L\u2080 sim (Tf \u2013 T)\u3b6 with \u3b6 = 1. Our measurements yield \u3b6 48 1.NRC publication: Ye