Sodium nitrate -- a difficult discontinuous phase transition

Capacitance dilatometry was used to measure the relative thermal expansivity, ΔL/L 0 , of single crystals of sodium nitrate (NaNO 3 ) as a function of temperature. From these measurements, the phase transition at T f = 550 K shows characteristics of a discontinuous transition. A theory based on the competition between long-range interactions and the entropy gained by rotating NO 3 – groups at high temperatures predicts a discontinuous transition at T = T f with ΔL/L 0 ∼ (T f – T) ζ with ζ = 1. Our measurements yield ζ ≈ 1. PACS Nos.: 23.23.+x, 56.65.Dy