Sodium nitrate -- a difficult discontinuous phase transition

Abstract

Capacitance dilatometry was used to measure the relative thermal expansivity, ΔL/L0, of single crystals of sodium nitrate (NaNO3) 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– groups at high temperatures predicts a discontinuous transition at T = Tf with ΔL/L0 ∼ (Tf – T)ζ with ζ = 1. Our measurements yield ζ ≈ 1. PACS Nos.: 23.23.+x, 56.65.Dy