Anomalous thermal expansion behaviour of Na8[AlSiO4]6(NO3)2-sodalite: P4̅3n to Pm3̅n phase transition by untilting and contraction of TO4 units

Abstract

Abstract X-ray single crystal, X-ray powder and specific heat investigations were carried out on alumosilicate NaNO3-sodalite between 290 K and 1100 K. Single crystal structure determinations at 295 K, 505 K and 720 K reveal the sodalite structure in its partially collapsed form in space group P4̅3n whereas at 935 K the z coordinate of the framework oxygen is shifted to z = 0.5 according to an untilted sodalite framework in Pm3̅n. The sodium ions are located on 8e (x, x, x) position in P4̅3n and become statistically distributed on 16i (x, x, x) in Pm3̅n. The displacements of the nitrate atoms are extraordinarily enlarged at 935 K compared to the values at the lower temperatures. Analysis of the X-ray powder data show a discontinuity in the thermal expansion behaviour at T c = 930 K with the temperature dependence of the lattice parameter described by a calc = a 1 — A(T c — T)0.5, a 1 = BT + a 0 (A = 0:57 pm K–0.5, B = 5:84 · 103 pm K–0.5, a 0 = 909.9 pm, T in K) which is related to a square root decrease of the volume strain. The anomalous thermal expansion compromises a gradual contraction of TO4 units and the disappearence of structural tilt with increasing temperature. The refined data reveal a change of z coordinate of O1 proportional to (T c-T)1/4 in P4̅3n which is considered as an effective order parameter in a Landau tricritical description of the phase transition triggered by the displacement of the cage filling ions.