On the crystal structure and low-temperature behaviour of davyne : a single-crystal X-ray and neutron diffraction study

The crystal structure of a natural davyne [(Na3.98K 1.79Ca1.89Mg0.02Fe0.01) Σ7.69(Al5.99Si6.01O24)(SO 4)0.65Cl1.96, a = 12.7908(5) Å, c = 5.3469(2) Å and V = 757.59(6) Å3, space group P6 3], its thermoelastic behaviour and low-temperature induced structural evolution have been investigated by single-crystal neutron diffraction and in situ single-crystal X-ray diffraction within the range 110 ≤ T (K) ≤ 293. X-ray and neutron structure refinements provided a better picture of the disordered configuration of the SO4-groups and of the cation sites into the [0 0 0 1]-channel. No evidence of phase transition or change of the thermoelastic behavior have been observed within the T-range investigated. The volume thermal expansion coefficient is αV = 4.2(4) * 10-5 K-1, and the axial thermal coefficients are αa = 1.4(1) *10-5 K -1 and αc = 1.6(1) *10-5 K -1. A series of structure refinements based on X-ray intensity data collected between 293 and 110 K showed that the structural evolution of davyne is driven by the rotation of the rigid TO4 framework tetrahedra. The main deformation mechanisms of framework and extraframework population, in response to the applied temperature, are described. A comparison between the low-temperature structure evolution of the isotypic davyne, balliranoite and cancrinite is carried out.