Struktur, Dynamik und Phasenumwandlungen
anorganischer plastischer Kristalle

Phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na3PO4-Na2SO4

The phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na3PO4-Na2SO4 have been investigated using a combination of in-situ XRD, neutron powder diffraction, and molecular dynamics simulation. Pure Na3PO4 undergoes a first-order phase transition from the tetragonal a phase to the cubic g phase at 325°C. The transition is driven by dynamic reorientation of the PO43- tetrahedra, which is thought to contribute to the high ionic conductivity of g (paddle-wheel effect). Molecular dynamics simulations are used to derive a simple model for the average structure of g, in which P atoms are octahedrally coordinated by O atoms directed along the crystallographic a, b, and c axes. The simulations predict that tetrahedrally-coordinated Na is relatively tightly bound within its interstitial site, whereas octahedrally-coordinated Na shows enhanced thermal vibration and often performs large amplitude excursions towards neighbouring interstitial sites. These observations are confirmed by structure refinements of g, which show mean-square displacement parameters for octahedral Na a factor of 5 larger than for tetrahedral Na.

The solubility of Na2SO4 in a is very low (<< 1%). Samples of the solid solution exist as a mixture of a and g phases, with the (a + g) -> g transition temperature decreasing linearly with increasing Na2SO4 content. Kinetic stabilisation of g to room temperature was achieved by substitution of 10% Na2SO4. True stabilisation of g is estimated to occur at around 23% Na2SO4. The behaviour of samples containing less than 10% Na2SO4 is complicated by sluggish kinetics and the presence of two intermediate phases with triclinic and monoclinic symmetry (bt and bm, respectively). These new phases are closely related to g, with the g -> b transition being kinetically favoured over the g -> (a + g) transition on cooling. The structure of b is unknown, but is thought to be related to ordering of cations and/or vacancies, consistent with the presence of superstructure reflections in the diffraction patterns.

Beteiligte Wissenschaftler:

Dr. Richard Harrison (until 30.06.01), Dr. Hinrich-Wilhelm Meyer (as of 01.07.01), Prof. Andrew Putnis

Veröffentlichungen:

Harrison R.J., Putnis A., Kockelmann W.: Phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na3PO4-Na2SO4 (in preparation)