Sonderforschungsbereiche
Sonderforschungsbereich 458 - Ionenbewegung in Materialien mit ungeordneten Strukturen -
vom Elementarschritt zum makroskopischen Transport -
B1 - Prof. Dr. A. Putnis

Struktur, Dynamik und Phasenübergänge anorganischer plastischer Kristalle

Phase relations and kinetics in the fast-ion conductor system Na₃PO₄-Na₂SO₄

Analysis of in-situ XRD and neutron powder diffraction data for different compositions in the Na₃PO₄-Na₂SO₄ system established that the solubility of Na₂SO₄ in the low-temperature phase of Na₃PO₄ is very low. Samples of the Na₃PO₄-Na₂SO₄ solid solution display a mixture of high and low-temperature phases. The transition temperature of the reaction from high to low-temperature phase decreases linearly with increasing Na₂SO₄ content. Extrapolation implies that the low-temperature g phase should be stabilized by addition of ~23 % Na₂SO₄. Therefore, the experimentally observed stabilization of g in the 10 % sample is a purely kinetic effect. There are two reasons for these cumbersome kinetics. Firstly, there is a significant energy barrier associated with formation of a within a matrix of g. This is not surprising given the large difference in unit cell volume for the two phases. Secondly, taking into account the low solubility of Na₂SO₄ in a, any ordering of the PO₄ tetrahedra requires either diffusion of S away from the nucleus or the pre-existence of a Na₃PO₄ rich cluster.

Samples containing less than 10 % Na₂SO₄ in x-ray diffraction experiments display prominent a and g peaks with additional peaks corresponding to the occurrence of two additional phases named bm and bt. The similarity in lattice parameters implies that b is more closely related to g than a. The transition g -> b is first-order in character and kinetically favoured over the g -> (a + g) transition on cooling. It is not, therefore, a simple displacive distortion of g and will involve diffusion of cations and possibly anion rotation.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft

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. Physical Chemistry Chemical Physics, 4, 3252 - 3259 (2002).

Meyer, H.-W., Putnis, A., Harrison, R. J.: Symmetry and kinetics of intermediary phases in the Na₃PO₄-Na₂SO₄ system. Eur. J. Mineralogy, 14, 109 (2002).