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

Abstract

The phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na₃PO₄–Na₂SO₄ have been investigated using a combination of insitu XRD, neutron powder diffraction, and molecular dynamics simulation. 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 the cubic (γ) phase, which show mean-square displacement parameters for octahedral Na a factor of 5 larger than for tetrahedral Na. The solubility of Na₂SO₄ in the tetragonal (α) phase is very low (≪1%). Samples of the solid solution exist as a mixture of α and γ phases, with the (α + γ) → γ transition temperature decreasing linearly with increasing Na₂SO₄ content. Kinetic stabilisation of γ to room temperature was achieved by substitution of 10% 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 (β_t and β_m, respectively). These new phases are closely related to γ, with the γ → β transition being kinetically favoured over the γ → (α + γ) transition on cooling.