Ionic Diffusion in the Solid Solution Na3-XKxPO4 (0 ≤ x ≤ 3): Experimental Results and First Principle Calculations

Na₃PO₄ is a well-known ionic conductor [1,2] and serves in this work as a starting point to investigate the solid solution Na_3-xK_xPO₄with 0 ≤ x ≤ 3. The aim is to elucidate the impact of potassium ions on the physical properties: crystalline phases, band gaps, activation energies and ionic conductivities.

Various compositions in this solid solution are synthesised by annealing the starting materials at 1200°C according to the Na₃PO₄-K₃PO₄phase diagram [3]. Then the crystalline phases, band gaps, activation energies and ionic conductivities are determined experimentally and measured.

Using density functional theory [4] the ground state energies and band gaps are calculated for the various phases. Energy barriers are calculated for various paths of the cations Na⁺ and K⁺by using the nudged elastic band (NEB) method [5].

The theoretical and experimental results, especially the simulated energy barriers and the equivalent activation energies, are compared and discussed.

We think that our approach can be applied to more complex solid electrolytes with polyanionic structures like NASICON [6,7].

References:

[1] A. Hooper, P. McGeehin, K. T. Harrison, B. C. Tofield, J. Solid State Chem. 24 (1978) 265-275

[2] J. T. S. Irvine, A. R. West, Solid State Ionics 28-30 (1988) 214-219

[3] I. B. Markina, N. K. Voskresenskaya, Russ. J. Inorg. Chem. 14 (1969) 1188-1192

[4] P. E. Blöchl, Phys. Rev. B 50 (1994) 17953-17979

[5] G. Henkelman, B. P. Uberuaga, H. Jónsson, J. Chem. Phys. 113 (2000) 9901-9904

[6] H.Y.P. Hong, Mat. Res. Bull. 11 (1976) 173-182

[7] H.Y.P. Hong, J.B. Goodenough, J.A. Kafalas, Mat. Res. Bull. 11 (1976) 203-220