Electronic structure and sodium-ion diffusion in glaserite-type A3−хNa1+х(MoO4)2 (A = Cs, K) studied with first-principles calculations

Abstract

Here we present an ab initio insight into the electronic structure and sodium diffusion in A3−хNa1+х(MoO4)2 with trigonal (A = Cs) and monoclinic (A = K) glaserite-type structures. Our DFT calculations predict the stability of К3−xNa1+x(МoO4)2 to high sodium content and a significantly smaller homogeneity region for Cs3−xNa1+x(МoO4)2 in accordance with experimental findings. These molybdates are wide-gap insulators with a band gap of 4.2 eV, which is very weakly dependent on the sodium content. The calculated electric field gradients at the Na and A sites were related to the anisotropy of electronic charge distribution and used to predict the quadrupole frequencies of the 133Cs, 39K and 23Na NMR lines in Cs3−хNa1+х(MoO4)2 and K3−хNa1+х(MoO4)2. We examined the possible diffusion paths of Na+ ions and showed that the direct Na−Na migration is unlikely due to a large energy barrier in both molybdates. Instead, the Na+ ions can migrate through the A positions with much lower barriers in A3−xNa1+x(МoO4)2. Therefore, designing stable non-stoichiometric compositions may be a way to reach good sodium-ion diffusion in double molybdates with the glaserite-type structure.