Effects of A3+ cations on hydration in A2M3O12 family materials: A first-principles study

Abstract

By using first-principles calculations, we have systematically investigated the hydration in A2M3O12 (A=Y, Er, Yb, Lu, In, Sc; M=Mo, W) family materials, focusing on the role of the ionic radius and electronegativity of A3+ cations on the hydration of the materials. Our calculations showed that O end of the inserted H2O prefers to bind to A3+ or M6+ cations depending on the ionic radius and electronegativity of A3+ cations. As the radius of A3+ cations decreasing from R(Y3+)=0.90 to R(Sc3+)=0.75Å, the binding site of H2O is shifted from the position near the A3+ cations to that between the A3+ and the M6+ cations, finally to the position between the two M6+ cations. The binding energy is generally decreased as R decreasing (exception for A=Sc). Meanwhile, in all the cases two H of H2O tend to point to the nearby bridge O atoms sharing by AO6 octahedra and MO4 tetrahedra, and form hydrogen bonds, which play a key role in the binding between H2O and the materials. It was also found that the tungstates generally exhibit weak hygroscopicity comparing to the corresponding molybdates. Our results revealed the relationship between the hygroscopicity and the ionic radius of the A3+ cations, and provided a theoretical guidance in synthesis of A2M3O12 family materials with weak hygroscopicity.