Ab initio study of RaWO4: Comparison with isoelectronic tungstates

Abstract

We performed first-principles calculations to study several physical properties of the scheelite RaWO4 tungstate at ambient pressure. This work provides new information that contributes to a better understanding of a compound that is very difficult to synthesize experimentally due to the radioactivity of radium. According to our results from elastic constants and phonon dispersion relation, RaWO4 meets the criteria to be elastically and dynamically stable. RaWO4 behaves as a wide band-gap semiconductor with an energy band-gap value of Eg ​= ​4.65 ​eV, close to other AWO4 tungstates and many other ABO4 compounds. Our results demonstrate that RaWO4 presents a ductile behavior with relatively small values of B, G, E, and HV, which are similar to those reported for BaWO4. Also, we performed the same characterization for isoelectronic tungstates AWO4 (A ​= ​Ca, Sr, Ba) to compare with RaWO4. We found an increasing linear trend of structural parameters, electronic band-gap, and elastic anisotropy with the ionic radius of the cation A. Such behavior occurs due to the increment of AO8 polyhedra volume. In contrast, we observed the opposite behavior in the elastic constants and mechanical properties. The calculations showed that as the ionic radius of cation A increases, the Laplacian of charge density (∇2ρ) of A–O bonds decreases, which means that A–O bonds become weaker, at the same time the ionicity decreases from CaWO4 to RaWO4. In addition, we computed the Bader charges to complement our study and analyze the behavior of the internal stretching modes (ν1 and ν3) of the WO4 polyhedra.