β-K2SO4-type isomorphs:: Prediction of structures and refinement of Rb2CrO4

The crystal structures of members within a group of isostructural compounds may be successfully predicted. This is demonstrated for the beta-K2SO4 group isomorphs with the general formula M2XO4, which were chosen as a family of very closely related compounds nearly all with accurately refined crystal structures. The unit-cell parameters and the fractional atomic coordinates are shown to exhibit systematic variations with both cation and anion size, as well as the Mulliken charge on the O atom in the tetrahedral anion. This allows the prediction of the crystal structures of members in the series, with only the chemical composition of the compound being known. The agreement is good, except for an early structure determination of Rb2CrO4. The now refined structure gives excellent agreement with that predicted.