Flux crystal growth of rubidium–iron silicates and germanates and their ion-exchange using alkali nitrate salts

Abstract

Single crystals of two new rubidium ferrites, RbFeSiO4 and RbFeGeO4, were grown out of a eutectic RbCl–RbF melt. Both compounds crystallize in the noncentrosymmetric orthorhombic space group Ima2 and exhibit a three-dimensional porous framework structure composed of statistically disordered (Fe/T)O4 (T = Si, Ge) tetrahedra that corner-share to generate large channels running down the c-axis. These channels are occupied by Rb ions to maintain charge balance. RbFeSiO4 can also be prepared via a solid-state reaction by mixing and heating stoichiometric amounts of Rb2CO3, Fe2O3 and elemental Si; however, a similar approach for preparing RbFeGeO4 yielded a mixture of RbFeGeO4 and RbFeGe2O6. Powder second harmonic generation (SHG) measurements of RbFeSiO4 determined that the material is SHG-active with an intensity of 0.5 times of α-SiO2. First principles calculations in the form of density-functional theory indicated that it would be possible to ion exchange the Rb cation for other alkali metal cations. Applying a molten alkali nitrate salt-bath treatment at low soak temperatures of 350 °C–450 °C and short soak times of ∼16 h resulted in the almost complete replacement of rubidium with potassium and the partial replacement of rubidium with cesium, yielding K0.92Rb0.08FeSiO4, K0.91Rb0.09FeGeO4, and Cs0.48Rb0.52FeGeO4. No ion-exchange was observed when RbFeSiO4 was soaked in molten CsNO3; however, it was possible to force the exchange by using a CsCl melt at 680 °C to yield Cs0.13Rb0.87FeSiO4.