Second-order nonlinear optical and linear ultraviolet-visible absorption properties of the type-II multiferroic candidates RbFe ( A O 4 ) 2 ( A = Mo , Se , S )

Motivated by the search for type-II multiferroics, we present a comprehensive optical study of a complex oxide family of type-II multiferroic candidates: $\mathrm{RbFe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}, \mathrm{RbFe}{(\mathrm{Se}{\mathrm{O}}_{4})}_{2}$, and $\mathrm{RbFe}{(\mathrm{S}{\mathrm{O}}_{4})}_{2}$. We employ rotational-anisotropy second harmonic generation spectroscopy (RA SHG), a technique sensitive to point symmetries, to address discrepancies in literature-assigned point/space groups and to identify the correct crystal structures. At room temperature we find that our RA SHG patterns rotate away from the crystal axes in $\mathrm{RbFe}{(A{\mathrm{O}}_{4})}_{2}$ ($A=\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{S}$), which identifies the lack of mirror symmetry and in-plane two-fold rotational symmetry. Also, the SHG efficiency of $\mathrm{RbFe}{(\mathrm{Se}{\mathrm{O}}_{4})}_{2}$ is two orders of magnitude stronger than $\mathrm{RbFe}{(A{\mathrm{O}}_{4})}_{2}$ ($A=\mathrm{Mo},\mathrm{S}$), which suggests broken inversion symmetry. Additionally, we present temperature-dependent linear optical characterizations near the band edge of this family of materials using ultraviolet-visible absorption spectroscopy. Included is experimental evidence of the band gap energy and band gap transition type for this family. Subband gap absorption is also presented, which reveals prominent optical transitions, some with an unusual central energy temperature dependence. Furthermore, we find that by substituting the $A$ site in $\mathrm{RbFe}{(A{\mathrm{O}}_{4})}_{2}$ ($A=\mathrm{Mo},\mathrm{Se},\mathrm{S}$), the aforementioned transitions are spectrally tunable. Finally, we discuss the potential origin and impact of these tunable transitions.