  Motivated by the search for type-II multiferroics, we present a comprehensive
optical study of a complex oxide family of type-II multiferroic candidates:
RbFe(MoO4)2, RbFe(SeO4)2, and RbFe(SO4)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 RbFe(AO4)2
(A = Se, S), which identifies the lack of mirror symmetry and in-plane two-fold
rotational symmetry. Also, the SHG efficiency of RbFe(SeO4)2 is two orders of
magnitude stronger than RbFe(AO4)2 (A = Mo, 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 (UV-VIS) absorption spectroscopy. Included is experimental
evidence of the band gap energy and band gap transition type for this family.
Previously unreported sub-band 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 RbFe(AO4)2
(A = Mo, Se, S), the aforementioned transitions are spectrally tunable.
Finally, we discuss the potential origin and impact of these tunable
transitions.
