Second-order nonlinear optical and linear UV-VIS absorption properties of type-II multiferroic candidates RbFe(AO4)2 (A = Mo, Se, S)

TL;DR

This study investigates the optical properties and crystal symmetries of RbFe(AO4)2 (A = Mo, Se, S) multiferroic candidates using SHG and UV-VIS spectroscopy, revealing symmetry details, band gaps, and tunable optical transitions.

Contribution

It provides the first detailed optical and symmetry analysis of RbFe(AO4)2 compounds, clarifying their structures and optical transition behaviors, with implications for multiferroic applications.

Findings

RA SHG patterns reveal lack of mirror symmetry in RbFe(SeO4)2 and RbFe(SO4)2

SHG efficiency is two orders of magnitude higher in RbFe(SeO4)2

Sub-band gap absorption transitions are tunable by A-site substitution

Abstract

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.