Hybrid Organic–Inorganic Halide Derivatives of the 2H Hexagonal Perovskite Structure

Abstract

Four quaternary hybrid halide perovskites have been synthesized in hydrohalic acid solutions under hydrothermal conditions. The structures of (CH 3 NH 3 ) 2 AgRhX 6 and (CH 3 NH 3 ) 2 NaRhX 6 (X = Cl − , Br − ) consist of infinite one-dimensional chains of face-sharing metal halide octahedra. The structure is closely related to the 2H hexagonal perovskite structure, but the space group symmetry is lowered from hexagonal P6 3 /mmc to trigonal P3m1 by site ordering of the Rh 3+ and Ag + /Na + cations. All compositions demonstrate broad-spectrum visible light absorption with optical transitions arising from rhodium d-to-d transitions and halide-to-rhodium charge transfer transitions. The bromides show a 0.2 eV red-shift in the optical transitions compared to the analogous chlorides. Crystal field splitting energies were found to be 2.6 and 2.4 eV for the chloride and bromide compositions, respectively. Band structure calculations for all compositions give rather flat valence and conduction bands, suggesting a zero-dimensional electronic structure. The valence bands are made up of crystal orbitals that are almost exclusively Rh 4d−Cl 3p (Br 4p) π* in character, while the conduction bands have Rh 4d−Cl 3p (Br 4p) σ* character.

Exploring the Electronic Dimensionality of Ternary and Quaternary Rhodium Halides

“…The presence of Ag 5s and/or 4d valence orbitals near the Fermi energy raises the possibility of wider bands and higher dimensionality, provided that the symmetry is conducive for Ag–Cl–Rh orbital mixing. The spectrum of (MA) 2 AgRhCl 6 across the visible region is very similar to the compositions discussed above, suggesting that its electronic structure is also zero-dimensional, as was concluded previously . In contrast, the spectra of Cs 2 AgRhCl 6 are significantly red-shifted with respect to the chloride compounds discussed thus far.…”

“…Despite the presence of one-dimensional chains of face-sharing octahedra, the absorption spectra mirrored the localized absorption characteristics of the [RhX 6 ] 3– complex ions, suggesting a zero-dimensional electronic structure. The calculated band structures agreed with this description, revealing relatively flat bands near the Fermi energy …”

Molybdenum chloride double perovskites: dimensionality control of optical and magnetic properties

“…34 Moving to 4d metals increases the importance of spin–orbit coupling (SOC) in exotic physics of unpaired spins such as magnetic frustration in layered α-RuCl 3 , 35,36 α-MoCl 3 , 37 and other heavy transition metal halides. 38 High SOC metal ions such as Ru III , and Rh III have been incorporated in 1-D chain, 39,40 and 2-D layered double perovskites. 41 The effect of SOC on the magnetic properties has been studied in these low dimensional systems.…”

Organic–Inorganic Hybrid Hexagonal Perovskites (4-Methylpiperidinium)MnX3 (X = Cl and Br) with High-Temperature Dielectric Phase Transitions and Photoluminescent Properties

“…6 Among the derivatives, the 2H hexagonal perovskite, characterized by face-sharing octahedra and hexagonal packing, is adaptable for the largest tolerance factors. 7 This versatile structure allows for the incorporation of various elements, giving rise to a diverse family of perovskites with tunable properties. 8−12 Recently, organic−inorganic hybrid perovskites have sprung up as a fascinating subclass, integrating the benefits of organic molecules and inorganic materials that enable tunable properties and multifaceted functionalities.…”

“…To accommodate ionic radii constraints, the structure is sliced into lower-dimensional derivatives, generating a family that is typically known as hexagonal perovskites, with varying degrees of broken corner connectivity . Among the derivatives, the 2H hexagonal perovskite, characterized by face-sharing octahedra and hexagonal packing, is adaptable for the largest tolerance factors . This versatile structure allows for the incorporation of various elements, giving rise to a diverse family of perovskites with tunable properties. − Recently, organic–inorganic hybrid perovskites have sprung up as a fascinating subclass, integrating the benefits of organic molecules and inorganic materials that enable tunable properties and multifaceted functionalities. − Furthermore, the appearance of CH 3 NH 3 PbI 3 has brought about a significant revolution in the realm of photovoltaics owing to its exceptional ability to facilitate long-range transport of charge carriers and exhibits a remarkably low exciton binding energy, which has paved the way for advancements in energy harvesting, enabling efficient and low-cost photovoltaic devices. − Notably, the integration of organic components enhances the material processibility and allows for precise manipulation over the electronic property of the resulting multifunctional hybrid perovskites.…”