Perovskite-related hybrid noble metal iodides: Formamidinium platinum iodide [(FA)2Pt I6] and mixed-valence methylammonium gold iodide [(MA)2Au Au I6]

Anion···Anion [AuI4]-···[AuI2]- Complex Trapped in the Solid State by Tetramethylammonium Cations

A discrete π-hole···σ-hole dimer is synthesized and X-ray characterized. It presents a perfect thumbtack geometry where the σ-hole of the linear [AuI₂]^- anion points to the π-hole located above the central Au-atom of the [AuI₄]^- anion. Such discrete π-hole···σ-hole dimers are unprecedented in literature, since all mixed-valence gold(I/III) iodide compounds reported to date form infinite ···([AuI₄]^-···[AuI₂]^-) _n ·· chains in the solid state. If an excess of iodine is used for the synthesis, triiodide [I₃]^- ions are partially incorporated into the [AuI₂]^- sites, forming infinite chains. The nature of the anion···anion interaction has been studied considering two possibilities: (i) a π-hole coinage bond or (ii) σ-hole halogen bond using high-level density functional theory calculations, the quantum theory of atoms in molecules, and the noncovalent interaction plot index.

BiVO4/Cs2PtI6 Vacancy-Ordered Halide Perovskite Heterojunction for Panchromatic Light Harvesting and Enhanced Charge Separation in Photoelectrochemical Water Oxidation

Photoelectrochemical water oxidation is a challenging reaction in solar water splitting due to the parasitic recombination process, sluggish catalytic activity, and electrode stability. Oxide semiconductors are stable in an aqueous medium but show huge charge carrier recombination. Creation of a heterojunction is found to be effective for extracting the photogenerated electrons/holes before they recombine to the ground state. In this work, we created a heterojunction of BiVO₄ with vacancy-ordered halide perovskite Cs₂PtI₆ and used it as a photoanode in PEC water oxidation. Cs₂PtI₆ is the only halide perovskite that is found to be extremely stable even in strong acids and bases. We utilized the stability of this material and its panchromatic visible light absorption property and made the first unprotected heterojunction dual-absorber photoanode for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO₄ gave 0.6 mA cm^-2 photocurrent density, whereas the BiVO₄/Cs₂PtI₆ heterojunction shows 0.92 mA cm^-2. With the addition of IrO_x cocatalyst, at 1.23 V (vs RHE), the heterojunction gave ∼2 mA cm^-2. To obtain 2 mA cm^-2 photocurrent, pure BiVO₄ requires 560 mV overpotential, whereas the heterojunction requires 250 mV. The increase in the photocurrent arises from the increase in the efficiency of charge separation from BiVO₄ to Cs₂PtI₆ and the complementary absorption offered by the latter.

Organic-Inorganic Hybrid Gold Halide Perovskites: Structural Diversity through Cation Size

Crystal structures of a series of organic-inorganic hybrid gold iodide perovskites, formulated as A₂ [Au^I I₂ ][Au^III I₄ ] [A=methylammonium (MA) (1) and formamidinium (FA) (2)], A'₂ [I₃ ]_1-x [Au^I I₂ ]_x [Au^III I₄ ] [A'=imidazolium (IMD) (3), guanidinium (GUA) (4), dimethylammonium (DMA) (5), pyridinium (PY) (6), and piperizinium (PIP) (7)], systematically changed depending on the cation size. In addition, triiodide (I₃ ^- ) ions were partly incorporated into the AuI₂ ^- sites of 3-7, whereas they were not incorporated into those of 1 and 2. Such a difference comes from the size of the organic cation. Optical absorption spectra showed characteristic intervalence charge-transfer bands from Au^I to Au^III species, and the optical band gap increased as the size of the cation became larger.

Widely used hardly known. An insight into electric and dynamic properties of formamidinium iodide

The simple organic crystal formamidinium iodide (FAI) appeared to be a novel semiconducting material in a wide temperature range. The electric properties of FAI and the role of formamidinium cation (FA⁺) in the molecular mechanism of the solid-to-solid phase transitions (at 345 K (III → II) and 388 K (II → I)) were analysed. The creation of the ferroelastic domain structure in phases III and II was proved on the basis of observation under a polarizing microscope. Moreover, the molecular arrangement of dipolar organic FA⁺ was studied by ¹H NMR (spin-lattice relaxation time) and vibrational spectroscopy supported by density functional theory. The theoretical results show a good agreement with the experimental data. The infrared spectrum in a harmonic approximation was calculated and a comparative vibrational analysis was performed. All used techniques showed that the prototypic phase I exhibits the feature of plastic phase.

The simple organic crystal formamidinium iodide (FAI) appeared to be a novel semiconducting material in a wide temperature range.