Regulation of the order–disorder phase transition in a Cs₂NaFeCl₆ double perovskite towards reversible thermochromic application

Abstract

Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields. Herein, an environmentally-friendly, low-cost, high-throughput Cs2NaFeCl6 single crystal with exceedingly high thermal stability is designed and grown. It obtains a cubic lattice system in the temperature range of 80–500 K, accompanied by a completely reversible chromatic variation ranging from yellow to black. Importantly, the intriguing thermochromism is proved to own extremely high reproducibility (over 1000 cycles) without a hysteretic effect, originating from its structural flexibility that including (i) the noteworthy distortion/deformation of [NaCl6]5− and [FeCl6]3− octahedra; (ii) order–disorder arrangement transition of [NaCl6]5− and [FeCl6]3− octahedra as the function of temperature. This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs2NaFeCl6 perovskite.

Lattice Dynamics and Electron–Phonon Coupling in Double Perovskite Cs2NaFeCl6

“…For the first theoretical curve (see the orange curves), the quasi-harmonic (QH) approximation is applied, where the temperature-induced frequency shift is evaluated only by expanding the volume of the structure, following the measured thermal expansion. 22 Even though this approximation can reasonably describe the experimentally measured frequency shifts of the A 1 g , E g , and T 2 g _ Cs modes, it predicts a strong downward shift of the T 2 g mode (see the orange squares in Figure 3 b), which is not observed experimentally. For the second theoretical curve (see the green curves), we include, in addition to the volumetric thermal expansion, the effect of intrinsic anharmonicity, i.e., phonon–phonon interactions, using a self-consistent phonon (SCPH) methodology 37 − 39 (see also Methods and Supporting Information ).…”

“…We show two separate theoretical curves. For the first theoretical curve (see the orange curves), the quasi-harmonic (QH) approximation is applied, where the temperature-induced frequency shift is evaluated only by expanding the volume of the structure, following the measured thermal expansion . Even though this approximation can reasonably describe the experimentally measured frequency shifts of the A 1 g , E g , and T 2 g _ Cs modes, it predicts a strong downward shift of the T 2 g mode (see the orange squares in Figure b), which is not observed experimentally.…”

“…One of the promising lead-free double perovskite materials is the novel Cs 2 NaFeCl 6 compound. Most recent studies have shown that this material is thermally stable and exhibits exceptionally strong and completely reversible thermochromism within a wide temperature range that is caused by a strong temperature dependence of its bandgap energy. , Since its onset energy of optical absorption lies within the visible spectral range (at around 2.1 eV at room temperature), Cs 2 NaFeCl 6 could potentially be used in displaying devices, including smart windows, visual thermometers, and thermal sensors. Furthermore, the presence of transition-metal Fe 3+ ions with an unpaired electron spin suggests interesting magnetic properties attractive for spintronic applications.…”

Low Temperature Optical Properties of Novel Lead‐Free Cs2NaFeCl6 Perovskite Single Crystals

“…Recently, cesium sodium iron chloride (Cs 2 NaFeCl 6 ) single crystals were introduced as a new promising lead‐free double perovskite material for thermochromic applications. [ 12 ] It has been shown that this perovskite has a bandgap of 2.07 eV at room temperature (RT) and exhibits promising absorption in the visible region. [ 13 ] Furthermore, doping Cs 2 NaFeCl 6 with small amounts of silver (Ag) resulted in significant changes in structural and optoelectronic properties.…”

Halogen-Regulated Atomically Thin Two-Dimensional Organic–Inorganic Perovskites with Tunable Fluorescence Emission

“…Based on the above, five regulation modes were commonly made to accommodate the optical properties of organic–inorganic hybrid perovskites: temperature regulation, , chemical composition regulation, − ion-doping regulation, − structural dimension regulation, ,− and heterostructure regulation. − However, despite some progress that has been made in studying the regulation of the optical properties of organic–inorganic hybrid perovskite materials, the present regulation strategies of optical properties are mainly limited to changing the chemical composition and ion doping. As for structural dimension regulation, it still has great potential to investigate.…”