Angew. Chem. Int. Ed., 2021, 60(21): 11592-11603, https://doi.org/10.1002/anie.202011833

Lead-free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead-based counterparts. Among them, double perovskites with Cs₂AgInCl₆ composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self-trapped exciton (STEs) emission and dopant-induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono- and trivalent cations other than Ag⁺ and In³⁺. In this Minireview we describe the recent developments on Cs₂AgInCl₆ bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character.

Adv. Opt. Mater., 2021, 9, 14, 2101333, https://doi.org/10.1002/adom.202101333

Organic–inorganic metal halides (OIMHs) with unique structural flexibility possess excellent photoelectric properties. They are regarded as next-generation photovoltaic materials, phosphors, semiconductors, and ferroelectrics. The metal-halide units in OIMHs are good microscopic building blocks of nonlinear optical crystals for laser wavelength conversion. However, most OIMHs are absent from nonlinear optics owing to their macroscopic nonlinear optical (NLO)-inactive centrosymmetric crystal structure. In this study, two new lead-free OIMHs, (TMEDA)SbI₅ and (TMEDA)BiI₅ (where TMEDA²⁺ is N,N,N′-trimethylethylenediammonium), having 1D structure, crystallized in the orthorhombic system with a non-centrosymmetric P2₁2₁2₁ space group, are synthesized. Remarkably, upon 2090 nm laser irradiation, both compounds possess a strong infrared (IR) nonlinear optical response of the same magnitude as AgGaS₂, which is a benchmark semiconductor-type nonlinear optical crystal. In addition, under the excitation of ultraviolet and visible lights, both compounds produce self-trapped exciton-induced red-light emission. First-principles electronic structure calculations reveal that the optical properties originate from the electronic transitions within the inorganic metal-halide group. The obtained results indicate that both compounds are potential photoelectric materials for laser frequency conversion and fluorescence, and the observation of NLO effect in these two compounds verifies that OIMHs are also good candidates for NLO crystals.

Chem. Mater., 2021, 33, 20, 8106–8111, https://doi.org/10.1021/acs.chemmater.1c02896

Recent discoveries in organic−inorganic metal halides reveal superior semiconducting and polarization properties. Herein, we report three organic–inorganic metal halides, (PBA)₄BiBr₇·H₂O, (PBA)₄BiI₇·H₂O, and (PBA)₄InBr₇·H₂O [(PBA)⁺ = C₆H₅(CH₂)₄NH₃⁺], with band gaps of ∼3.52, ∼2.29, and ∼4.05 eV, respectively. They possess zero-dimensional structures containing the inorganic octahedra [MX₆]^3– (M = Bi, In, X = Br, I) and unbound X^– ions and crystallize in the C2 space group. (PBA)₄BiI₇·H₂O shows a second-harmonic-generation (SHG) response in the infrared region, approximately 1.3 times that of AgGaS₂; (PBA)₄BiBr₇·H₂O and (PBA)₄InBr₇·H₂O show SHG responses in the ultraviolet region, approximately 0.4 and 0.6 times that of KH₂PO₄, respectively. The large SHG responses are attributed to the synergistic contribution of the octahedral distortion of [MX₆]^3– (M = Bi, In, X = Br, I) and the ordered arrangement of the benzene ring-containing organic cation PBA⁺. Upon ultraviolet and visible-light excitations at room temperature, (PBA)₄BiBr₇·H₂O, (PBA)₄BiI₇·H₂O, and (PBA)₄InBr₇·H₂O exhibit broad red-light luminescence with large Stokes shifts of 290, 237, and 360 nm, respectively, due to self-trapped exciton emission. All of these properties demonstrate that this series of metal halides are potential multifunctional optoelectronic materials.

Adv. Funct. Mater., 2021: 2110771. https://doi.org/10.1002/adfm.202110771

Stimulus-responsive photoluminescent materials have attracted extensive research attention in recent years owing to their potential application in information storage and switch devices. It is important to further explore such bistable materials as well as the underlying transformation mechanisms. Herein, the syntheses and mechanically tunable “on–off” photoluminescence (PL) of two organic–inorganic hybrid metal halides, (Bmpip)₉Pb₃Zn₂Br₁₉ and (Bmpip)₉Pb₃Cd₂Br₁₉ (Bmpip⁺ = 1-butyl-1-methyl-piperidinium, C₁₀H₂₂N⁺), are reported. Both as-obtained compounds are nonemissive under UV light at ambient conditions but exhibit bright PL upon grinding or under hydrostatic pressure. Interestingly, the PL is quenchable by short-time annealing or storage in air for 1 week, and the process is repeatable. Through a combination of extensive structural and spectral analyses, the crucial role of the organic cations interacting with inorganic chromophores in the “on–off” PL behavior of the title compounds is revealed. Moreover, pressure-induced PL and PL-enhancement phenomena are observed in both compounds, which are similar to but slightly different than the above-mentioned mechano-PL. Finally, proof-of-concept devices are fabricated to demonstrate the potential applications of the title compounds in message recording and force sensing.

Inorg. Chem., 2021, 60, 15, 11429–11434. https://doi.org/10.1021/acs.inorgchem.1c01440

Low-dimensional organic–inorganic metal halides (OIMHs), as emerging light-emitting materials, have aroused widespread attention owing to their unique structural tunability and photoelectric characteristics. OIMHs are also promising materials for optoelectronic equipment, light-emitting diodes, and photodetectors. In this study, (C₃H₁₂N₂)₂Sb₂Cl₁₀ (C₃H₁₂N₂²⁺ is an N-methylethylenediamine cation), a new zero-dimensional OIMH, has been reported, and (C₃H₁₂N₂)₂Sb₂Cl₁₀ possesses a P2₁/n space group. The (C₃H₁₂N₂)₂Sb₂Cl₁₀ structure contains [Sb₂Cl₁₀]^4– dimers (composed of two edge-sharing [SbCl₆]^3– octahedra) that are surrounded by C₃H₁₂N₂²⁺ cations. The experimental band gap of (C₃H₁₂N₂)₂Sb₂Cl₁₀ is 3.80 eV, and density functional theory calculation demonstrates that (C₃H₁₂N₂)₂Sb₂Cl₁₀ possesses a direct band gap, with the edge of the band gap mainly contributed from the inorganic units. (C₃H₁₂N₂)₂Sb₂Cl₁₀ exhibits good ambient and thermal stability. Under 395 nm excitation at room temperature, (C₃H₁₂N₂)₂Sb₂Cl₁₀ exhibits a broad emission with a full width at half-maximum of ∼114 nm, peaking at 480 nm, and the broad emission was ascribed to self-trapped exciton emission.

Inorg. Chem., 2021, 60, 1, 325-333. https://doi.org/10.1021/acs.inorgchem.0c02966

Transition-metal-based chalcogenides are a series of intriguing semiconductors with applications spanning various fields because of their rich structure and numerous functionalities. This paper reports the crystal structure and basic physical properties of a new quaternary chalcogenide In₄Pb_5.5Sb₅S₁₉. The crystal structure of In₄Pb_5.5Sb₅S₁₉ was determined by both powder and single-crystal X-ray diffraction techniques. In₄Pb_5.5Sb₅S₁₉ crystallizes in the monoclinic system with I2/m space group, and the structure parameters are a = 26.483 Å, b = 3.899 Å, c = 32.696 Å, and β = 111.86°. The polyhedral double chains of Sb³⁺ and Sb/Pb²⁺ as the main cations are parallel to each other and form a Jamesonite-like mineral structure through the short chain links of the distorted In, Pb, and Sb polyhedron. In₄Pb_5.5Sb₅S₁₉ exhibits a moderate experimental band gap of 1.42 eV, indicating its potential for application in solar cells and photocatalysis. In addition, In₄Pb_5.5Sb₅S₁₉ exhibits good ambient stability, and differential scanning calorimetry tests demonstrate that it is stable up to 892 K in a nitrogen atmosphere. Moreover, In₄Pb_5.5Sb₅S₁₉ exhibits extremely low thermal conductivity (0.438–0.478 W m^–1 K^–1 ranging from 300 to 700 K) compared with binary counterparts such as PbS and In₂S₃. Future chemical manipulation via elemental doping or defect engineering may make the title compound a potential thermoelectric or thermal insulating material.

Mater. Chem. Front., 2021, 5(3): 1283-1294. https://doi.org/10.1039/D0QM00662A

Themoelectric materials exhibit great potential in alleviating the energy shortage and environmental pollution. The development of homologous series is helpful for understanding the relationship between structure and properties, thereby providing new strategies for seeking high-performance thermoelectric materials. Among the various structure prototypes, pavonite is a rising star and has received increasing attention as a potential n-type thermoelectric material owing to their diverse structures and extremely low thermal conductivity. In this review, we summarized the structural characteristics of pavonite and introduced the relationship between structure and thermoelectric performance. The pavonite structure consists of two alternating slabs with separately tunable thicknesses, and has wide adaptability for elemental substitution. Specifically, the participation of heavy atoms in the pavonite structure results in large unit cell volume and Grüneisen parameters, and thus extremely low lattice thermal conductivity. Finally, we briefly discussed the potential of pavonite compounds in thermoelectric applications.

Int. J. Miner. Metall. Mater., 29(2022), No. 6, pp. 1286-1294. http://ijmmm.ustb.edu.cn/article/doi/10.1007/s12613-021-2363-6

Cr3+-activated far-red and near-infrared phosphors have drawn considerable attention owing to their adjustable emission wavelengths and wide applications. Herein, we reported a series of Cr3+-doped phosphors with β-Ca3(PO4)2-type structure, of which Ca9Ga(PO4)7:Cr3+ possessed the highest far-red emission intensity. At an excitation of 440 nm, the Ca9Ga(PO4)7:Cr3+ phosphors exhibited a broad emission band ranging from 650 to 850 nm and peaking at 735 nm, and the broadband superimposed two sharp lines centering at 690 and 698 nm. The optimal sample Ca9Ga0.97(PO4)7:0.03Cr3+ had an internal quantum efficiency of 55.7%. The luminescence intensity of the Ca9Ga0.97(PO4)7:0.03Cr3+ phosphor obtained at 423 K could maintain 68.5% of that at room temperature, demonstrating its outstanding luminescence thermal stability. A phosphor-conversion light-emitting diode was fabricated, indicating that the Ca9Ga(PO4)7:Cr3+ phosphor has potential applications in indoor plant cultivation.

Sci. China Mater. 65, 748–756 (2022).https://doi.org/10.1007/s40843-021-1785-6

For healthy lighting, daily lighting that considers both visible light and near-infrared (NIR) light is necessary. However, at ∼900 nm, the extensively used solar-like phosphor-converted light-emitting diodes (pc-LEDs) are limited by a lack of high-performance NIR luminescent materials. We report a broadband NIR phosphor Sr₂ScSbO₆:Cr³⁺ with a double perovskite-type structure, thus simultaneously demonstrating high luminescence efficiency and good thermal stability. Under 550-nm excitation, Sr₂ScSbO₆:Cr³⁺ demonstrates broadband NIR emission centered at ∼890nm with luminescence internal/external efficiencies of 82.0%/35.7%, respectively. Furthermore, the luminescence integrated intensity at 430 K remains at ∼66.4% of the initial intensity. We successfully fabricated pc-LED devices using a 465-nm-sized blue chip and other commercial phosphors, presenting a relatively complete solar-like spectrum from blue to NIR light and is expected to be used in solar-like lighting.

Chem. Mater. 2021, 33, 21, 8360–8366 https://pubs.acs.org/doi/10.1021/acs.chemmater.1c02734

Energy transfer (ET) between optically active ions usually leads to luminescent concentration quenching and thermal quenching. Toward luminescence enhancement, it is very challenging to control the ET path. Herein, we demonstrated a strategy for selectively controlling ET pathway through the structural confinement effect for activated ions. In the Yb3+-doped Sr9Cr(PO4)7 (SCP) compound, Cr3+ ions are well separated from each other (≥8.97 Å), but they are close to Yb3+ ions (3.70–5.29 Å) due to structural confinement. Therefore, ET is depressed between Cr3+ ions but induced from Cr3+ to Yb3+ ions. On increasing Yb3+ concentration, the thermal stability of near-infrared emission is significantly improved. The emission intensity of the SCP:0.15Yb3+ phosphor at 375 K can keep 100% of that at 80 K. Finally, we show the potential applications of SCP:Yb3+ phosphor in food analysis and nondestructive examination fields. This study provides a new strategy for enhancing luminescence.