Structural and Magnetic Properties of RMO3 (R = Pr,Nd and M = Fe,Co) Perovskites

Journal of Superconductivity and Novel Magnetism - In this work, the RMO3 (R?=?Pr, Nd and M?=?Fe, Co) perovskites had been synthesized by hydrothermal method. The structural...     

General Solvothermal Synthesis Method for Complete Solubility Range Bimetallic and High‐Entropy Alloy Nanocatalysts

Nanoalloys (NAs) have extraordinary catalytic properties, but metals are often immiscible giving compositional limits on catalytic design. It is generally believed that solution‐based chemical synthesis is inadequate for obtaining NAs, and often exotic shock synthesis or severe decomposition or reduction reactions are required. However, such methods only work on the laboratory scale making real‐world applications difficult. Here, a general solvothermal method is reported to obtain phase‐pure bimetallic and high‐entropy nano‐alloys across the entire composition range. Tuning of solvent chemistry and precursors leads to six different bimetallic NAs: Pd_xRu_1‐x, Pt_xRu_1‐x, Ir_xRu_1‐x, Rh_xRu_1‐x, Ir_1‐xPt_x, and Rh_1‐xPt_x, without immiscibility regions. All samples have face‐centered‐cubic crystal structures, which have not previously been observed for the ruthenium‐based systems. Additionally, quaternary and quinary systems are produced, demonstrating the ability to obtain medium‐ and high‐entropy NAs. The method described herein provides a simple, general production method of previously unknown solid solutions throughout their entire composition range potentially allowing for detailed tuning of nanocatalyst properties.     

Tracking Structural Phase Transitions in Lead‐Halide Perovskites by Means of Thermal Expansion

The extraordinary properties of lead‐halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX₃ (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high‐resolution capacitive dilatometer to determine the phase transition temperatures. For δ‐FAPbI₃, two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr₃ that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X‐ray diffraction. Moreover, it is demonstrated that transport under steady‐state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead‐halide perovskite materials.     

Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs₂AgBiBr₆ has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs₂AgBiBr₆ and alkali-metal-substituted (Cs_1−xY_x)₂AgBiBr₆ (Y: Rb⁺, K⁺, Na⁺; x = 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron–acoustic phonon scattering is found compared to Cs₂AgBiBr₆. The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.