High-Entropy Perovskites as Multifunctional Metal Oxide Semiconductors: Synthesis and Characterization of (Gd_0.2Nd_0.2La_0.2Sm_0.2Y_0.2)CoO₃

Abstract

Single-phase multicomponent perovskite-type cobalt oxide containing five cations in equiatomic amounts on the A-site, namely, (Gd0.2Nd0.2La0.2Sm0.2Y0.2)­CoO3, has been synthesized via the modified coprecipitation hydrothermal method. Using an original approach for heat treatment, which comprises quenching utilizing liquid nitrogen as a cooling medium, a single-phase ceramic with high configuration entropy, crystallizing in an orthorhombic distorted structure was obtained. It reveals the anomalous temperature dependence of the lattice expansion with two weak transitions at approx. 80 and 240 K that are assigned to gradual crossover from the low- via intermediate- to high-spin state of Co3+. The compound exhibits weak ferromagnetism at T ≤ 10 K and signatures of antiferromagnetic correlations in the paramagnetic phase. Ab initio calculations predict a band gap Δ = 1.18 eV in the ground-state electronic structure with the dominant contribution of O_p and Co_d orbitals in the valence and conduction bands, respectively. Electronic transport measurements confirm the negative temperature coefficient of resistivity characteristic to a semiconducting material and reveal a sudden drop in activation energy at T ∼ 240 K from E a ∼ 1 eV in the low-temperature phase to E a ∼ 0.3 eV at room temperature. The possibility of fine tuning of the semiconducting band gap via a subtle change in A-site stoichiometry is discussed.

Stabilization of Lead-Reduced Metal Halide Perovskite Nanocrystals by High-Entropy Alloying

“…Traditional alloying usually refers to the addition of relatively small amounts of secondary elements to a primary element. Intriguingly, recent advance in high-entropy alloys (HEAs) suggests that the combination of multiple principal elements in high concentrations could increase the configurational entropy of mixing that overcomes the enthalpies of compound formation, thereby stabilizing the HEAs. − Very recently, A-site doping of lead halide perovskite (LHP) NCs has shown to improve the optical properties and chemical stability due to the entropy of mixing. , Back to the 2010s, high-entropy perovskite materials, including high-entropy perovskite oxides (HEPOs) and high-entropy perovskite fluorides (HEPFs) had emerged. − These high-entropy perovskite compounds have demonstrated outstanding catalytic properties, serving as efficient electrocatalysts in the oxygen evolution reaction. , The preparation of HEPOs, however, demands a process temperature of greater than 1000 °C. − , Clearly, it is desirable to develop less energy-consuming approaches for the synthesis of high-entropy perovskite materials.…”

Ionic liquid-hydroxide-mediated low-temperature synthesis of high-entropy perovskite oxide nanoparticles

“…13 Introducing the high-entropy concept to perovskite oxides can expand the selection range for material design and tailor the functional properties of materials, providing a new platform for broad applications such as catalysis, 14 energy storage, [15][16][17] electronics 18,19 and magnetics. 20,21 Recently, a wide range of approaches for the synthesis of HEPOs have been reported, including the solid phase sintering method, 22 nebulised spray pyrolysis method, 23 reactive spark plasma sintering method, 24 sol-gel method, 25 coprecipitation hydrothermal method 26 and pulsed laser deposition method. 27 However, the available methods have many disadvantages, for example, the solid phase sintering method requires an ultrahigh reaction temperature (1300-1500 °C) and a long reaction time (≥10 h) due to the difficulty of iondiffusion between solid interfaces, while composition segregation often occurs in nebulised spray pyrolysis, sol-gel, coprecipitation hydrothermal processes so that a high reaction temperature (≥1000 °C) and/or vacuum conditions are still needed to prevent the formation of secondary phases.…”

Structure and Electronic Effects from Mn and Nb Co-doping for Low Band Gap BaTiO3 Ferroelectrics

“…The Ti 4+ ion off-centers from its centrosymmetric position in the unit cell, resulting in large inherent polarization ( P = 24.1 μC/cm 2 ). In addition to such a high P value, BaTiO 3 exhibits a rich structural phase diagram, high chemical stability, and wide doping tunability of the perovskite structure ABO 3 which provides an ideal framework for tuning and exploring multifunctionalities. − However, the biggest challenge in realizing BaTiO 3 ferroelectrics as photovoltaics stems from its typically large bulk band gap (3.2 eV) which limits its access mostly to the UV range, as commonly observed , for all d 0 -ferroelectric perovskites (e.g., KNbO 3 and BaZrO 3 ). This is primarily due to the requirement of partial d-occupancy on the B-site cation to reduce the optical band gap that tends to remove the ferroelectric distortion, eventually stabilizing the prototypical high-symmetry phase. − Work on improving overall performance focuses mainly on narrowing the band gap through modification in compositions and the connection between polar order and photovoltaic effect .…”