Local A-Site Layering in Rare-Earth Orthochromite Perovskites by Solution Synthesis

In-depth study on the structures and properties of rare-earth-containing perovskite materials

Rare-earth-containing perovskite (RECP) materials have been extensively studied in various fields for their outstanding optical, electrical, magnetic and catalytic properties. In order to understand the clear relationship between structures and functions of RECP materials, the high-level and effective characterization technologies and analytic methods are absolutely necessary. Normally, diversiform measurement methods should be used simultaneously to analyze RECP materials clearly from different aspects, such as the phases, structures, morphologies, compositions, properties and performances. Therefore, this review will introduce the features and advantages of different analytic technologies and discuss their significances for the research on RECP materials. We hope that this review will provide valuable suggestions for researchers to promote the further research and development of RECP functional materials in the future.

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

Perovskite oxides with general composition ABO₃ are a large group of inorganic materials that can contain a variety of cations from all parts of the Periodic Table and that have diverse properties of application in fields ranging from electronics, energy storage to photocatalysis. Solvothermal synthesis routes to these materials have become increasingly investigated in the past decade as a means of direct crystallisation of the solids from solution. These methods have significant advantages leading to adjustment of crystal form from the nanoscale to the micron-scale, the isolation of compositions not possible using conventional solid-state synthesis and in addition may lead to scalable processes for producing materials at moderate temperatures. These aspects are reviewed, with examples taken from the past decade's literature on the solvothermal synthesis of perovskites with a systematic survey of B-site cations, from transition metals in Groups 4-8 and main group elements in Groups 13, 14 and 15, to solid solutions and heterostructures. As well as hydrothermal reactions, the use of various solvents and solution additives are discussed and some trends identified, along with prospects for developing control and predictability in the crystallisation of complex oxide materials.

Mild Hydrothermal Crystallization of Heavy Rare-Earth Chromite RECrO3 (RE = Er, Tm, Yb, Lu) Perovskites and Magnetic Properties

Crystallization of perovskite structure chromites (ACrO₃) in aqueous conditions is difficult owing to the amphoteric nature of the Cr³⁺ in the alkaline-mediated reaction conditions. This is especially true for the small metal cations at the A-site with large distortion angle of CrO₆ octahedral and small Goldschmidt tolerance factors. Here, we performed a progressive dehydration crystallization strategy to synthesize four RECrO₃ with the smallest radii of rare earth elements (Er, Tm, Yb, Lu) in mild hydrothermal conditions. Profile refinement of the high-resolution powder X-ray diffraction results indicated slightly longer unit cell parameters of  a and c in our samples with a higher distorted angle of CrO₆ octahedral units along ⟨010⟩ direction. All of the samples show rounded rectangle plate morphology with uniform distribution of particle sizes. These four RECrO₃ crystals can only form in a very narrow mineralization temperature range, i.e., 260-280 °C and 4.45-6.24 M of KOH. HRTEM results indicated that the normal crystallographic direction is ⟨001⟩, and the lattice of steps at the edge of elliptic rounded crystal is consistent with the bulk, which demonstrated single crystalline nature of the as-obtained crystals. Room-temperature Raman and FT-IR spectra reveal a continuous symmetry mode shift-dependent on the radii of A-site rare-earth cations. Temperature-dependent magnetization curves of RECrO₃ show typical antiferromagnetism to paramagnetism transition with Néel temperature of 93, 90, 86, and 83 K for ErCrO₃, TmCrO₃, YbCrO₃, and LuCrO₃, respectively. Samples of YbCrO₃ and LuCrO₃ show clear magnetization reversal and exchange bias phenomena below their Néel points. This paper indicates that the coupling of magnetic exchange in perovskite structure oxides could be tailorable in mild hydrothermal condition, towards the exploration of new magnetic and other physical properties.

Hydrothermal synthesis, morphology, structure, and magnetic properties of perovskite structure LaCr1−xMnxO3 (x = 0.1, 0.2, and 0.3)

We report the synthesis of LaCr_1−xMn_xO₃ (x = 0.1, 0.2, and 0.3) single crystal microcubes via a mild hydrothermal method.

Correlating the Influence of Two Magnetic Ions at the A-Site with the Electronic, Magnetic, and Catalytic Properties in Gd1-x Dy x CrO3

Considering the absence of reports dealing with the perovskite-structured orthochromites containing two A-site magnetic rare-earth ions, GdCrO₃ and progressively Dy³⁺-substituted samples of the series Gd_1-x Dy _x CrO₃ have been synthesized employing the epoxide-mediated sol-gel procedure. The samples were characterized extensively using high-resolution powder X-ray diffraction, thermal analysis, Fourier transform infrared, Raman, and UV-visible spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. Monophasic samples possessing an orthorhombic perovskite structure emerged by calcining the xerogels formed by the reaction of rare-earth nitrates, chromium(III)chloride, and propylene oxide at 800 °C for 2 h. Uniform presence of wormlike morphology was observed in both the field emission SEM (FE-SEM) and TEM images of the samples. Zero-field and field-cooled magnetic measurements using a SQUID magnetometer down to 4 K showed that the Neel temperature of Gd_0.5Dy_0.5CrO₃ was 155 K, more or less midway between the values observed for GdCrO₃(169 K) and DyCrO₃ (146 K). For the Gd_0.5Dy_0.5CrO₃ sample, a spin reorientation was observed at ∼38 K when measured under an applied field. Because the optical band gap, determined by Kubelka-Munk function, of these chromites was around 3 eV, their application as a catalyst for the photodegradation of the aqueous rhodamine-6G dye solution was demonstrated, in which the percentage of the total dye that was degraded varied with the average ionic radius of A-site ions. A similar systematic trend was observed even for the catalytic oxidation of the XO dye in the presence of H₂O₂, with DyCrO₃ influencing the reaction to a greater extent followed by Gd_0.5Dy_0.5CrO₃ and GdCrO₃. Both the photocatalytic and catalytic reactions followed pseudo-first-order kinetics.

Local A-Site Layering in Rare-Earth Orthochromite Perovskites by Solution Synthesis

Cation size effects were examined in the mixed A‐site perovskites La_0.5Sm_0.5CrO₃ and La_0.5Tb_0.5CrO₃ prepared through both hydrothermal and solid‐state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation‐radius variance in La_0.5Tb_0.5CrO₃ results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid‐state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La³⁺ and Tb³⁺. The A‐site layering in hydrothermal La_0.5Tb_0.5CrO₃ is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route.